Walter Scott, Jr. College of Engineering

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