GPT related publications: The following list contains all publications directly or indirectly related to the GPT project in reverse chronological order:

[2016] [2015] [2014] [2013] [2012] [2011] [2010] [2009]  [2008]  [2007]  [2006]  [2005]  [2004]  [2003]  [2002]  [2001]  [2000]  [1999]  [1998]  [1997]  [1996]

Correction of the deterministic part of space–charge interaction in momentum microscopy of charged particles
Ultramicroscopy 159 · May (2015)
DOI: 10.1016/j.ultramic.2015.05.015

G. Schönhense, K. Medjanik, C. Tusche, M. de Loos, B. van der Geer, M. Scholz, F. Hieke, N. Gerken, J. Kirschner, W. Wurth

Ultrahigh spectral brightness femtosecond XUV and X-ray sources like free electron lasers (FEL) andtable-top high harmonics sources (HHG) offer fascinating experimental possibilities for analysis oftransient states and ultrafast electron dynamics. For electron spectroscopy experiments using illumi-nation from such sources, the ultrashort high-charge electron bunches experience strong space–chargeinteractions. The Coulomb interactions between emitted electrons results in large energy shifts andsevere broadening of photoemission signals. We propose a method for a substantial reduction of theeffect by exploiting the deterministic nature of space–charge interaction. The interaction of a givenelectron with the average charge density of all surrounding electrons leads to a rotation of the electrondistribution in6Dphase space. Momentum microscopy gives direct access to the three momentumcoordinates, opening a path for a correction of an essential part of space–charge interaction. In afirstexperiment with a time-of-flight momentum microscope using synchrotron radiation at BESSY, the ro-tation in phase space became directly visible. In a separate experiment conducted at FLASH (DESY), theenergy shift and broadening of the photoemission signals were quantified. Finally, simulations of arealistic photoemission experiment including space–charge interaction reveals that a gain of an order ofmagnitude in resolution is possible using the correction technique presented here Correction of the deterministic part of space–charge interaction in momentum microscopy of charged particles.

Concept of a laser-plasma based electron source for sub-10 fs electron diffraction
Applied Physics Letters 104, 264102 (2014)
DOI: 10.1103/PhysRevAccelBeams.19.021302

J. Faure, B. van der Geer, B. Beaurepaire, G. Gallé, A. Vernier, and A. Lifschitz

We propose a new concept of an electron source for ultrafast electron diffraction with sub-10-fs temporalresolution. Electrons are generated in a laser-plasma accelerator, able to deliver femtosecond electronbunches at 5 MeV energy with a kilohertz repetition rate. The possibility of producing this electron sourceis demonstrated using particle-in-cell simulations. We then use particle-tracking simulations to show thatthis electron beam can be transported and manipulated in a realistic beam line, in order to reach parameterssuitable for electron diffraction. The beam line consists of realistic static magnetic optics and introduces notemporal jitter. We demonstrate numerically that electron bunches with 5-fs duration and containing 1.5 fCper bunch can be produced, with a transverse coherence length exceeding 2 nm, as required for electron diffraction.

Laser-Free RF-Gun as a Combined Source of Thz and Ps-Sub-Ps X-Rays
23rd Conference on Application of Accelerators in Research and Industry, CAARI 2014
DOI: 10.1016/j.phpro.2015.05.021

R. Agustsson, S. Boucher, O. Finn, J. Hartzell, M. Ruelas, A. V. Smirnov, S. Storms, Z. Ning, A. Murokh, T. Campese, L. Faillace, A. Verma, Y. Kimb, P. Buaphad, A. Andrews, B. Berls, C. Eckman, K. Folkman, A. Knowles-Swingle, C. O'Neill, M. Smith , T. Grandsaert, B. van der Geer, M. de Loos, W. J. Berg, N. S. Sereno, Y. Sun, and A. A. Zholents

A coherent, mm-sub-mm-wave source driven by a RF electron gun is proposed for wide research applications as well as auxiliary inspection and screening, safe imaging, cancer diagnostics, surface defectoscopy, and enhanced time-domain spectroscopy. It allows generation of high peak and average THz-sub-THz radiation power provided by beam pre-bunching and chirping in the RF gun followed by microbunching in magnetic compressor, and resonant Cherenkov radiation of an essentially flat beam in a robust, ~inch-long, planar, mm-sub-mm gap structure. The proof-of-principle has been successfully demonstrated in Phase I on a 5 MeV beam of L-band thermionic injector of Idaho Accelerator Center. The system can also deliver an intense, ps-sub-ps bursts of low-to-moderate dose of relativistic electrons and X-ray radiation produced by the same beam required for pulsed radiolysis as well as to enhance screening efficiency, throughput and safety.

An ultrashort pulse ultra-violet radiation undulator source driven by a laser plasma wakefield accelerator
Applied Physics Letters 104, 264102 (2014)
DOI: 10.1063/1.4886997

M. P. Anania, E. Brunetti, S. M. Wiggins, D. W. Grant, G. H. Welsh, R. C. Issac, S. Cipiccia, R. P. Shanks, G. G. Manahan, C. Aniculaesei, S. B. van der Geer, M. J. de Loos, M. W. Poole, B. J. A. Shepherd, J. A. Clarke, W. A. Gillespie, A. M. MacLeod and D. A. Jaroszynski

Narrow band undulator radiation tuneable over the wavelength range of 150–260 nm has been produced by short electron bunches from a 2 mm long laser plasma wakefield accelerator based on a 20 TW femtosecond laser system. The number of photons measured is up to 9 × 106 per shot for a 100 period undulator, with a mean peak brilliance of 1 × 1018 photons/s/mrad2/mm2/0.1% bandwidth. Simulations estimate that the driving electron bunch r.m.s. duration is as short as 3 fs when the electron beam has energy of 120–130 MeV with the radiation pulse duration in the range of 50–100 fs.

Space-charge effects in ultrafast electron diffraction patterns from single crystals
Ultramicroscopy 116 (2012) p. 86–94
DOI: 10.1016/j.ultramic.2012.03.001

Robert P.Chatelain, Vance Morrison, Chris Godbout, Bas van der Geer, Marieke de Loos, Bradley J.Siwick

The impact of electron–electron interactions in the post-specimen region of ultrafast electron diffraction and dynamic transmission electronmicroscopy instruments has been studied. Specifically, space-charge induced distortions of ultrafast electron diffraction patterns from single crystal specimens and their dependence on electron bunch-charge, beamenergy, energyspread, focusing conditions and speciment hickness have been investigated using the General Particle Tracer code. We have found that these space-charge interactions lead to significant broadening and displacement of the Bragg spots at currently realizable electron beam illumination conditions. These impacts increase in severity with beam brightness and are reduced with increasing (relativistic) beam energies. The primary mechanism for the distortions has been determined to bespace-charge interactions between the scattered beamlets and the main unscattered beam. Overall, these results suggest that creative post-specimen electron optical design, relativistic beam energies and post-processing of diffraction patterns to correct for space-charge distortions hould be explored as routes to make good use of any futuree nhancements to beam brightness in UED and DTEM instruments.

Design and implementation of a flexible beamline for fs electron diffraction experimentss
NIM-A: Volume 691, 1 November 2012, Pages 113–122
DOI: 10.1016/j.nima.2012.06.057

Giulia Fulvia Mancini, Barbara Mansart, Saverio Pagano, Bas van der Geer, Marieke de Loos, Fabrizio Carbone

Ultrafast Electron Diffraction (UED) has been widely used to investigate the structural dynamics of molecules and materials. Femtosecond (fs) electron bunches are used to obtain diffraction images of a specimen upon photo-excitation by a temporally delayed light pulse. The high cross-section of electrons makes it a very flexible tool for the study of light elements, monolayers and surfaces; at the same time, electrons can travel down to few nanometers (nm) and structural information from the bulk can also be retrieved. In this article, we discuss the design and implementation of a flexible beamline for fs electron diffraction experiments in transmission or reflection geometry. By the use of a radiofrequency (RF) compression cavity synchronized to our laser system, in combination with a set of electron optics, we demonstrate that we can control the beam properties in terms of charge per pulse, transverse spot-size on the sample and temporal duration of the bunches. The characterization of the beam is performed via a light-electrons cross-correlation experiment and we demonstrate an overall temporal resolution around 300 fs for bunches containing up to 10^5 electrons at a repetition rate of 20 kHz.

Optimization of the current extracted from an ultracold ion source
New Journal of Physics 14 (2012) 083011
DOI: 10.1088/1367-2630/14/8/083011

N Debernardi, R W L van Vliembergen, W J Engelen, K H M Hermans, M P Reijnders, S B van der Geer, P H A Mutsaers, O J Luiten and E J D Vredenbregt

Photoionization of trapped atoms is a recent technique for creating ion beams with low transverse temperature. The temporal behavior of the current that can be extracted from such an ultracold ion source is measured when operating in the pulsed mode. A number of experimental parameters are varied to find the conditions under which the time-averaged current is maximized. A dynamic model of the source is developed that agrees quite well with the experimental observations. The radiation pressure exerted by the excitation laser beam is found to substantially increase the extracted current. For a source volume with a typical root-mean-square radius of 20 μm, a maximum peak current of 88 pA is observed, limited by the available ionization laser power of 46 mW. The optimum time-averaged current is 13 pA at a 36% duty cycle. Particle-tracking simulations show that stochastic heating strongly reduces the brightness of the ion beam at higher current for the experimental conditions.

Microwave TM(010) cavities as versatile 4D electron optical elements
Ultramicroscopy, Volume 127, April (2013), Pages 19–24
DOI: 10.1016/j.ultramic.2012.07.011

P.L.E.M. Pasmans, G.B. van den Ham, S.F.P. Dal Conte, S.B. van der Geer, O.J. Luiten

The realization of high quality ultrashort pulsed beams requires ultrafast time-dependent electron optics. We present derivations of closed expressions both for the longitudinal and transverse focusing powers of resonant microwave TM(010) cavities. The derived expressions are validated by particle tracking simulations using realistic cavity fields. For small field amplitudes, in which case the "weak lens" approximation holds, the focusing powers obtained from simulations are in good agreement with the derived expressions. Furthermore, the required phase and temperature stability for synchronization of electron bunches generated by femtosecond photoemission are discussed.

Experimental validation of a radio frequency photogun as external electron
injector for a laser wakefield accelerator

Journal of Applied Physics 110, 024910 (2011)
DOI: 10.1063/1.3610509

X. F. D. Stragier, O. J. Luiten, S. B. van der Geer, M. J. van der Wiel, and G. J. H. Brussaardt

A purpose-built RF-photogun as external electron injector for a laser wakefield accelerator has been thoroughly tested. Different properties of the RF-photogun have been measured such as energy, energy spread and transverse emittance. The focus of this study is the investigation of the smallest possible focus spot and focus stability at the entrance of the plasma channel. For an electron bunch with 10 pC charge and 3.7 MeV kinetic energy, the energy spread was 0.5% with a shot-to-shot stability of 0.05%. After focusing the bunch by a pulsed solenoid lens at 140 mm from the middle of the lens, the focal spot was 40 micrometer with a shot-to-shot stability of 5 micrometer. Higher charge leads to higher energy spread and to a larger spot size, due to space charge effects. All properties were found to be close to design values. Given the limited energy of 3.7 MeV, the properties are sufficient for this gun to serve as injector for one particular version of laser wakefield acceleration, i.e., injection ahead of the laser pulse. These measured electron bunch properties were then used as input parameters for simulations of electron bunch injection in a laser wakefield accelerator. The arrival time jitter was deduced from measurements of the energy fluctuation, in combination with earlier measurements using THz coherent transition radiation, and is around 150 fs in the present setup. The bunch length in the focus, simulated using particle tracking, depends on the accelerated charge and goes from 100 fs at 0.1 pC to 1 ps at 50 pC. When simulating the injection of the 3.7 MeV electron bunch of 10 pC in front of a 25 TW laser pulse with a waist of 30 micrometer in a plasma with a density of 0.7*10^24/m3, the maximum accelerated charge was found to be 1.2 pC with a kinetic energy of ~900 MeV and an energy spread of ~5%. The experiments combined with the simulations show the feasibility of external injection and give a prediction of the output parameters that can be expected from a laser wakefield accelerator with external injection of electrons.

Measurement of the temperature of an ultracold ion source using time-dependent electric fields
Journal of Applied Physics 110, 024501 (2011)
DOI: 10.1063/1.3605555

N. Debernardi, M. P. Reijnders, W. J. Engelen, T. T. J. Clevis, P. H. A. Mutsaers, O. J. Luiten,
and E. J. D. Vredenbregt

We report on a measurement of the characteristic temperature of an ultracold rubidium ion source, in which a cloud of laser-cooled atoms is converted to ions by photo-ionization. Extracted ion pulses are focused on a detector with a pulsed-field technique. The resulting experimental spot
sizes are compared to particle-tracking simulations, from which an effective source temperature T=(3+/-2) mK and the corresponding transversal reduced emittance er=1.4*108 m rad sqrt(eV) are determined. Space charge effects that may affect the measurement are also discussed.

An RF photogun for external injection of electrons in a laser wakefield accelerator
Proc. SPIE 8079, 80790W (2011)
DOI: 10.1117/12.891404

Xavier F. D. Stragier, Bas van der Geer, Marnix J. van der Wiel, Jom Luiten and Seth Brussaard

We have developed a 2.5 cell, 3 GHz RF accelerator specifically to inject electrons in a laser wakefield accelerator (LWA). The electron bunches are accelerated to around 3.5 MeV and focused at 1.14 m from the cathode of the accelerator using a pulsed solenoid. Bunches between 0 and 33 pC were focused onto a phosphor screen at the position of the entrance of a plasma channel. The (RMS) bunch size was 32 μm at 1 pC and increases to 61 μm at 33 pC. The energy of the bunches at the chosen settings was measured to be 3.71 MeV with 0.02 MeV energy spread (at 10 pC). Energy fluctuations were less than 2 keV. The pointing stability of the focused electron bunches was determined from 100 consecutive shots at 1 Hz to be 5 μm (RMS). GPT (General Particle Tracer) simulations have been performed using the measured bunches as input for LWA. The simulations show that up to 1 pC of charge can be accelerated to energies of around 1 GeV using realistic plasma and laser parameters. The measured bunch parameters in combination with the simulations show how external injection of pre-accelerated electrons can be a viable alternative to other injection mechanisms.

Time-dependent manipulation of ultracold ion bunches
Journal of Applied Physics 109, 033302 (2011).
DOI: 10.1063/1.3544009

M. P. Reijnders, N. Debernardi, S. B. van der Geer, P. H. A. Mutsaers, E. J. D. Vredenbregt, and O. J. Luiten

The combination of an ultracold ion source based on photoionization of a laser-cooled gas and time-dependent acceleration fields enables precise manipulation of ion beams. We demonstrate reduction in the longitudinal energy spread and transverse (de)focusing of the beam by applying time-dependent acceleration voltages. In addition, we show how time-dependent acceleration fields can be used to control both the sign and strength of the spherical aberrations. The experimental results are in close agreement with detailed charged particle tracking simulations and can be explained in terms of a simple analytical model.


Compression of subrelativistic space-charge-dominated electron bunches for single-shot femtosecond electron diffractionon
Physical Review Letters, 105 (26), art. no. 264801 (2010).
DOI: 10.1103/PhysRevLett.105.264801

Van Oudheusden, T., Pasmans, P.L.E.M., Van Der Geer, S.B., De Loos, M.J., Van Der Wiel, M.J., Luiten, O.J.

We demonstrate the compression of 95 keV, space-charge-dominated electron bunches to sub-100 fs durations. These bunches have sufficient charge (200 fC) and are of sufficient quality to capture a diffraction pattern with a single shot, which we demonstrate by a diffraction experiment on a polycrystalline gold foil. Compression is realized by means of velocity bunching by inverting the positive spacecharge- induced velocity chirp. This inversion is induced by the oscillatory longitudinal electric field of a 3 GHz radio-frequency cavity. The arrival time jitter is measured to be 80 fs.

High quality electron beams from a laser wakefield accelerator
Plasma Physics and Controlled Fusion, 52 (12), art. no. 124032 (2010).
DOI: 10.1088/0741-3335/52/12/124032

Wiggins, S.M, Issac, R.C, Welsh, G.H, Brunetti, E, Shanks, R.P, Anania, M.P, Cipiccia, S, Manahan, G.G, Aniculaesei, C, Ersfeld, B, Islam, M.R, Burgess, R.T.L, Vieux, G, Gillespie, W.A, MacLeod, A.M, Van Der Geer, S.B, De Loos, M.J, Jaroszynski, D.A

High quality electron beams have been produced in a laser-plasma accelerator driven by femtosecond laser pulses with a peak power of 26 TW. Electrons are produced with an energy up to 150 MeV from the 2 mm gas jet accelerator and the measured rms relative energy spread is less than 1%. Shot-to-shot stability in the central energy is 3%. Pepper-pot measurements have shown that the normalized transverse emittance is ~1π mm mrad while the beam charge is in the range 2–10 pC. The generation of high quality electron beams is understood from simulations accounting for beam loading of the wakefield accelerating structure. Experiments and self-consistent simulations indicate that the beam peak current is several kiloamperes. Efficient transportation of the beam through an undulator is simulated and progress is being made towards the realization of a compact, high peak brilliance free-electron laser operating in the vacuum ultraviolet and soft x-ray wavelength ranges.

Ultracold electron source for single-shot diffraction studies
Europhysics Letters, 91 ( 4 ) , art. no. 46004 (2010).
DOI: 10.1209/0295-5075/91/46004

Taban, G., Reijnders, M.P., Fleskens, B., Van Der Geer, S.B., Luiten, O.J., Vredenbregt, E.J.D.

Ultracold electron sources, which are based on near-threshold photo- and fieldionization of a cloud of laser-cooled atoms, offer the unique combination of low emittance and extended size that is essential for achieving single-shot, ultrafast electron diffraction of macromolecules. Here we present measurements of the effective temperature of such a pulsed electron source employing rubidium atoms that are magneto-optically trapped at the center of an accelerator structure. Transverse source temperatures ranging from 200K down to 10K are demonstrated, controllable with the wavelength of the ionization laser. Together with the 50 μm source size, the achievable temperature enables a transverse coherence length of ≈ 20nm for a 100 μm sample size.

Phase-space manipulation of ultracold ion bunches with time-dependent fields
Physical Review Letters, 105 (3), art. no. 034802 (2010).
DOI: 10.1103/PhysRevLett.105.034802

Reijnders, M.P., Debernardi, N., Van Der Geer, S.B., Mutsaers, P.H.A., Vredenbregt, E.J.D., Luiten, O.J.

All applications of high brightness ion beams depend on the possibility to precisely manipulate the trajectories of the ions or, more generally, to control their phase-space distribution. We show that the combination of a laser-cooled ion source and time-dependent acceleration fields gives new possibilities to perform precise phase-space control. We demonstrate reduction of the longitudinal energy spread and realization of a lens with control over its focal length and sign, as well as the sign of the spherical aberrations. This creates new possibilities to correct for the spherical and chromatic aberrations which are presently limiting the spatial resolution.

Longitudinal phase space characterization of the blow-out regime of rf photoinjector operation
Phys. Rev. ST Accel. Beams 12, 070704 (2009).
DOI: 10.1103/PhysRevSTAB.12.070704

J. T. Moody, P. Musumeci, M. S. Gutierrez, J. B. Rosenzweig, and C. M. Scoby

Using an experimental scheme based on a vertically deflecting rf deflector and a horizontally dispersing dipole, we characterize the longitudinal phase space of the beam in the blow-out regime at the UCLA Pegasus rf photoinjector. Because of the achievement of unprecedented resolution both in time (50 fs) and energy (1.0 keV), we are able to demonstrate some important properties of the beams created in this regime such as extremely low longitudinal emittance, large temporal energy chirp, and the degrading effects of the cathode image charge in the longitudinal phase space which eventually leads to poorer beam quality. All of these results have been found in good agreement with simulations.

Single-shot ultrafast electron diffraction with a laser-accelerated sub-MeV electron pulse
Appl. Phys. Lett. 95, 111911 (2009).
DOI: 10.1063/1.3226674

Shigeki Tokita, Shunsuke Inoue, Shinichiro Masuno, Masaki Hashida, and Shuji Sakabe

We have demonstrated single-shot measurements of electron diffraction patterns for a single-crystal gold foil using 340 keV electron pulses accelerated by intense femtosecond laser pulses with an intensity of 2 1018 W/cm2. The measured electron beam profile is faithfully reproduced by the numerical simulation of the electron trajectory, providing evidence that the electron pulse spontaneously expands in time owing to the velocity spread  produced in the acceleration process, but is not distorted in an irreversible nonlinear manner. This study shows that the laser acceleration is promising for the development of pulse compression methods for single-shot femtosecond electron diffraction.

Ultracold Electron Source for Single-Shot, Ultrafast Electron Diffraction
Microscopy and Microanalysis 15, p. 282-289 (2009).
DOI: 10.1017/S143192760909076X

S.B. van der Geer, M.J. de Loos, E.J.D. Vredenbregt, and O.J. Luiten

Ultrafast electron diffraction (UED) enables studies of structural dynamics at atomic length and timescales, i.e., 0.1 nm and 0.1 ps, in single-shot mode. At present UED experiments are based on femtosecond laser photoemission from solid state cathodes. These photoemission sources perform excellently, but are not sufficiently bright for single-shot studies of, for example, biomolecular samples. We propose a new type of electron source, based on near-threshold photoionization of a laser-cooled and trapped atomic gas. The electron temperature of these sources can be as low as 10 K, implying an increase in brightness by orders of magnitude. We investigate a setup consisting of an ultracold electron source and standard radio-frequency acceleration techniques by GPT tracking simulations. The simulations use realistic fields and include all pairwise Coulomb interactions. We show that in this setup 120 keV, 0.1 pC electron bunches can be produced with a longitudinal emittance sufficiently small for enabling sub-100 fs bunch lengths at 1% relative energy spread. A transverse root-mean-square normalized emittance of εx=10 nm is obtained, significantly better than from photoemission sources. Correlations in transverse phase-space indicate that the transverse emittance can be improved even further, enabling single-shot studies of biomolecular samples.

Effects of timing and stability on laser wakefield acceleration using external injection
Phys. Rev. ST Accel. Beams 12, 051304 (2009)

W. van Dijk, J.M. Corstens, S.B. van der Geer, M.J. van der Wiel, and G.J.H. Brussaard

The effects of experimental variations in the synchronization, laser power, and plasma density on the final beam parameters of externally injected electrons accelerated in a plasma wave are studied using a hybrid model. This model combines a relativistic fluid description of the plasma wave generated by the laser pulse with particle tracking of the accelerated electrons. For cases in which the effects of beam loading and laser depletion can be neglected, the two parts can be separated, allowing a significant reduction in computational power needed compared to particle in cell codes. Two different approaches to externally injecting electrons into plasma waves are studied: In the first case, the electrons are injected behind a laser pulse with a0=0.32. In the second case, electrons are injected in front of the laser pulse in three different laser regimes a0=0.32, a0=0.56, and a0=1.02, ranging from linear to nonlinear. For these four cases, the effects of expected experimental variations in synchronization (±500 fs), laser power (±10%), and plasma density (±30%) are studied. From these simulations, it becomes clear that in some cases, even a small variation in one of these parameters can create a large change in the final energy, energy spread, and trapped charge. For lower laser intensities, the method of injecting behind the laser pulse is the least sensitive to fluctuations while injection in front of the laser pulse becomes less sensitive at higher intensities.

Design construction and operation of the Dutch rf-photoguns
Proceedings of PAC 2009, Vancouver, Canada.

S.B. van der Geer, M.J. de Loos, Pulsar Physics, The Netherlands
W.P.E.M. Op 't Root, W. van Dijk, W. van Hemmen, G.J.H. Brussaard, O.J. Luiten, Eindhoven University of Technology, The Netherlands
W. Knulst, M.J.W. Vermeulen, Delft University of Technology, The Netherlands

Three different S-band RF-photoguns have been constructed by Eindhoven University of Technology in the Netherlands: A 1.5-cell, a 100-Hz 1.6-cell, and a 2.6-cell. They share a design concept that differs from the ‘standard’ BNL-gun in many aspects: Individual cells are clamped and not brazed, saving valuable manufacturing time and allowing damaged parts to be replaced individually. The inner geometry employs axial incoupling, inspired by DESY, to eliminate any noncylindrically symmetric modes. Elliptical irises, identical to a 2.6-cell design of Strathclyde University, reduce the maximum field on the irises and thereby reduce electrical breakdown problems. The manufacturing process uses single-point diamond turning based on a micrometerprecise design. The overall precision is such that the clamped cavities are spot-on resonance and have nearperfect field balance without the need for any postproduction tuning. Operational performance of the three Dutch RF-photoguns will be presented.

Low-Energy-Spread Ion Bunches from a Trapped Atomic Gas
Physical Review Letters 102, 034802 (2009)
DOI: 10.1103/PhysRevLett.102.034802

M. P. Reijnders, P. A. van Kruisbergen, G. Taban, S. B. van der Geer, P.H.A. Mutsaers, E. J. D. Vredenbregt, and O. J. Luiten

We present time-of-flight measurements of the longitudinal energy spread of pulsed ultracold ion beams, produced by near-threshold ionization of rubidium atoms captured in a magneto-optical atom trap. Well-defined pulsed beams have been produced with energies of only 1 eVand a root-mean-square energy spread as low as 0.02 eV, 2 orders of magnitude lower than the state-of-the-art gallium liquid-metal ion source. The low energy spread is important for focused ion beam technology because it enables milling and ion-beam-induced deposition at sub-nm length scales with many ionic species, both light and heavy. In addition, we show that the slowly moving, low-energy-spread ion bunches are ideal for studying intricate space charge effects in pulsed beams. As an example, we present a detailed study of the transition from space charge dominated dynamics to ballistic motion.

Transport of ultra-short electron bunches in a free-electron laser driven by a laser-plasma wakefield accelerator
Proc. SPIE, Vol. 7359, 735916 (2009)

M. P. Anania, D. Clark, S. B. van der Geer, M. J. de Loos, R. Isaac, A. J. W. Reitsma, G. H. Welsh, S. M. Wiggins, and D. A. Jaroszynski

Focussing ultra-short electron bunches from a laser-plasma wakefield accelerator into an undulator requires particular attention to be paid to the emittance, electron bunch duration and energy spread. Here we present the design and implementation of a focussing system for the ALPHA-X beam transport line, which consists of a triplet of permanent magnet quadrupoles and a triplet of electromagnetic quadrupoles.

Picosecond electron deflectometry of optical-field ionized plasmas
Nature Photonics, Vol 2., (2008)
DOI: 10.1038/nphoton.2008.77

Martin Centurion, Peter Reckenthaeler, Sergei A. Trushin, Ferenc Krausz and Ernst E. Fill

Optical-field ionized plasmas are of great interest owing to their unique properties and the fact that they suit many applications, such as the study of nuclear fusion, generation of energetic electrons and ions, X-ray emission, X-ray lasers and extreme-UV attosecond pulse generation. A detailed knowledge of the plasma dynamics can be critical for optimizing a given application. Here we demonstrate a method for real-time imaging of the electric-field distribution in optical-field ionized plasmas with ultrahigh temporal resolution, yielding information that is not accessible by other methods. The technique, based on electron deflectometry, yields images that reveal a positively charged core and a cloud of electrons expanding far beyond the Debye length.

Beam transport of Ultra-Short Electron Bunches
35th EPS Conference on Plasma Phys. Hersonissos, 9 - 13 June 2008 ECA Vol.32D, P-1.147 (2008)

M. P. Anania, S. B. van der Geer, M. J. de Loos, A. J. W. Reitsma, D. A. Jaroszynski

Focussing of ultra-short electron bunches from a wakefield accelerator into an undulator requires particular attention to be paid to the emittance, electron bunch duration and energy spread. We present a design of a focussing system for the ALPHA-X transport section, which consists of a triplet of permanent magnet quadrupoles. The design has been carried out using the GPT (General Particle Tracer) code [1], which considers the space charge effects and allows us to obtain a realistic estimate of the electron beam properties inside the undulator and therefore the properties of synchrotron emission and self-amplified spontaneous free-electron laser action.

Note: The following publication is not at all GPT related, it is not even about physics, but interesting nevertheless:
The origin of Homo floresiensis and its relation to evolutionary processes under isolation
Anthropological Science, Vol. 117 (2009) , No. 1 pp.33-43
DOI: 10.1537/ase.080411

G.A. Lyras, M.D. Dermitzakis, A.A.E. van der Geer, S.B. van der Geer and J. de Vos

Since its first description in 2004, Homo floresiensis has been attributed to a species of its own, a descendant of H. erectus or another early hominid, a pathological form of H. sapiens, or a dwarfed H. sapiens related to the Neolithic inhabitants of Flores. In this contribution, we apply a geometric morphometric analysis to the skull of H. floresiensis (LB1) and compare it with skulls of normal H. sapiens, insular H. sapiens (Minatogawa Man and Neolithic skulls from Flores), pathological H. sapiens (microcephalics), Asian H. erectus (Sangiran 17), H. habilis (KNM ER 1813), and Australopithecus africanus (Sts 5). Our analysis includes specimens that were highlighted by other authors to prove their conclusions. The geometric morphometric analysis separates H. floresiensis from all H. sapiens, including the pathological and insular forms. It is not possible to separate H. floresiensis from H. erectus. Australopithecus falls separately from all other skulls. The Neolithic skulls from Flores fall within the range of modern humans and are not related to LB1. The microcephalic skulls fall within the range of modern humans, as well as the skulls of the Neolithic small people of Flores. The cranial shape of H. floresiensis is close to that of H. erectus and not to that of any H. sapiens. Apart from cranial shape, some features of H. floresiensis are not unique but are shared with other insular taxa, such as the relatively large teeth (shared with Early Neolithic humans of Sardinia), and changed limb proportions (shared with Minatogawa Man).

Parameter study of acceleration of externally injected electrons in the linear laser wakefield regime
Physics of Plasmas 15, 093102 (2008)
DOI: 10.1063/1.2977765

W. van Dijk, S. B. van der Geer, M. J. van der Wiel, and G. J. H. Brussaard

A parameter study for laser wakefield acceleration is presented, in which externally injected electrons are accelerated in low amplitude plasma waves, represented by an analytical two-dimensional description. Results have been obtained for plasma densities up to 2.6x1024 m−3, plasma lengths up to 300 mm, laser intensities up to 3.5x1021 W/m2, and injection of Gaussian model bunches at energies up to 12 MeV. For the range of parameters studied, effects of laser depletion and the influence of the electron bunch on the plasma can be ignored. In the parameter space, a region is identified where final energies of over 100 MeV are reached, at an energy spread of less than 5% and a rms emittance of a few micrometers.

Benchmarking of 3D space charge codes using direct phase space measurements from photoemission high voltage dc gun
Phys. Rev. ST Accel. Beams 11, 100703 (2008)
DOI: 10.1103/PhysRevSTAB.11.100703

Ivan V. Bazarov, Bruce M. Dunham, Colwyn Gulliford, Yulin Li, Xianghong Liu, Charles K. Sinclair, and Ken Soong, Fay Hannon

We present a comparison between space charge calculations and direct measurements of the transverse phase space of space charge dominated electron bunches from a high voltage dc photoemission gun followed by an emittance compensation solenoid magnet. The measurements were performed using a double-slit emittance measurement system over a range of bunch charge and solenoid current values. The data are compared with detailed simulations using the 3D space charge codes gpt and parmela3d. The initial particle distributions were generated from measured transverse and temporal laser beam profiles at the photocathode. The beam brightness as a function of beam fraction is calculated for the measured phase space maps and found to approach within a factor of 2 the theoretical maximum set by the thermal energy and the accelerating field at the photocathode.

Calculation of coherent synchrotron radiation in General Particle Tracer
EPAC 2008, p. 118

Ivan V. Bazarov, Tsukasa Miyajima

General Particle Tracer (GPT) is a particle tracking code, which includes a 3D space charge effect based on a non-equidistant multigrid Poisson solver or a point-to-point method. It is used to investigate beam dynamics in ERL and FEL injectors. We have developed a new routine to simulate coherent synchrotron radiation (CSR) in GPT based on the formalism of Sagan. The routing can calculate the 1D-wake functions of arbitrary beam trajectories as well as CSR shielding effects. In particular, the CSR routine does not assume ultrarelativistic electron beam and is therefore applicable at low beam energies in the injector. Energy loss and energy spread caused by CSR effect were checked for a simple circular orbit to obtain more accurate results in bending magnets. In addition, we enhanced the 3D space charge routine to obtain more accurate results in bending magnets.

Note from Pulsar Physics: Please contact us for a new version of the spacecharge3Dmesh element that solves Poissons’ equation in a frame that is aligned with the particle bunch. This additional rotation significantly speeds up convergence in cases where the primary axes of the bunch, as measured in the co-moving frame, are not aligned with the Cartesian axes used in the tracking. The main use of this new element is spacecharge calculations inside bend magnets.

Pancakes versus beer-cans in terms of 6D phase-space density
EPAC 2008, p. 151

S. B. van der Geer, M.J. de Loos, O. J. Luiten

Uniformly filled ellipsoidal (waterbag) electron bunches can be created in practice by space charge blow out of transversely tailored pancake bunches. Ellipsoidal bunches have linear self fields in all dimensions, and will not deteriorate in quality under linear transport and acceleration. There is a discussion if such a bunch is better than a conventional beer-can shape. This paper compares the two approaches in terms of usable phase-space density. Detailed GPT simulations of a simplified setup show that although the pancakes approach requires less charge, it is the application that is decisive.

Simulated performance of an ultracold ion source
Journal of Applied Physics 102, 094312 (2007)
DOI: 10.1063/1.2804287

S. B. van der Geer, M. P. Reijnders, M. J. de Loos, E. J. D. Vredenbregt, P. H. A. Mutsaers, and O. J. Luiten

At present, the smallest spot size which can be achieved with state-of-the-art focused ion beam (FIB) technology is mainly limited by the chromatic aberrations associated with the 4.5 eV energy spread of the liquid-metal ion source. Here we numerically investigate the performance of an ultracold ion source which has the potential for generating ion beams which combine high brightness with small energy spread. The source is based on creating very cold ion beams by near-threshold photoionization of a laser-cooled and trapped atomic gas. We present ab initio numerical calculations of the generation of ultracold beams in a realistic acceleration field and including all Coulomb interactions, i.e., both space charge effects and statistical Coulomb effects. These simulations demonstrate that with existing technology reduced brightness values exceeding 105 A m−2 sr−1 V−1 are feasible at an energy spread as low as 0.1 eV. The estimated spot size of the ultracold ion source in a FIB instrument ranges from 10 nm at a current of 100 pA to 0.8 nm at 1 pA.

Electron source concept for single-shot sub-100 fs electron diffraction in the 100 keV range
Journal of Applied Physics 102, 093501 (2007)
DOI: 10.1063/1.2801027

T. van Oudheusden, E. F. de Jong, S. B. van der Geer, W. P. E. M. Op ’t Root, and O. J. Luiten, and B. J. Siwick

We present a method for producing sub-100 fs electron bunches that are suitable for single-shot ultrafast electron diffraction experiments in the 100 keV energy range. A combination of analytical estimates and state-of-the-art particle tracking simulations show that it is possible to create 100 keV, 0.1 pC, 30 fs electron bunches with a spot size smaller than 500 µm and a transverse coherence length of 3 nm, using established technologies in a table-top setup. The system operates in the space-charge dominated regime to produce energy-correlated bunches that are recompressed by radio-frequency techniques. With this approach we overcome the Coulomb expansion of the bunch, providing a single-shot, ultrafast electron diffraction source concept.


Ultracold Electron Sources
International Journal of Modern Physics A, Vol 22, Issue 22, p. 3882 - 3897 (2007)

O.J. Luiten, B.J. Claessens, S.B. van der Geer, M.P. Reijnders, G.Taban, and E.J.D. Vredenbregt

Ultra-cold plasmas with electron temperatures of ~10 K can be created by photo-ionization just above threshold of a cloud of laser-cooled atoms. Recently it was shown 7 by GPT particle tracking simulations that an ultra-cold plasma has an enormous potential as a pulsed bright electron source. Here we discuss these results in the framework of normalized 6D brightness, which allows us to make a proper comparison both with the performance of pulsed, radio-frequency photo-emission sources and with the performance of continuous, needle-like field-emission sources. In addition we speculate on the possibility of using ultra-cold plasmas to realize quantum degenerate electron beams, constituting the ultimate limit in electron beam brightness.

Design of a 2 kA, 30 fs RF-photoinjector for waterbag compression
International Journal of Modern Physics A, Vol 22, Issue 22, p. 4000 - 4005 (2007)
DOI: 10.1142/S0217751X07037573

S.B van der Geer, O.J. Luiten, M.J. de Loos

Because uniformly filled ellipsoidal ‘waterbag’ bunches have linear self-fields in all dimensions, they do not suffer from space-charge induced brightness degradation. This in turn allows very efficient longitudinal compression of high-brightness bunches at sub or mildly relativistic energies, a parameter regime inaccessible up to now due to detrimental effects of non-linear space-charge forces. To demonstrate the feasibility of this approach, we investigate ballistic bunching of 1 MeV, 100 pC waterbag electron bunches, created in a half-cell rf-photogun, by means of a two-cell booster-compressor. Detailed GPT simulations of this table-top set-up are presented, including realistic fields, 3D space-charge effects, path-length differences and image charges at the cathode. It is shown that with a single 10MW S-band klystron and fields of 100 MV/m, 2kA peak current is attainable with a pulse duration of only 30 fs at a transverse normalized emittance of 1.5 μm.

Design considerations for table-top, laser-based VUV and X-ray free electron lasers
Appl. Phys. B. 86, 431-435 (2007)

F. Grüner, S. Becker,  U. Schramm, T. Eichner, M. Fuchs, R. Weingartner, D. Habs, J. Meyer-ter-vehn, M. Geissler, M. Ferrario, L. serafini, B. van der geer, H. backe, W. Lauth, S. Reiche,

A recent breakthrough in laser-plasma accelerators, based upon ultrashort high-intensity lasers, demonstrated the generation of quasi-monoenergetic GeV electrons. With future Petawatt lasers ultra-high beam currents of ~ 100 kA in ~ 10 fs can be expected, allowing for drastic reduction in the undulator length of free-electron-lasers (FELs). We present a discussion of the key aspects of a table-top FEL design, including energy loss and chirps induced by space-charge and wakefields. These effects become important for an optimized table-top FEL operation. A first proof-of-principle VUV case is considered as well as a table-top X-ray-FEL which may also open a brilliant light source for new methods in clinical diagnostics.

Radial bunch compression: Path-length compensation in an rf photoinjector with a curved cathode
Phys. Rev. ST Accel. Beams 9, 084201 (2006)

M. J. de Loos, S. B. van der Geer, Y. M. Saveliev, V. M. Pavlov, A. J. W. Reitsma, S. M. Wiggins, J. Rodier, T. Garvey, and D. A. Jaroszynski

Electron bunch lengthening due to space-charge forces in state-of-the-art rf photoinjectors limits the minimum bunch length attainable to several hundreds of femtoseconds. Although this can be alleviated by increasing the transverse dimension of the electron bunch, a larger initial radius causes path-length differences in both the rf cavity and in downstream focusing elements. In this paper we show that a curved cathode virtually eliminates these undesired effects. Detailed numerical simulations confirm that significantly shorter bunches are produced by an rf photogun with a curved cathode compared to a flat cathode device. The proposed novel method will be used to provide 100 fs duration electron bunches for injection into a laser-driven plasma wakefield accelerator.

Front-to-end simulations of the design of a laser wakefield acceleratoron with external injection
DOI: 10.1063/1.2195382

W. H. Urbanus, W. van Dijk, S. B. van der Geer, G. J. H. Brussaard, and M.J. van der Wiel, Eindhoven University of Technology

We report the design of a laser wakefield accelerator (LWA) with external injection by a rf photogun and acceleration by a linear wakefield in a capillary discharge channel. The design process is complex due to the large number of intricately coupled free parameters. To alleviate this problem, we performed front-to-end simulations of the complete system. The tool we used was the general particle-tracking code, extended with a module representing the linear wakefield by a two-dimensional traveling wave with appropriate wavelength and amplitude. Given the limitations of existing technology for the longest discharge plasma wavelength (~50 µm) and shortest electron bunch length (~100 µm), we studied the regime in which the wakefield acts as slicer and buncher, while rejecting a large fraction of the injected bunch. The optimized parameters for the injected bunch are 10 pC, 300 fs at 6.7 MeV, to be injected into a 70 mm long channel at a plasma density of 7×1023 m–3. A linear wakefield is generated by a 2 TW laser focused to 30 µm. The simulations predict an accelerated output of 0.6 pC, 10 fs bunches at 90 MeV, with energy spread below 10%. The design is currently being implemented. The design process also led to an important conclusion: output specifications directly comparable to those reported recently from "laser-into-gas jet" experiments are feasible, provided the performance of the rf photogun is considerably enhanced. The paper outlines a photogun design providing such a performance level.

Longitudinal phase-space manipulation of ellipsoidal electron bunches in realistic fields
Phys. Rev. ST Accel. Beams 9, 044203 (2006)
DOI: 10.1103/PhysRevSTAB.9.044203

S. B. van der Geer, M. J. de Loos, T. van Oudheusden, W. P. E. M. op ’t Root, M. J. van der Wiel, and O. J. Luiten, Eindhoven University of Technology

Since the recent publication of a practical recipe to create “pancake” electron bunches which evolve into uniformly filled ellipsoids, a number of papers have addressed both an alternative method to create such ellipsoids as well as their behavior in realistic fields. So far, the focus has been on the possibilities to preserve the initial “thermal” transverse emittance. This paper addresses the linear longitudinal phase space of ellipsoidal bunches. It is shown that ellipsoidal bunches allow ballistic compression at subrelativistic energies, without the detrimental effects of nonlinear space-charge forces. This in turn eliminates the need for the large correlated energy spread normally required for longitudinal compression of relativistic particle beams, while simultaneously avoiding all problems related to magnetic compression. Furthermore, the linear space-charge forces of ellipsoidal bunches can be used to reduce the remaining energy spread even further, by carefully choosing the beam transverse size, in a process that is essentially the time-reversed process of the creation of an ellipsoid at the cathode. The feasibility of compression of ellipsoidal bunches is illustrated with a relatively simple setup, consisting of a half-cell S-band photogun and a two-cell booster compressor. Detailed GPT simulations in realistic fields predict that 100 pC ellipsoidal bunches can be ballistically compressed to 100 fs, at a transverse emittance of 0.7 μm, with a final energy of 3.7 MeV and an energy spread of only 50 keV.


Laser wakefield acceleration: the injection issue. Overview and latest results
Philosophical Transactions of the Royal Society A: Volume 364, Number 1840, (2006), p. 679 - 687
DOI: 10.1098/rsta.2005.1731

M.J. van der Wiel, O.J. Luiten, G.J.H. Brussaard, S.B. van der Geer, W.H. Urbanus, W. van Dijk, Th. van Oudheusden, Eindhoven University of Technology

External injection of electron bunches into laser-driven plasma waves so far has not resulted in ‘controlled’ acceleration, i.e. production of bunches with well-defined energy spread. Recent simulations, however, predict that narrow distributions can be achieved, provided the conditions for properly trapping the injected electrons are met. Under these conditions, injected bunch lengths of one to several plasma wavelengths are acceptable. This paper first describes current efforts to demonstrate this experimentally, using state-of-the-art radio frequency technology. The expected charge accelerated, however, is still low for most applications. In the second part, the paper addresses a number of novel concepts for significant enhancement of photo-injector brightness. Simulations predict that, once these concepts are realized, external injection into a wakefield accelerator will lead to accelerated bunch specs comparable to those of recent ‘laser-into-gasjet’ experiments, without the present irreproducibility of charge and final energy of the latter.

Radiation sources based on laser–plasma interactions
Philosophical Transactions of the Royal Society A: Volume 364, Number 1840, (2006), p. 689 - 710
DOI: 10.1098/rsta.2005.1732

D.A. Jaroszynski, R. Bingham, E. Brunetti, B. Ersfeld, J. Gallacher, B. van der Geer, R. Issac, S.P. Jamison, D. Jones, M. de Loos, A. Lyachev, V. Pavlov, A. Reitsma, Y. Saveliev, G. Vieux, S.M. Wiggins, University of Strathclyde

Plasma waves excited by intense laser beams can be harnessed to produce femtosecond duration bunches of electrons with relativistic energies. The very large electrostatic forces of plasma density wakes trailing behind an intense laser pulse provide field potentials capable of accelerating charged particles to high energies over very short distances, as high as 1GeV in a few millimetres. The short length scale of plasma waves provides a means of developing very compact high-energy accelerators, which could form the basis of compact next-generation light sources with unique properties. Tuneable X-ray radiation and particle pulses with durations of the order of or less than 5fs should be possible and would be useful for probing matter on unprecedented time and spatial scales. If developed to fruition this revolutionary technology could reduce the size and cost of light sources by three orders of magnitude and, therefore, provide powerful new tools to a large scientific community. We will discuss how a laser-driven plasma wakefield accelerator can be used to produce radiation with unique characteristics over a very large spectral range.

Ultracold Electron Source
Phys. Rev. Lett. 95, 164801 (2005)
DOI: 10.1103/PhysRevLett.95.164801

B. J. Claessens, S. B. van der Geer, G. Taban, E. J. D. Vredenbregt, and O. J. Luiten, TU-Eindhoven

We propose a technique for producing electron bunches that has the potential for advancing the state-of-the-art in brightness of pulsed electron sources by orders of magnitude. In addition, this method leads to femtosecond bunch lengths without the use of ultrafast lasers or magnetic compression. The electron source we propose is an ultracold plasma with electron temperatures down to 10 K, which can be fashioned from a cloud of laser-cooled atoms by photoionization just above threshold. Here we present results of simulations in a realistic setting, showing that an ultracold plasma has an enormous potential as a bright electron source.

Raw 1 GV/m results

3D Space-charge model for GPT simulations of high-brightness electron bunches
Computational Accelerator Physics 2002; Editors: M. Berz and K. Makino, 101, Inst. of Physics Conf. Series Number 175.

S. B. van der Geer, O.J. Luiten, TU-Eindhoven
M.J. de Loos, Pulsar Physics
G. Pöplau, U. van Rienen, Rostock University, Germany

For the simulation of high-brightness electron bunches, a new 3D space-charge model is being implemented in the General Particle Tracer (GPT) code. It is based on a non-equidistant multigrid solver, allowing smooth transitions from a high to a low-aspect ratio bunch during a single run. The algorithm scales linearly in CPU time with the number of particles and the insensitivity to aspect ratio ensures that it can be used for a variety of applications. Tracking examples and field comparisons with an analytical model will be shown.

Multigrid parameters

A multigrid based 3D space-charge routine in the tracking code GPT
Computational Accelerator Physics 2002; Editors: M. Berz and K. Makino, 281, Inst. of Physics Conf. Series Number 175

G. Pöplau, U. van Rienen, Rostock University, Germany
M.J. de Loos, TU-Eindhoven
S. B. van der Geer, Pulsar Physics

Fast calculation of 3D non–linear space–charge fields is essential for the simulation of high–brightness charged particle beams. We report on our development of a new 3D space–charge routine in the General Particle Tracer (GPT) code. The model is based on a non–equidistant multigrid Poisson solver that is used to solve the electrostatic fields in the rest frame of the bunch. Since the multigrid Poisson solver depends only linearly on the number of mesh points for the discretized electrostatic problem the space–charge routine scales linearly with the number of particles in terms of CPU time. This performance allows over a million particles to be tracked on a normal PC. The choice of the routine parameters for an optimal performance will be discussed with the model of a spherical bunch.

How to Realize Uniform Three-Dimensional Ellipsoidal Electron Bunches
Phys. Rev. Lett. 93, 094802 (2004)
DOI: 10.1103/PhysRevLett.93.094802

O. J. Luiten, S. B. van der Geer, M. J. de Loos, F. B. Kiewiet, and M. J. van der Wiel, TU-Eindhoven

Uniform three-dimensional ellipsoidal distributions of charge are the ultimate goal in charged particle accelerator physics because of their linear internal force fields. Such bunches remain ellipsoidal with perfectly linear position-momentum phase space correlations in any linear transport system. We present a method, based on photoemission by radially shaped femtosecond laser pulses, to actually produce such bunches.

Multigrid algorithms for the fast calculation of space-charge effects in accelerator design
IEEE Transactions on magnetics, Vol 40, No. 2, (2004), p. 714.
DOI: 10.1109/TMAG.2004.825415

Gisela Pöplau, Ursula van Rienen, Bas van der Geer, and Marieke de Loos

Numerical prediction of charged particle dynamics in accelerators is essential for the design and understanding of these machines. Methods to calculate the self-fields of the bunch, the so-called space-charge forces, become increasingly important as the demand for high-quality bunches increases. We report on our development of a new three-dimensional (3-D) space-charge routine in the general particle tracer (GPT) code. It scales linearly with the number of particles in terms of CPU time, allowing over a million particles to be tracked on a normal PC. The model is based on a nonequidistant multigrid Poisson solver that has been constructed to solve the electrostatic fields in the rest frame of the bunch on meshes with large aspect ratio. Theoretical and numerical investigations of the behavior of SOR relaxation and PCG method on nonequidistant grids emphasize the advantages of the multigrid algorithm with adaptive coarsening. Numerical investigations have been performed with a wide range of cylindrically shaped bunches (from very long to very short) occuring in recent applications. The application to the simulation of the TU/e DC/RF gun demonstrates the power of the new 3-D routine.

Performance of the TU/e 2.6 cell rf-photogun in the 'pancake' regime
Proceedings of EPAC 2004, Luzern, Switserland, p. 2128.

S.B. van der Geer, M.J. de Loos, O.J. Luiten, G.J.H. Brussaard, M.J. van der Wiel, TU-Eindhoven
G. Pöplau, Rostock University, Germany

Controlled plasma acceleration requires electron bunches to be injected into the plasma channel with a length of a fraction of the plasma wavelength. Taking into account parameters of a realistic plasma channel, this sets the requirements for the bunch at the entrance of the channel to a transverse size of about 30 micrometer and a duration in the order of 100 femtoseconds. The production of such bunches requires state-of-the art accelerator technology. In this paper we present GPT simulation results of the 2.6 cell rf-photogun currently in operation at Eindhoven University of Technology. Calculations are presented in the low-charge short-pulse regime with emphasis on bunch lengthening due to path length differences and space-charge effects. The numerical challenge is tackled using high-precision field-maps and the newly developed 3D mesh-based space-charge model of GPT. It is shown that with the present injector bunches can be produced that are suitable for injection into the planned experiment for controlled acceleration in a plasma-wakefield accelerator.

Progress in 3D space-charge calculations in the GPT code
Proceedings of EPAC 2004, Luzern, Switserland, p. 2592..

G. Pöplau, U. van Rienen, Rostock University, Germany
S.B. van der Geer, TU-Eindhoven
M.J. de Loos, Pulsar Physics

The mesh-based 3D space-charge routine in the GPT (General Particle Tracer, Pulsar Physics) code scales linearly with the number of particles in terms of CPU time and allows a million particles to be tracked on a normal PC. The crucial ingredient of the routine is a non-equidistant multigrid Poisson solver to calculate the electrostatic potential in the rest frame of the bunch. The solver has been optimized for very high and very low aspect ratio bunches present in state-of-the-art high-brightness electron accelerators. In this paper, we introduce a new meshing strategy based on a wavelet decomposition of the space-charge density. The numerical results show that the number of particles with large numerical error, typically located at the edges of the bunch, can be reduced with this new approach enormously.

Ideal waterbag electron bunches from an rf photogun
Proceedings of EPAC 2004, Luzern, Switserland, p. 725.

Jom Luiten, Bas van der Geer, Marieke de Loos, Fred Kiewiet, Marnix van der Wiel, TU-Eindhoven

The use of femtosecond photoemission laser pulses in high-gradient RF photoguns enables the production of electron bunches whose rest-frame bunch length is much smaller than the bunch radius (so-called ’pancake’ bunches) during a significant part of the acceleration path. Recently, we have shown for a constant and uniform acceleration field that by proper radial shaping of the photoemission laser pulses, a pancake bunch can be created that will evolve automatically into a uniformly filled 3D ellipsoid, i.e. into the ideal bunch. In this paper we show that the same holds for a realistic, non-uniform and time-dependent, RF acceleration field with magnetic focusing.

A Fast Beam Chopper for Next Generation High Power Proton Drivers
Proceedings of EPAC 2004, Luzern, Switserland, p. 1449.

M.A. Clarke-Gayther, CCLRC/RAL/ASTeC

The identification and development of a successful beam chopper design is regarded as key for the European Spallation Source (ESS), and for all next generation high intensity proton driver schemes that adopt the linac-accumulator ring configuration. A description is given of refinements to the beam line design of a 'Tandem' chopper system, developed to address the requirements of the ESS. Particle tracking using the 'General Particle Tracer' (GPT) code has enabled efficient optimisation of beam apertures, and the analysis of beam power density distributions on chopper beam dumps. Preliminary results of 'proof of principle' testing on prototype fast, and slower transition high voltage pulse generators, are presented.


A 3D Particle tracking technique for FEL start-up and saturation effects
Nucl. Instr. and Meth. A, Vol. 507, Issues 1-2 , P. 97-100
DOI: 10.1016/S0168-9002(03)00846-5

M.J. de Loos, C.A.J van der Geer, Pulsar Physics
S.B. van der Geer, TU-Eindhoven
A.F.G. van der Meer, D. Oepts, FOM-Rijnhuizen
R.Wünsch, Forschungzentrum Rossendorf

Self-consistent simulation of a linac driven fel by time-domain particle tracking can give very detailed results on both the produced radiation and the evolution of the electron bunch. We show that when special subsets are tracked, instead of individual macro-particles, only a few of these subsets are required to obtain converging results. The subsets used are short longitudinal arrays of macro-particles, of the order of a few ponderomotive waves, distributed longitudinally in such a way that they are almost only sensitive to stimulated emission. This new approach has been carried out with the 3D General Particle Tracer (GPT) code and a set of axisymmetric Gaussian waves propagating in free space. Due to the from-first-principles approach, it can be used for a variety of radiation problems, including studies of FEL start-up and saturation e ects. The model and two applications will be presented.

New Elements of the GPT Code to Simulate a Resonator Free-Electron Laser
Start-Up Simulations of the Spectral and Spatial Evolution of the ELBE FEL
Simulations of Limit Cycle Oscillations in the U27 FEL

R. Wünch, C.A.J. van der Geer, S.B. van der Geer and M.J. de Loos
Jahresbericht FZR (2003).

Effect of Undulator-Field Irregularities
P. Gippner, W. Seidel, W. Wohlfarth, A. Wolf, R. Wünch and C.A.J. van der Geer
Jahresbericht FZR (2003).


Non-linear space-charge
Fast Calculation of Space Charge in Beam Line Tracking by Multigrid Techniques
Proceedings of SCEE 2002, Eindhoven, 2002

G. Pöplau, U. van Rienen, Rostock University, Germany
M.J. de Loos, TU-Eindhoven
S. B. van der Geer, Pulsar Physics

Numerical prediction of charged particle dynamics in accelerators is essential for the design and understanding of these machines. The calculation of space charge forces influencing the behaviour of a particle bunch is still a bottleneck of existing tracking codes. We report on our development of a new 3D space–charge routine in the General Particle Tracer (GPT) code. It scales linearly with the number of particles in terms of CPU time, allowing over a million particles to be tracked on a normal PC. The model is based on a non–equidistant multigrid Poisson solver that is used to solve the electrostatic fields in the rest frame of the bunch. A reliable multigrid scheme for the tracking of particles should be very fast, stable and show good convergence for a great variety of meshes. Numerical results demonstrate the effect of the choice of the multigrid components. Further, the values of physical quantities show good agreement compared to the values calculated by a well–tested 2D routine in the GPT code.

A Fast 3D Multigrid Based Space-Charge Routine in the GPT Code
Proceedings of EPAC 2002, Paris, France, p. 1658.

G. Pöplau, U. van Rienen, Rostock University, Germany
S. B. van der Geer, Pulsar Physics
M.J. de Loos, TU-Eindhoven

Fast calculation of 3D non-linear space-charge fields is essential for the simulation of high-brightness charged particle beams. We report on our development of a new 3D space-charge routine in the General Particle Tracer (GPT) code. It scales linearly with the number of particles in terms of CPU time, allowing over a million particles to be tracked on a normal PC. The model is based on a non-equidistant multigrid Poisson solver that is used to solve the electrostatic fields in the rest frame of the bunch. Bunch lengthening and emittance growth calculations in a low-energy short electron bunch are chosen as an example of non-linear space-charge effects in a high-brightness photo-injector.

3D Multi-Frequency FEL Simulations with the General Particle Tracer code
Proceedings of EPAC 2002, Paris, France, p. 849.

M.J. de Loos, C.A.J. van der Geer, Pulsar Physics
S.B. van der Geer, TU-Eindhoven

The 3D General Particle Tracer (GPT) code has been extended to perform multi-frequency multi-pass free-electron laser (FEL) simulations. The new model is based on axisymmetric multiple Gaussian waves propagating in free space, without averaging over the undulator period. The field equations are integrated simultaneously with the equations of motion of the particles, making the model ideally suited to calculate electron beam phase-space distributions during and after FEL interaction. Due to the from-first-principles approach of the model, it is useful to a variety of radiation problems. The model and a proof-of-principle test are presented.

A 1 GV/M Laser-triggered compact accelerator
Proceedings of EPAC 2002, Paris, France, p. 989.

S.B. van der Geer*, M.J. de Loos, G.J.H. Brussaard, O.J. Luiten, M.J. van der Wiel, TU-Eindhoven

A novel design for a compact multistage electron injector is presented. The new method should exceed state of the art photocathode rf guns for the production of high-brightness, ultra-short electron bunches by one order of magnitude. It is based on 1 GV/m stepwise acceleration using a switched 2 MV, 1 ns pulsed power supply. Switching the consecutive acceleration stages on ps timescales is accomplished by instantaneous ionization of laser-triggered spark-gaps. Colliding pulses are used to double the acceleration field and eliminate the magnetic deflection field in the acceleration gaps. Using this scheme, an average field of over 600 MV/m is produced. Simulation results using the GPT code show that it is possible to generate 12 MeV, 100 pC, 0.7 kA bunches with an emittance below 1 p mm mrad and a length of 100 fs, without magnetic compression. The ultra-short electron bunches can be used to produce short XUV/X-ray pulses or can be further accelerated in a laser wakefield accelerator.

A high-brightness pre-accelerated rf-photo injector
Proceedings of EPAC 2002, Paris, France, p. 1831.

M.J. de Loos*, S.B. van der Geer, F.B. Kiewiet, O.J. Luiten, M.J. van der Wiel, TU-Eindhoven

At Eindhoven University of Technology a project has started aiming at the production of 100 fs, 100 pC electron bunches, with an emittance below 1 p mm mrad. These bunches are compatible with the requirements for a plasma wakefield accelerator or can be used to generate ultra-short XUV/X-ray pulses. The device currently under construction consists of two stages: A photo-excited DC 1 GV/m pre-accelerator, directly followed by a state-of-the-art S-band rf-booster. The field in the first stage is sufficiently high to avoid space-charge explosion at low energies. Therefore magnetic compression is not needed and this in turn eliminates undesirable radiative collective effects, which spoil the emittance. The 1 GV/m field is created in a 2 mm acceleration gap, powered by a 2 MV, 1 ns pulse generator. The second stage increases the energy to 10 MeV using a 100 MV/m 2.5 cell standing wave cavity with axial symmetric incoupling. Simulation results using the GPT code for the combined setup are presented.

Nonlinear electrostatic emittance compensation in kA, fs electron bunches
Phys. Rev. E 65, 046501 (2002)
DOI: 10.1103/PhysRevE.65.046501

S.B. van der Geer, M.J. de Loos, Pulsar Physics.
J.I.M. Botman, O.J. Luiten, M.J. van der Wiel, TU-Eindhoven

Nonlinear space-charge effects play an important role in emittance growth in the production of kA electron bunches with a bunch length much smaller than the bunch diameter. We propose a scheme employing the radial third-order component of an electrostatic acceleration field, to fully compensate the nonlinear space-charge effects. This results in minimal transverse root-mean-square emittance. The principle is demonstrated using our design simulations of a device for the production of high-quality, high-current, subpicosecond electron bunches using electrostatic acceleration in a 1 GV/m field. Simulations using the GPT code produce a bunch of 100 pC and 73 fs full width at half maximum pulse width, resulting in a peak current of about 1.2 kA at an energy of 2 MeV. The compensation scheme reduces the root-mean-square emittance by 34% to 0.4 pi  mm mrad.

Thesis: The General Particle Tracer code: Design, implementation and application

S.B. van der Geer, M.J. de Loos

Charged particle beams are important tools for scientific, industrial and medical applications. The design and understanding of new charged particle accelerators rely on numerical simulations to predict beam behavior. To aid in the design of these machines, we developed the General Particle Tracer (GPT) code. GPT is a general-purpose particle-tracking code and is currently being used in many institutes worldwide for a variety of applications. The code solves the 3D equations of motion of sample particles in time-dependent electromagnetic fields. The self-fields of the beam known as space-charge, are also calculated. GPT contains an efficient high-order tracking algorithm with variable accuracy, a large number of modules to represent beam line components and various space-charge models. Furthermore GPT can be adapted to specific needs, an essential feature in a research environment.

A follow-up of the FOM fusion FEM for 1 MW, 1 s
Fusion Engineering and Design, Vol. 53, Issues 1-4 , (2001), p. 577-586
DOI: 10.1016/S0920-3796(00)00536-6

A. G. A. Verhoeven, W. A. Bongers, V. L. Bratman, S. Brons, G. G. Denisov, C. A. J. van der Geer, S. B. van der Geer, O. G. Kruijt, M. J. de Loos, P. Manintveld, A. J. Poelman, J. Plomp, A. V. Savilov, P. H. M. Smeets and W. H. Urbanus

Experiments have been performed with the free-electron maser (FEM) at Rijnhuizen, a high-power mm-wave source. A unique feature of the FEM is the possibility to tune the frequency over the entire range from 130 to 260 GHz at an output power exceeding 1MW. In the so-called inverse set-up, where the electron gun is mounted inside the high-voltage terminal, a peak power of 730 kW was measured at 200GHz and of 350kW at 167GHz [1,2]. Furthermore, we made the design work to extend the pulse-length to 1s. Detailed thermal behavior of the critical components is studied. Both the cavity mirrors and the depressed-collector electrodes seem to have adequate cooling.

Simulation of laser - electron beam interaction in the optical klystron of a free electron laser
Proceedings of EPAC 2000, Vienna, Austria, p. 776.

C.A. Thomas, J.I.M. Botman, Eindhoven University of Technology, The Netherlands,
C.A.J. van der Geer, FOM Institute for Plasma Physics, “Rijnhuizen”, The Netherlands,
M.E. Couprie, CEA/SPAM Lure, Orsay, France

The particle optics code GPT has been applied to study the single pass electron beam interaction with an external laser field in the undulator system of a free electron laser. In particular, this code has been used to simulate the laser - electron beam interaction in an optical klystron. Micro-bunching as a result of the interaction of the electron bunch with an electromagnetic pulse is presented. The energy exchange during the interaction calculated with GPT gives the gain curve of the optical klystron.

A solver for the General Particle Tracer package
Proceedings of EPAC 2000, Vienna, Austria, p. 1411.

S.B. van der Geer, M.J. de Loos, Pulsar Physics

The General Particle Tracer (GPT) code has been extended with a multi-dimensional optimizer and solver to automate the final stages of a design process. The new solver can be used for all GPT simulations, including 3D space-charge and particle-wave interaction. The internal algorithms and two examples are presented in this paper.

Coupling sections, emittance growth, and drift compensation in the use of bent solenoids as beam transport elements
Phys. Rev. ST Accel. Beams 2, 054001 (1999)

J. Norem, ANL

Bent solenoids can transmit charged particle beams while providing momentum dispersion.While less familiar than quadrupole and dipole systems, bent solenoids can produce superficially simple transport lines and large acceptance spectrometers for use at low energies. Design issues such as drift compensation and coupling sections between straight and bent solenoids are identified, and aberrations such as shears produced by perpendicular error fields are discussed. Examples are considered which provide the basis for the design of emittance exchange elements for the cooling system of a muon collider.

Effect of the drift gap between the undulator sections on the operation of the Fusion-FEM
Nucl. Instr. and Meth. A, Vol 445, Issues 1-3, (2000), p. 187-191.
DOI: 10.1016/S0168-9002(00)00063-2

C.A.J. van der Geer, B.L. Militsyn, W.A. Bongers, V.L. Bratman, G.G. Denisov, P. Manintveld, A.V. Savilov, A.A. Varfolomeev, A.G.A. Verhoeven, W.H. Urbanus, FOM Institute for Plasma Physics "Rijnhuizen", Russian Research Centre "Kurchatov Institute", Institute of applied physics Nizhny Novgorod.

To be published in Nuclear Instruments and Methods in Physics Research, A. Editorial reference: TO3 Mo-P-51. Our reference: NIMA 23036

Production of ultra-short, high charge, low emittance electron bunches using a 1 GV/m diode-gap
Proceedings of PAC 1999, New York, p. 3266.

M.J. de Loos, S.B. van der Geer, Pulsar Physics
J.I.M. Botman, O.J. Luiten, M.J. van der Wiel, TU-Eindhoven

Advanced acceleration schemes, for example those based on wake fields of laser pulses traveling through plasma, require the injection of very high quality relativistic femtosecond electron bunches. Such bunches can be produced by a photoexcited RF gun followed by longitudinal bunch compression. Currently we are investigating a different pre-acceleration scheme, which avoids the necessity of magnetic compression and the associated potential emittance growth due to coherent synchrotron radiation. Instead of an RF cavity, we propose 1 GV/m DC acceleration of laser excited electrons across a 2 mm gap, following recent developments at Brookhaven Nat. Lab. The gun is powered by a 2 MV, 1 ns pulse. Simulation results using the General Particle Tracer (GPT) code show that with the DC gun scheme a 100 pC bunch can be accelerated to 2 MeV with a final bunch length of 70 fs and an emittance well below 1 π mm mrad.

3D-Design of the Fusion-FEM Depressed Collector using the General Particle Tracer (GPT) code
Proceedings of PAC 1999, New York, p. 2462.

S.B. van der Geer, M.J. de Loos, Pulsar Physics
A.G.A. Verhoeven, W.H. Urbanus, FOM Institute for Plasma Physics "Rijnhuizen"

The "Rijnhuizen" Fusion Free-Electron Maser (FEM) is the pilot experiment for a high power, mm-wave source, tunable in the range 130-260 GHz. The FEM has generated 730 kW output power during 10 m s pulses. To increase the overall efficiency to over 50 % and to reach a pulse length of at least 100 ms, an electron beam charge and energy recovery system is currently being designed and installed. This system consists of an electrostatic decelerator, which decels the beam from 2 MeV to an average of 200 keV, and a depressed collector. The EM-wave interaction inside the undulator can result in an energy spread of 300 keV behind the decelerator. The multi-stage collector is designed so that electrons fall on the backside of one of three electrodes, thus ensuring that secondary particles will immediately be accelerated back towards the electrodes. However, scattered primary electrons can cause back streaming, hereby reducing the efficiency and possibly damaging the machine. To reduce this back streaming to below a tolerable 0.1 %, the General Particle Tracer (GPT) code is being used to calculate primary and scattered particle trajectories inside the collector. It will be shown that an off-axis bending scheme, using a rotating perpendicular magnetic field lowers the back streaming and hereby increases the pulse length of the machine. The bending scheme also improves the power dissipation in the collector.

Hamiltonian calculations on particle motion in linear electron accelerators
Proceedings of EPAC 1998, Stockholm, Sweden, p. 716.

A.F.J. Hammen, J.M. Corstens, J.I.M. Botman, H.L. Hagedoorn, W.H.C. Theuws, TU-Eindhoven

A Hamiltonian theory, in which electromagnetic space waves and longitudinal electric fields are incorporated by means of their vector potentials, is used to calculate particle motion in linear electron accelerators. In particular these calculations have been applied to the Eindhoven 10 MeV travelling-wave linac as well as to the Eindhoven racetrack microtron accelerating cavity. The calculations are in good agreement with simulations performed by particle-tracking codes.

General Particle Tracer: A 3D code for accelerator and beamline design
Proceedings of EPAC 1998, Stockholm, Sweden, p. 1245.

S.B. van der Geer, M.J. de Loos, Pulsar Physics

The General Particle Tracer (GPT) code is a well established simulation platform for the study of charged particle dynamics in electromagnetic fields. The code is completely 3D, including the space-charge model. Because of its modern implementation, GPT can be conveniently customized without compromising its ease of use, accuracy or simulation speed. In this paper we will present the latest version of GPT, version 2.40.
This newest release is twice as fast, is capable of simulating different types of particles simultaneously and includes many new elements. The new integration method is based on a fifth order embedded Runge-Kutta method with adaptive stepsize control to ensure both accuracy and speed in solving the particle’s equations of motion in time domain. Furthermore any additional differential equations can be solved while tracking the particles.
GPT features also include complete freedom in the initial particle distribution and the flexibility to position and orient all beam line components. Separate utility programs calculate macroscopic quantities, produce ASCII and graphical output and automate parameter scans.
In this paper we report on the internal structure of the General Particle Tracer. In addition various pre- and postprocessors, combined in the integrated Windows 95/NT based graphical user interface, will be described.

The General Particle Tracer code applied to the Fusion Free-Electron Maser
Nucl. Instr. and Meth. B, Volume 139, Issues 1-4, (1998), p. 481-486
DOI: 10.1016/S0168-583X(98)00053-6

M.J. de Loos, S.B. van der Geer; Pulsar Physics
C.A.J. van der Geer, A.G.A. Verhoeven, W.H. Urbanus; FOM Institute for Plasma Physics "Rijnhuizen"

The fusion Free-Electron Maser (FEM) is the prototype of a high power, electrostatic mm-wave source, tunable in the range 130-260 GHz. In order to achieve a high overall efficiency, the charge and energy of the spent electron beam, i.e. the beam which leaves the undulator after interaction with the EM-wave, has to be recovered. A 50% overall efficiency is achieved, even for the maximum energy spread of 320 keV generated in the undulator, using a collection system consisting of a decelerator and a depressed collector.
The General Particle Tracer code (GPT) is being used as the major design tool for the whole Fusion FEM beam line, from the accelerator to the depressed collector. The high accuracy, ability to include FEL interaction and full 3D treatment make GPT the ideal choice for such a project. An overview of the separate sections and the use of GPT for each part of the FEM is presented. GPT is currently being applied to the design of the energy recovery system of the Fusion FEM. The first simulation results, including a 3D off-axis bending scheme and scattered incident electrons, are shown.

Applications of the General Particle Tracer code
Proceedings of PAC 1997, Vancouver, Canada, p. 2577.

S.B.van der Geer, M.J. de Loos, Pulsar Physics

The General Particle Tracer (GPT) code provides a new 3D simulation package to study charged particle dynamics in electromagnetic fields. Because of its modern implementation, GPT can be conveniently customized without compromising its ease of use, accuracy or simulation speed.
The most common use of GPT is accelerator and beam line design, especially the calculation of non-linear 3D space-charge effects. A typical example is the study of the effect of different bunch charges in a bend system.
One of the advanced GPT features is the possibility to solve additional differential equations while tracing particles. Using this mechanism, the generated EM wave power spectra in an undulator and the effect of beam-loading in a traveling wave linac are calculated self-consistently with the particle trajectories.
GPT is currently being applied to the design of the energy recovery system of the 2 MeV, 12 A Free Electron Maser under construction at FOM-Rijnhuizen. The required 99.8 % beam recovery and the initial energy spread of 300 keV ensure this to be a challenging GPT project.

General Particle Tracer: A new 3D code for accelerator and beamline design
Proceedings of EPAC 1996, Sitges, Spain, p. 1241.

M.J. de Loos, S.B. van der Geer, Pulsar Physics

We report on the particle tracking package General Particle Tracer (GPT). GPT is a new computer code to study charged particle dynamics in electromagnetic fields. To ensure both accuracy and speed, a fifth order Runge- Kutta method with adaptive stepsize control is used to solve the particle’s equations of motion in the time domain. Any additional differential equations can be solved while tracing the particles. The code is completely 3D, including the space-charge model. GPT uses an input file language supporting variables and expressions to describe the simulated set-up. Additional features include the complete freedom in initial particle distribution and the flexibility to position and orient all beam line components individually. Separate utility programs are used to calculate macroscopic quantities, produce ascii/graphical output and automate parameter scans. GPT is written in ANSI-C for portability and runs on Unix platforms and PC’s. Applications demonstrating the capabilities of GPT are presented.