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DESIGN & DEVELOPMENT OF 37.8MHZ, 2.0KW RF AMPLIFIER FOR
REBUNCHER OF RIB PROJECT
H.K.Pandey#, T. K. Mandi, Y. Kumar, D.P. Dutta, S. Basak, S.K. Thakur,
A. Bandopadhyay, VECC, 1/AF Salt Lake, Kolkata, India
K.P.Ray, SAMEER, IIT campus, Powai, Mumbai, India
Abstract
A 2 kW amplifier has been indigenously designed and
developed for a 37.8 MHz re-buncher. The buncher is
presently installed in the RIB beam line between the RFQ
and LINAC I. The amplifier has two stages of
amplification: a 100 W solid state amplifier (SSA) stage
followed by a triode based amplifier, all operating at 37.8
MHz. A DC power supply system has also been designed
and developed for the biasing of the amplifier stages. This
paper describes the design and development of the 2 kW
amplifier and the high power DC power supply.
INTRODUCTION
The Rare Ion Beam (RIB) facility being developed at
VECC has a RFQ post-accelerator followed by IHLINACs [1-2]. The re-buncher is a 4 gap λ/4 RF structure
operating at 37.8 MHz [3]. It helps to match the
longitudinal bunch width of the beam to the acceptance of
the downstream IH LINAC. The 2 kW transmitter has
been designed and developed for powering the rebuncher, although it may be used for powering other
cavities operating at the same frequency as well. The 2
kW transmitter is a two stage RF amplifier consisting of a
100 W SSA Driver stage and a triode based final stage.
Figure 1 shows the schematic diagram of the RF
transmitter. The final stage amplifier is a class AB
amplifier designed using triode 3CX5000A7 in grounded
grid configuration. Input and output matching networks
were designed to match this impedance to 50.
Grid (-100V/0.5A) and Filament (10V/24A) of tube have
been designed and tested successfully. The system control
and all safety interlocks have been incorporated.
Table1: Amplifier Specification
Parameters
RF Section
Frequency
Tunable; 35-42MHz
Output Power
2.0kW (Max.)
Instantaneous BW
2 MHz
Harmonic Level
<50dB
Max Anode Voltage
4.5 KV
Max Anode Current
3.0 A
Gain
13dB
Driving Power
120W
It is also necessary for the amplifier to have a stable
power output and excellent harmonic suppression, which
has been taken care of in the design. This paper describes
the detailed design, test and measurement report of this
indigenously developed amplifier.
DRIVER STAGE AMPLIFIER
The driver stage is a 100 W Solid State Amplifier
(SSA) operating at a centre frequency of 37.8 MHz. It has
been developed using a CA2832C pre-amplifier stage
followed by power FETs in push-pull configuration
housed within a single Gemini package [4]. Figure 3
shows a picture of the 100 W solid state amplifier. A
supply voltage of 28.3 V has been used for drain bias.
Transmission line transformers have been used along with
other lumped components for input and output impedance
matching. This has resulted in a large bandwidth of the
amplifier ranging from 14 MHz to 84 MHz as shown in
figure 4.
Figure 1: Schematic Diagram of the Amplifier
The triode cage and forced air cooling arrangement
with RF filter blocks have been designed and
implemented. The power supplies for Anode (4.5kV/3A),
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Figure 3: Photograph of the Solid State Amplifier
Figure 4: Measured Dynamic Response and Frequency
Response of the SSA
Figure 5 shows the picture of the final stage of the
amplifier system. The parameters for the matching
networks have been calculated and the analysis of this
amplifier has been carried out using ABCD parameters
method. It uses a linear small-signal AC model of the
triode with its matching circuits. The analytical as well as
measured results have been already presented [6]. The
matching networks are single LC networks with variable
L and C, resulting in a tuning range of 36 MHz to 43
MHz. Figures 6.a and 6.b show the measured S11
parameter for the input and output matching circuits of
the final stage. The amplifier is being tested and
necessary tuning of the input and output matching circuits
is being done. Initially, low-level RF measurements of the
input and output matching networks were done. These
tests were followed by DC tests of the amplifier by
holding the triode electrodes at their respective biasing
potentials. Using 4 kW dummy load, RF tests are being
done at present along with further fine tuning and
adjustments
The amplifier has a gain of about 57 dB at the
frequency of operation. It has been operated up to a
maximum of 98 W CW power with dummy load. A drift
of 3 W over a period of 7 hrs of operation was observed
while operating at 90 W. The 2nd, 3rd and 4th harmonics
are at levels of -28 dBc, -68 dBc and -49 dBc respectively
at 38 MHz.
FINAL STAGE AMPLIFIER
The final stage is a class AB triode amplifier in the
grounded grid configuration [5]. The grounded grid acts
as a shield and prevents coupling between the input and
output circuits. Hence, at high frequencies and high
power, this configuration of using the triode results in a
more stable amplifier operation. An air cooled
3CX5000A7 triode tube has been used as the active
device of the amplifier, preceded and followed by
appropriate matching networks. The triode tube along
with the matching networks are housed inside an
aluminium enclosure with RF filter blocks, developed in
VECC workshop and cooled using a centrifugal fan. The
proper air flow and pressure drop of tube cage have been
measured. The 4 kV bias for the anode is applied using a
HV feed-through in the enclosure.
Figure 5: Assembly of the final stage of the 2kW
amplifier
Figure 6.a: Input matching of final stage
Figure 6.b: Output matching of final stage
DC REGULATED POWER SUPPLY
The necessary high voltage power supply (4.5 kV, 3 A
DC) with fast (~microsecond) crowbar protection circuit
for biasing the anode electrode of the triode tube, other
power supplies to bias grid (0-100 V, 0.5 A DC), and
filament (10 V, 24 Amp AC) of the triode tube were
designed, developed and commissioned with the amplifier
stages. Figure 7 shows the cabinet for housing the DC
power supplies and the two amplifier stages along with
the controls and indications on the front panel.
The anode power supply is designed with an input of
415 V, 3-phase, 50Hz AC fed to the three primary
terminals of a step-up transformer through three numbers
of series AC inductor and three anti-parallel pair of SCRs.
Forced air cooled 3-Phase full wave bridge rectifier
assembly is used in secondary of Transformer to achieve
3.2 kV DC output. A shunt is used for sensing over
current/short circuit current and fed to a crowbar
protection circuit [7].
Figure 7: The cabinet for the amplifier system
After sensing the over-current, the crowbar circuit
gives signal to a series of SCRs, connected across output
DC bus capacitor, and discharge the same. At the same
time, signals are sent to thyristor triggering card to stop
the pulses to the AC side SCRs. Thus the output is
quickly discharged to zero. The circuit diagram for the
anode power supply is shown in figure 8.
Figure 8: Circuit diagram of anode power supply
The Grid power supply is a linear regulated DC power
supply, which is designed with 230V, 1-phase, 50Hz AC
mains. The AC input voltage is fed to the primary of an
air-cooled transformer and the secondary is rectified by
full wave bridge rectifier and filtered by electrolytic
capacitors. This filtered DC link is fed to a transistor as
series element for voltage regulation. The circuit diagram
of the grid power supply is shown in figure 9.
Figure 9: Circuit diagram of grid power supply
RF and microwave tubes are prone to internal arc that
can lead to a permanent damage if excessive energy is
dissipated. So, a shunt diverter topology has been chosen
for the anode supply in which stored energy in electrical
system is diverted to the ground by quick shorting the
output terminals of power supply through series of SCRs,
connected across output DC bus capacitor. The DC
regulated power supply has been tested using high power
resistive loads.
For the safety of the amplifier, power supply units and
the operator, various interlocks and protection
mechanisms have been incorporated into the design of the
DC power supply. These include over-current and overvoltage protection and timer for gradual increase in
filament voltage. The triode electrodes need to be
powered up in a proper sequence starting with the
filament, grid and then anode. This has been taken care of
in the design of interlocks where the power supply for an
electrode will not turn ON if the sequence violated.
Besides these, door interlocks are provided for the safety
of the operator.
SUMMARY
In this paper, the design and the status of development
of the 2 kW RF amplifier has been presented. The driver
stage and final stage of the amplifier, along with the DC
regulated power supply has been designed and developed
with the aim of using it as the power source for the 37.8
MHz re-buncher. The amplifier has been tested up to
500W with 13 dB gain and is being tuned to achieve
higher power levels. The tests and tuning for 2 kW CW
operation, stable power output, minimum drift and good
harmonic suppression will be done before commissioning
the system.
ACKNOWLEDGEMENTS
The authors would like to thank VECC Workshop and
Shri M W Siddiqui of AC Division for their help and
support.
REFERENCES
[1] A Chakrabarti, Nucl. Intr. & Meth. B261 (2007) 1018.
[2] A Bandyopadhyay et al., Proceedings of PAC09,
Vancouver, FR5REP117, p. 5053; http://www.JACoW.org
[3] S Dechoudhury et al., PRAMANA, Vol. 75, No. 3, (2010)
p. 485-499.
[4] H O Granberg, “Wideband RF Power Amplifier”, A313, R
F Design, (Feb. 1988).
[5] H K Pandey et al., IEEE MICROWAVE 2008, Jaipur,
India, Nov. 2008, p. 119-121.
[6] H K Pandey et al., CODEC 2012, Kolkata, India, Dec.
2012, p. 1-4.
[7] S K Thakur et al., CYCLOTRONS 2010, Lanzhou, China,
Sep. 2010, MOPCP009, p. 60.