Ccollector design

Introduction: The "Rijnhuizen" Fusion Free-Electron Maser (FEM) is the prototype of a high power, electrostatic mm-wave source, tunable in the range 130-260 GHz. The FEM has generated 730 kW output power during 10 micro-s pulses.
FEM design parameters: Typical value
Gun voltage 80 kV
Electron beam current 12 A
Electron beam energy 1.35-2 MeV
Rms xx’ emittance <10 p mm mrad
Pulse length 100 ms
Microwave frequency 130-260 GHz
Microwave net power 1 MW
Target system efficiency > 50%
Target current losses 20 mA
Linear gain per pass 7-10
Gain at saturation 3.5
Waveguide dimension 15x20 mm2
Waveguide mode HE11
Undulator period 40 mm
Undulator periods 20+14
Undulator field 0.20+0.16 T

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.

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.

part2.gif (6KB)
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.

Collector (23KB)
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".