MO P1-27.pdf - University of Maryland

density of 31 A/cm 2 and eliminate requirements for beam
compression. This dramatically simplified the gun design and
improved the beam quality. The estimated lifetime exceeds
90,000 hours.
Figure 3. Photograph of 15-beam electron gun with CPR
cathodes operating at 30 A/cm 2
Figure 4 shows a solid model of the segmented MIG being
built for KIT. The gun will be used to study the effects of nonuniform
cathode emission on
gyrotron performance. The
current emitted from each
segment will be externally
controlled.
Photocathode research is
focused on extending the
reservoir concept to
photocathode materials with
higher quantum efficiency.
The Figure 6 shows current Figure 4. Solid model of
performance of photocathodes,
including the recent advance
for cesiated tungsten. CPR
segmented MIG using CPR
cathodes
photocathodes use a modified version of the configuration
shown in Figure 2. Because of the increased volatility of
cesium, the reservoir is capped by a section of sintered
tungsten powder. This is followed by a mixing section and
controlled porosity material for uniform cesium distribution
over the surface. The unique feature of this configuration is
that the cesium diffusion can be precisely controlled to match
the surface evaporation rate.
Current research is investigating evaporative and atomic
layer deposition to deposit higher quantum efficiency
materials onto the CPR tungsten material. Experiments will
measure both the quantum efficiency of the photocathodes and
the evaporation rate of cesium. This will allow estimation of
the lifetime. The goal is to increase the lifetime of high
quantum efficiency materials by two orders of magnitude or
more.
Cathode and photocathode designs will be describes as well
as available experimental results.
Figure 6. Quantum efficiency and lifetime for common
photocathode materials. Some cathodes have longer
reported lifetimes under high vacuum, so the graph is
normalized to a nanoTorr, assuming contaminationlimited
lifetime scales linearly with chamber pressure.
Figure 5. CPR configuration for cesiated photocathodes
IV. SUMMARY
CPR cathodes provide increased performance for both
thermionic and photocathodes. This will lead to improved
electron guns for electron beam devices
V. ACKNOWLEDGEMENTS
This research was supported by U.S. Department of Energy
grants Nos. DE-FG02-04ER83918, DE-SC0006208, DE-
SC0004570, and DE-SC0009583, and by the U.S. Army
Research Office under contract W911NF-08C-0051 as part of
the DARPA HiFIVE program.
REFERENCES
[1] “Controlled porosity elevates performance, lifetime of cathodes,” R&D
Magazine, Vol. 53, No. 5, September 2011.
[2] R. Lawrence Ives, Senior Member, IEEE, Louis R. Falce, George
Miram, George Collins, “Controlled Porosity Cathodes for High Current
Density Applications,” IEEE Trans. Plasma Sci., Special Edition on
High Power Microwave Sources, Vol. 38, No. 6, pp. 1345-1353, June
2010.
[3] Eric Montgomery, et al, “Enhanced Lifetime Hybrid-Diffuser Cesium
Reservoir Photocathode,” Advanced Accelerator Concepts Workshop,
Austin, TX June 2012.
[4] R.L. Ives, L.R. Falce, S. Schwartzkopf, and R. Witherspoon, “Controlled
Porosity Cathodes from Sintered Tungsten Wires, IEEE Trans. On
Electron Devices, Vol. 52, No. 12, pp. 2800-2805, December 2005.