DESIGN AND CHARACTERIZATION OF RF DRIVER TO DRIVE
HIGH POWER KLYSTRON
Kiran Thakur* , Vaibhav Jadhav, K. Naresh Kumar, S N Pethe , R. Krishnan
*Society for Applied Microwave Electronics Engineering and Research (SAMEER), Research and
Development Institution under Department of Electronics and Information Technology, I.I.T.
Campus, Powai, Mumbai,
400 076, India.
Address
*E- mail: [email protected] and [email protected]
Abstract— Linear accelerator (Linac) is one of the
important fields of study in modern technology due its
wide applications for medical and industrial purposes in
addition to research in physics. A klystron is used as an
RF power source in high-energy Linac. RF Driver power
module is a critical component in any klystron Powered
accelerator. The RF Driver of Linac system contributes to
effectively get the capability of producing an electron
beams with an energy of 15 MeV. The Driver power
module consists of low power RF source and a multistage, fully solid state power amplifier with isolated
outputs. In this paper, the status of RF driver for 15 MeV
electrons LINAC is presented, which includes the system
design and operation of it. The RF drive system with 140
watt high power solid-state amplifiers was installed and
tested at SAMEER. The results of measurement and
operation have indicated that the RF drive system satisfied
the requirement of the 15 MeV Linac. The RF drive
system has operated successfully for over the years.
Keywords— Linear Accelerator, RF Driver, Klystron,
by a temperature controlled low loss RF cavity, resonant
in a high-order mode.
The major components of RF driver are Dielectric
Resonating Oscillator (DRO), RF switch and amplifier
stages to achieve required amplitude level of 200 Watts.
RF driver specifications are shown in table 1.
Table 1: Specifications of RF driver developed at
SAMEER.
Electrical
Parameter
Peak Power
Frequency
Pulse width
Pulse repetition rate
Sync pulse voltage
AC Supply
Units
Value
Watt
MHz
µs
Hz
Vp-p
200
2998
12
250
1
RF Driver
and Dielectric Resonating Oscillator (DRO).
INTRODUCTION
The klystron is an amplifier, a RF driver is required to
drive the klystron. The RF Driver power module is a
critical component in any Klystron-powered accelerator
[1]-[3]. This uses a solid state RF source for reliability and
stability. It is modular and adaptable, providing superior
performance with a straight forward design. The RF
Driver produces low-level microwave power which
powers a klystron amplifier, providing high power
microwaves that can be used to produce high energy
electron beam by Linac. The RF Driver frequency can be
controlled internally or externally by an AFC circuit. The
RF pulses from RF driver unit are fed to klystron
amplifier, to produce 6.5MW microwave pulses. A
klystron is used as an RF power source in high-energy
LINAC that employ a standing waveguide. The RF driver
pulse power output needs to be around 140 Watts for the
VKS-8262H klystron to produce 6.5 Megawatts of RF
pulse power used in LINAC system. A microwave planar
triode can be used as an RF driver, stabilized in frequency
Tuning
Voltage
Klystro
n
SYNC
Pulse
Control Console Unit
Figure 1: Block Diagram of RF Driver System.
The RF driver has many features for maintainability
and availability has been obtained as follows: Front panel
frequency display, power adjustment through variable
attenuator.
KEY DESIGN CONSIDERATIONS AND
DESIGN DECISIONS.
The block diagram of 200 Watt RF driver system is
shown in Figure 1. DRO is designed at 2.998 GHz which
gives about 15 dBm of CW power as shown in figure 2.
The pulses from RF driver unit are fed to klystron
amplifier. It has a coarse (mechanical) and fine frequency
tuning VT arrangements. VT voltage is adjusted manually
from the control panel to tune the Linac frequency. This
voltage can be controlled automatically by switching it to
Automated Frequency Control mode (AFC) from control
console with proper corresponding voltage for frequency
error [1].
RF switch used at the output of DRO is to get pulsed
RF power of oscillator. It is required to switch the CW
power obtained from DRO. Necessary synchronization is
obtained by providing trigger pulses (Received from
control console through front panel connector ‘SYNC’)
[2]. This is done by Mini-circuits RF switch ZFSWA 2-46.
The pulsed signal is shown in figure 3.
The overall gain is more than 53 dBm which is
achieved by three stages of amplification. The outputs of
first and second stage amplifier are 1 Watt and 20 Watt,
respectively. The output stage consists of two 135 Watt
amplifier modules which are combined. The first stage is
1 Watt BJT & FET transistors. The second stage transistor
is a 20 Watt power transistor followed by a third stage of
two 135 Watt transistors. A separate housing is provided
for these amplifiers with proper heat sink arrangement to
remove the dissipated heat.
designed to require gain at 3 GHz. 20 Watt Amplifier is a
class C pulsed amplifier provides approx. 8.2 dB gain at
2.998 GHz.
20 W & 135 W amplifiers are two identical arms used
to amplify output received from port A & B of the power
divider. Individual arm gives 135 watts of power output in
phase. It consists of PH2931-135S NPN silicon
Microwave power transistor, operated in class C with
Common Base configuration.
Figure 4: RF driver module
Figure 2: DRO output in Spectrum Analyser.
Figure 5: Bench setup of RF driver
RESULTS
Figure 3: RF burst signal at RF Switch
A photograph of outline of the RF driver is shown in
figure 4. Its test bench set up is shown in figure 5.
Performance parameters were measured. The ideal
measuring result is obtained and detail presented in table 2.
A Copy of peak power waveform of RF driver is shown in
figure 6.
A two way power divider/combiner (Wilkinson) micro
strip couplers is used to excite and collect power form
parallel output stage transistors. It consists of low cost
network that is fabricated in a high dielectric constant,
low loss RT/Duroid 5880 microstrip board.
2:1 Power divider/combiner is a Wilkinson type that
yields very low loss, low VSWR, and excellent phase and
amplitude balance. The insertion loss for both divider and
combiner was measured 0.5 dB and VSWR is 1.24.
1W amplifier is a class A amplifier stage consisting of
BJT (A 42035 NPN) and GaAs FET (ATF 44101)
medium power amplifier.
GaAs FET amplifier is
Figure 6: RF driver output in spectrum analyser
RF driver is tested for thermal stability in test bench
and real time and it is found dielectric resonating
oscillator and amplifiers are thermally stable.
Table 2: Measured parameters of RF diver
Parameters
Frequency
Peak Power
Pulse Width
Trigger
Pulse repetition
rate
Unit
MHz
Watts
µsec
Volt
Hz
Achieved Value
2998
200
12
1
250
CONCLUSION
In this work, it was reported the Design, development and
performance of RF driver for 15 MeV Linac. The simulated
results show a good agreement with experimental results.
The RF Driver is capable of driving Klystron to deliver the
6.19 MW peak power with 6μs pulse width at 250 Hz
repetition rate. This equipment achieves high reliability,
enhanced performance which is required for our Linac.
This equipment serves for 15 MeV and 18 MeV Linacs over
years, its operating stats is good.
REFERENCES
[1] AR305 ,Building push-pull VHF power amplifier,
Motorola RF device data vol.II
[2] Mini-Circuits, New York
[3] C.J. Karzmark, Craig S. Nunan, Eiji Tanabe,
Medical Electron Accelerator, New York, McGrawHill.
[4] G. N. Glasoe and J.V. Lebascqz, Pulse Generator,
New York: McGraw-Hill, 1948.
[5] R. Krishnan et. al., “S band linac tube development
work in SAMEER”, FR5RREP083, PAC09,
Vancouver, Canada.