Beam optics in the FXR 1.5 MeV, 4 kA injector

The 20 MeV, 4 kA linear induction accelerator (FXR), currently under construction at LLNL, will use a cold cathode injector. The foil cathode consists of a 0.03 mm tantalum ribbon wound in a spiral with successive turns spaced 0.8 mm apart. The emitted current will be 15 to 20 kA but will be reduced to 4 kA by collimating immediately beyond the anode. This will minimize the beam emittance and maintain a good impedance match with the low impedance driving source. Computer simulation was used extensively to optimize the configuration of the beam forming electrodes, the emitter geometry, and the overall focusing and beam transport system in the injector region. This paper will discuss the approach used and the problems encountered in the design of the diode geometry and of the overall beam optics. Preliminary experimental results will be presented and will be compared with the computer model predictions