DESIGN OF RECTANGULAR RADIO FREQUENCY CAVITY FOR MINIATURE
MULTIBEAM KLYSTRON
A.K.Singh, A.K.Bandyopadhyay, L.M.Joshi, D.Kant, B. Kumar and R.Meena.
CENTRAL ELECTRONICS ENGINEERING RESEARCH INSTITUTE (CEERI-CSIR)
Pilani (Rajasthan)-333031, India
Abstract:
This paper is concerned with the design of
rectangular radio frequency (RF) cavities for a miniature
multiple beam Klystron. The proposed multiple beam
klystron will have four beamlets, each carries 64mA
current. The RF cavities under design will be compatible
with a multiple beam electron gun under development in
CEERI. This work is in continuation of the work
reported in the previous paper [1] where design of the
cylindrical cavity was presented. The possibility of using
rectangular cavities instead of cylindrical ones has been
explored in this work and a comparison of their
performance has been done. The rectangular cavity has
been designed to match the RF characteristics with that
of cylindrical cavity designed earlier. The rectangular
cavity designed has following advantages:
a) Fabrication of rectangular cavity is easier compared to
cylindrical counterpart of it.
b) Coupling of rectangular waveguide with rectangular
cavity is easier compared to the case of cylindrical one.
The RF cavities play an important role in
deciding the RF performance of the tube, such as its
gain, bandwidth, efficiency etc. Eventually proper choice
of cavity parameters including its resonant frequency,
quality factor, shunt resistance which depend on the
geometry of the cavity are of utmost importance to
decide the ultimate performance of the Klystron. The RF
cavity geometry of a four beam Klystron working in Jband frequency range has been optimized for its
frequency, R/Q etc. The shape and size of the central
conductor and the nose cones have been optimized to get
the desired performance of the RF cavity. A step
impedance transformer is also designed to check the
symmetric coupling over all the beams in the input and
output sections at the design frequency. The state of art
electromagnetic simulation tool CST microwave studio
has been used for the design and operation of the cavity.
Different cavity parameters such as the quality factor,
shunt impedance, etc. have been estimated with the help
of the simulation tool.
Keywords: Multibeam Klystron,
Transformer, Slot Coupling
Step
Impedance
Introduction:
Klystrons are high power vacuum tube power amplifiers.
In modern systems they are used from UHF (hundreds of
MHz) up through hundreds of GHz. Klystrons can be
found at work in radars, satellites, wideband high-power
communication, medicine, and high energy physics. RF
section is an important part of Klystron. The exchange of
energy between the accelerated electron beam and RF
signal takes place at the gap of the cavity. There are
conflicting requirement in deciding the gap diameter and
spacing, while for good interaction both the parameters
should be small compared with the transit time of the
electron through the gap, a too small gap diameter
imposes severe complications in gun design. Also the
small gap spacing increases the gap capacitance resulting
in a reduced gain bandwidth product. There is a
possibility of multipactor phenomenon in the small gap.
Therefore both parameters are to be decided keeping the
set specification in mind. Also the proper coupling irises
are required for input and output cavities so that tube
will look approximately matched at the input to the
driver and would be properly loaded by the matched
output line.
Parameter
Frequency Band
Beam Voltage
Beam Current
No. of Beams
Specifications
17-18 GHz
4KV
64mA
4
Table I: Design Specifications of the MBK
Design Approach:
The electric field distribution of the operating mode
inside the RF cavity discussed in the earlier paper [1] is
shown in the Fig.1. We started by taking the diameter of
the cylindrical cavity as the diagonal of the rectangular
cavity and further optimized the later cavity to get the
desired resonant frequency of 17.6GHz. The electric
field distribution of the
other. Although the electric field distribution of all these
modes were same.
Figure 2: Electric field distribution of the operating
mode of the rectangular RF cavity
Figure 1: Electric field distribution of the operating
mode of the cylindrical RF cavity
rectangular RF cavity is shown in Fig.2. We can see
from Fig.2 that the maximum field is in the gap between
the drift tubes as it is in the case of cylindrical RF cavity.
The comparison of design parameters of cylindrical and
rectangular cavity is given in Table II.
Parameters
Resonant
Frequency
Quality Factor
R/Q along four
gap center
Cylindrical
Cavity
17.5944 GHz
3279
53,51,53,50
Rectangular
Cavity
17.6041 GHz
To couple power to the cavity we went for slot coupling.
The dimensions of the slot in the cavity had been
optimized to get the desired operating frequency. Then
as the width of the standard waveguides [4] for the
desired frequency band was found larger we went for
half to full width four step impedance transformer [2, 4]
to match the impedance of the slot and the standard
waveguide. The simulation model of the impedance
transformer is shown in Fig.3 and its return loss
calculated using CST Microwave Studio is shown in
Fig.4.
3398
66,32,66,32
Table II: Comparison of design parameters of
Cylindrical and Rectangular RF Cavity
While designing the rectangular cavity it was found that
the first four modes had resonant frequencies too close to
each other and their field distribution were also same. To
separate the mode frequencies further apart various
central conductor geometries were tried and finally we
got the geometry (Fig.2) for which the first two modes
were having the same resonant frequency and the next
two modes had the same resonant frequency and these
resonant frequencies were 300 MHz apart from each
Figure 3: Four step impedance transformer
From Fig.5 we see coupling of magnetic field from the
cavity to the impedance transformer which was required.
Conclusions:
The rectangular RF cavity of a multi-beam Klystron
operating at 17.6 GHz has been designed with CST
Microwave Studio. Several cavity parameters like the the
Q-factor, R/Q etc have been compared with the same in
case of a cylindrical cavity. A four step impedance
transformer has also been designed for a half to full
width waveguide transition. The impedance transformer
has also been simulated together with the RF cavity to
optimize the slot coupling for feeding the cavity.
Figure 4: S-parameters of the impedance transformer
From Fig.4 we can see that it has flat response in the
desired frequency band and the return loss is also good.
After that whole geometry of cavity with impedance
transformer was simulated to check the symmetric
coupling of power over all the beams.
Acknowledgements:
The authors are thankful to Dr. Chandra Shekhar,
Director CEERI for encouragement. The authors are also
thankful to all team members at CEERI and the
concerning academic persons of The University of
Burdwan.
References:
1.
2.
3.
4.
S Maity, AK Bandyopadhyay, D Kant, R Meena,
SK Jhangid, N Devi and LM Joshi, ”Design of RF
cavity for a J-band multibeam Klystron”,
proceedings of NCRA-MTDCS-2011.
David M. Pozar, “Microwave Engineering”, 2 nd ed.,
John Wiley and Sons Inc.
R.E. Collin, “Theory and design of wide-band
multisection
quarter-wave
transformers”,
proceedings of IRE
Samuel Y. Liao, “Microwave Devices and Circuits”,
PHI Private
Figure 5: Magnetic field distribution and S-parameter
of the operating mode of the rectangular cavity along
with the waveguide coupler
Limited