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DEVELOPMENT OF A PULSE MODULATOR TO DRIVE 6.1 MW
KLYSTRON FOR 15MeV ELECTRON LINAC
Kiran Thakur # , R Krishnan, S N Pethe, Rahul Patil
#
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
and # [email protected]
# [email protected]
AbstractWe present the Design, construction, and
characterization of a line-type pulse high-voltage
modulator system capable of generating 140-kV, 6 µs
pulses width at 150 pulses per second into a Klystron as
load. The modulator is used to energize a variety of highpower microwave devices requiring voltage stability and
reproducibility. Voltage ripple is less than 0.4% during
the flat top, with a shot-to-shot voltage variation of less
than 0.2 %. The primary circuit consists of six-stage
tuneable pulse-forming networks (PFN’s). The PFN is
charged by a highly stable charging power supply. The
total energy stored is released through a CX-1559 E2V
make thyratron into a 1: 11 pulse transformer, which
generates 140 kV, 6 µs pulses. The flat-top voltage
generated by the modulator is highly desirable for driving
RF sources requiring high quality electron beams. Linac is
one of the important fields of study in modern technology
due its wide applications for Medical and Industrial
purposes in addition to physics research.
Keywords—Line-type Modulator, Klystron, PFN, Pulse
Modulator, Pulse Transformer, Linear accelerator.
INTRODUCTION
A microwave tube Klystron or Magnetron requires high
voltage pulsed power supplies, commonly referred as
Modulator. The important parameters of the modulator are
Pulse power, Pulse duration, duty ratio and impedance.
The 13 MW compact pulse modulator is designed and
tested with S-Band Klystron. The modulator has a careful
selection and placement of components. The modulator is
built with safety interlock like over current and impedance
mis-match faults. The mechanism of De-Qing of charging
inductor is used to regulate the pulse to pulse voltage
regulation. The paper discusses development of a
modulator and compares the design values with the
measured. Table 1. shows the main specifications of
tested modulator at SAMEER. The S-band Klystron is
used of CSI make, Model-VKS-8262H. Table 2- shows
the Specification of Klystron VKS-8262H.
MODULATOR AND KLYSTRON
SPECIFICATIONS
Table 1. Shows the specifications of Modulator developed
at SAMEER.
Electrical Parameter
Units
Peak power output
Average power output
PFN charging voltage
PFN impedance
Pulse width
Pulse rise time
Pulse Repetition rate
Pulse transformer ratio
Thyratron peak current
Thyratron peak voltage
MW
kW
kV
Ω
µs
µs
Hz
A
kV
Achieved
Value
13
11.7
26
12.5
6
0.5
0 to 150
1:11
1000
13.2
Table 2- Specification of CPI make Klystron VKS8262H
Electrical
Parameter
VKS-8262H
Peak RF output
power
Klystron Beam
voltage
Klystron Beam
current
Pulse width (RF)
Peak Drive power
Duty ratio
Frequency
Units
Value
MW
6.19
kV
135
A
98.4
µs
W
8.5
120
0.0026
2996-3001
MHz
KEY DESIGN CONSIDERATIONS AND
DESIGN DECISIONS
Figure 1: Simplified diagram of Modulator
The 14 kV DC supply was designed to charge the PFN
capacitor. Taking into consideration the voltage and
current of the klystron, which act as load, is calculated as,
Impedance of Klystron = Zo =
132kV / 91A
= 1450.5 Ω
Pulse transformer turns ratio = N= Zo / Zin
The pulse transformer used is having the turn ratio of
1:11.This transformer is having bi- filler secondary
through which the klystron filament supply is fed.
Commercially available tri-axial cables RG8-A of 50 Ω
impedance is used to transfer energy from modulator to
input of pulse transformer. The four cables are used in
parallel, so the input impedance is Zin = 12.5 Ω is
matching.
There are many topologies that can be used to form a
Pulse Forming Network (PFN). However a PFN with
mutual coupling, which is called the Guillemin type E
network, is used to improve the pulse characteristic. In
Guillemin type E network, the value of capacitance and
inductance are same in each section and there is a mutual
inductance introduced between adjacent inductors. PFN
with mutual coupling around 12-20 % between the
adjacent inductors will improve the desired pulse shape
and keeping the last inductor’s value with 25 % higher
reduces the overshoot and ringing in the discharge pulse.
The PFN is designed for 6 µs pulse width with 12.5 Ω
impedance. The total capacitance Cn and total inductance
Ln is of PFN is calculated as,
Cn =
T
2 Zin
and
Ln =
B- is length of coil in inches= 24.5
N- is number of turns of coil= 69
L- is inductance in µH
The PFN capacitors are charged up-to 26 kV, hence
CSI make capacitor 0.047µF/35 kV are used for making
the PFN.
The charging choke is used to charge the PFN. Higher
the inductance of charging choke better the isolation for
thyratron from EHT side, but slower becomes the rate of
charging PFN. So compromise is made between the
charging time and isolation. The inductance of charging
inductor is calculated by considering the charging time 1.5
ms.
T = π LCn
WhereL- Inductance of charging choke
Cn- Total capacitance of PFN network
The main pulse transformer primary current goes up to
the order of 1000 A. So the switching device of E2V
Technology, Model- CX -1559 is used. The modulator is
protected from over current and mis-match faults. The
klystron beam current waveform at 91A is as shown.
IV.
EXPERIMENTAL RESULTS AND
ACHIVED PERFORMANCE
Figure 2: Klystron Beam Current Waveform.
TZin
2
The calculated Cn is 0.248µF and inductance is 45 µH.
The PFN inductance coil is made from cupper tube of Ø
6 mm. The coil is air core single layer. The coil is
designed by formula-
Where –
A- is mean diameter of coil in inches = 2.75
Figure.3: Charging Current waveform of charging reactor
with De-Qing.
are also focused
interference.
to
reduce
the
electromagnetic
Figure 4: Klystron beam current (Yellow) & forward
power (Red) at directional coupler placed next to
Klystron.
Figure 5: Peak Beam voltage verses peak output power of
Klystron.
Figure 6: View of the Modulator
CONCLUSIONS
In this work, it was reported design, construction and
characterization of the line-type pulse modulator. The
simulated results showed a good
agreement with
experimental results. The modulator is capable of driving
Klystron to deliver 6.19 MW peak power with 6 µs pulse
width at 150 Hz repetition rate. The input peak drive
power fed to Klystron is 130 watt. Hence, the modulator
design is suitable for driving high power Klystron. Efforts
REFERENCES
[1] www.ga-esi.com
[2] G.N. Glasoe and J.V. Lebascqz, Pulse Generator,
New York: McGraw-Hill,1948.
[3] Medical Electron Accelerator by C.J. Karzmark