Development of a 10 MW, 91 GHz
Gyroklystron
R. Lawrence Ivesa, Wesley Lawsonb, Jeff Neilsona, Michael Reada, Max
Mizuharaa, Bart Hoganb, David Marsdena, Tom Robinsona
(a) Calabazas Creek Research, Inc.
20937 Comer Drive, Saratoga, CA 95070-3753
(408) 741-8680, Fax: (408) 741, Email:[email protected]
(b) Institute for Research in Electronics and Applied Physics
University ofMaryland,223 Paint Branch Road
College Park, MD 20742
Abstract. A 10 MW gyroklystron is being developed for W-Band accelerator research by
Calabazas Creek Research, Inc. through a grant from the U.S. Department of Energy. The
device will operate at 91.386 GHz and be tested at Stanford Linear Accelerator Center. The
gyroklystron will operate at 500 kV and produce 1.5 microsecond pulses at a 120 Hz repetition rate. Predicted gain and efficiency are 56 dB and 36%, respectively. The circuit consists
of six cavities, two operating in the fundamental TE01 mode and the remaining four at the
second harmonic TE02 mode. The output is converted to a combination of modes that allow
propagation across a 1 cm gap between the body and collector, allowing depressed collector
operation. The mode combination also allows implementation of a right angle mirror to redirect the output power into a vertical window and protect the window ceramic from electron
impact. The gyroklystron is currently being constructed for initial testing. Progress to date is
reported.
Introduction
An international effort is underway to design advanced generations of linear
electron-positron colliders with anticipated center of mass energies of 0.5 TeV and
beyond. While it appears conventional state-of-the-art klystrons at 11.424 GHz may
be suitable for the 0.5 TeV energy level [1], the expected performance requirements
for RF drivers of linear colliders with energies above 1 TeV are well beyond the
state-of-the-art in amplifier technology. Many believe, for example, that a 5 TeV collider extrapolated from current RF-based structures will require a drive frequency
somewhere between Ka-Band (e.g. 35 GHz) and W-Band (e.g. 91 GHz) [2].
Several novel source concepts are under development; however, none of these
devices have demonstrated all the necessary driver requirements for advanced colCP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli
© 2002 American Institute of Physics 0-7354-0102-0/02/$19.00
408
liders.
liders. Of
Of these
these requirements,
requirements, the
the most
most difficult
difficult appear
appear to
to be
be the
the ability
ability to
to produce
produce
the
the required
required peak
peak power
power and
and efficiency.
efficiency. To
To keep
keep the
the operating
operating costs
costs of
of the
the linear
linear colcollider
lider manageable,
manageable, itit is
is expected
expected that
that the
the minimum
minimum acceptable
acceptable wall
wall plug-to-acceleraplug-to-accelerator
tor input
input energy
energy conversion
conversion efficiencies
efficiencies for
for RF
RF drivers
drivers will
will be
be atat least
least 40%
40% [3].
[3].
Included
Included in
in this
this efficiency
efficiency calculation
calculation are
are losses
losses in
in the
the power
power supply,
supply, magnet
magnet supsupplies,
plies, beam
beam transport,
transport, RF
RF interaction,
interaction, pulse
pulse compression,
compression, and
and microwave
microwave transport.
transport.
Because
the
best
RF
interaction
efficiencies
are
typically
near
Because the best RF interaction efficiencies are typically near 50%,
50%, there
there isis little
little
room
room for
for loss
loss in
in the
the other
other systems.
systems. Consequently,
Consequently, this
this efficiency
efficiency requirement
requirement may
may
well
well lead
lead to
to the
the use
use of
of DC
DC supplies,
supplies, gridded
gridded or
or modulated
modulated cathodes,
cathodes, permanent
permanent
(PPM
(PPM or
or solenoidal)
solenoidal) or
or superconducting
superconducting magnets,
magnets, and
and energy
energy recovery
recovery (e.
(e. g.
g.
depressed
collector)
systems.
While
these
technologies
are
fairly
well
developed
depressed collector) systems. While these technologies are fairly well developed for
for
low-power,
low-power, CW,
CW, linear-beam
linear-beam tubes,
tubes, they
they have
have not
not been
been generally
generally applied
applied to
to highhighpower,
power, short-pulse
short-pulse systems.
systems. Furthermore,
Furthermore, depressed
depressed collectors
collectors have
have only
only recently
recently
been
been applied
applied to
to gyrotron
gyrotron and
and other
other rotating-beam
rotating-beam devices.
devices. These
These requirements
requirements on
on
efficiency
efficiency and
and peak
peak power,
power, along
along with
with the
the other
other requirements
requirements for
forhigh
highgain,
gain,stability,
stability,
tube
tube lifetime,
lifetime, average
average power,
power, phase
phase noise,
noise, etc.,
etc., combine
combine to
to make
make the
the driver
driver design
design aa
formidable
formidable problem.
problem.
This
This paper
paper describes
describes the
the status
status of
of aa program
program to
to develop
develop aa high
high power
power W-Band
W-Band
gyroklystron
capable
of
depressed
collector
operation,
including
details
gyroklystron capable of depressed collector operation, including details of
of the
the conconstruction
now
underway.
A
solid
model
of
the
gyroklystron
is
shown
in
Figure
struction now underway. A solid model of the gyroklystron is shown in Figure 1.1.
Descriptions
Descriptions of
of the
the design
design of
of the
the components
components was
was presented
presented in
in the
the AAC2000
AAC2000publipublication^]
and
will
not
be
repeated
here.
cation[4] and will not be repeated here.
Figure
Figure 1.
1. Solid
Solid Model
Model of
of 91
91 GHz
GHz
gyroklystron
gyroklystronwith
with 66 foot
footmodel.
model.
409
Magnet System
The gyroklystron requires an axial magnetic field of approximately 28 kG which
is obtained using a superconducting magnet manufactured by Cryomagnetics, Inc.
The acceptance test for the magnet system, which includes all the power supplies
and control electronics, was successfully completed in March 2002, and the magnet
is now being stored at Stanford Linear Accelerator Center, where the gyroklystron
will be tested.
In addition to the superconducting coils, three room temperature coils surround
the gyroklystron collector to aid in distributing the spent electron beam. It is not
anticipated that all three coils will be needed, but they provide considerable flexibility in controlling spent beam electrons to minimize power densities on the collector
surfaces. These magnets were built by Stanganese Industries, Inc. and were delivered in March 2002.
Magnetron Injection Gun
The double-anode MIG operates at 500 kV and produces 55 amps. The target
beam power is 27.5 MW, and the magnetic compression is 35. The control anode
voltage is adjusted to produce the target average velocity ratio of 1.6. EGUN was
used to design the MIG and characterize its performance. The required velocity ratio
was achieved with an axial velocity spread of 3.23% (from ray optics considerations) and an average beam radius in the circuit of 1.65 mm. The beam radius provides for a beam clearance in the drift tunnels of 0.35 mm, which is typical for WBand gyroklystrons. At the nominal current, the velocity ratio can be varied from 1
to 2 by changing the control voltage from 47 kV to 55 kV with the corresponding
value of velocity spread remaining below 5.5%. The axial velocity spread stays
below 10% for currents from nearly 40 A to 80 A.
The electron gun is 95% complete and should be ready for installation into the
gyroklystron in August 2002. A photograph of the gun stem is shown in Figure 2.
The program purchased two cathode assemblies, and both were successfully fired in
a bell jar to the anticipated operating temperature.
Once assembly of the electron gun is completed, it will be fired again in the bell
jar to ensure proper operation before insertion into the gyroklystron.
Microwave Circuit
An idealized schematic of the circuit layout is shown in Figure 3. Six cavities are
used to achieve a large signal gain in excess of 55 dB. The input cavity and the first
buncher cavity interact at the first harmonic in the TE011 mode; all other cavities
interact near the second harmonic in the TE02i mode. The walls of the first five cavities are formed by abrupt radial transitions. Mode conversion in the three harmonic
buncher cavities from the TE02 mode to the TE01 mode is minimized by adjusting
the cavity length to provide destructive interference. This is required because the
410
Figure
Figure 2.
2. Gun
Gun stem
stem for
for 10
10 MW
MW gyroklystron
gyroklystron
TE01
mode
modeisisnot
not cut
cut off
off in
in the
the drift regions at 91 GHz.
frequency
TE
GHz. The
The nominal
nominal drive
drive frequency
frequency
01 mode
is
45.696
GHz.
The
cavities
are
stagger-tuned
to
increase
the
efficiency
and
45.696 GHz. The cavities are stagger-tuned to increase the efficiency and bandisis 45.696
bandwidth.
The
mode at the
width.The
The drift
drift tube
tube radius
radius is
is 0.3175
0.3175 cm
cm between all
all cavities.
TE01
width.
cavities. The
The TE
01
01 mode at the
drive
frequency
and
the
TE
mode at
drift
drive frequency
frequency and
and the
the TE
TE02
drive
at the
the output
output frequency
frequency are
are cut
cut off
off in
in the
the drift
drift
02 mode
02
tubes
by
25%
and
15%,
respectively.
tubesby
by25%
25% and
and 15%,
15%, respectively.
respectively.
tubes
The
circuit
assembly
was
cold
The circuit
circuit assembly
assembly was
was cold
cold tested
The
tested using
using facilities
facilities provided
provided by
by the
the Naval
Naval
Research
Laboratory.
This
proved to
Research Laboratory.
Laboratory. This
This proved
difficult
Research
to be
be more
more difficult
difficult than
than anticipated
anticipated due
due to
to the
the
nature
of
coupling
to
overmoded
natureof
ofcoupling
coupling to
to overmoded
overmoded cavities.
cavities. Special
nature
cavities.
Special probes
probes were
were designed
designed to
to couple
coupleto
to
the
desired
mode
without
exciting
excessive
amounts
of
parasitic
modes.
the
desired
mode
without
exciting
excessive
A
photothe desired mode without exciting
amounts of parasitic modes. A photograph
of
the
cold
test
setup
is shown
shown
graphof
ofthe
thecold
cold test
test setup
setup is
shown in
graph
in Figure
Figure 4.
4. Following
Following the
the cold
cold tests,
tests, the
theresults
results
of
the
measurements
were
used
as
input
to
the
analytical
programs
to
predict
of
the
measurements
were
used
as
input
analytical
of the measurements were used as input to the analytical programs to predict perforperfor-
Radial
location
(mm)
Radial
location
(mm)
6
6
5
5
4
4
3
3
2
2
1
1
0
00
0
20
20
40
40
40
60
60
81
80
60
Axial location
location
(mm) 80
Axial
(mm)
Axial location (mm)
100
100
100
120
120
120
3. Simplified
Simplified schematic diagram of the six
Figure 3.
Figure
3. Simplified schematic diagram of the six
cavity circuit.
circuit.
cavity
cavity circuit.
411
Figure 4. Setup for cold tests
Figure
tests of
of gyroklystron
gyroklystron
circuit. Mode converters on each end of the
circuit.
the
setup allow coupling of the Network
setup
Network Analyzer
Analyzer to
to
the overmoded
overmoded circuit.
mance based
based on
on the
the “as-built”
"as-built" circuit. The results
mance
results of
of these
these simulations
simulations are
are shown
showninin
Table 1.1. The
The gyroklystron
gyroklystron is now predicted to operate with slightly
Table
slightly lower
lower efficiency
efficiency
andgain,
gain, but
but still
still meet
meet the output power requirement. A 200
and
200 W
W TWT
TWT is
is available
available for
for
driving the
the gyroklystron,
gyroklystron, so
so the
the increased
increased input
driving
input power
power requirement
requirement will
will not
not be
be aa
problem. The
The cavity
cavity assembly
assembly is
is now
now being
problem.
being completed
completed for
for insertion
insertion into
into the
the
gyroklystron.
A
photograph
of
the
assembly
is
shown
in
Figure
5.Output
gyroklystron. A photograph of the assembly is shown in Figure 5.Output
Waveguide, Collector,
Collector, and
and Output
Output Window
Window
Waveguide,
In
the
91.4
GHz
tube,
the
collector
serves as
In the 91.4 GHz tube, the collector serves
as the
the output
output waveguide
waveguide for
forthe
theRF
RFcircircuit
and
the
beam
collection
region
for
the
spent
electron
beam.
A
schematic
cuit and the beam collection region for the spent electron beam. A schematicof
ofthe
the
collector design
design is
is shown
shown in
in Figure
Figure 6.An
6.An uptaper
collector
uptaper from
from the
the RF
RF circuit
circuit to
tothe
thediameter
diameter
of the
the collector
collector incorporates
incorporates aa mode
mode converter
converter that
2 mode
mode
of
that generates
generates aa TEoi/TE
TE01/TE002
mixture.
This
mode
mixture
allows
incorporation
of
gaps
in
the
waveguide
wall
mixture. This mode mixture allows incorporation of gaps in the waveguide wallwith
with
less than
than 0.1%
0.1% loss
loss of
of RF
RF power.
power. The
The collector
collector design
less
design incorporates
incorporates gaps
gaps between
betweenthe
the
body of
of the
the gyroklystron
gyroklystron and
and the
the output
output window
body
window to
to allow
allow for
for voltage
voltage depression
depressionof
of
the collector.
collector. This
This would
would allow
allow partial
partial recovery
the
recovery of
of energy
energy remaining
remaining in
in the
the electron
electron
beam following
following interaction
interaction in
in the
the circuit,
circuit, thereby
beam
thereby increasing
increasing the
the overall
overall efficiency.
efficiency.
Table
1:
Comparison
of
Design
Value
and
Predicted
"As-built"
Performance
Table 1: Comparison of Design Value and Predicted “As-built” Performance
Parameter
Prediction
Desig
Parameter
Desig
Prediction based
based on
on
nn Value
cold
test
measurements
Value
cold test measurements
Frequency (GHz)
(GHz)
91.386
91.386
Frequency
91.386
91.386
Drive Power for Saturation
17.0
23
Drive Power for Saturation
17.0
23
(watts)
(watts)
38.2
Efficiency (%)
36
Efficiency (%)
38.2
36
Output Power (MW)
10.5
10
Output Power (MW)
10.5
10
Gain (dB)
57.9
56
Gain (dB)
57.9
56
412
Figure
Cavity
Figure 5.
5. Gyroklystron
Gyroklystron Cavity
Assembly
Assembly
The
handlingthe
thehigh
highpeak
peakpower
powerdensity.
density.
Theprincipal
principaldesign
design issue
issue of
of the collector was handling
As
made to
to allow
allow depression
depression ofofthe
thecollector
collector
As indicated
indicated above,
above, provision
provision was made
potential
efficiency.
Thiswas
wasdone
donevia
single
efficiency. This
viaaasingle
potentialininorder
order to
to improve
improve the gyroklystron efficiency.
break
Good RF
RF transmission
transmission across
acrossthe
thebreak
break
breakand
and insulator
insulator as
as shown
shown in Figure 6.
6. Good
was
/TE022 mode mixture.
wasinsured
insured by
by use
use of
of the
the TE
TE011/TE0
TE0
mixture. The
The current
current program
programdoes
doesnot
not
provide
operation of
providefunding
fundingfor
for depressed
depressed collector operation
of the
the tube.
tube.
Generationof
of the
the mode
mode mixture
mixture at a 1.3
Generation
cm radius
radius was
wasachieved
achievedby
anoptimized
optimized
1.3 cm
byan
one-period,unfolded,
unfolded, sinusoid
sinusoid wall, mode converter[7]
one-period,
converter[7] followed
followedby
conventional
byaaconventional
one-periodconverter.
converter. The
The initial
initial converter was
one-period
was unfolded
unfoldedto
toincorporate
incorporatethe
changeinin
thechange
radiusfrom
fromthe
thecavity
cavity output
output radius (0.635 cm)
radius
cm) to
finaldesired
1.3cm.
cm.
to the
the final
final
desiredradius
radiusofof1.3
Thisconverter
converter achieved
achieved the
the desired conversion
conversion with
This
less than
with less
than 0.1%
0.1%power
powerininspurispuriousmodes.
modes. Return
Return loss
loss from
from the taper section
,,
ous
section is
is higher
higher than
than 60
60 dB
dBfor
forthe
0101
theTE
TE
TE
and
TE
modes.
The
TE
and
TE
modes
had
greater
than
50
dB
return
loss
TE
02and TE
03modes. The TE
11
TE02
TE03
TE11
TE12
02
03
n and TE
12 modes had greater than 50 dB return loss
12
Beam Collection
Beam Collection
Region
Region
Elbow
Elbow
TE01/02
TE01/02
Converter
Converter
Single disk alumina window
Single disk alumina window
Figure 6. Schematic of gyroklystron collector,
6. Schematic
of gyroklystron
gyroklystron collector,
Figure
including
output window.
including output
output window.
413
while the TE13 and TE14 had 40 and 28 dB return loss.The mode converter assembly
the TEand
TE14 had
40 gyroklystron.
and 28 dB return loss.The mode converter assembly
13 and
iswhile
completed
installed
in the
is completed and installed in the gyroklystron.
The window consists of a single 3.50 cm diameter ceramic disk made from
Thepure
window
consists
of a single
3.50 cmconsists
diameterof ceramic
disk made from
99.5%
Alumina
(Al-995).
The design
a 3/2 wavelength
thick
99.5%
pure
Alumina
(Al-995).
The
design
consists
of
a
3/2
wavelength
thick
ceramic disk. The average power deposition in the window is only 0.07 watts,
so
ceramic
disk.
The
average
power
deposition
in
the
window
is
only
0.07
watts,
so
thermal issues are not an issue. Mechanical stresses due to pressure loading are also
thermal
are of
notthe
an material’s
issue. Mechanical
stresses due
pressure
loading
are also
less
than issues
one third
tensile strength.
Thetooutput
window
is installed
less
than
one
third
of
the
material's
tensile
strength.
The
output
window
is
installed
on the gyroklystron and is shown in Figure 7.
on the gyroklystron and is shown in Figure 7.
Current Status
Current Status
The gyroklystron assembly is nearing completion. Only the cavity assembly and
The gyroklystron assembly is nearing completion. Only the cavity assembly and
electron gun are still being completed. It is anticipated that the gyroklystron will be
electron gun are still being completed. It is anticipated that the gyroklystron will be
ready for bakeout in August 2002. Following bakeout, it will be transported to Stanready for bakeout in August 2002. Following bakeout, it will be transported to Stanford Linear Accelerator Center for testing. Testing is scheduled for the fall of 2002.
ford Linear Accelerator Center for testing. Testing is scheduled for the fall of 2002.
Acknowledgements
Acknowledgements
This
Business InnovaInnovaThisprogram
program isis funded
funded by
by U.S.
U.S. Department
Department of
of Energy
Energy Small
Small Business
tive
Research
grant
number
DE-FG03-99ER82754.
Support
was
also
provided
by
tive Research grant number DE-FG03-99ER82754. Support was also provided by
the
Vacuum
Electronics
Division
of
the
Naval
Research
Laboratory,
Communicathe Vacuum Electronics Division of the Naval Research Laboratory, Communications
Center.
tionsand
andPower
PowerIndustries,
Industries, Inc.,
Inc., and
and Stanford
Stanford Linear
Linear Accelerator
Accelerator Center.
Figure7.7.Photograph
Photograph of
of output
output window
Figure
installed on
ongyroklystron.
gyroklystron. Also shown is the
installed
rightangle
angle elbow.
elbow.
right
414
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The NLC Design Group: Zeroth Order Design Report for the Next Linear Collider, SLAC Report
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"R.L. Ives, W Lawson, J.M. Neilson, and M. Read, "Development of a 10 MW, 91 GHz
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