MO P1-27.pdf - University of Maryland

Controlled Porosity Reservoir Cathodes and Photocathodes
R. Lawrence Ives a , Eric Montgomery b , Blake Riddick b , Zhigang Pan b ,
Lou Falce a David Marsden a , George Collins a
a Calabazas Creek Research, San Mateo, CA 94404 USA
b University of Maryland, College Park, 20742 MD USA
Abstract—Research is continuing to develop controlled
porosity reservoir cathodes and photocathodes. Advances in
design tools and fabrication techniques will be presented as well
as available test results for thermionic cathodes and
photocathodes.
I. INTRODUCTION AND BACKGROUND
HE development of material with a uniform array of
Tmicron scale pores is providing new opportunities for
improved cathodes and photocathodes. Reservoir cathodes
provide significantly greater lifetime than can be achieved
with traditional impregnated cathodes. The tungsten material
with uniform, user-defined porosity provides the reservoir cap
whose structure controls the rate at which work function
reducing material diffuses to the emission surface. The
uniformity of the pore array ensures unprecedented uniformity
of electron emission.
The significance of this development for thermionic
cathodes was recognized with a 2011 R&D 100 award [1].
Research clearly demonstrated the potential for dramatically
improved thermionic cathodes [2]. Research is continuing to
extend the applicability of controlled porosity reservoir (CPR)
technology using segmented cathodes and improved design
and fabrication processes.
The potential for photocathodes was demonstrated by recent
experiments at the University of Maryland, where the lifetime
of cesiated tungsten photocathodes was increased by more
than two orders of magnitude [3]. Photocathode lifetime is a
major issue for electron guns using laser initiated electron
emission. This presentation will describe recent research on
CPR cathodes and photocathodes.
demonstrating the concept and confirming emission
performance and lifetime. More recent research focused on
improving both the design tools and the fabrication
procedures.
CPRCd is a free computer design tool that calculates emitter
and reservoir specifications to achieve a desired emission
current density and lifetime. It calculates Knudsen Flow
through the emitter material at the temperature required for the
specified emission current density.
Figure 1. Sintered wire material for cathode emitters
Research also focused on improving the fabrication and
brazing of the emitter material and new assembly processed
for very small cathodes as well as larger cathodes for
magnetron injection guns (MIGs) and high power klystrons.
Current research is focused on developing new electron guns
for advanced electron beam devices.
II. FABRICATION
CPR cathodes utilize a uniform porosity tungsten emitter
over a reservoir of work function reducing material [4]. For
thermionic cathodes, this is usually barium calcium aluminate.
For photocathodes, this is typically cesium chromate -
titanium. The emitter material is created by winding 20 micron
diameter tungsten wire around a molybdenum form and
sintering to bond the wires. The material is then sliced
perpendicular to the grain to form the cathode emitters. Figure
1 shows a microphotograph of an emission surface.
The cathodes are formed by sealing a reservoir of barium or
cesium at one end with the cathode/photocathode emitter and
the other end with a heater. A schematic diagram is shown in
Figure 2.
Initial research utilized thermionic cathodes designed for
traditional Life Test Vehicles. This research focused on
Figure 2. Schematic of controller porosity reservoir
cathode
III. CATHODES AND PHOTOCATHODES
Fabrication of both a magnetron injection gun (MIG)
cathode and high power gridded cathodes is in progress. The
MIG cathode will be implemented in a 28 GHz gyrotron at
Karlesruhe Institute of Technology, and the high power
klystron cathode is targeted for a 5 MW, X-Band klystron.
CCR recently completed a 15-beam electron gun for an X-
Band klystron. The cathodes operate with an emission current

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.