1. International Journal of Electronics and Communication Engineering & Technology (IJECET),
INTERNATIONAL JOURNAL OF ELECTRONICS AND
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Special Issue (November, 2013), pp. 49-53
© IAEME: www.iaeme.com/ijecet.asp
Journal Impact Factor (2013): 5.8896 (Calculated by GISI)
www.jifactor.com
IJECET
©IAEME
Design, Fabrication and Characterization of RF Cavities for
250 kW CW C-Band Klystron
Richa Badola1, O S Lamba2, Meenu Kaushik3, Suman4, Monika Jangir5, LM Joshi6
Microwave Tubes Division
CSIR-Central Electronics Engineering Research institute Pilani(Raj)
1badola6@gmail.com, 2osl@ceeri.ernet.in
ABSTRACT: The paper presents the design and development of cylindrical and rectangular reentrant cavities for a 5 GHz, 250 kW CW C-band klystron being developed at CEERI Pilani.
Simulation tools like CST MICROWAVE STUDIO, SUPERFISH, AJ DISK, MAGIC-2D have been
used for design of the cavities. Cavities are designed using 1D code Superfish. By using
Superfish , value of R/Q, Stored energy, Quality factor, field contours, is calculated. Further the
distances between the cavities are calculated using AJ disk software. Then it is validated by
using Magic 2D software and power distribution, field contour, intensity of field along the
direction of propagation is calculated. Based on the simulated results the cavities are designed.
The fabrication of cavities was carried out through machining of piece parts, brazing, vacuum
leak testing and characterized through cold testing. The paper will describe the design
procedure, mechanical fabrication processes and cold measurement of frequency and Q of the
cavities. These re-entrant cavities are widely used in klystron. They play a prominent role in
deciding the performance of the klystron.
KEYWORDS: Klystron; RF cavity; Tuner; Quality factor (Q); R/Q
I.
INTRODUCTION
Klystron is a vacuum tube used to amplify small signals up to high power levels applicable in
radar, satellite communication and coherent RF power sources in application like linear
particle accelerators. Gain, Efficiency, stability can be increased by increasing the bandwidth of
the klystron. Analogy with conventional staggered tuned multiple resonant circuit amplifiers
suggested that this could be achieved by the use in a klystron of a larger number of cavities
suitably loaded and tuned. The theoretical analysis of multi-cavity klystron operation has met
with a number of difficulties owing to the complicated nature of modulation processes
involved. Account must be taken, for example, of the interaction between non adjacent cavities.
As a result, the design criteria for optimum performance, such as the number of cavities
required, their spacing, design and detuning from the band center, the effects of space charge
and the quality of electron beam are only now being established. Our aim is to design a cavities
and RF Section (Integration of cavities) in order to achieve the desired power. Cavity is
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
Page 49

2. International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
designed using Superfish Software and value of R/Q, stored energy, and Quality factor is
calculated. Then it is validated by using Magic 2D software. Based on the simulated results,
fabrication of cavities has been done.
II.
SIMULATED RESULT OF RF CAVITIES
Computer aided simulation tool like CST, SUPERFISH, MAGIC -2D CODE has been used for
designing of cavities. These tools will provide the required design parameters and give
information about the resonance frequency, quality factor, field contours, power distribution
etc.
A. Modeling aspects of re-entrant cavities
The design aspects of cavities are being discussed. The optimum design of cavity follows the
choice of the beam current, voltage, perveance and diameter .The main parameters involve in
the design cavity resonators are the gap length, drift tube radius, diameter and height of outer
concentric cylinder, shunt impedance and quality factor of the cavity and finally the coupling.
Re-entrant type resonators cavity oscillates with wavelength λ/4, 3λ/4, 5λ/4 etc. so height of
re-entrant cavity resonator is chosen to be slightly λ/4 for the frequency range. Here λ is free
shape wave length.
B. Design of RF cavities using Superfish 1D code
Poisson’s Superfish is a collection of programs for calculating static magnetic and electric fields
and radio-frequency electromagnetic fields in either 2-D Cartesian coordinates or axially
symmetric cylindrical coordinates. The programs generate a triangular mesh fitted to the
boundaries of different materials in the problem geometry. Plotting programs and other
postprocessor codes present the results in various forms. We design individual Cavities and
calculate their respective R/Q in Poisson’s Superfish.
Fig 1: Design of input cavity
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
Page 50

3. International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
Fig 2: Design of output using Superfish
parameters
Input cavity
Output cavity
Height of cavity
15 MM
15MM
Radius of cavity
13.75 MM
13.48 MM
Height of Drift Tube
5.75 MM
6.00 MM
Frequency (GHz)
5.008
5.003
R/Q
105
103
K
250
260
Table 1: Parameters of input and output cavity
C. Simulation of cavity using MAGIC 2D PIC code
MAGIC is particle-in-cell code developed by Mission Research Corporation, USA. The code is
available both in 2D and 3D the outlines of RF cavities, drift tubes and pole pieces on closely
spaced grids pushing thousands if simulated electrons through these simulated fields and
iterating to convergence. The code is capable of designing electron gun and cavity as well.
Following figures show the result of cavity simulation using versions. It simulates actual
electric and magnetic fields by tracing MAGIC code.
Fig 3: Design of cavity in Magic 2D
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
Page 51

4. International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
Fig. 4: R/Q of input cavity
Fig. 5: Frequency of input cavity using Magic 2D
Fig. 6: Fabricated cavities
Fig. 7: Fabricated output cavity
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
Page 52

5. International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME
III.
INTEGRATION OF RF CAVITIES USING AJ DISK SOFTWARE
It is an ideal code for quickly estimating the basic design of klystron. Input needed to run the
program include resonant frequency, Qs, R/Q and gap voltage of each cavity, axial distance
between the gap centers, operating beam voltage, current and drive power. The output of the
program includes gain, output power, cavity voltages, phase diagram and velocity dispersion
diagram.
Fig. 8: Integrated RF Cavities using AJ Disk
IV.
CONCLUSION
Re-entrant cavity resonator parameters of resonance frequency, Q, R/Q , shunt impedance,
tuning range , stored energy, field profiles, are computed using various code like as CST
MICROWAVE STUDIO, HFSS and MAGIC. These results are validated using MAGIC code. RF
cavities fabricated and experimental measurements carried out. . The computed results are
well matched with the measured value of the cavities.
REFERENCES
[1] Liao, Samuel Y, Microwave devices and circuits, third edition Prentice Hall of India Private
Limited.
[2] Gilmour, A.S, Microwave Tubes, Artech House, 1986.
[3] Robert M. Philips and Daryl W. Sprelin, High power klystrons for next linear colliders,
Proceeding of I EEE, Vol 87, No.5, May 1999.
[4] Kosmahl, H.G. and Branch, G.M. Jr., Generalized representation of electric fields in
interaction gaps of klystrons and TWTs, IEEE Trans. ED-20, pp.621-629,(1973).
[5] Kosmahl, H.G. and Albers, L.U., Three dimensional evaluation of energy extraction in output
cavities of klystron amplifiers, IEEE Trans. ED-20, pp.883-890, (1973).
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India
October 18-20, 2013
Page 53