Download Design and development of high voltage Marx modulator

DESIGN AND DEVELOPMENT OF HIGH VOLTAGE MARX
MODULATOR TECHNOLOGY FOR LONG PULSE APPLICATION
Mahesh Acharya, Purushottam Shrivastava, PHPMS, RRCAT, India.
Abstract
High power pulse modulators are used for powering the
RF amplifier like klystrons. This paper describes the
development of a 10 kV, 10 A, 1 ms Marx modulator for
technology demonstration. The modulator is developed
using four no. of main modules each of 2.5 kV. To reduce
the over sizing factor of capacitors, the allowed drop of
main Marx cell is 9%. A droop compensation circuit has
been developed to reduce the output pulse voltage droop
from 9% to within ±1%. Droop compensation consists of
10 numbers of corrector modules each of 200V. A
microcontroller based trigger circuit was used for
simultaneous triggering of main modules and for
staggered triggering of corrector modules. A 25 kV, 10 A,
1 ms Marx modulator is being developed. The advantages
of this scheme are oil free design, low DC voltage,
adjustable pulse width, adjustable rise time/fall time and
modular design etc.
INTRODUCTION
In the accelerators, high power RF pulse is required to
accelerate the particle. High power RF pulse is supplied
by the klystrons which amplify low power RF to high
power RF. Modulator is used to generate pulses of
specified time duration for klystron. Next generation
accelerators e.g. LINAC for proton synchrotron requires
long RF pulses i.e. 800 µs. So for this purpose long pulse
modulator is required.
Resonant
converter
current
source
driver
card
driver
card
driver
card
driver
card
driver
card
driver
card
driver
card
driver
card
0
Load
Timing & trigger card
0
Figure 1: Schematic diagram of Marx modulator
There are different types of topologies for the
development of modulator. A conventional PFN (pulse
forming network) type modulator have good operation for
short pulses (e.g. ~10-20 µs), But for long pulses it is not
an optimum solution. Because a well designed pulse
transformer is required for long pulses. The inductor, used
for PFN also becomes bulky for long pulse operation.
The single switch topology is the quickest approach to
obtain a working Klystron modulator due to its simplicity.
The pulse transformer has to be specially designed to the
pulse length and pulse power, the switch must be tailor
made to the voltage and current level, and the eventual
correction bouncer must be fine tuned to fit the load
perfectly.
Figure 2: 10kV Marx modulator without correction circuit
In the last decade, advancement of the solid state
switches created a path for more controllable Marx
generator which is called Marx modulator. In a Marx
modulator, stack of capacitors are charged slowly in
parallel to a given voltage by a power supply through
solid state charging switches. After charging, the
capacitors are discharged in series by firing of solid state
discharging switches. The Marx modulator will then
generate an output pulse with a voltage equal to the
individual cell voltage times the number of cells for a
duration of discharging pulse. A solid state Marx
modulator is used to generate high voltage pulse with
relatively low voltage power supply. The Marx modulator
has many advantages over other type of modulator e.g. oil
free design, low DC voltage, modular topology, adjustable
pulse width, adjustable rise time/fall time and higher
machine availability.
Marx modulator has low voltage Marx cells. A Marx
cell employs an energy storage capacitor, a charging
IGBT switch, a discharging IGBT switch, driver cards
and bypass diodes. Marx cells float at high voltage during
output pulse, therefore their driver circuits should be
isolated to each other as well as ground. A compensation
circuit is required to reduce the size of energy storage
capacitors.
A 10kV solid state Marx modulator has been developed
for realization of this topology. In this modulator, four
2.5kV main Marx cells were used to produce 10kV, 10A,
1ms pulse.
DESIGN & TESTING OF MODULATOR
Fig. 1 shows the schematic of the Marx modulator,
which consists of 4 main Marx cells, rated 2.5kV. The
capacitors, charging IGBTs, discharging IGBTs and
bypass diodes are connected as depicted in Fig. 1. Driver
cards for IGBTs were developed using totem pole
transistor circuit. Since the driver cards float on high
voltage during pulse output, an isolated auxiliary power
supply was required. It was developed using the single
turn ferrite core transformer. A resonant converter power
supply was used for the primary of transformer and the
secondary of transformer was rectified and used as an
isolated auxiliary power supply for driver cards.
eventually increase the cost. Oversizing factor depends
upon the droop d and it is equal to the 1/ (2d – d2). If the
accepted droop of is about 8% or more, then capacitor
oversizing factor is drastically reduced.
Figure 4: Oversizing Factor
Fig. 4 shows the general relationship between the
capacitor over sizing factor and the accepted voltage
droop. So it may be feasible to reduce the size of the
capacitor and compensate the exceeding voltage droop by
a separate compensation circuit. However, the Marx
modulator offers a possibility to compensate the droop
internally. If the modulator has a surplus of cells, the extra
cells can be added sequentially during the pulse so that
they compensate the droop. The diode bypass in a Marx
modulator, allow us to add the cell in the circuit at any
time. When a Marx cell is bypassed, it adds no voltage in
the output pulse. When the output pulse drops to the
minimum specified voltage for flattop specification, an
additional call can be turned on, this step the voltage up
by that cell’s voltage.
Figure 3: 10kV pulse without correction
A timing and trigger card based on 89S52
microcontroller was developed for providing ON/OFF
signal to charging & discharging IGBTs. Optical fiber
cable was used for transmitting each signal to driver card
to isolate it from timing and trigger card.
Capacitors were charged in parallel by switching on the
charging IGBTs. At the end of charging, a delay is set
between charging and discharging. After this delay,
discharging IGBTs were switched on for 1ms. The
capacitors were connected in series for 1ms and produce
10kV, 10A, 1ms output pulse with a droop of 8-9% as
shown in Fig. 3.
Pulse modulator is required to give the flat top pulse
with a maximum droop of ±1%. One solution to achieve
this required droop is to increase the capacitance. The
oversizing factor i.e. the output pulse energy divided by
total stored energy is increased by increasing the
capacitance for the same output pulse which will
Figure 5: 10kV Marx modulator with correction circuitry
Droop compensation was done by adding 10 corrector
cells, each of 200V with the main Marx cells as shown in
Fig. 5. A corrector cell consists of an energy storage
capacitor, charging IGBT, discharging IGBT and bypass
diode. These corrector cells were connected with the
existing main Marx cells. The corrector capacitors were
charged in parallel to 200V with the main Marx cells.
During the start of output pulse, these corrector cells are
bypassed by the diodes. When the output pulse drops by
1%, the discharging IGBT of 1st corrector cell is switched
on; this connects the 1st corrector cell in series of main
Marx cells, which shoots up the output pulse voltage by
1%. Again when pulse drops by 1%, 2nd corrector cell is
connected in series with the main Marx cell and output
voltage pulse is shoot up and so on. So the droop of the
resultant pulse comes within ±1%. Fig. 6 shows the
modulator generating a 10kV pulse by delay firing of
corrector cells. By increasing no. of corrector cells, droop
can be reduced considerably.
Figure 6: 10kV pulse with correction
CONCLUSION
A solid state Marx modulator topology was presented.
The operation, features and design consideration were
illustrated and verified with the experimental result of a 4section 10kV modulator. The prototype test has shown to
be good and validate the theoretical analysis. The output
pulse is 1ms flattop pulse with droop within ±1%. The
modulator is also tested at 1Hz, and it can operate at
higher frequency. This topology does not limit high
frequency operation fundamentally. The Marx modulator
can be operated at high repetition rate by charging
capacitors fast, during defined low charging time. A 25kV,
10A, 1ms pulse modulator is being designed and
developed which will be eventually upgraded to 100kV,
20A, 1ms pulse output. For the 25kV output pulse, 10
main Marx cell each of 2.5kV will be used and 20
corrector cells will be used for the droop compensation.
ACKNOWLEDGEMENT
The authors wish to acknowledge the fruitful
discussions with Shri T. Reghu, Shri J.K. Mulchandani
and technical support of Shri Hargovind Singh and Shri
J.Y. Parate.
REFERENCES
[1] G. E. Leyh, The Marx Modulator Development
Program for the International Linear Collider, SLACPUB-11868, June, 2006.
[2] J. Casey, R. Ciprian, I. Roth, M. Kempkes, M. P.J.
Gaudreau, F. Arntz, Diversified Technologies, Inc.,
35 Wiggins Avenue, Bedford, MA 01730 USA, Marx
Bank Technology for Accelerators and Colliders,
EPAC08, Genoa, Italy.
[3] Michael A. Kempkes, Ian Roth, Marcel P.J.
Gaudreau, Floyd O. Arntz, Jeffrey A. Casey,
Diversified Technologies, Inc., Marx Bank
Technology for the International Linear Collider,
PAC07, Albuquerque, New Mexico, USA