Undergraduate Research at Jefferson Lab

Creation of Jefferson Lab 200kV Electron Gun Upgrade

Student: Elliott Holliday

School: NC State University

Mentored By: Carlos Hernandez-Garcia

At Jefferson Lab, electrons are accelerated to near the speed of light using the Continuous Electron Beam Accelerator Facility (CEBAF). The electron beam is generated using an electron gun composed of a highly polarized GaAs/GaAsP superlattice photocathode with an operating voltage of 130kV in an ultra-high vacuum with an internal pressure of approximately 10^-12 torr. The electron gun is geometrically designed in such a way to limit the electric field below 10 MV/m at every point in the gun. Our objective is to construct and install an upgraded electron gun capable of running at 200kV in an ultra-high vacuum with zero field emission. We started the design of the gun by focusing on a specific geometry of the electrode to prevent high voltage cable breakdown. We went through a multi-step vacuum creation process that included three different types of pumping, a bake at 250° C, and the activation of the non-evaporable getters (NEGs). Afterwards, we did a high voltage conditioning of the gun where we slowly raised the voltage until we no longer saw electron field emission. Using the vacuum protocols, we achieved a vacuum of 10^-11 torr, which is reasonably acceptable, but not quite the desired value. During our high voltage conditioning, we saw slight field emission around the operating voltage of 200 kV, making the gun unfit for installation into CEBAF. This can be improved with further conditioning at higher voltages, but due to the limits of our power supply, we did not feel we had adequate head room to continue voltage conditioning. With this upgraded gun, we can give the accelerator higher beam quality and extended photocathode lifetime.