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DESIGN & DEVELOPMENT OF 37.8MHZ, 2.0KW RF AMPLIFIER FOR REBUNCHER OF RIB PROJECT H.K.Pandey#, T. K. Mandi, Y. Kumar, D.P. Dutta, S. Basak, S.K. Thakur, A. Bandopadhyay, VECC, 1/AF Salt Lake, Kolkata, India K.P.Ray, SAMEER, IIT campus, Powai, Mumbai, India Abstract A 2 kW amplifier has been indigenously designed and developed for a 37.8 MHz re-buncher. The buncher is presently installed in the RIB beam line between the RFQ and LINAC I. The amplifier has two stages of amplification: a 100 W solid state amplifier (SSA) stage followed by a triode based amplifier, all operating at 37.8 MHz. A DC power supply system has also been designed and developed for the biasing of the amplifier stages. This paper describes the design and development of the 2 kW amplifier and the high power DC power supply. INTRODUCTION The Rare Ion Beam (RIB) facility being developed at VECC has a RFQ post-accelerator followed by IHLINACs [1-2]. The re-buncher is a 4 gap λ/4 RF structure operating at 37.8 MHz [3]. It helps to match the longitudinal bunch width of the beam to the acceptance of the downstream IH LINAC. The 2 kW transmitter has been designed and developed for powering the rebuncher, although it may be used for powering other cavities operating at the same frequency as well. The 2 kW transmitter is a two stage RF amplifier consisting of a 100 W SSA Driver stage and a triode based final stage. Figure 1 shows the schematic diagram of the RF transmitter. The final stage amplifier is a class AB amplifier designed using triode 3CX5000A7 in grounded grid configuration. Input and output matching networks were designed to match this impedance to 50. Grid (-100V/0.5A) and Filament (10V/24A) of tube have been designed and tested successfully. The system control and all safety interlocks have been incorporated. Table1: Amplifier Specification Parameters RF Section Frequency Tunable; 35-42MHz Output Power 2.0kW (Max.) Instantaneous BW 2 MHz Harmonic Level <50dB Max Anode Voltage 4.5 KV Max Anode Current 3.0 A Gain 13dB Driving Power 120W It is also necessary for the amplifier to have a stable power output and excellent harmonic suppression, which has been taken care of in the design. This paper describes the detailed design, test and measurement report of this indigenously developed amplifier. DRIVER STAGE AMPLIFIER The driver stage is a 100 W Solid State Amplifier (SSA) operating at a centre frequency of 37.8 MHz. It has been developed using a CA2832C pre-amplifier stage followed by power FETs in push-pull configuration housed within a single Gemini package [4]. Figure 3 shows a picture of the 100 W solid state amplifier. A supply voltage of 28.3 V has been used for drain bias. Transmission line transformers have been used along with other lumped components for input and output impedance matching. This has resulted in a large bandwidth of the amplifier ranging from 14 MHz to 84 MHz as shown in figure 4. Figure 1: Schematic Diagram of the Amplifier The triode cage and forced air cooling arrangement with RF filter blocks have been designed and implemented. The power supplies for Anode (4.5kV/3A), ____________________________________________ # [email protected] Figure 3: Photograph of the Solid State Amplifier Figure 4: Measured Dynamic Response and Frequency Response of the SSA Figure 5 shows the picture of the final stage of the amplifier system. The parameters for the matching networks have been calculated and the analysis of this amplifier has been carried out using ABCD parameters method. It uses a linear small-signal AC model of the triode with its matching circuits. The analytical as well as measured results have been already presented [6]. The matching networks are single LC networks with variable L and C, resulting in a tuning range of 36 MHz to 43 MHz. Figures 6.a and 6.b show the measured S11 parameter for the input and output matching circuits of the final stage. The amplifier is being tested and necessary tuning of the input and output matching circuits is being done. Initially, low-level RF measurements of the input and output matching networks were done. These tests were followed by DC tests of the amplifier by holding the triode electrodes at their respective biasing potentials. Using 4 kW dummy load, RF tests are being done at present along with further fine tuning and adjustments The amplifier has a gain of about 57 dB at the frequency of operation. It has been operated up to a maximum of 98 W CW power with dummy load. A drift of 3 W over a period of 7 hrs of operation was observed while operating at 90 W. The 2nd, 3rd and 4th harmonics are at levels of -28 dBc, -68 dBc and -49 dBc respectively at 38 MHz. FINAL STAGE AMPLIFIER The final stage is a class AB triode amplifier in the grounded grid configuration [5]. The grounded grid acts as a shield and prevents coupling between the input and output circuits. Hence, at high frequencies and high power, this configuration of using the triode results in a more stable amplifier operation. An air cooled 3CX5000A7 triode tube has been used as the active device of the amplifier, preceded and followed by appropriate matching networks. The triode tube along with the matching networks are housed inside an aluminium enclosure with RF filter blocks, developed in VECC workshop and cooled using a centrifugal fan. The proper air flow and pressure drop of tube cage have been measured. The 4 kV bias for the anode is applied using a HV feed-through in the enclosure. Figure 5: Assembly of the final stage of the 2kW amplifier Figure 6.a: Input matching of final stage Figure 6.b: Output matching of final stage DC REGULATED POWER SUPPLY The necessary high voltage power supply (4.5 kV, 3 A DC) with fast (~microsecond) crowbar protection circuit for biasing the anode electrode of the triode tube, other power supplies to bias grid (0-100 V, 0.5 A DC), and filament (10 V, 24 Amp AC) of the triode tube were designed, developed and commissioned with the amplifier stages. Figure 7 shows the cabinet for housing the DC power supplies and the two amplifier stages along with the controls and indications on the front panel. The anode power supply is designed with an input of 415 V, 3-phase, 50Hz AC fed to the three primary terminals of a step-up transformer through three numbers of series AC inductor and three anti-parallel pair of SCRs. Forced air cooled 3-Phase full wave bridge rectifier assembly is used in secondary of Transformer to achieve 3.2 kV DC output. A shunt is used for sensing over current/short circuit current and fed to a crowbar protection circuit [7]. Figure 7: The cabinet for the amplifier system After sensing the over-current, the crowbar circuit gives signal to a series of SCRs, connected across output DC bus capacitor, and discharge the same. At the same time, signals are sent to thyristor triggering card to stop the pulses to the AC side SCRs. Thus the output is quickly discharged to zero. The circuit diagram for the anode power supply is shown in figure 8. Figure 8: Circuit diagram of anode power supply The Grid power supply is a linear regulated DC power supply, which is designed with 230V, 1-phase, 50Hz AC mains. The AC input voltage is fed to the primary of an air-cooled transformer and the secondary is rectified by full wave bridge rectifier and filtered by electrolytic capacitors. This filtered DC link is fed to a transistor as series element for voltage regulation. The circuit diagram of the grid power supply is shown in figure 9. Figure 9: Circuit diagram of grid power supply RF and microwave tubes are prone to internal arc that can lead to a permanent damage if excessive energy is dissipated. So, a shunt diverter topology has been chosen for the anode supply in which stored energy in electrical system is diverted to the ground by quick shorting the output terminals of power supply through series of SCRs, connected across output DC bus capacitor. The DC regulated power supply has been tested using high power resistive loads. For the safety of the amplifier, power supply units and the operator, various interlocks and protection mechanisms have been incorporated into the design of the DC power supply. These include over-current and overvoltage protection and timer for gradual increase in filament voltage. The triode electrodes need to be powered up in a proper sequence starting with the filament, grid and then anode. This has been taken care of in the design of interlocks where the power supply for an electrode will not turn ON if the sequence violated. Besides these, door interlocks are provided for the safety of the operator. SUMMARY In this paper, the design and the status of development of the 2 kW RF amplifier has been presented. The driver stage and final stage of the amplifier, along with the DC regulated power supply has been designed and developed with the aim of using it as the power source for the 37.8 MHz re-buncher. The amplifier has been tested up to 500W with 13 dB gain and is being tuned to achieve higher power levels. The tests and tuning for 2 kW CW operation, stable power output, minimum drift and good harmonic suppression will be done before commissioning the system. ACKNOWLEDGEMENTS The authors would like to thank VECC Workshop and Shri M W Siddiqui of AC Division for their help and support. REFERENCES [1] A Chakrabarti, Nucl. Intr. & Meth. B261 (2007) 1018. [2] A Bandyopadhyay et al., Proceedings of PAC09, Vancouver, FR5REP117, p. 5053; http://www.JACoW.org [3] S Dechoudhury et al., PRAMANA, Vol. 75, No. 3, (2010) p. 485-499. [4] H O Granberg, “Wideband RF Power Amplifier”, A313, R F Design, (Feb. 1988). [5] H K Pandey et al., IEEE MICROWAVE 2008, Jaipur, India, Nov. 2008, p. 119-121. [6] H K Pandey et al., CODEC 2012, Kolkata, India, Dec. 2012, p. 1-4. [7] S K Thakur et al., CYCLOTRONS 2010, Lanzhou, China, Sep. 2010, MOPCP009, p. 60.