2 kW, 31.613 MHz Indus-1 and Booster RF Systems

Indus-1, Synchrotron Radiation Source (SRS) facility with stored electron energy of 450 MeV, is operational at RRCAT. It emits radiation in VUV range and is being used for research in Physics and Chemistry. Indus-1 SRS complex consists of 20 MeV injector microtron, Booster synchrotron and 450 MeV storage ring. In all these machines the energy to the accelerating particle i.e. electrons, is imparted by the RF voltages developed across RF cavities. Booster Synchrotron accelerates electron beam from 20MeV to 450MeV and 550MeV for Indus-1 and Indus-2 respectively. The role of RF system, in SRS, is to compensate for Synchrotron Radiation losses suffered by circulating electrons through the bending magnets and insertion devices to keep the beam in equilibrium orbit. The RF system also provides energy for accelerating the beam from injection energy to final energy. Booster synchrotron and Indus-1 are relatively small rings with perimeters of 28.449 m and 18.996 m respectively. The operating frequency of both the rings is chosen as 31.613 MHz mainly to avoid ion trapping problems. The booster synchrotron is operating at harmonic number of three and storage ring is operating with harmonic number of two. Owing to same RF frequency, the cavities in both the rings are similar. The RF system for the storage ring consists of a synthesized signal generator, the low level RF system and Solid State RF amplifier.

Indus-1 31.6 MHz Solid State RF Amplifier

The high power RF amplifiers of Booster Synchrotron and Indus1 RF Systems at 31.6 MHz were earlier designed using vacuum tube devices. Due to non-availability of Vacuum tetrode tubes from M/S BEL India, development of solid state RF amplifiers at 31.6 MHz was taken up. 1kW Solid State RF Power amplifier for Booster Synchrotron and 2kW Solid State RF Power amplifier for Indus-1 storage ring were developed, tested and commissioned. The deployment of solid state RF power amplifiers has resulted in improved performance with elimination of the high voltage problems, higher efficiency and less maintenance. In Booster RF System 800 watts RF power at 31.6 MHz is required for 10 mA beam power and generating 20 kV accelerating gap voltage in RF cavity. Similarly in Indus-1 RF System 1400 watts RF power at 31.6 MHz is required for 150 mA stored beam current with 22 kV accelerating gap voltage in the RF cavity. Hence, 1kW Solid State RF Power amplifier for Booster RF system and 2kW Solid State RF Power amplifier for Indus1 RF system were designed. The basic building block in both the amplifier systems is MRF 141 G MOSFET based 300 watts RF power module. Output of four and eight such modules is combined together using Wilkinson type power combiner to get 1 kW and 2 kW RF power respectively for Booster synchrotron and Indus-1 SRS.


[Fig. 1 : Block diagram of 31.6 MHz Solid State RF Amplifier]

At low frequency of 31.6 MHz, circulators are not easily available for protection of individual modules. Hence, high power combiners are designed to absorb the reflected power coming from mismatched load and protect the amplifier modules. Amplitude and phase matching of all RF power modules is done to improve the combining efficiency. Interlock unit is also developed for the protection of the high power solid state RF amplifier along with the other subsystems of the RF system.


[Fig.2: 1 kW, 31.6 MHz Solid State RF amplifier for Booster Synchrotron]

All the components like Pre-driver amplifier, Driver amplifier, Lumped power splitters , RF filters, Lumped power combiners and a 4:1 Coaxial line high power combiner used in these amplifiers, were developed in-house. The RF amplifier modules and combiners are water cooled to have efficient cooling and hence higher output power. This also increases the life of the amplifier by reducing failure rate of the MOSFET devices. Proper shielding was done to ensure radiation safety and RF radiation measurements were done at different stages of the development and installation of the RF amplifiers.


[Fig.3 : 2 kW 31.6 MHz Solid State RF amplifier installed in Indus-1]

1 kW solid state RF amplifier in Booster Synchrotron and 2kW solid state RF amplifier in Indus-1 SRS were installed and commissioned in March 2010 and March 2012 respectively. Since their inception both the systems are running successfully with better performance and having less down time. Indus-1 is being regularly operated at 450 MeV/125 mA with this amplifier in round the clock shift operation.

Indus-1 low level RF System

Low level RF control system consists of synthesized signal source,0-360˚ phase shifter, feedback loops for amplitude, phase and frequency control, Coaxial RF switch to put RF on & off and Limiter. Synthesized signal generator giving outputs at 31.613 MHz is developed which is also used to get synchronized RF drive signal for Indus-2. Amplitude and phase control feedback loops are incorporated to maintain the amplitude and phase of the cavity gap voltage within 1% and 1˚ respectively for proper operation of the machine. The phase loop compares the phase of the RF signal at the feed of RF cavity with the reference signal phase. The error signal obtained is used to drive the variable phase shifter which corrects the phase changes in the amplifier chain. To get the phase information at different operating power levels, limiter circuits have been used. The amplitude control loop measures the accelerating field in the cavity sampled through small coupling loop. This sampled signal is compared with a reference (amplitude set) signal. The correction is made by driving the RF attenuator with the error signal, which in turn controls the signal drive to solid state amplifier. The tuning loop together with amplitude control loop compensates for the beam loading effect and temperature change. The PLC based tuning loop maintains the resonance frequency by comparing the phases of RF signals at feed port and sense port of the RF cavity. The phase detector gives equivalent error signal proportional to phase difference between the two signals. Resonance control is achieved by driving the plunger tuners with the help of stepper motors. Total tuning range provided is 40 KHz. During operation of both the RF systems manual access is not possible hence, both RF amplifier systems are made to operate in local mode and remote mode from control room. All the parameters like cavity gap voltage, forward and reflected powers at cavity, tuner position, cavity tuning error etc. are monitored and interlocks are provided for safe operation of the machine and human safety.

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