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DESIGN & DEVELOPMENT OF POWER SUPPLIES FOR HIGH POWER IOT BASED RF AMPLIFIER Yashwant Kumar*, S. Kumari, M.K. Ghosh, A. Bera, A. Sadhukhan, S. S. Pal, V.K. Khare, T.P. Tiwari, S.K. Thakur, S. Saha Variable Energy Cyclotron Centre, 1/AF Bidhannagar, Kolkata, India Abstract Design, development, circuit topology, function of system components and key system specifications of different power supplies for biasing electrodes of Thales Inductive Output Tube (IOT) based high power RF amplifier are presented in this paper. A high voltage power supply (-30 kV, 3.2A dc) with fast (~microsecond) crowbar protection circuit is designed, developed and commissioned at VECC for testing the complete setup. Other power supplies for biasing grid electrode (300V, 0.5A dc) and Ion Pump (3kV, 0.1mA dc) of IOT are also designed, developed and tested with actual load. A HV Deck (60kV Isolation) is specially designed in house to place these power supplies which are floating at 30kV. All these power supplies are powered by an Isolation Transformer (5kVA, 60kV isolation) designed and developed in VECC. INTRODUCTION The development of multi-cell medium- Superconducting RF linac cavity has been taken up at VECC as a R&D project in line with various accelerator programmes (Indian SNS, ADSS and advanced RIB) of DAE.As a part of this project, the design and development of high power RF amplifier has been done based on IOT. The state-of-the-art technology of IOT based high power RF amplifier developed and tested at VECC is the first of its kind in India. The amplifier can be operated at 704 MHz/650 MHz at maximum output RF power of 60 kW at 50 Ohm load with proper tuning of its primary and secondary cavities. The necessary high voltage power supply (-30 kV, 3.2A dc) with fast (~µs) crowbar protection circuit for biasing Cathode of the IOT is designed, Installed and commissioned. The IOT fed by the -30kV/3.2A power supply, is required to be protected against the internal arcing. The inductive output tubes are prone to internal arc that can lead to a permanent damage of it, if stored energy is not removed or diverted through a switch to ground after occurrences of fault. Thus to bypass the energy, a crowbar system (with shunt diverter topology) is used. Other power supplies for biasing Grid, Ion Pump and Filament electrodes are connected with respect to the cathode of the IOT and floating at high voltage (30kV). A HV Deck is specially designed in house to place these power supplies which are floating at 30kV. Inside the HV Deck, these power supplies are powered through an Isolation transformer. All these power supply are *[email protected] controlled by a PLC, which communicates through an Optical Fibre link to the PC. DESIGN AND TOPOLOGY Anode Power Supply This is a -30kV, 3.2A primary regulated (by SCR) DC power supply. It incorporate two numbers of power transformers, each rated at 100 kVA, 415V/12kV, 3phase, 50Hz, vector group Dy11- d0 and force air cooled operating in 12 pulse mode. The 3-phase power is connected to each of two transformers via air core choke and back to back SCR in each line through fast interrupting circuit breakers. The SCR bank at the input of the Transformer is used in closed loop as a primary regulating device. Rectified outputs (15 kV each) of both transformer’s secondary are connected in series and then a LC filter (400uH, 1.9uF/60kV capacitor) is used to keep the ripple in desired limit. The special feature of this power supply is Crowbar Protection system against internal arcing in the IOT by using high voltage Ignitron. Other protections i.e. over current, over voltage, phase failure, over temperature etc. have been incorporated and tested with the IOT. The schematic diagram and photographs of anode power supply shown in Fig. 1and Fig. 2 respectively. Figure1: Schematic of anode power supply Figure 2: Photographs of anode power supply Development of Crow bar Protection System for HV anode Power Supply The crowbar protection system, which is an integral part of the anode power supply was designed and developed in VECC. It involves ignitron NL7703EHV as a switching device rated at 50kV and current transformer having sensitivity of 0.01V/A with response time 20ns installed in ground line of power supply for sensing rate of change of current during fault. When IOT draws the more current or arc occurs, CT senses the rate of change of current and fed to the electronic control card which gives a tripping signal. This signal triggers a SCR through optical fiber to discharge a charged capacitor (~5J energy) through a pulse transformer for the generation of fast rising pulse of 2-3 kV peak. This is used to trigger the ignitron which short circuit the power supply to protect the IOT. The same tripping signal is used to trip main circuit breaker and withdraw the gate pulse of SCR to protect the power supply. Isolation between control circuit and power circuit has been done by using optical fiber. Fig. 3and Fig. 4 shows the schematic diagram and photographs of crowbar protection system. switched at high frequency (20 kHz) by push-pull inverter circuit to give high frequency AC. This AC voltage is fed to primary terminals of ferrite core transformer to step-up from 0-50V to 0-1.5kV AC. This is further multiplied to achieve 3kV DC by using Cockcroft-Walton voltage multiplier. The voltage regulation is achieved by comparing the sample of the output voltage with stable reference voltage. The error signal is fed to the built in error amplifier of PWM IC (SG3525) which change the duty cycle of pulse according to error signal. These pulses are fed to gates of MOSFETs which used as a push-pull inverter. Fig. 5 and Fig. 6 shows schematic diagram and photo graphs of ion pump power supply respectively. Figure 5: Schematic of Ion pump power supply Figure 3: Schematic diagram of Crowbar Protection System Figure 4: Photographs of crowbar protection system Figure 6: Photographs of Ion Pump Power Supply Ion pump Power Supply Grid Power Supply This is voltage regulated switch mode power supply for ion pump of IOT and works on the principle of pulse width modulation (PWM) technique. The power supply is operated at high frequency (20 kHz) hence the size and weight of magnetic component, filter capacitors are reduced with improved efficiency. A single phase line voltage (230AC, 50Hz) is fed to step down transformer to achieve 40 V AC. This AC voltage rectified and filtered to get around 50 V DC. The rectified dc output is then This power supplies provide negative bias voltage (adjustable from 0 to -300V) to the control grid of IOT. In this power supply 230V AC is stepped up to 350V AC by a transformer. The transformer secondary voltage is rectified and filtered. Regulation is achieved by using IGBT in common emitter configuration. This power supply is kept on high voltage deck and tested with actual load and running smoothly without problem. Fig. 7: shows schematic of Grid power supply. Figure 7: Schematic of Grid power supply High Voltage Deck with Isolation Transformer Isolation transformer design & developed for auxiliary power supplies placed in high voltage deck for biasing IOT. This is a single phase, 4.5 kVA, 230V/230V, dry type, air cooled resin cast transformer design at flux density of 1.25 Tesla. Both the coils wound on fibre glass bobbin at current density of 1.3A/mm2. Low current density is taken to keep temperature rise of the transformer within the limit. A 5 mil Nomex paper is provided for inter layer insulation and total coil wrapped by two layer of Kapton tape to give extra reinforcement of the coil. To achieve 60kV isolation between secondary to primary and core, secondary coil encapsulated by dr. beck brand dobefil -60 & hardener 758 epoxy resins of 20mm thick. This dr. beck brand epoxy resin has good thermal conductivity, high dielectric strength and better for continuous running of the transformer. We conducted routine test viz. open circuit test, short circuit tests and percentage impedance test of the transformer. Herewith we have specially checked dielectric strength of the transformer up to 80kV as per IS 2026 with precision grade testing equipments. Fig. 8 shows photographs of high voltage deck with isolation transformer. CONCLUSION The high power IOT-based amplifier along with power supplies and interlocks etc. has been installed and already tested up to 40 kW with 50 ohm water-cooled dummy load at 704 MHz/650MHz. ACKNOWLEDGEMENT The authors acknowledge the support of Shri Sumit Som and Shri Aditya Mandal of RF Division, VECC, Kolkata. REFERENCES [1] Samit Bandyopadhyay, Anirban De, Yashwant Kumar, M.L.V Krishnan, T.P. Tiwari, V.K. Khare, S. Sinha, S.S. Pal, A. Sadhukhan, S.K. Thakur, Manoranjan Das, Subimal Saha,” “Power Supplies for the RF-System of K=130 Cyclotron at VECC, Kolkata”, Proc. of Indian Particle Accelerator Conference (InPAC) - 2011, IUAC, Delhi, Feb 1518, 2011. [2] S. K. Thakur, T. P. Tiwari, J. S. Prasad, A. De, Y Kumar, S. Som, S. Saha, R. K. Bhandari. “Development of Power Supplies for 3-Ф, 240 KW RF System with Crowbar protection for Superconducting Cyclotron at VECC”. MOPCP009 2010. [3] Yashwant Kumar, J.S.Prasad, S.K.Thakur, T.P.Tiwari, S.Saha. “Design and Development of HV power supply with Crowbar Protection for screen electrode of RF amplifier”. Presented in AE-BRNS Indian Particle Accelerator conference- 2009,INDIA. Figure 8: Photographs of high voltage deck with isolation transformer.