Graduate Exam Abstract

Jonathan Edelen

Ph.D. Final

May 27, 2015, 1:00 pm - 3:00 pm

ERC Electronic Classroom (A210)

Theory, Simulation, and Mitigation of Electron Back-bombardment in Radio-Frequency Thermionic Cathode Electron Guns

Abstract: Photocathode RF guns are currently the standard for high- power, low-emittance beam generation in free electron lasers. These devices require the use of high-power lasers (which are bulky and expensive to operate) and high-quantum- efficiency cathodes (which have short lifetimes requiring frequent replacement). The use of RF-gated thermionic cathodes enables operation without a large drive laser and with long lifetimes. One major limitation of RF-gated thermionic cathodes is that electrons emitted late in the RF period will not gain enough energy to exit the gun before being accelerated back towards the cathode by the change in sign of the RF field. These electrons deposit their kinetic energy on the cathode surface in the form of heat, limiting the ability to control the output current from the cathode. This dissertation is aimed at understanding the fundamental design factors that drive the back-bombardment process and at exploring novel techniques to reduce its impact on a high-current system. This begins with the development of analytic models that predict the back-bombardment process in single cell guns. These models are compared with simulation and with measurements taken at other facilities published in the literature. This is followed by the development of analytic models that predict the effects of space charge on back- bombardment. This shows that the longitudinal space charge forces decrease the effective accelerating field while the transverse space charge field decreases the effective area of the beam that impacts the cathode. An analysis of how the addition of multiple cells will impact the back-bombardment process shows that the addition of a second cell that is independently phased can be tuned to not increase the back- bombardment power while at the same time increasing the average energy output of the gun. Finally, a two-frequency gun is studied for its ability to mitigate the back- bombardment process.

Adviser: Stephen Milton
Co-Adviser: Sandra Biedron
Non-ECE Member: Thomas Johnson, Environmental and Radiological Health Sciences
Member 3: Branislav Notaros, Electrical and Computer Engineering
Addional Members: N/A

J. P. Edelen et al, Electron Back-bombardment and mitigation in a short gap thermionic cathode RF Gun IEEE Transactions in Nuclear Science, Volume 61 Issue 2

J. P. Edelen et al, Theory and simulation of backbombardment in single-cell thermionic-cathode electron guns. Physical Review Special Topics – Accelerators and Beams (Volume 18 – Issue 4)

Program of Study:
PHY 963
GSTR 600
ECE 581A3
ECE 641
ECE 680A3
ECE 799
ENGR 697
ECE 795