INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

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1 INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET) International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN (Print) ISSN (Online) Volume 4, Issue 7 (2013), pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJECET I A E M E Design, Fabrication and Characterization of RF Cavities for 250 kw CW C-Band Klystron Richa Badola 1, O S Lamba 2, Meenu Kaushik 3, Suman 4, Monika Jangir 5, LM Joshi 6 Microwave Tubes Division CSIR-Central Electronics Engineering Research institute Pilani(Raj) 1 badola6@gmail.com, 2 osl@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 B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 49

2 designed using Superfish Software and value of R/Q, stored energy, and Quality factor is calculated. Then it is validatedd 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 Fig 2: Design of output using Superfish parameters Input cavity Output cavity Height of cavity 15 MM 15MM Radius of cavity MM MM Height of Drift Tube 5.75 MM 6.00 MM Frequency (GHz) R/Q K 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 B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 51

4 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 B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 52

5 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 Prenticee Hall of India Private Limited. [2] Gilmour, A.S, Microwave Tubes, Artech House, [3] Robert M. Philips and Daryl W. Sprelin, High power klystrons for next linear colliders, Proceeding of I EEE, Vol 87, No.5, May [4] Kosmahl, H.G. and Branch, G.M. Jr., Generalized representation of electric fields in interaction gaps of klystronss and TWTs, IEEE Trans. ED-20, pp ,(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 , (1973). International Conference on Communication Systems (ICCS-2013) B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India October 18-20, 2013 Page 53