Projection ablation lithography cathode for high-current, relativistic magnetron

Abstract

Initial results are presented of an innovative cathode operating in a relativistic magnetron powered by an accelerator with parameters: −0.3 MV−0.3MV, 1–10 kA1–10kA, and 0.5 μs0.5μs pulse length. This cathode is fabricated by ablating a pattern on the cathode using a KrF laser. This projection ablation lithography (PAL) cathode has demonstrated fast current turn-on and microwave startup times have decreased from an average of 193 to 118 ns193to118ns. The pulselength of 1 GHz1GHz microwave oscillation has increased from a 144 ns144ns average to 217 ns217ns. With these improvements in microwave startup and pulse length, the microwave power has approximately remained the same compared to the previously used cloth cathodes. A new triple-azimuthal emission region is tested as means of prebunching the electrons (“cathode priming”) into the three spokes desired for pi mode operation in a six-cavity magnetron. The Tri-PAL cathode priming results in the fastest startup and highest efficiency of relativistic magnetron microwave generation. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/70745/2/RSINAK-75-9-2976-1.pdf

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Projection ablation lithography cathode for high-current,
relativistic magnetron
M. C. Jones, V. B. Neculaes, R. M. Gilgenbach,a)W. M. White,
M. R. Lopez, and Y. Y. Lau
Nuclear Engineering and Radiological Sciences Department, Intense Energy Beam Interaction Lab,
University of Michigan, Ann Arbor, Michigan 48109-2104
T. A. Spencer
Air Force Research Laboratory, Phillips Research Lab, Kirtland AFB, New Mexico
D. Price
Titan Corporation
(Received 5 March 2004; accepted 29 May 2004; published 14 September 2004)
Initial results are presented of an innovative cathode operating in a relativistic magnetron powered
by an accelerator with parameters: −0.3 MV, 1–10 kA, and 0.5 ?s pulse length. This cathode is
fabricated by ablating a pattern on the cathode using a KrF laser. This projection ablation
lithography (PAL) cathode has demonstrated fast current turn-on and microwave startup times have
decreased from an average of 193 to 118 ns. The pulselength of 1 GHz microwave oscillation has
increased from a 144 ns average to 217 ns. With these improvements in microwave startup and
pulse length, the microwave power has approximately remained the same compared to the
previously used cloth cathodes. A new triple-azimuthal emission region is tested as means of
prebunching the electrons (“cathode priming”) into the three spokes desired for pi mode operation
in a six-cavity magnetron. The Tri-PAL cathode priming results in the fastest startup and highest
efficiency of relativistic magnetron microwave generation. © 2004 American Institute of Physics.
[DOI: 10.1063/1.1784561]
I. INTRODUCTION
There currently exist two types of microwave tube tech-
nology:
(a)
commercial,
cathode, repetitively-pulsed, or cw tubes, and
experimental, high-power-microwave (HPM) tubes
which operate single pulse, with poorer vacuum (typi-
cally
?10−6Torr)
and
cathodes.
high-vacuum,long-lifethermionic-
(b)
expendable
(cloth/fabric)
The device presented in this article strictly falls into the sec-
ond category. However, experimental results reported here
use a ceramic insulator (base vacuum on the 10−8Torr scale)
and all-metal cathode (no cloth), which represent the first
steps in a research program at The University of Michigan
(UM) to apply the principles of commercial tubes to experi-
mental HPM devices.
One of the most challenging requirements of high power
microwave sources is high-current, cold cathodes. Extensive
previous research has been performed on such cathodes over
the past 30 years.1–7A variety of cathodes were investigated
in the past, including:
(1) carbon fiber tufts,1,6–9
(2) velvet (carbon, cotton or polyester),4,6
(3) carbon felt,10
(4) plasma,5and
(5) bare metal.2
The ideal cathode for an HPM device would be one that
exhibits the following properties:
(a)
(b)
high current densities ?kA/cm2?;
rapid current turn on at moderate electric fields
??100 kV/cm?;
little or no plasma production (after conditioning or
cleaning), resulting in no diode impedance collapse
over microsecond pulselengths; and
survivability under intense, microsecond current gen-
eration, electron and ion backbombardment, as well as
postpulse arcs.
(c)
(d)
Until recently, these ideal cathode properties appeared unat-
tainable. However, two recent cathode developments have
shown that these ideal cathode goals may be technically fea-
sible. The first type of cathode is Schiffler’s carbon–fiber,
cesium iodide-coated cathode.8,9This cathode has been
shown to generate long-pulse, high power microwaves in a
relativistic magnetron. The Schiffler cathode generates high
current densities and survives even under repetitively pulsed
operation.
The second, innovative new type of cathode, demon-
strated in these preliminary experiments at UM is the Projec-
tion Ablation Lithography (PAL) cathode. The PAL cathode
utilizes microtexturing of a solid metal cathode substrate to
a)Electronic mail: rongilg@umich.edu
REVIEW OF SCIENTIFIC INSTRUMENTSVOLUME 75, NUMBER 9SEPTEMBER 2004
0034-6748/2004/75(9)/2976/5/$22.002976© 2004 American Institute of Physics

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provide field enhancement. The advantages of this cathode
are that, unlike microtips which can overheat, explode and
create plasma, the PAL cathode emission regions can be mi-
cromachined with limited electric field enhancement while
the emission regions are “heat-sinked” to the base cathode
material. Furthermore, the PAL cathode consists of pure
metal so after an initial bakeout or plasma cleaning (or a few
conditioning shots) there is negligible gas emission (unlike
fabric cathodes).
We have demonstrated for the first time that by ablating
three azimuthal regions of active current emission on the
cathode (Tri-PAL), that the electron beam can be pre-
bunched into three spokes. This process is defined here as
“cathode priming” of the pi mode for a six-cavity magnetron.
II. EXPERIMENTAL CONFIGURATION
The relativistic magnetron is driven by the Michigan
Electron Long Beam Accelerator Ceramic (MELBA-C), with
operating parameters: voltage flattop of −300 kV, entrance
current of 3–9 kA, and a pulse length of 300 ns. The ce-
ramic insulator results in a base vacuum on the 10−8Torr
scale, a factor of 100 lower than most HPM devices. Rela-
tivistic magnetron experiments at UM have been performed
using a six-vane Titan Pulses Sciences tube [Fig. 1(a)]. The
Titan tube is based on an unstrapped A6 geometry, with
rounded stainless-steel vane tips. The pi mode frequency is
about 1.04 GHz and the 2/3 pi mode resonant frequency is
980 MHz.11Three L-Band (WR-650) waveguides are at-
tached to the magnetron for extraction of microwaves as seen
in Fig. 1(a). Figure 1(b) shows the cross sectional view of the
magnetron with the cathode orientation. Also shown are the
two electromagnets, which provide an axial magnetic field;
(same value for all shots: 3 kG). The cathode is shown in
Fig. 1(c), with the emission region centered inside the mag-
netron vanes. Endcaps are located at the end of the cathode
and before the magnetron to help prevent electron end-loss
current.
Microwaves are extracted out of three of the six cavities
in the magnetron, each spaced 120° from one another; this
favors pi-mode operation. The microwaves then travel
through Lucite windows into L-band waveguide. In one of
the waveguides a loop-coupler extracts power and the micro-
wave signal is transmitted to the screen room through RG-
214/U cable. A zero-area B-dot loop is located on the wave-
guide. The remaining two waveguides are terminated in
water loads. Microwave signals are heterodyned with a local
oscillator at 1.3 GHz. This heterodyne signal is sent to a fast
oscilloscope and data are processed in a time frequency
analysis program to give the frequency of the microwaves as
a function of time.12Incoming diode current, endloss elec-
tron current, and voltage traces are also measured and sent to
the screen room.
Previous cathodes used in the relativistic magnetron at
UM were made of an aluminum alloy (Al 6061); after the Al
rod was sanded, either carbon or cotton fibers were glued
onto the cathode in the middle of the interaction region.
Problems with these cathodes included outgassing of the fi-
bers and glue that held them on the cathode, resulting in
short microwave pulse lengths. To alleviate problems with
the carbon/cotton cathodes, the new technique was employed
wherein a pattern was ablated onto the aluminum cathode to
provide the field enhancement without the outgassing of
cloth.
III. FABRICATION TECHNIQUE
To fabricate the cathode, a pattern needed to be projected
onto the cathode at sufficiently high laser intensity to ablate
the surface. In order to accomplish this, a 600 mJ/pulse KrF
laser was employed. The cathode stalk (Al 6061 alloy,
1/2 in. diameter) was sanded smooth using 600, 1500, and
then 2500 grit sandpaper. After being sanded the cathode rod
was cleaned using acetone and methanol. Figure 2 shows the
experimental setup of the KrF laser, focusing lens (25 cm
focal length), projection mask (wire mesh with 914 ?m wire
diameter with 52% open area), and the cathode orientation.
The ablation laser is a KrF laser (Lambda-Physik Compex
FIG. 1. Experimental configuration of: (a) the A6 magnetron structure with
orientation of the cathode and the three WR-650 waveguides, (b) cross
section of the magnetron, cathode, and the magnet coil orientation, and (c) a
close up the PAL cathode with the endcaps, emission region, and the non-
emitting regions.
FIG. 2. Experimental configuration used in fabricating the cathodes.
Rev. Sci. Instrum., Vol. 75, No. 9, September 2004Projection ablation lithography cathode2977

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205) operating at 20 Hz, 400 mJ, 248 nm wavelength, and a
20 ns pulse length. The ablated pattern on the cathode has a
size of 9.1 mm2. After the laser was fired for one minute
(1200 shots) the Al cathode rod was rotated by 45° and the
laser was fired for another minute; after rotating 360° the
cathode was shifted 1 mm along its axis, and the process was
repeated. The total area of the pattern on the cathode was
initially 4 cm2(PAL-I) and then changed to 6 cm2(PAL-II)
to increase the emitted current.
A cross-sectional view of the cathode is shown in Fig.
3(a). This view shows the ablated regions on the cathode.
Figures 3(b) and 3(c) show scanning electron microscope
(SEM) images of the ablated pattern. From Fig. 3(b) the
trench width is approximately 180 ?m. The depth of the ab-
lated regions is approximately 50 ?m. Figure 3(c) shows the
particles that have been redeposited on the high points of the
cathode because the cathodes were processed in atmospheric
air. These redeposited particles may contribute to the en-
hancement of the electric field.
Three different PAL cathodes have been used to date as
seen in Fig. 4. PAL-I, shown in Fig. 4(a), had the smallest
area of 4 cm2, the axial length was 1 cm. Figure 4(b) shows
the PAL-II cathode, in which the area was increased by 50%
to 6 cm2and the surface was polished smooth with a com-
pound after sanding, but before laser ablation. Tri-PAL, seen
in Figs. 4(c) and 4(d), has the same area as PAL-II, 6 cm2,
but utilizes three emission regions, spaced 120° apart to gen-
erate the desired three-electron spokes, to help promote
faster microwave startup time for the pi mode in the six-
cavity magnetron; “cathode priming.”
No in situ bakeout was employed in these experiments.
Note that the emitting region of the cathode is heated to
vaporization by the laser ablation.
IV. EXPERIMENTAL DATA AND ANALYSIS
The initial projection ablation lithography (PAL-I) cath-
ode tested had an emission area of 4 cm2. The observed elec-
tron current was too low for the generation of high-power
microwave oscillation; the current averaged 2.8 kA. Previous
work using a cotton cathode showed that there needed to be
between 4 and 6 kA electron beam current to generate high-
power microwaves. The power on the first shot of PAL-I was
100 MW, however, the average power of the remaining 16
shots was 15 MW. The increased current and power on the
first shot was probably due to initial release of surface con-
taminants. Although the source of plasma is believed to be
primarily hydrogen, no residual gas analyzer (RGA) was
used in the present experiments. Previous UM experiments
utilizing an RGA with metal cathodes and a plastic insulator
showed that the most abundant contaminant was hydrogen.13
FIG. 3. Laser ablated pattern on the cathode: (a) shows the cross section
where the trench width is 180 ?m and the depth is approximately 50 ?m.
(b) and (c) are SEM images of the ablated pattern which shows the trench
created by the laser and the particles which were redeposited on the sample.
FIG. 4. Ablated pattern orientation on the three cathodes. (a) and (b) were a
solid pattern around the circumference of the rod, with areas of 4 and 6 cm2.
(c) Tri-PAL cathode which has three emission regions around the cathode,
shown in (d); which is a cross section of the cathode showing the emission
regions spaced 120° apart.
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Plasma cleaning experiments at UM showed that this H con-
taminant could be reduced or eliminated if the plasma clean-
ing was operated until within a few seconds of the HPM
pulse.2The heterodyned microwave signal for the first shot is
shown in the upper half of Fig. 5. This heterodyned signal,
after being processed in the time frequency program, shows
the frequency versus time, shown in the lower half of Fig. 5.
The frequency for this shot was 1.014 GHz, which is the pi
mode frequency.
To increase the electron current, PAL-II cathode was
fabricated having a larger emission area, increased from
4 to 6 cm2. The new average current was 3.7 kA. This in-
crease in current led to the microwave power increasing by a
factor of 2.5, from 15 to 40 MW. Figure 6(a) shows the
entrance current and voltage for a typical PAL-II cathode
shot; the voltage has a flattop around 300 kV, and the current
ramps from 3 to 6 kA on the voltage flattop. The average
microwave startup time was 126 ns, with a standard devia-
tion of 22 ns. The microwave startup is defined here as the
time microwave oscillation starts relative to the time at 10%
of the voltage pulse maximum amplitude. The average pulse
length was the longest for the PAL-II cathode with the mi-
crowave oscillation lasting 288 ns.
The Tri-PAL cathode was made with three emission re-
gions to promote faster startup (cathode priming) by preb-
unching the electrons into three spokes to generate the pi
mode inside the six-cavity magnetron.14,15The average mi-
crowave startup time decreased to 110 ns with the Tri-PAL
cathode and the standard deviation of the startup was re-
duced to 9.6 ns. This is nearly the fastest microwave startup
time possible with this voltage pulse, since the voltage rise
time is now on the order of the microwave startup time.
Another important property of the Tri-PAL cathode is that in
approximately 40% of the shots, the current followed the
voltage with no measurable plasma closure, as seen in Fig.
6(b).
Closure velocities of the plasma were calculated for the
three PAL cathodes for comparison to shots of the previous
cotton cathodes. The closure velocity is found by plotting
1/?P, where P is the perveance, and comparing this to the
measured values. The perveance is found from Eq. (1) and is
derived from Langmuir16and Dow.17This equation is simply
the Child–Langmuir law in one dimension (1D) for currents
limited by space charge between coaxial cylinders
I ? 14.68? 10−6LV3/2
r??2,
where L is the effective length of the emission region, rpis
the anode radius, and ? is a function of the anode radius and
the cathode radius which varies with time and the closure
velocity.
Figure 7 plots 1/?P for the two shots shown in Fig. 6.
Figure 7(a) shows the case for the single-emission region
cathode in which plasma closure existed, with a velocity of
2.35 cm/?s. Figure 7(b) shows the Tri-PAL cathode case
when the current followed the voltage trace; as seen in Fig.
6(b), the closure velocity here was found to be 0.5 cm/?s.
The average closure velocity of the cotton cathode was
2.7 cm/?scomparedtoPAL-I
2.1 cm/?s, and PAL-II closure velocity of 2.9 cm/?s.Acru-
cial finding for the Tri-PAL cathode is that there were two
distinct regions for the closure velocity. Region one occurred
when plasma closure existed, which led to an average veloc-
ity of 1.9 cm/?s; whereas in the second region the closure
was extremely low, averaging 0.2 cm/?s, with some shots
showing zero plasma closure (shots in which the slope of
1/?P is positive).
?1?
closurevelocityof
V. DISCUSSION
Figure 8 compares the performance of two types of PAL
cathodes, single, and triple emission regions, as well as the
cotton fiber cathode. (Data are not compared to previous car-
bon fiber cathode; since that cathode was utilized in a two-
waveguide extraction and the table only compares three-
waveguide extraction.) From the figure it is shown that the
microwave startup time and microwave pulselength improve
dramatically with PAL cathodes, while the single-waveguide
power levels have roughly remained constant. Power mea-
FIG. 5. (Top) Heterodyned microwave signal for PAL-I cathode. (Bottom)
Time frequency analysis of heterodyned microwave signal, showing a peak
frequency of 1014 MHz, which corresponds to the pi mode frequency.
FIG. 6. Voltage and current traces for cathodes with: (a) single emission
region and (b) triple emission regions (Tri-PAL).
Rev. Sci. Instrum., Vol. 75, No. 9, September 2004Projection ablation lithography cathode2979

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surements for the single emission PAL cathodes are not
shown in Fig. 8. The microwave power signals were ex-
tremely noisy and therefore could not be compared directly
with other cathodes using the same 60 MHz low-pass filter.
Peak microwave electronic efficiency ?Pmicrowaves/IV? was
13% for the cotton cathode, 9% for the single emission PAL
cathode, and was increased to 17% for the Tri-PAL cathode.
The efficiency calculations assume that all three waveguides
are extracting the same microwave power, and therefore the
microwave power signals is multiplied by three.
In conclusion, a projection ablation lithography (PAL)
high-current cathode has been developed and demonstrated
to generate high-power microwaves in a relativistic magne-
tron. This cathode demonstrates rapid-current turn-on, low
vacuum base-pressure and no significant degradation over
10’s of shots. A new triple-emission region (Tri-PAL) cath-
ode has been shown to provide a technique for “cathode
priming”of a magnetron to prebunch the electrons into the
desired three spokes for the pi mode in a six-cavity magne-
tron. The microwave starting time for the Tri-PAL cathode is
the lowest of the cathodes tested and the electronic efficiency
is the highest.
Future research will focus on the fundamental electron
emission mechanisms of the PAL cathode and the physics of
shots that exhibit zero closure and correlation to the plasma.
Future experiments will utilize an RGA to further study gas
emission from PAL cathodes. Also the angular orientation of
the Tri-PAL cathode represents an important future area of
interest. Processing of PAL cathodes in vacuum will reduce
the redeposited particles.
ACKNOWLEDGMENTS
This research has been supported by the Air Force Office
of Scientific Research and by the Air Force Research Lab
(AFRL). The authors also acknowledge equipment loans
from AFRL.
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FIG. 7. Plots of 1/?P of the same shots as in Fig. 6, where P represents the
perveance. (a) Single emission case when there is plasma closure, with a
velocity of 2.35 cm/?s. (b) Triple emission case (Tri-PAL) when the plasma
closure velocity is reduced to 0.5 cm/?s, e.g., when the current nearly fol-
lows the voltage.
FIG. 8. Comparison of the PAL cathode, single emission region, and triple
emission region (Tri-PAL) with the previous cotton fiber cathode. Micro-
wave power measurements are from a single-waveguide; total extracted mi-
crowave power is expected to be a factor of 3 higher than shown.
2980Rev. Sci. Instrum., Vol. 75, No. 9, September 2004Jones et al.