Ultrafast Electron Diffraction (UED) sources

Introduction: Ultrafast Electron Diffraction (UED) is an indispensible tool for the study of molecular dynamics and ultrafast chemistry. Despite several advantages of the use of electrons over the use of x-rays, UED has one crucial disadvantage: Coulomb interactions limit attainable pulse duration and dilute beam quality. Several schemes are being investigated with our GPT code to study and reduce the deteriorating effects of Coulomb interactions in various UED sources.

Megavolt electron energies to reduce space-charge forces by relativistic effects

Simulation of this approach with GPT is basically plug-and-play. All standard features borrowed from the high-energy use of GPT --such as import of rf-field maps, external lenses and accelerator structures-- are directly applicable. The Particle In Cell (PIC) space-charge model of GPT developed in collaboration with DESY and Rostock University is both efficient and accurate. A sample publication employing GPT to the 'MeV-UED' approach is listed below.

Ultrafast time-resolved electron diffraction with megavolt electron beams
Applied Physics Letters 89, 184109 (2006)
DOI: 10.1063/1.2372697

J. B. Hastings, F. M. Rudakov, D. H. Dowell, J. F. Schmerge, J. D. Cardoza, J. M. Castro, S. M. Gierman, H. Loos, P. M. Weber

Use only a few electrons per pulse, combined with a high repetition rate

The PIC space-charge model of GPT is useless in the case of far less than a thousand electrons per bunch. The relativistic point-to-point model is the method of choice to calculate space-charge and stochastic effects between just a few electrons. A sample publication is listed below, where the authors chose to model the rf-fields by analytical expressions.

Hybrid dc–ac electron gun for fs-electron pulse generation
New Journal of Physics 9 (2007) 451
DOI:10.1088/1367-2630/9/12/451

L. Veisz, G. Kurkin, K. Chernov, V. Tarnetsky, A. Apolonski, F. Krausz and E. Fill

We present a new concept of an electron gun for generating subrelativistic few-femtosecond (fs) electron pulses. The basic idea is to utilize a dc acceleration stage combined with an RF cavity, the ac field of which generates an electron energy chirp for bunching at the target. To reduce space charge (SC) broadening the number of electrons in the bunch is reduced and the gun is operated at a megahertz (MHz) repetition rate for providing a high average number of electrons at the target. Simulations of the electron gun were
carried out under the condition of no SC and with SC assuming various numbers of electrons in the bunch. Transversal effects such as defocusing after the dc extraction hole were also taken into account. A detailed analysis of the sensitivity of the pulse duration to various parameters was performed to test the realizability of the concept. Such electron pulses will allow significant advances in the
field of ultrafast electron diffraction.

Reversal of a 'ellipsoidal' Coulomb explosion by rf-techniques

This method relies on space-charge and well-chosen initial conditions to create an approximately ellipsoidal bunch. In theory this bunch can be recompressed by a downstream rf-cavity to conditions matching the photo-emission process. In practice pulse duration and beam quality are limited by an accumulation of small effects due to non-linear space-charge forces and non-linear external fields. In the paper listed below, the DC electrostatic field, solenoids and the rf-cavity are all imported into GPT as cylindrically symmetric field-maps, calculated by superfish. The time-consuming task of finding the optimal settings for all beamline components was partly automated with the GPT multi-dimensional optimizer.

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.

Collaboration: This project is commissioned by the Technical University of Eindhoven (TUE), Department of Physics, The Netherlands.

Future developments