R.F.AND HIGH VOLTAGE SYSTEMS FOR IR-FEL (CUTE FEL) IN INDIA Umesh Kale Beam Physics & FEL Lab Raja Ramanna Centre for Advanced Technology, Indore

Pulsed thermionic GUN 476MHz 2856MHz 2856MHz Undulator Beam Dump Pulser + Power supplies 50kW, 476MHz RF Source 10MW,2856MHz RF system LLRF & HPM INJECTOR For CUTE FEL MODULATOR Terahertz FEL Wave length region : 80 um, 10ps pulses Peak power : 1MW, Average power 1W Beam Energy : 10MeV Already made Under way

Pulsed thermionic GUN 476MHz 2856MHz 2856MHz Undulator Beam Dump Pulser + Power supplies 50kW, 476MHz RF Source 10MW,2856MHz RF system LLRF & HPM INJECTOR For CUTE FEL MODULATOR Terahertz FEL Wave length region : 80 um, 10ps pulses Peak power : 1MW, Average power 1W Beam Energy : 10MeV

PWT LINAC: 2856MHz BUNCHER : 2856MHz PRE BUNCHER :476MHZ GUN : 36.6 MHz

Pre - Buncher A single cell re-entrant type pill box cavity with nose cone at 476MHz. It prebunches the beam to minimize current loss. RF power for the pre-buncher : ~ 476MHz Cavity developed put in the beam line. Procurement of Planner triode based multistage amplifier is underway. (JPAW,USA) Buncher A 4-cell PWT ( Plane wave transformer) LINAC developed indigenously. Around 3MW RF 2856MHz for ~4MeV energy gain. 10MW RF 2856MHz delivers power to both Buncher & Accelerator. Accelerator A 8-cell PWT ( Plane wave transformer) LINAC developed indigenously. Around 8MW RF 2856MHz for 7.5MeV energy gain. RF system for Buncher & Accelerator ready.

PULSED THERMIONIC GUN ( CCR, USA InC.) Thermionic gun with grided dispenser cathode Beam energy : 90keV, Charge : 1nC & Pulse width : 1ns FWHM Pulse separation : 27.3 ns MHz) dictated by the one round trip time of the optical pulse in 4.1m long optical cavity. Normalized emittance : 15 pi-mm-mrad ( rms) & Energy spread : <1% Macro-pulse duration : 10Hz ( Max) Operating cathode voltage is -90kV, grid is negatively biased with -150V and a fast pulser applies 200V to drive 1A peak micro pulse current. Spellman make -130kV,0.1mA HVDC power supply with 1nF intermediate energy storage capacitor bank supply 1A peak current. A isolation transformer supply power to Filament P/S, BIASING P/s & pulser floating at -90kV. Employed a crow bar system for the protection Gun in events of arcing. (maximum energy that can be dumped in the event of arc is 1J ). But stored energy is 4J. COMMISSIONING IS UNDERWAY.

LLRF ( Low level RF ) Synchronized operation of GUN, modulator & low power RF SG1 operated with internal in-built 10MHz source & SG-2 operated from SG-1 externally. SRS delay generator triggers the SG-1,gun controller & modulator. Pulse modulation of 2856MHz done internally in SG-1. Pulse modulated low power RF pulse is set inside the high voltage pulse in flat portion to avoid false triggering, any phase change. Complete system is operated remotely.

VA1 DG 535 GUN controller Modulator TTL, 1-10Hz MHz 2856MHz SG-2 SG-1 90kV thermionic gun ( procured from CCR Inc.,USA) Low power RF line for present set up ( pre-buncher is not energized)

Specification of 10MW klystron Specification of 25MW line type modulator RF System & Modulator

Circulator High power RF WR284 based transport line. Presently RF transport line is pressurized with N2 at 4 bar. Circulator has provision for installing arc detector. (AFT make) Each line has one circulator, phase shifter and dual directional coupler for protection,phase adjustment and power measurement.

N R Y B C1R1 R2 C2 L130HD1 L2 R3 L3 C3 U4 TX4 12 L3 Reservior Heater Chiller Filament Heater Ref 0-10V HVDC P/S, EHT xmer, 12 pulse rectifier + LC filter Driver Micro -controller Analog/Digital control TTL from SRS delay generator High power RF line Filament P/s Interlockedfor Low flow Droor interlock Scram Micro -controller Analog/Digital control Core bias P/s Modulator Block diagram

Features Line type modulator, PFN with mutual inductance Resonant charging through charging inductor. 15kV, 1A HVDC power supply, reg. <1%, Ripple <1% HVDC power supply has two 6 pulse rectifier in series forming 12 pulse rectifier with LC filter. 15 section PFN with mutually coupled inductors of 6 Ohm characteristic impedance. Total capacitance : 0.8 uF E2V make 40kV, 10kA thyratron switch ( CX2609) used for discharging PFN. Pulse transformer ( ratio 1:6 ) with reset core employed. Provision to power filament through transformer secondaries. Remotely operated using distributed control architecture 8 bit micro controllers placed locally using supervisory PCs in control room. Lab-view based GUI

Interlocks for experimental hall door,Vacuum of klystron, wave guide arcing, scram, low flow provided. All safety interlocks are relay based. PFN voltage, klystron filament current, HVDC voltage are monitored remotely. Modulator tested with klystron and dummy water load up to 7MW peak power. Modulator is operational since Preliminary acceleration trials with 4-cell PWT performed. Electron beam form 2us,40keV DC thermionic gun accelerated up to 3.5MeV consuming ~ 4MW of RF power. Typical wave shape across the klystron

Using the existing set up we have done the following experiments  Conditioning of 4 cell buncher.  Acceleration up to 4MeV using 4 cell PWT and a 2us DC thermionic gun.  Transmission through undulator experiment. ( 60 % transmission measured) Problems encountered during commissioning  Ringing on flat top of pulse due to stray inductance, capacitance. ( eliminated by putting an RC network across Pulse Transformer. However rise time affected)  Damaging spikes through ground path. ( all components insulated from cabinet then grounded to ensure single point grounding), used MOVs, filters, faraday shield transformer for the electronics)  Cabinet door with RF contact fingers helped in reduction of EMI generated noise.  Tuning of PFN is difficult with this design.

These power supplies are basically SMPS buck converters with 100ppm stability developed in RRCAT

Some photographs 25MW Pulse Modulator Pulse transformer Brazed components of PWT3

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