1. Up gradation of Indus-1 Magnet Power Supplies:

Up gradation work of various magnet power supplies of Indus-1 storage ring, TL1, TL2, microtron, booster has been initiated with the aim of better performance. Ongoing activities in this area are briefed as follows.

• TL2 Quadrupole Magnet Power Supplies:

These power supplies are rated for 80 A / 25 V with output current stability within ± 500 ppm. The power converter is operating on 415 V, 50 Hz three-phase AC mains and is based on two-switch forward converter operating at 50 kHz. All semiconductor components are water-cooled. A zeranin shunt is used to sense the output current for feedback regulation. The feedback control scheme consists of two loops: the inner fast voltage loop (bandwidth~2 kHz) corrects input line variation and outer slow current loop (bandwidth~10 Hz) caters for temperature variation in load. Each power supply is capable of being operated from local front panel or in remote mode from central computer interface via a 25-pin sub-D connector. The remote interface is directly compatible with the existing provision in Indus-1. Power supply design is standardized in a standard 4U rack. One of these power supplies has been installed in Indus-1 magnet power supply hall for regular operation.

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Photographs of TL2 qudrupole magnet power supply. (a) Top view. (b) Front view

• TL1 Quadrupole and Dipole Magnet Power Supplies:

These power supplies are rated for 5 A / 12 V with output current stability within ± 500 ppm. The power converter is an off-line SMPS (operating on 230 V, 50 Hz AC mains) using two-switch forward converter, which is chosen for its simple configuration, ruggedness, high efficiency and compact size. It is free-from shoot-though failures and returns energy stored in the leakage as well as magnetizing inductance of the transformer to source, thereby clamping maximum switch voltage to the source voltage. A zeranin shunt is used to sense the output current for feedback regulation. The feedback control scheme consists of two loops: the inner fast voltage loop (bandwidth~2 kHz) corrects input line variation and outer slow current loop (bandwidth~10 Hz) caters for temperature variation in load. Each power supply is capable of being operated from local fascia panel or in remote mode from central computer interface via a 25-pin sub-D connector provided on the fascia plate. The remote interface is directly compatible with the existing provision in Indus-1. Power supply design is standardized on a standard 6U card with full-function feedback control and local-remote operation interface electronics on the same card. Two numbers of TL1 quadrupole magnet power supplies have been successfully developed and thoroughly tested.

Photographs of TL1 quadrupole magnet power supply card. (a) Power supply card, and, (b) Two power supply cards assembled in a 19-inch 6U rack.

• TL2 Dipole and Indus-1 Quadrupole Magnet Power Supplies:

These power supplies are rated for 300 A / 60 V, 80 V and 120 V with output current stability within ± 500 ppm. The power converter is operating on 415 V, 50 Hz three-phase AC mains and is based on phase-shifted PWM converter operating at 25 kHz. The power converter scheme and designed has been standardized as a 300 A / 60 V module with series-connected output for higher load voltage requirement. All semiconductor components are water-cooled. A zeranin shunt is used to sense the output current for feedback regulation. The feedback control scheme consists of two loops: the inner fast voltage loop (bandwidth~2 kHz) corrects input line variation and outer slow current loop (bandwidth~10 Hz) caters for temperature variation in load. Each power supply is capable of being operated from local front panel or in remote mode from central computer interface via a 25-pin sub-D connector. The remote interface is directly compatible with the existing provision in Indus-1. Power supply design is standardized in a standard 4U rack. Prototype power supply is designed, fabricated and tested.

• Transmission line type extraction kicker for booster synchrotron

The existing pulser for booster extraction kicker can deliver a maximum peak current of 800A. The rise time of this pulse is in excess of 55 nsec. This prevents extraction of the three electron bunches from the booster, and one bunch is lost as a result. Upgraded pulser has been designed with maximum peak current of 1200A and a much smaller rise time of 25-30 nsec, and tested the prototype version at low charging voltages upto 10 kV with load shorted. This upgraded power supply uses six RG217 cables (12 meters) as PFL. The latter will be charged up to 30kV and discharged into series connection of HV resistor and the load magnet to produce trapezoidal current pulse. New thyratron and cable termination assembly has been designed to achieve faster rise of the current pulse. The testing was done with lumped magnet as well.

2. Development of Prototype Power Supplies for Proton Synchrotron:

Rapid Cycling Synchrotron (RCS) requires dc biased sinusoidal excitation for electromagnets. Power supplies based on resonant schemes are best suited for such applications, as only the losses of the resonant network are drawn from the mains and the magnets are energized by resonating its inductance with external energy storage elements. The actual magnets as proposed in the preliminary design of Proton Synchrotron are quite big. It is plan to design, fabricate, and test a scaled-down version of complete white circuit network. This would include magnets arranged in different groups connected in a single White circuit operating in synchronism. Apart from this study on scaled-down or low power systems, it is proposed to develop real scale full current resonant system for powering of one actual size magnet.

Various studies for optimization of resonant network for one full scale dipole and scale down version( 1/5^th and 1/10^th) for different combination of magnet in series were carried out. The effect of changing the magnet inductance on the current and other parameters of the white circuit network were studied and provided to magnet section, these input would help them to design and fabricate full scale dipole and scale down dipole magnets.

Excitation of magnets in parallel resonant excitation scheme of White Circuit can be carried out either with a pulsed ac source or continuous ac source, the latter being advantageous. Development of the continuous ac source (inverter) has been initiated with two independent approaches: sinusoidal output inverter and square-wave output inverter. While the former features sinusoidal output waveform and possibility for magnet current waveform manipulation, the latter is a simpler approach in which it is possible to achieve a high quality magnet current waveforms without high frequency switching in the inverter stage.

3. Development of Powering Scheme of HPIA:

The scheme for generating high voltage in the high-power ELV-type electron accelerators (also termed as high-power industrial accelerator, the HPIA) is based on air-core, multi-secondary step-up transformer. Each secondary has voltage doubler rectifier and filter, the outputs of which are connected in series to generate the high voltage. As opposed to the conventional transformer, the air-core transformer has large leakage inductance (L_s) and small magnetizing inductance (L_m), poor regulation and it draws a large reactive power from the source. Suitable compensation scheme must therefore be employed in the powering scheme to minimize these undesirable effects. High voltage generator with air core transformer is therefore the integral part of the powering scheme as shown in the block diagram.

Block diagram of powering scheme of HPIA

The frequency converter converts DC voltage (obtained from three phase ac mains using SCR rectifier) to square-wave ac voltage of required frequency (430 Hz). The compensation network is designed in such a way that near-unity power factor operation and nearly constant output voltage is obtained under all loading conditions.

4. Development of High-Voltage DC Power Supply:

Development of high voltage DC power supply of -20 kV / 1 A output rating has been initiated. It is based on LCL-T resonant converter, which exhibits constant-voltage to constant-current conversion characteristics, which in turn is advantageous for phase-shifted parallel operation of modules without any need for current equalization feedback control and safe operation under arcing and partial discharges. The power supply is designed to be a three-phase LCL-T resonant converter which is free-running at constant (resonant) frequency. The output is controlled by controlling the input dc voltage of the resonant converter using another front-end DC-DC converter

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(a) Three-phase ferrite core high-voltage transformer, and, (b) assembly of three-phase LCL-T RC under tests.

5. Line Side Active Filter:

A line side active filter for reduction of line current harmonics is under development in lab. Three line currents have been sensed with the help of isolation amplifiers. Accurate sensing of these currents is of prime importance for calculation of line current harmonics using DSP. Real time calculation of line current harmonics is done using DSP. Sensed three phase voltages and three line currents are the inputs to DSP for online calculation of current harmonics. All the required interfacing electronics for DSP, a control rack has been developed. Measurement, calibration and integration of DSP derived current ripple information with power circuit is being carried out.

Photograph of control rack under development for line side active filter