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High Power RF System for KOMAC RFQ HIGH POWER RF SYSTEM FOR KOMAC RFQ*
Y. S. Cho , H. J. Kwon, J. M. Han and B. H. Choi
KAERI, Daejeon 305-353, KOREA
Abstract
KOMAC (Korea Multipurpose Accelerator Complex)
Test Facility (KTF) has been prepared to develop
accelerator technologies and low energy proton beam
applications with a 20 MeV proton accelerator. For the
KTF accelerator, a 3 MeV 350 MHz RFQ has been
developed. The required RF power for the RFQ is 417 kW
CW, and the RF system of 1 MW CW has been developed.
The RF system for the RFQ consists of 1 MW 350 MHz
klystron, 100 kV, 20 A high voltage power supply, 1 MW
circulator, 1 MW RF dummy load, WR 2300 waveguide
and 2 MW cooling system. The details of the high power
RF system development are reported.
1 INTRODUCTION
As a first stage of the KOMAC project whose final goal
is to develop a 1 GeV, 20 mA proton accelerator, the KTF
has been developed at KAERI (Korea Atomic Energy
Research Institute). The KTF consists of 50 keV proton
injector [1], 3 MeV 350 MHz RFQ [2] and 20 MeV, 700
MHz CCDTL [3].
Two types of RFQ have been developed. The one is
0.45 MeV RFQ whose purpose is to check the basic RFQ
technologies such as tuning, beam matching and so on,
the other is 3 MeV main RFQ which was fabricated and
vacuum tight checked already. The required RF power for
0.45 MeV RFQ is 110 kW CW and for 3 MeV is 417 kW
CW respectively. The 1 MW, 350 MHz RF system has
been developed to deliver a RF power to the RFQ. The
high power RF system consists of klystron, circulator, RF
window, various waveguide components, klystron power
supplies and cooling system. Recently all components of
the RF system were prepared and are being tested. The
status of the KTF accelerator, which includes RF system,
is shown in Figure 1.
2 RF COMPONENTS DESCRIPTIONS
2.1 Klystron
The KTF RFQ RF system utilizes a 1 MW CW klystron
at 350 MHz. The klystron was manufactured by THALES
Electron Devices and is shown in Figure 2. The klystron
has a triode type electron gun to control the beam current
and is capable of dissipating the full beam power (1800
kW). It could deliver RF power up to 1.1 MW into the
load of a VSWR 1.2:1 at 350 MHz during the acceptance
test at THALES Electron Devices last year. It is now
installed at KAERI KTF site and being high power tested.
Figure 1: Status of KTF accelerator
Figure 2: 1 MW 350 MHz Klystron
2.2 Klystron Power Supplies
The specifications of the high voltage power supplies
for the KTF klystron are 100 kV 20 A with the conditions
that the voltage peak to peak ripple and the voltage
regulation are less than 1 %, and energy deposition in the
klystron at the tube arc is less than 20 J. The power
supply that meets the above conditions has been designed,
________________________________________
*Supported by the Korea Ministry of Science and Technology. choys@kaeri.re.kr
Proceedings of EPAC 2002, Paris, France
2302 manufactured and tested by KAERI. The schematic
circuit diagram of the power supplies that includes high
voltage power supply, modulating anode power supply
and heater power supply is shown in Figure 3. The main
components of the high voltage power supply are IVR
(Induction Voltage Regulator), transformer & rectifier
tank for 12 pulses rectification, L-C filter, and ignitron
crowbar. Voltage dividing resistors and a tetrode tube
were used to provide variable high voltage to modulating
anode of the klystron. The variable voltage modulating
anode power supply gives the flexibility of klystron
operation. The manufactured power supply is shown in
Figure 4.
Figure 3: Circuit diagram of high voltage power supply
Figure 4: High voltage power supply
(Left: IVR, transformer & rectifier, filter capacitor
Right: modulating anode power supply, crowbar)
2.3 Circulator
A 350 MHz Y-junction type circulator, which can
deliver 1.3 MW CW RF power for forward direction and
permit 1.3 MW reverse power at any phase, was used. The
circulator has been manufactured by Advanced Ferrite
Technology (AFT). It uses temperature compensating
system to compensate the temperature dependent ferrite
saturation magnetization. It was installed at KTF site as
shown in Figure 5.
Figure 5: 1.3 MW, 350 MHz Y-junction circulator
2.4 Waveguide Components
The WR2300 waveguide components, which were
manufactured by Dielectric communications, have been
used. It consists of straight, E-plane, H-plane sweep bend,
flexible section, and full height to half height step
transition. In addition to those purchased from Dielectric,
waveguide components were manufactured in a domestic
company. Those includes E-plane, H-plane mitre bend,
harmonic filter and are being tested.
2.5 RF Dummy Load
A 1 MW at 350 MHz RF dummy load manufactured by
Dielectric communications was used. It is about 4 m long
and uses water and ethylene glycol mixture as a coolant.
In the KTF RFQ RF system, the RF dummy load is used
as a dummy load not only for klystron full power test but
also for absorbing reflected RF power from RFQ through
circulator.
2.6 RF Window
A RF window manufactured by THALES Electron
Devices was used. It uses half height WR2300 waveguide
flange at both sides, and can transmit RF power of 750
kW CW at 350 MHz on a load with a VSWR 1.3 : 1.
The window is shown in Figure 6. The RF window will
be installed with the RFQ input coupler, which is
WR2300 waveguide to coaxial type.
Figure 6: RF window
Proceedings of EPAC 2002, Paris, France
2303 2.7 Cooling System
A 2 MW DI water cooling system for KTF RFQ and
RF system was prepared already. The required cooling
loops for RF system are circulator, RF load, RF window
cooling loops and body, cavity, collector cooling loops for
klystron. Because the coolant of the RF load is a mixture
of water and ethylene glycol, a separate cooling loop for
RF load with storage tank, pump and heat exchanger was
installed. Also pump for pressurization was installed in
klystron body cooling loop, because the pressure of the
DI water cooling loop was too low to supply enough flow
to that cooling loop. The schematic diagram of the
cooling loop of the RF system is shown in Figure 7.
Figure 7: Schematic diagram of the cooling loop
2.8 LLRF (Low Level RF)
The LLRF consists of a 350MHz signal generator, a
160W solid state amplifier, amplitude/phase control loops,
and RF interlocks. The designed field stability in the
RFQ cavity is within
±
1% amplitude and
±
1.4
°
phase
using feedback control loops in the LLRF. For frequency
control, another tuner controller module was used. The
RF interlock signals comes from excessive reflected RF
power, circulator arcs and window arcs.
3 TEST RESULTS
The klystron power supplies were tested. At first,
electrical breakdown at the high voltage cable was
occurred. After the cable was replaced, the high voltage
test up to 95 kV had shown no problems. A crowbar test
using ignitron was carried out. Wire test using 0.3 mm
diameter and 200 mm long copper wire to confirm that
the energy deposition in the klystron at the tube arc is
within the limit value, that the dissipation energy at the
load was less that 20 J. Interlocks for high voltage power
supply and LLRF were installed and RF tests are being
carried out. In RF tests, parts of waveguide section that
connected circulator with RFQ were removed and
shorting plate was installed at the end of the open section.
In these test scheme, klystron and circulator can be tested
simultaneously, because the RF power flows from
klystron to shorting plate through circulator, then reflects
from shorting plate, and finally is absorbed in dummy
load through the circulator. Using these test schemes, the
high power RF system was tested up to 100 kW CW RF
power. RF power was measured by calorimetric method
with RF dummy load and simultaneously by power meter
with directional coupler. Two data shows good agreement
within 5%.
4 CONCLUSIONS AND FUTURE WORKS
The RF system for KTF RFQ has been developed.
Each components and overall system are being tested. The
high power RF system was tested up to 100 kW CW RF
power. During the test, it was pointed out that there were
RF power reflection from the dummy load that is sensitive
to coolant temperature and RF leakages between the
waveguide joint.
After the correction of the above problems, this RF
system will supply the RF power of 100kW level for RF
window and low energy RFQ test. With the completion of
the low energy RFQ experiment, it will be used as RF
system of 500kW level for the 3MeV RFQ experiment,
that is the original role of this RF system.
5 ACKNOWLEDGEMENT
The authors would like to express their thanks to Dr.
Sang-hyun Park at SLAC for his warm advice about high
power RF system.
This work was supported by the Korea Ministry of
Science and Technology.
6 REFERENCES
[1] Y. S. Cho, et al., High-current ion source
development for KOMAC, Rev. Sci. Inst., P609,
Vol.71, No. 2, 2000.
[2] J. M. Han, et al., A 350 MHz CW RFQ Linac for the
KOMAC/KTF, Proc. Of EPAC2000, Vienna, P812,
200