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Graduate Exam Abstract


Jonathan Edelen

Ph.D. Preliminary

November 3, 2014, 9:00 am - 11:00 am

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 analysis of a specific gun design under a wide range of operating parameters. Space-charge effects in this design are then studied to determine under what conditions the gun operation will deteriorate as a result of space-charge-limiting current. Following this, a more general first-principles theory that is applicable to any single- cell thermionic gun is derived and compared with simulations. The use of a harmonic frequency for back- bombardment mitigation is then studied through simulation. Finally, extensions of the current theory to allow for a more general treatment of the RF fields are proposed. These theories are aimed at producing a framework for predicting back-bombardment mitigation performance in a multi- frequency gun, traveling-wave gun, or a multi-cell single- frequency gun. These theories will be benchmarked against simulation and, if possible, compared to measurements of existing thermionic cathode systems.

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

Publications:
J. 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)


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