Walter Scott, Jr. College of Engineering

Graduate Exam Abstract

Jonathan Edelen
Ph.D. Preliminary
Nov 03, 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
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
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