Indus-2 SRS RF system

RF System for 2.5GeV,200mA Synchrotron Radiation Source Indus-2 is developed. The role of RF System is to boost the electron energy from 600 MeV to 2.5 GeV and compensates the SR losses by the circulating particles in the bending magnets and insertion devices. The system is designed to generate an accelerating voltage of 1500 kV at 505.812 MHz which gives sufficiently high quantum and Touschek lifetime. The RF system employs four numbers of elliptical cavities (Fig. 4),to generate 1500 kV accelerating RF voltage. Three such RF cavities are powered by three 75 kW Solid state RF amplifiers respectively. The fourth cavity is operated on Klystron based RF amplifier.Each cavity along with high power RF amplifier and LLRF system is termed as RF station. Each RF station is equipped with a dedicated interlock unit to safeguard RF components viz. RF cavity, klystron, circulator, solid state power amplifiers, high power transmission lines etc. against the over drive of different electrical and mechanical parameters.

[Fig. 4: Four numbers of elliptical normal conductingcavities in Indus-2]

Klystron RF Amplifier

The Klystron RF amplifier is based on 64 kW multi-beam, integral-cavity klystron KY400, with dispenser type of cathode. The auxiliary power supplies for its filament, ion pump and mod-anode are floating at beam supply voltage of 20 kV. The current and voltage signals floating at beam voltage are monitored by an optical fibre interface. For RF transmission system max. VSWR of 1.07 and insertion loss less than 0.4dB were obtained at the operating frequency. This system was realized using 61/8” EIA coaxial lines and coaxial line components, which operate at normal atmosphere pressure. The 20 kV, 5.5 Amp HVDC power supply feeds power to 64 kW klystron amplifier (Fig. 5). To cater to the wide varying input conditions and to meet the high voltage requirements of possible loads, these power supplies were controlled through six SCRs in 3-Φ AC regulator scheme. Different primary control schemes were analysed and the configuration having either by Delta connected primary or Star connected primary without neutral, of the main transformer, avoiding 3rd harmonics in the line, were chosen.

[Fig. 5: 64 kW multi-beam, integral-cavity 505.8 MHz klystron and its power supply]

A three phase linear inductor is intentionally kept at the primary side of each power supply unit to reduce the fault current level, to limit the higher order harmonics and to limit the worst-case di/dt subjected to semiconductor devices employed in these power supplies. Various protection circuits like over voltage circuit, over current circuit, shunt trip from klystron, phase failure/reversal circuit, spark and arc control circuit, transformer oil top and bottom float (level), SCR temperature high, oil temperature high etc., are also incorporated in them. A line harmonic filter is employed, for this supply, not only to keep the input power factor near unity but also to keep the line harmonics within the limit specified by IEEE Std-519, 1992. The filter components are intentionally tuned to 228 Hz, to avoid parallel resonance with the source. When one HVPS is ON, its corresponding line filter bunch will also be made ON. Several protection features like over load, reactor core temp high etc., are also incorporated to disconnect these filter bunches, in case any unforeseen resonance conditions occurs. Klystron tubes are highly sensitive to arcing, hence crowbar protection is provided, which operates within few microseconds under arcing conditions. In the event of arcing of the klystron, the energy dissipated in the klystron is limited to below 20 Joules by the crowbarring.

75 kW (X4) Solid State RF Amplifier for Indus-2 machine

RRCAT has designedand developed four 75 kW Solid State Amplifiers. Presently three 75 kW solid state RF amplifiers, delivering total RF output of 225 kW operating at 505.8 MHz, have been successfully commissioned in Indus-2 Synchrotron Radiation Source to achieve beam current more than 200 mA at 2.5 GeV. The fourth 75 kW amplifier is being used as a high power test bench and also as a spare unit for Indus-2 RF system. These Solid state amplifiers were developed indigenously at RRCAT, starting from scratch for the first time in the world at frequency greater than 500 MHz.Few years back, the availability of the klystrons, previously used in RF system of Indus-2 synchrotron radiation source, was uncertain. Hence the development of solid state high power amplifier was taken up as an alternate RF power source. These amplifiers are modular combination of 12.5 kW amplifier units operating at 505.8 MHz. Each unit is housed in a customised cabinet and employs 32 numbers of 500W RF amplifier modules, 2 power dividers, 2 power combiners, phase shifter and high power directional coupler. All these components, developed indigenously, were tested and integrated with safety interlocks, FPGA based data acquisition system and water cooling circuit to make a complete high power amplifier.

[Fig. 6: Indus-2 RF gallery with 64 kW klystron and three 75 kW solid state RF amplifiers]

The experience gained, will be useful for the development of high power solid state amplifiers for Spallation neutron source (SNS) and accelerator driven systems (ADS).

Digital Low level RF Control for Indus-2

Earlier the LLRF system of Indus-2 was based on analogue technology and had its inherent limitations. To improve the performance of Indus-2 RF system DLLRF system was developed. LLRF control system is required in each station to keep amplitude and phase of RF field stable in the RF cavity. Digital Low level RF (LLRF) control system is used to keep the amplitude and phase of the RF field stable in the accelerating structures. LLRF system could be Analogue or Digital in nature. Digital LLRF system offers inherent advantages of digital system like flexibility, adaptability, good repeatability and reduced long time drift errors as compared to analogue system. Digital LLRF systems have been developed and commissioned for three RF stations.

This development involves use of latest technologies of Digital Signal Processing, RF signal Processing and Fast Feedback Control Systems based on FPGAs. RF signal processing unit is developed for down conversion, synchronized clock generation and amplitude and phase control of RF signal. Digital feedback controller is realized using FPGA with suitable codes prepared in VHDL and LabVIEW. A Real time RF cavity simulator is developed to test and optimized the DLLRF system in Lab. DLLRF systems have been installed and successfully commissioned in Indus-2 for 2.5 GeV operation. These systems provide 20 dB dynamic range and 360° phase correction with amplitude and phase stability of 0.5% and 0.5° respectively.

[Fig. 7: Digital LLRF control System in Indus-2, with a view of single DLLRF rack]

In each RF station the interlockunit safeguards precious RF components viz. poor vacuum, inadequate water flow, forward and reflected RF power over drive, inadequate coolant air pressure, tuner plungers out of limits and over temperature. RF interlock unit is housed in Main RF Signal Distribution Rack‟ of Indus-2 RF system. This interlock unit breaks the supply of the common RF signal being fed to the inputs of all 4 RF stations in case of occurrence of over drive of any parameter. Undulators safety interlock signals have also been incorporated in this interlock unit.

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