Cathode Fabrication for UH Manoa's Electron Gun*

We report on the fabrication and testing of five thermionic cathode stem (heater and cathode button) prototypes for the microwave electron gun of the University of Hawaii at Manoa’s Free Electron Laser (FEL). These designs followed the general instruction written by Jeremy Kowalczyk and John Madey. Using LaB₆ cathode crystals, spare parts, and new tools, we were able to fabricate five stems. Two of these stems passed powering tests in which we extrapolated resistance and temperature over the course of increasing the input power. At first, the temperature of the cathode was estimated by analyzing the change of resistance in the heater coil as the power ramp-up proceeded. A pyrometer was later purchased and installed in the beamline to procure a more accurate temperature reading. Furthermore, modeling and simulation programs are being used to test the fabricated cathodes to alter designs for better optimization. The heater coil’s geometry has a strong effect on powering, temperature distribution, and consistency. So far, there are six 3D models that are being prepared for simulation. The last fabricated cathode was installed into the gun cavity and is currently being used in the recommissioning of the linear accelerator (linac) and FEL.