Development of a long pulsed RF test stand and its applications for performance studies of 1 MW CW klystron
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Sådhanå (2020)45:36 Ó Indian Academy of Sciences
https://doi.org/10.1007/s12046-020-1281-z Sadhana(0123456789().,-volV)FT
3](0123456789().,-volV)
Development of a long pulsed RF test stand and its applications
for performance studies of 1 MW CW klystron
PURUSHOTTAM SHRIVASTAVA, T REGHU* , J MULCHANDANI, V MANDLOI,
P MOHANIA, A MAHAWAR, HARGOVIND SINGH and VIKAS RAJPUT
Pulsed High Power Microwave Division, Raja Ramanna Centre for Advanced Technology (RRCAT),
Indore 452013, India
e-mail: traghu@rrcat.gov.in
MS received 22 February 2019; revised 23 December 2019; accepted 24 December 2019
Abstract. Research and development activities of high power microwave sources for powering RF cavities of
Indian Spallation Neutron Source and Accelerator Driven Subcritical Systems are underway at RRCAT. Front
end accelerating structures such as Radio Frequency Quadrupoles and Drift Tube Linac demand pulsed RF
power up to 1 MW. A 1 MW pulsed RF system based on TH 2089 klystron amplifier at 352.2 MHz with pulse
width capability up to 1.5 ms has been developed and tested. A compact 100 kV, 20 A converter type modulator
with pulse width capability up to 1.6 ms has been used to energize the klystron of RF test stand. The perfor-
mance of the klystron in pulsed mode operation has been studied and presented. The variation in the RF output
power was measured and it is within ±0.75%. The phase variation of RF output power within the pulse and the
pulse to pulse is less than ±2.5°.
Keywords. Pulsed RF test stand; high power klystron; converter modulator; long pulsed modulator.
1. Introduction problem of the droop, pulse transformer needs to be
carefully designed. In hard switched modulators there is no
High power klystron amplifiers are widely used for pow- concern of the ripple and only the droop need to be man-
ering Radio Frequency (RF) cavities of high energy particle aged by careful design of the pulse transformer and the
accelerators [1]. They are available in both continuous bouncer circuit [7].
wave (CW) and pulsed mode operating conditions. High Another important concern with regard to the life of the
voltage pulsed sources are required for powering the kly- klystron is the fault energy of the modulator in case of
stron amplifier and such sources are known as modulators. internal arc in the klystron. Typically, 10 J is the allowable
Conventionally, line type and hard switched modulators energy limit and beyond this limit, it can cause severe
based on tube switches are used for the generation of pulsed damage to the klystron [8, 9]. Line type modulators are
high voltages [2, 3]. Line type modulators are normally inherently current limited due to the presence of impedance
used for short and fixed output pulse width (typically less of the PFN and leakage inductance of the pulse transformer
than 20 ls) applications. For the longer pulse width, in series with the klystron. In the worst case, the maximum
modulator system would be very large as the width of pulse current will be twice of the rated current. In the case of hard
is directly proportional to number of PFN stages [4]. Hard switched modulators, entire energy stored in the input
switched modulators are used for both short and long pulse storage capacitor gets dissipated in the klystron. To avoid
applications. The performance of klystron with line type this situation, a suitable crowbar circuit is incorporated in
and hard switched modulators has been well studied as parallel with the energy storage capacitor of the modulator
these topologies are in use for the past many decades [5]. [3]. In the event of an arc, the klystron current is sensed and
Other than rise and fall times which directly affect the it generates a trigger to crowbar switch, thus diverting and
efficiency of RF system, the droop and the ripple in the dissipating the energy stored in the storage capacitor in an
output pulse are other major concerns as they introduce alternate circuit through the crowbar switch. The
phase delay in the RF output of klystron [6]. A well advancement in the technology of semiconductor switches
designed line type modulator system with large number of with high voltage, high current, high switching frequency
PFN stages will considerably reduce the ripple but for the capabilities leads to the development of advanced solid
state modulators [1, 10]. Solid state switches gradually have
*For correspondence replaced thyratron switches of the conventional line type
36 Page 2 of 7 Sådhanå (2020)45:36
modulators. Recently new type of modulators viz., the high voltage long pulses. The desired output pulse
converter and Marx type are gaining popularity for long width is achieved by gating the gate drives by the required
pulse (up to 5 ms pulse width) and high average power pulse width. The modulator system is modular and there are
applications. Among the two, converter type modulators are two identical modules and each generates -50 kV, thus
more popular for driving klystrons for the development of overall output is -100 kV. Each module consists of high
high energy particle accelerators for spallation neutron frequency switched inverters (fs = 20 kHz), high voltage
sources [11]. Additionally converter type modulators are high frequency transformer and fast six pulse rectifier. The
compact, inherently energy limited and there is no need of control circuits, droop correction circuits and low pass p-
any high voltage crowbar switch [12]. section filter are common to both the modules. The full
Pulsed High Power Microwave Division of Raja bridge inverter works on parallel loaded resonant (PLR) LC
Ramanna Centre for Advanced Technology, Indore, India, topology. PLR topology inherently offers higher gain of
has been pursuing research and development (R&D) on the output voltage depending on the overall quality factor
long pulsed klystron modulators [12–14]. Other than the (Q) of the topology. This is advantageous in lowering the
popular topologies for the generation of high voltage long turns-ratio of the transformer. The non-idealities of the
pulse, we have an R&D program on dc-dc converter based transformer are also effectively used as a part of the reso-
modulators also. In the past, design and development of a nant circuit.
33 kV, 20 A, 1.6 ms long pulse modulator has already been The high voltage and high frequency transformer has
reported for driving low voltage klystrons and inductive been developed using Fe based nanocrystalline core mate-
output tube (IOT) applications [12]. Very recently work on rial (VITROPREM 800 of Vacuumschmelze) [16]. The flux
the development of 100 kV, 20 A, 1.6 ms long pulse density is chosen as 0.6 T. Primary and secondary windings
modulator for driving 1 MW klystron has been reported are made using a litz wire. Since the secondary winding
[15]. Such modulators are relatively new and their effects needs high voltage insulation, a specially fabricated solid
on the performance of the continuous wave (CW) klystrons Teflon bobbin has been used. Fast recovery epitaxial diodes
are not well studied. In order to study the pulsed mode are used in the six pulse rectifier. Each arm is a series
performance of a klystron driven by long pulsed modulator combination of many diodes. A p-section low pass filter has
and to test various RF components of the accelerator sub- been used to filter out the 120 kHz ripple. All high voltage
systems, an RF test stand has been developed. In this paper, components are assembled in a stain less steel tank filled
we will describe the details of test stand and performance with transformer oil for insulation and cooling. The ‘phase
characteristics of the klystron amplifier in pulsed mode shift control’ in tandem with feed forward correction
operation. The operational parameters of the RF test stand scheme has been employed for droop correction of the
are given in table 1. output pulse. With this technique, a droop of less than ±1%
has been achieved. Finer details about the modulator are
given in the reference [15].
2. Pulsed RF test stand The klystron amplifier, TH 2809, (Thales Electron
Devices make) is a high power linear beam vacuum tube
The simplified block diagram of RF test stand is shown in device that amplifies RF signal [17]. It has 1.1 MW output
figure 1. It consists of a 2 MW (-100 kV, 20 A) pulsed power capability at nominal frequency of 352 MHz with
modulator, 1 MW klystron, WR 2300 waveguide trans- -1 dB bandwidth of 1 MHz. The tube has five integrated
mission system, RF measurement system, solid state power cavities, two integral electromagnets for focusing the beam
amplifier driver and water loads to dissipate the microwave within the klystron and a modulating anode to adapt
power. The pulsed modulator system is based on DC–DC specific operating conditions. When the electron beam
converter topology and employs IGBT switches which passes through the resonant cavities of klystron there is a
operate at lower supply voltage and higher switching fre- strong beam-RF interaction. While passing through the
quency. High frequency, high voltage transformers in cavities, the energy of the electron beam amplifies the input
conjunction with a series of fast recovery diodes generate signal applied to the first cavity. The catcher cavity at the
other end of the tube delivers the amplified output. The
electrons are finally captured by an anode cooled by low
Table 1. Operational parameters of RF test stand.
conductivity water (LCW).
RF output power (peak) 1 MW Solid state pulsed amplifier based on Laterally Diffused
Pulse width (max) 1.5 ms RF MOSFET (LDMOS) devices has been developed to
RF frequency (nom) 352.2 MHz energize input of the klystron. The amplifier consists of two
Droop ±0.75% stages, a driver and a high power stage, and operates in
Phase stability (in pulse) ±2.5° class AB mode. It has gain of *47 dB and generates
Phase stability (pulse to pulse) ±2.5°
pulsed power in excess of 200 W. A Rhode and Schwarz
Modulator output power 2 MW
make, precision signal generator, SMB 100 A, has been
PRR capability 1–30 Hz
used for driving the amplifier. The output of the solid state
Sådhanå (2020)45:36 Page 3 of 7 36
Figure 1. A simplified block diagram RF test stand.
amplifier is fed to the input port of the klystron. The output common point of heater and cathode to avoid emission
RF power from klystron is coupled from its output ceramic from back of the cathode surface. The Sputter Ion Pump
window and fed to a water (LCW) load through a WR2300 (SIP) of the klystron has been powered by Vacion make
waveguide network. An Advanced Ferrite Technology 5 kV, 50 mA power supply. The coils of focusing elec-
make three port circulator suitable for 352 MHz and high tromagnets of the klystron have been powered by 250 V, 20
power operation has been used for isolation between the A DC power supplies. The supply to the modulating anode
source and load and it has a -40 dB isolation. In the of the klystron has been derived from same cathode pulsed
waveguide network, full height to half height tapered sec- power supply using a resistive divider.
tion has been used to reduce the overall length of the net- The modulating anode plays major role in the CW kly-
work and to couple E- plane half height magic tee. 1 MW strons mainly for output power control or pulsed mode
pulsed RF power is finally dumped into four numbers of operation. In this case the cathode voltage itself is in the
250 kW water loads. These four loads are connected at the pulsed mode, therefore, for maximum output power the
output ports of magic tees. One more 250 kW load has been modulating anode voltage is kept at 71% of the cathode
connected at the termination port of the circulator to absorb voltage. A resistive divider has been used for the generation
the reflected power, if any. A higher order mode filter is of modulating anode voltage and it is kept in an oil tank for
connected in the network to filter out the higher order providing cooling and high voltage insulation. The klystron
modes generated at high power levels. Directional coupler collector, body and RF output window are cooled by cir-
has been used at the klystron output to measure both the culating LCW. The klystron body has been electrically
forward and the reflected powers. The forward and reflected grounded by low impedance ground. The LCW flow circuit,
coupling factors are -50.1 dB and -44.8 dB, respectively. vacuum status of klystron, cathode heater, and focussing
Boonton make pulsed power meter, model 4542, has been coil power supplies are interlocked with the main modu-
used for measurement peak power and it has an RF fre- lator to avoid any failure of the former systems. A relay
quency range of 10 kHz to 40 GHz and pulse power based, hardwired interlocking circuit has been used for this
measurement range of -50 to ?20 dBm. purpose. The aerial view of the experimental set-up is
The cathode heater filament of klystron is powered by a shown in figure 2.
30 V, 35 A DC power supply that is 120 kV isolated as the
cathode is floating during the high voltage pulse [18]. The
high voltage portion of the power supply is kept in trans-
former oil and the feedback signals of the output voltage 3. Results of performance studies and discussion
and the current are taken through fiber optic link for dis-
play. DC powering to klystron heater is preferred so that Klystron is a nonlinear high power microwave amplifier
there is no AC magnetic field generated by AC heater and the output power not only depends on cathode voltage
current near cathode area. Such AC magnetic field extends and RF input power as in the conventional amplifiers but it
to the cathode surface and perturbs the beam trajectories also depends on other parameters. The variation of output
causes variation in the beam current [17]. The negative power with respect to cathode voltage has been studied and
terminal of heater power supply is connected to one of the plotted in figure 3. The output RF power has a following
heater terminal and positive terminal is connected to relation with cathode voltage,
36 Page 4 of 7 Sådhanå (2020)45:36
function relationship between the output power and the
cathode voltage. Output power of 968 kW at 352.6 MHz
has been extracted from the klystron. The cathode voltage
beyond 96 kV is not tried due to chance of klystron arcing.
Figures 4 and 5 show the temporal shape of the cathode
voltage, beam current and RF power at maximum cathode
voltage of 96 kV. It is observed that the variation in the RF
power is within ±0.75% as shown in figure 6.
The variation in RF output power with respect to drive
frequency has been studied around the centre frequency of
352.21 MHz. The klystron has a -1 dB bandwidth of
1 MHz.
The drive frequency is varied from 351.7 MHz to
353 MHz keeping other inputs such as cathode voltage,
input RF power and currents of focusing magnets constant.
The output power increases as the drive frequency
approaches the centre frequency. An unusual behaviour has
been observed with increase in drive frequency beyond the
centre frequency the output power continues to increase as
opposed to the expected behaviour. This could possibly be
due to a different centre frequency than the specified one.
When output went beyond 1 MW, arcing was occasionally
observed in the LCW based loads, hence, no further
increase in frequency was tried. Figure 7 shows the varia-
tion in RF output power with respect to drive frequency.
During this experiment all other parameters were kept
constant.
Figure 2. Aerial view of RF test stand.
The klystron has a diode like characteristic and beam
current varies as three-and-a-half power of the voltage. The
relation between beam current and cathode voltage is
given by
3
Ib ¼ KV 2 ;
where Ib is the beam current, K is the perveance of the
klystron tube and V is the cathode voltage. Peak value of
Figure 3. RF output power versus cathode voltage.
5
P ¼ gPl 106 V 2 ;
where g is the efficiency, Pl is the microperveance and V is
the cathode voltage. Below -20 kV, the output power is
negligible and at lower voltages the output power increases
slowly due to low electron velocity [19]. At higher cathode Figure 4. Waveform of cathode voltage and beam current at
voltages the output power changes rapidly due to the power 96 kV.
Sådhanå (2020)45:36 Page 5 of 7 36
Figure 8. Beam current versus cathode voltage.
Figure 5. Temporal profile of the RF output of klystron.
Figure 6. Temporal profile of the RF output (expanded:
20 kW/div) of klystron.
Figure 9. RF output phase variation within the pulse.
Figure 7. RF Output power versus drive frequency.
the beam current has been recorded and plotted for different
values of cathode voltages up to 96 kV. Figure 8 shows the
observed relationship between beam current and cathode
voltage under pulsed operating conditions. Figure 10. RF output phase stability in pulse to pulse.
36 Page 6 of 7 Sådhanå (2020)45:36
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