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Development of High Voltage Surge Limiting Resistor for Protection of HV
Multiplier of 3MeV DC Accelerator
S. Dewangan*, D.K. Sharma, R.I. Bakhtsingh, R.N. Rajan, V. Sharma, R Barnwal,
S. R. Ghodke, S.K. Srivastava, R. Patel, S. Banerjee, Mahendra Kumar, S. Nayak, S. Gond, N.B.
Thakur, A. Waghmare, K.C. Mittal and L.M. Gantayet
Electron Beam Centre, Accelerator and Pulse Power Division, Bhabha Atomic Research Centre,
Sector-7, CBD Belapur, Navi Mumbai
*[email protected]
Abstract
A 3MeV, 10mA DC electron beam accelerator is
in commissioning stages at EBC, Kharghar, Navi
Mumbai. The accelerating potential of -3MV is generated
by a Parallel Coupled Voltage Multiplier (PCVM) scheme
using 74 stages of HV rectifier stacks in the 6kg/cm2 SF6
gas environment. The HV surges of order of 600kV,
42kA, 10ns is estimated across the rectifier stacks during
sparking in the multiplier column. To limit the surge
current and protect the rectifier diodes, a non inductive
thick film surge limiting resistor (SLR) and protective
spark gap is designed and developed. The rectifier stacks
with surge limiting resistors at both the ends and
protective spark gap in parallel has been successfully
tested in simulated surge condition at an impulse voltage
of 212kVp, 150ns FWHM and surge energy of 200J,
10ms, 20kV at 6kg/cm2 SF6 gas environment and found
satisfactorily. Subsequently the HV multiplier was
installed with this surge protection scheme and is being
tested at 1.2MeV level. This paper describes the design
features and test results of the non-inductive surge
limiting resistor.
rectifier diodes. For a 3MeV dc accelerator, the HV surges
of the order of 600kV, 42kA, 10ns is estimated across the
rectifier stacks during sparking in the multiplier column.
To limit the surge current in the rectifier diodes, a thick
film non inductive surge limiting resistor (SLR) rated for
3kΩ, 10W, 150J at 100ms has been designed and
developed. Rectifier stacks are connected across the
corona rings of a high Voltage multiplier. To protect the
rectifier diodes, spark gap operates across corona rings
whenever applied voltage exceeds 120 kVdc or 600kV
pulse within 10nsec time.
ANALYSIS OF HIGH VOLTAGE SURGE
The block schematic of the 3MV Multiplier column [1]
is shown in Fig.1. When there is a sparking between 3MV
terminal and ground, the voltage appearing across the
rectifier stack is of the order of 600kV with 10ns time.
INTRODUCTION
An accelerating potential in 3MeV dc accelerator is
generated by parallel feeding of radio frequency power to
a series connected rectifier stacks. The RF power is
provided by a 50 kW, 120 kHz triode tube based power
oscillator which is powered from three-phase 50 Hz
mains. The stabilized and variable 3Φ input is stepped up,
rectified and filtered to achieve variable 10kVDC, which
is inverted to 120kHz by a colpitts oscillator. An air-core
RF Transformer steps-up this voltage at 300kVp to feed
to a pair of semi cylindrical RF Electrodes surrounding
the HV multiplier column. The multiplier column is
configured in a cylindrical-geometry in SF6 gas medium
at 6kg/cm2 SF6 gas environment. The multiplier column
having 74 nos. of the rectifier stacks cascaded through 40
pairs of the corona guards generates -3MV DC. Each
rectifier stacks are rated for 304kV PIV, 500mA forward
current and 50ns reverse recovery time.
The high voltage dc accelerators are prone to HV surges
of the order of few kA, few nanosecond rise time while
conditioning & load changes. This can damage the HV
Fig.1: Schematic of 3MV Multiplier Column
The Spark channel resistance in Ohms is given by
kl
-------- (1)
R 
Q
where k = toepler constant (0.8x10-3 for air), l = spark
length between HV terminal and ground, in cm and Q =
charge transferred through spark channel, A.sec. The
Spark Channel Inductance in µH is given by
 2l 3 
--------- (2)
L  0.002l ln
 
4
 
Where  is the spark channel diameter in cm. For 3MeV
system, l = 33cm, dome to ground capacitance =100pF
and R= 88Ω. As a thumb rule, a spark channel inductance
of 15nH/cm gives 0.5µH and surge frequency of about
22MHz with peak current of 42kA.
Design Strategy of Surge Protection Scheme
The surge protection scheme for the rectifier stacks of
3MV multiplier should be able to limit the surge current
to 20A @ 8.33ms and absorb the surge energy of 150J for
100ms. Due to conditional sparking between two
consecutive rectifier stack, 0.7A current flows through
stack, these causes the dip in the HV column voltage and
system will trip within 100ms. The resistor has to
withstand a surge voltage of 600kV for 10ns time.
Considering this, the resistor is to be incorporated in
series with rectifier stacks at both the ends with suitable
terminals. The design involves optimization of various
electrical and thermal parameters such as self inductance
ls, Self capacitance Cs etc for the dimensions of resistor to
achieve minimum temperature rise and adequate high
voltage insulation. The equivalent circuit of the HV
multiplier column with rectifier diode is shown in Fig.1.
From the given RF input of 300kV, capacitive coupling
factor coupling factor, beam current and diode peak
current rating of 20A @ 8.33ms, the resistance value have
been finalised to 3kΩ for rms current of 31.4mA. From
these considerations, the design parameters have been
finalised as given in table 1.
Table 1. Design parameters of Surge Limiting Resistor
Parameter
Resistance Value
Resistance
Tolerance
Type of resistor
Self Capacitance
Self Inductance
Current through
resistors
Power
Dissipation
Surge rating
Voltage
Withstanding
Ambient
Temperature
Shape & Size
Working
environment
45C (Max)
Cylinder; Length: ≤ 50mm;
Both the ends to have brass cap
as terminal.
I.D = 20mm +/- 1/0mm;
O.D = ≤ 45mm
SF6 gas at 6kg/cm2 pressure
CONSTRUCTION DETAILS
At a surge frequency of 1- 100MHz, thick film based
resistor with uniform coating over the alumina base
cylinder are suitable to make non-inductive resistor. The
cylindrical geometry is decided for efficient heat
dissipation in compact size. The resistor outer surface is
coated with Silicone anti tracking coating for high voltage
insulation.
The resistor is modelled in ANSYS for thermal analysis.
The model is subjected to the heat load impulse for a
short duration of 100ms. Transient analysis is carried out
to visualise the temperature changes with respect to time
as shown in Fig 2.
Value
3kΩ
5%
Ceramic, high voltage, high
surge capability
≤ 1pF
<100nH
31.6mA-rms@100kHz (for a
load current of 10mAdc)
20A for 8.33msec
(for
transient Sparking condition)
10W (continuous)
150 Joules for 100msec.
300Vdc continuous
120kV pulse( For transient
Sparking condition 5us)
Fig.2.Impulse
power
Temperature(K) profile
(Watts)
Vs
Maximum
The base material of the resistor has been made of high
alumina ceramic bushes. Uniform coating of thick film
has been done over the alumina base to achieve required
resistance for better heat transfer and high voltage
insulation. The end terminal made with brass has been
soldered with film resistor. The resistor is connected in
the rectifier stack at both ends with spring touch contact.
The assembly photo of the rectifier stack has been shown
in Fig 3.
Charging time
Discharging Time
Fig. 5 Waveform of energy pulse applied to the resistor
Without SLR
With SLR
Fig. 6 Waveform of Impulse voltage generated by 300kV
UWB Marx Generator applied to the resistor
Fig.3. Rectifier stack assembled with surge limiting
Resistor at both the ends
PERFORMANCE EVALUATION
The surge limiting resistor has been fabricated as per the
design detail and the parameters were analyzed using the
parameters like R, self inductance using programmable
LCR meter 4284A-Agilent make. At 120kHz, the
measured resistance is 3kΩ with a tolerance of < 5%. Self
capacitance is measured to be < 2pF. A continuous 10 W
power is applied on resistor and kept it for long time till
steady state temperature is reached. Fig 4 shows the
variation of temperature with time at 10W power. The
resistor has successfully tested at a surge energy of 150J,
25ns rise time within 15ms as shown in the Fig.5. Fig. 6
shows the waveform of the energy pulse applied to the
resistor. The resistor has been tested at an impulse voltage
of 110kV/5ns rise time and 150ns FWHM in open air,
which is generated by 300kV, UWB Marx generator.
The rectifier stacks with surge limiting resistors
at both the ends and protective spark gap in parallel has
been successfully tested in simulated surge condition at
an impulse voltage of 212kVp, 150ns FWHM and surge
energy of 200J, 10ms, 20kV at 6kg/cm2 SF6 gas
environment and found satisfactorily.
CONCLUSION
The surge protection scheme for the protection of the
rectifier stacks of 3MV multiplier has been implemented
in the 3MeV dc accelerator and tested upto 1.5MeV in
SF6 gas at 6kg/cm2. The maximum beam power is 5kW.
The accelerator is functioning satisfactorily. The
accelerator is being prepared for the flue gas experiments
at 1MeV and 5mA electron beam.
ACKNOWLEDGEMENTS
Authors are very thankful to Dr. J. Bhattacharya, ECIL,
and his team for constant support and successful
fabrication of this high energy high voltage non-inductive
surge limiting resistor.
REFERENCES
Fig.4 Graph showing temperature on resistor at 10W
[1] K. Nanu, et al, “Design of 3MV/10mA DC power
source for E-Beam Accelerator” Proc. Indian Particle
Accelerator Conference, Centre for Advanced
Technology, Indore, Feb. 3–6 (2003), p-246.
[2] R. I. Bakhtsingh, et al, “High Voltage Surge
Protection System for Gun Power Supplies of
3MeV, 30kW DC Electron Beam Accelerator” Proc.
Indian Particle Accelerator Conference, Centre for
Advanced Technology, New Delhi, 2011.