AVS 62nd International Symposium & Exhibition
    Vacuum Technology Wednesday Sessions
       Session VT-WeM

Paper VT-WeM6
Design of a 250 KV DC Electron Gun Operating at Cryogenic Temperature

Wednesday, October 21, 2015, 9:40 am, Room 230B

Session: Accelerator and Large Vacuum Systems
Presenter: Xianghong Liu, Cornell University
Authors: X. Liu, Cornell University
I. Bazarov, Cornell University
B.M. Dunham, Cornell University
V.O. Kostroun, Cornell University
H. Lee, Cornell University
Correspondent: Click to Email
A photocathode DC electron gun is being built in our laboratory for ultrafast electron diffraction experiments. It is designed to operate at voltages up to 250 KV, with a maximum electric field of 12 MV/m in the cathode – anode gap. An inverted ceramic insulator is used for the high voltage insulation, making the gun relatively compact. In order to reduce the mean transverse energy of the electrons emitted from the photocathode, and hence increase the coherent length of the electron beam at diffraction, the photocathode is operated at 20 K instead of room temperature. A stainless steel thin wall tube connects the cathode holder to the insulator; it minimizes the heat load through conduction while providing a rigid support for the cathode holder. The cathode holder is cooled by a cold head through a sapphire rod of 225 mm length. The sapphire rod provides both sufficient electrical insulation and excellent thermal conduction at the temperature of interest. A load-lock system is attached to the back of the gun chamber for loading the photocathode to the gun. Photocathodes are transported into and out of the system through a vacuum suitcase. The load-lock system also provides storage for multiple photocathodes. The decay of the quantum efficiency of the photocathode is dominated by the existence of residual gases in the gun chamber, and thus vacuum level of 1x10-11 Torr or better is required to prolong the lifetime of the photocathode. The chamber is pumped by NEG pumps and ion pumps. We’ll describe details of the design and report initial test results.