Introduction and Goals Creation of Jefferson Lab 200 kV Electron Gun Upgrade Elliott G. Holliday, North Carolina State University, Raleigh, NC Carlos Hernandez-Garcia, Thomas Jefferson National Accelerator Facility, Newport News, VA Abstract Design Philosophy High Voltage Results (continued) 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 with an operating voltage of 130kV in an ultra-high vacuum with an internal pressure of approximately 10-12 torr. 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. Using the vacuum protocols, we achieved a vacuum of 10-11 torr, which is not 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. With this upgraded gun, we can give the accelerator higher beam quality and extended photocathode lifetime. High Voltage Conditioning Started from 50kV, slowly turning up the voltage in small increments in order to not damage the electrode and/or the high voltage cable and not to trip the power supply We saw our first sign of field emission at 181 kV Field emitter eliminated at 216 kV Continued to condition by letting the gun “soak” between 200 and 210 kV to get the gun used to operating voltage We wanted to design the gun in a way to reduce the electric field at every point in the gun Our maximum limit for the electric field at any point was 10 MV/m Learned by rigorous testing and experimentation Limiting the E-field at all points reduces the chances of electrical arcs and high voltage cable breakdown Results Figure 4. Ch. 8 Radiation vs. Voltage. Here, we were measuring the radiation from the field emission from a specific area of the gun. We can see a steep increase in radiation in the 200kV range, signifying the presence of a field emitter. Introduction and Goals Final Vacuum Pressure: 10-11 torr range Still slight field emission at 200 kV range that is getting worse over time Current electron gun in CEBAF operates: At an internal pressure of ~10-12 torr At 130kV No field emission at running voltage Requested upgraded gun requirements: Ultra-high vacuum (~10-12 torr) At 200kV Reasons for upgrade Higher beam quality Better signal-to-noise ratio and improved statistics Improved photocathode lifetime Conclusion We need to create a better vacuum in the gun chamber Need to take the gun to higher voltages in order to reduce the amount of field emission Due to our power supply limits, we didn’t feel comfortable going to higher voltages due to the lack of head room Figure 3. Turbo Ion Pump vs. Time. We didn’t turn on the pump until 19 hours after we started the bake. During the time of the bake, we did achieve low 10-9 torr pressure using the ion pump. Figure 2. Poisson simulation of the electric field inside the gun. Looking at the value of “E,” we see that the electric field doesn’t exceed 10 MV/m Vacuum Protocols Acknowledgements Ultra-high vacuum is required for two main reasons: Ionizing particles in the chamber will cause photocathode damage and decreased beam quality Electron-particle collisions could cause the beam to scatter before it leaves the gun chamber Vacuum Stages Mechanical – gets down to 0.1 torr Turbo – 10-8 torr Ion – 10-10 torr Bake and Full NEG Activation – Low 10-12 torr Thomas Jefferson National Accelerator Facility Dr. Carlos Hernandez-Garcia Donald (Bubba) Bullard Gabriel Palacios-Serrano Injector/Source Group Lisa Surles-Law Old Dominion University North Carolina State University National Science Foundation This work is made possible through support from NSF award 1659177 to Old Dominion University Figure 4. Anode Current vs. Voltage. We monitored this to make certain that no field emission was hitting the anode, which could decrease photocathode lifetime. We saw a general decrease with increasing time and voltage. Figure 1. Cross-sectional view of the upgraded electron gun.