1. 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 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 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: 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 – torr
Bake and Full NEG Activation – Low 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 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.