The electron beam line consists of an 80-keV, 12-A thermionic triode electron gun, a
2-MV electrostatic accelerator, an undulator and a waveguide resonator mounted in a
high-voltage terminal, an electrostatic decelerator and a depressed collector. The entire
system is enclosed in an SF6-tank of 11 m length for high voltage insulation. Frequency
tuning is done by variation of the accelerating voltage, i.e., the terminal voltage.
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.
See also: Home
page of this project
Publications:
[ Ecaart'97 ] [ Pac'99 ] [ FEL'99 ]
Our
contribution: To increase the overall efficiency to over 50% and to reach a
pulse-length of at least 100 ms, we have designed an electron beam charge and energy
recovery system. This system consists of a 2 MV electrostatic decelerator and a
depressed collector, separated by a magnetostatic guiding field to transport the
low-energy electron beam. The EM-wave interaction inside the undulator can result in an
energy range between 50 keV to 300 keV downstream the
decelerator.
Detailed GPT simulation result from the undulator exit till the collector
entrance. 6000 particles are used in combination with the built-in point-to-point
space-charge model.
Depressed collector: The multi-stage
collector is designed so 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 backstreaming, hereby reducing
the efficiency, decreasing the pulse length and possibly damaging the machine. To reduce
this backstreaming to below a tolerable 0.1 %, an off-axis bending scheme, using a
rotating perpendicular magnetic field, has been investigated using GPT.
Multi-stage FEM depressed collector with deflection coils
and sample electron trajectories.
Collaboration: This project is commissioned by the FOM Institute for Plasma Physics
"Rijnhuizen".
