Download development of high voltage nanosecond pulsar for pulsed

DEVELOPMENT OF HIGH VOLTAGE NANOSECOND PULSAR FOR
PULSED ELECTRON BEAM GENERATION
Ranjeet Kumar, Romesh Chandra, S. Mitra, D.K. Sharma, Archana Sharma, K.C. Mittal
Accelerator and Pulse Power Division,
Bhabha Atomic Research Centre,
Mumbai, India -400085
Abstract:
In pulse power system, generation
and measurement of fast rising, short duration,
high voltage pulses is a challenging job, because
of at high frequencies inductance of the circuit
elements
and their parasitic capacitances
becomes effective at such level that they can
change the desired characteristics of the
generated pulse waveforms. A 50kV PFL (pulse
forming line) based pulsar is developed to
produce 2nsecond duration pulses with
≤1nsecond rise time for single shot. Voltage is
measured across the matched load and at the
charging side of PFL. For the measurement in
nanosecond time region a fast response copper
sulphate aqueous solution voltage divider is
designed for voltage measurement and a
Nicrome wire based current shunt is designed for
current measurement.
Experiments are
performed with matched copper sulphate load.
Simulation is done by using PSpice and the result
is compared with experimental. Circuit diagram,
voltage & current waveforms, photos of
experimental setup & pulsar are presented.
Further this pulsar will be applied to drive an
electron gun for pulsed electron beam generation.
I.
Introduction
Pulsed power technology has got many
applications in different industrial, medical and
research areas such as food processing, medical
treatment, water treatment, HPM generation etc.
Here the pulsar is developed to produce pulsed
electron beam for neutron time of flight (NTOF)
application. NTOF is a method to measure the
energy of neutron, which will be generated from
some target impacted by intense electron beam.
In this article used electrical circuitry,
specifications the of components and results are
explained.
II.
Experimental Setup
Photos of experimental setup is given in fig.(01)
and (03).
Figure-(01) Photo of experimental setup
Electrical circuit diagram has been presented in
fig. 02.The source power supply used here is
a10mA, 30kVdc negative polarity. Further to
charge PFL up to 50kV, an air core transformer
is used. A capacitor C1 is charged with the power
supply through a charging resister, now when
the capacitor is charged up to desired level a
peaking switch S1 get closed, resulting the
capacitor discharge through the primary winding
of the air core transformer. At this moment a
stepped up high voltage pulse of few hundred
nanosecond appears across the secondary
winding terminal, there another peaking switch
S2 get closed and charging, switch S3 is bridges
and the energy of PFL is transferred to the
matched copper sulphate load. All switches used
here are spark gap electrode switches and their
gapes are optimized for peak voltage value. The
air core transformer used here, has the
transformation ratio 1:8 and the coupling
coefficient between primary and secondary
winding is 0.77. Although from reference [1] it
is known that the energy transfer coefficient of
transformer
will
be
maximum
As per above boundaries a 100kV dc coaxial
cable was selected which has the inductance
250nH /meter, Capacitance 100 pF/meter and
the characteristic impedance of 50Ω. According
to reference [2], length of PFL was selected for
required pulse duration by the
relation
Tpulse = 2l (LC) 1/2 second
Where, Tpulse is pulse duration, l is length of the
PFL and L&C are inductance and capacitance of
the selected length.
Characteristic impedance of the PFL is given by
the relation
Z0 = (L/C) 1/2 Ω
IV.
Figure-(02) Electrical Circuit
Experimental Result
In fig. 04 waveform of voltage across matched
load is presented. Measurement of load
when C1 and PFL capacitance will be equal, but
here capacitance of source capacitor C1
connected to the primary side is much higher
than the PFL capacitance; the reason is to
achieve doubled voltage charging of PFL.
27.50
13.75
Voltage (kV)
0.00
-13.75
-27.50
-41.25
Figure-(03) Photo of pulsar
A blocking inductor is used to stop the travelling
wave towards source from PFL. Value of this
inductor (13µH) is selected in such manner that it
offers low impedance to the PFL charging pulse
(500 ns duration, 200ns rise time) and a very
high impedance to the arriving pulse(2 ns
duration, 500ps rise time) from PFL. This
inductor also helps to achieve better voltage
waveform.
III.
Design Parameter
Required output pulse characteristics is as belowRise time ≥ 1ns
Pulse duration = 2ns
Pulse amplitude = 40kV
-55.00
-2
-1
0
1
2
3
4
Time(ns)
Figure-(04) Voltage pulse measured by 6 GHz Lecroy
oscilloscope
voltage was taken with a copper Sulphate
aqueous solution voltage divider, which has the
sensitivity of 0.25kV/V and 6 GHz Lecroy
oscilloscope. The amplitude of the voltage
across load is 42.9kV. Rise time and pulse
duration of voltage pulse are 700 ps and 2.4 ns
repectively. FWHM of the voltage pulse was 1.1
ns.
In fig.(5) voltage and current waveforms are
shown together. Here amplitude of voltage and
current are 40 kV and 880A. This waveform was
taken by 350MHz Agilent oscilloscope.
Figure-(07) Load & PFL chargingVoltage waveform
from smulation
V. Simulation Result and
Comparison with Experimental
Figure-(05) Voltage & current pulse measured by 350MHz
Agilent oscilloscope pulse
Fig.(06) presents the charging waveform of PFL
and load voltage & current. Its seen clearly that
PFL is getting charged exactly doubled of the
applied voltage. This PFL charging voltage was
measured by Nicrome wire wound resistive
voltage divider. Output voltage and current are
30kV and 630A.
Simulation circuit and result is been presented in
the following fig. (08), (09) and fig. (10).
10.5us
R1
10.54us
L4
100
10nH
2 1
U1
2 1
C5
13uH
1
C4
L1
21n
0
2
L5
1
10nH
L2
380uH
2
2 1
U3
2
C7
0.2p
2
3uH
COUPLING = 0.77
K1
T1
21
U2 C6
0.2p
0.2p
V1
-7kVdc
10.7us
L3
1
R2
50
1
K_Linear
K
0
Figure-(08) Simulation circuit
Waveform in fig. (08) shows the voltage across
matched load got in simulation. Rise time, pulse
duration and FWHM of the pulse are
consequently 300ps, 2ns and 1.25ns. Comparing
with experimental result in
Figure-(06)Load Voltage & current and PFL charging
profile measured by 350MGHz Agilent oscilloscope
Transformer output voltage waveform with PFL
charging and load voltage is given in fig. (07).
Transformer output pulse duration is 300ns and it
was measured by 60kVDC Tektronix HV probe.
Figure-(09) Load & PFL chargingVoltage waveform from
smulation
fig.(04 ), it’s concluded that voltage pulse has
some small differences in it’s parameter
such as difference in rise time is 400ps,
difference in duration is 0.4ns and difference in
FWHM is 0.15ns. In simulation, transformer
output voltage was found as in fig. (10). This is
clearly seen that in experimental result the
waveform is same as in simulation but charging
time is slightly small in experimental, which is
the only function of blocking inductor value.
Fi
gure-(10) Load , PFL charging and transformer output
voltage waveform from smulation
VI.
Conclusion
Pulse forming line (PFL) based high voltage,
nano-second pulsar is developed successfully.
Voltage is measured at various stages and current
also. The experimental result achieved here are
acceptable for the requirement and are also
similar to the simulation result with very small
differences. Further this pulsar will be used to
drive an electron gun for pulsed electron beam
generation.
REFERENCES
[1] “Theoretical study of pulse forming
line
charged by capacitance through transformer
considering the energy recuperation.” Jian hua
Yang, Jian de Zhang. 2008, 17th international
conference on “High power particle beams”
[2] “Lessons in electric circuit” Volume II – AC,
by Tony R. Kuphaldt
[3] “A compact source of subgigawatt
subnanosecond pulses.” Alex Pokryvailo, Yefim
Yankelevich and M. Shapira. IEEE Transactions
on Plasma Science, Vol. 32, No.5 October 2004.