2323

Proceedings of EPAC 2002, Paris, France
DEVELOPMENT OF AN 805-MHZ, 550 KW PULSED KLYSTRON FOR
THE SPALLATION NEUTRON SOURCE*
S. Lenci, E. Eisen, and B. Stockwell, CPI, Palo Alto, CA, USA
Abstract
The Spallation Neutron Source (SNS) is an
accelerator-based neutron source being built in Oak
Ridge, Tennessee, by the U.S. Department of Energy. The
SNS will provide the most intense pulsed neutron beams
in the world for scientific research and industrial
development. CPI has supported the effort by developing
2.5 MW and 550 kW pulsed klystrons. The 2.5 MW tube
met all performance requirements, but a 5 MW version
was chosen as the project evolved. Los Alamos National
Laboratory (LANL) has placed an order with CPI for 65
of the 550 kW klystrons for the super-conducting portion
of the accelerator. The primary output power
requirements are 550 kW peak, 49.5 kW average at 805
MHz, with an electron beam-to-rf conversion efficiency
of 65%. The prototype unit is schedule to be in test in
July. Performance specifications, computer model
predictions, and prototype operating results are
presented.
long pulse device. Great care is taken to ensure a wellbehaved beam is obtained.
The rf-circuits contain six cavities, including one tuned
slightly below the second harmonic of the operating
frequency. The designs are optimized to provide the
required efficiency and gain without compromising
bandwidth. The first two cavities are staggered around
the operating frequency to provide the bandwidth. Next is
the second-harmonic cavity followed by two inductively
tuned cavities to optimize the electron bunching. The
output cavity then extracts energy from the beam.
The rf-circuit is designed using 1-D and 2-D particlein-cell codes developed at CPI. Many years of
benchmarking the codes to measured results has lead to
high confidence in the results. SUPERFISH is used for
cavity design, while HFSS and MAFIA are used for the
output cavity, coupling loop, and output window design.
2.2 Mechanical Design
1 INTRODUCTION
CPI, formerly the Electron Device Group of Varian
Associates, has a long history of building high-power
pulsed UHF klystrons for many applications. In the early
1970’s, approximately 70 VA-862A klystrons were built
for the Los Alamos Meson Physics Facility (LAMPF, now
known as the Clinton P. Anderson Meson Physics
Facility). These tubes were rated for 1.25 MW peak, 150
kW average at 805 MHz.
CPI was awarded the contract by LANL to build one
2.5 MW klystron, VKP-8290A, in May 1998, with the
unit delivered in September 1999.
An order for 65 of the 550-kW tube, VKP-8291A, was
place in February 2001. The prototype unit had a
performance issue and the second unit is being assembled
with modifications. The contractual delivery rate is 2 per
month
2 DESIGN
2.1 Electrical Design
Figure 1: Comparison of SNS Klystrons
The electron gun design is primarily performed using
XGUN, starting with the electrostatic beam optics. Once
the performance is satisfactory, the design is refined with
magnetic field is applied. Care is taken to evaluate and
minimize the beam scallop down the drift tunnel.
Analyses are performed at various operating conditions.
The voltage gradients of the gun electrodes are analyzed
with a goal of a maximum gradient of 60 kV/cm for this
_________________________________________
*Work supported by US Department of Energy
Both klystrons were required to operate in a vertically
with the gun down. Figure 1 shows the tubes on the same
scale. The two buncher cavities and the two inductively
tuned cavities have stainless steel walls with copper
endwalls, with cavities 4 and 5 copper plated to reduce
resistive loss. The second harmonic and output cavities
have OFE copper walls. All cavities on the 2.5 MW
klystron, except the output, have one adjustable drift-tube
tip and an adjacent flexible cavity endwall to allow for
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Proceedings of EPAC 2002, Paris, France
beam voltage of 113 kV and a beam current of 41.5
amperes.
Additionally the klystron had to demonstrate stable
performance and achieve 85% of its rated power at six
equally spaced positions of a 1.5:1 mismatch. Figure 5
plots a transfer curve at different mismatch positions.
3000
Beam Voltage = 113 kV, Beam Current = 41.5 A
2500
-1 dB BW
2000
Output Power, kW
adjusting the tuning. The 550-kW tube incorporates
diaphragm tuners in the cavities.
The rf energy is extracted through a single window
with an alumina ceramic. The pillbox window is
designed around WR-975 waveguide.
The collector for both tubes is designed to dissipate the
entire beam energy. On the VKP-8290A, it is isolated
from the body to allow the monitoring of body current. It
is made with thick-walled OFE copper with drilled-holes
for the coolant to pass. The end-cap bolts on with an
o-ring seal. On the VKP-8291A, the collector is made
from thick-walled copper with grooves milled into the
outer wall for the coolant to pass. The water-jacket is part
of the brazed collector assembly. Both collectors were
proof tested at 200 psi (13.6 bar).
1500
Beam Voltage = 118 kV,
Beam Current = 39 A
Measured Output
Power, kW (9/23/99)
1000
Predicted Output
Power, kW
(Presented at CDR)
500
0
802.5
803
803.5
804
804.5
805
805.5
806
806.5
807
807.5
Frequency, MHz
Figure 3: VKP-8290A Bandpass Curve
3
Beam Voltage = 113 kV, Beam Current = 41.5 A
2.5
Beam Voltage = 118 kV, Beam Current = 39 A
Ou 2
tp
ut
Po
we 1.5
r,
k
W 1
Measured Output Power
(9/23/99)
Predicted Output Power
presented at CDR (11/3/98)
0.5
0
0.0
20.0
40.0
60.0
80.0
100.0
120.0
Drive Power, Watts
Figure 4: VKP-8290A Transfer Curve
3.000
#5
#6
#4
#1
2.500
#3
#2
O 2.000
ut
pu
t
1.500
Po
we
r,
k 1.000
W
Figure 2: VKP-8290A Klystron
3 TEST RESULTS
3.1 805-MHz, 2.5-MW Peak Klystron
0.500
The peak power (2.5 MW), efficiency (55%), gain (45
dB), and bandwidth (± .7 MHz) specification were all
achieved. The bandpass and transfer curves can be seen
in figures 3 and 4. The comparison to the predicted
performance is quite close. This data was taken at a
0.000
0.0
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Cathode Voltage = 113 kV
Mod Anode Voltage = 95.8 kV
Beam Current = 41.2 A
Frequency = 805 MHz
PRF = 60 Hz
RF Duty = 10%
20.0
40.0
60.0
80.0
100.0
Drive Power, Watts
Figure 5: VKP-8290A Transfer Curves
at Equally Spaced Mismatch Positions
120.0
Proceedings of EPAC 2002, Paris, France
3.2 805-MHz, 550-kW Peak Klystron
600
Peak Output Power (kw)
The prototype unit had a performance issue and the
second unit is being assembled with modifications. The
following picture shows the klystron. Figures 7 and 8
present the predicted performance of the VKP-8291A.
Actual test data is expected in July.
500
Max. Output Power Variation =
400
300
BW = +/- 1.3 MHz
200
Ek = 75 kV
Ik = 11.3 a
pd = 4.6 w
100
0
0.803
0.804
0.805
0.806
0.807
Frequency (GHz)
Figure 8: VKP-8291A Predicted Bandpass Curve
4 CONCLUSIONS
The measured results of the 2.5 MW klystron instill
high confidence in our simulation codes. It also
demonstrated a high degree of stability under various
operating conditions. Although a minor set-back was
experienced on the prototype 550-kW klystron, all
aspects of the specification are expected to be met.
Frequency
Peak Cathode Voltage
Peak Mod Anode Voltage
Peak Beam Current
Perveance
Peak Output Power
Efficiency
RF Duty Cycle
RF Pulse Length
Peak Drive Power
Gain
VKP-8290A
Measured
805 MHz
113 kV
95.8 kV
41.5 Amps
1.1
2,605 kW
55.5 %
10 %
1.67 msec
67 Watts
45.9 dB
VKP-8291A
Specification
805 MHz
75 kV
N/A
11.3 Amps
.55
550 kW
> 65 %
9%
1.5 msec
5.5 Watts
> 50 dB
Figure 6: Prototype VKP-8291A
Table 1: Performance Comparison
Peak RF Output Power
60
500
58
Sat. Point @ 805 MHz
po = 565.6 kw
η = 66.7 %
pd = 4.6 w
Gain = 51.22 dB
400
300
56
Gain (dB)
Peak Output Power (kw)
5 ACKNOWLEDGEMENT
Gain
600
54
200
52
Ek = 75 kV
Ik = 11.3 a
100
50
0
The authors would like to thank their co-workers at
CPI for their contributions throughout the development
of these products. They would also like to thank the CPI
management team for their support. Finally many thanks
go to the technical leaders and their colleagues at Los
Alamos National Lab, in particular Dan Rees and Paul
Tallerico.
48
0
1
2
3
4
5
6
7
8
RF Input Drive Power (w)
Figure 7: VKP-8291A Predicted Transfer Curve
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