Magnet Power Supplies for Indus-1

Indus-1 synchrotron radiation source (SRS) at RRCAT has an injector system consisting of a microtron and a booster synchrotron. The load specifications and number of the magnet power supplies required for microtron, booster, beam transport-lines (TL1 and TL2) and Indus-1 are listed in the following table:
 

The stability requirement of are stringent. Permissible ∆I/I in various power supplies is summarized in the following table:

The power supplies have been developed using different schemes since the ratings and stability requirements are diversified. A brief description of schemes used to develop these power supplies is as follows.

• Ramp Magnet Power Supplies
• Thyristor Controlled Power Supplies
• Transistor Series Pass Power Supplies
• Pre-regulator Followed by Transistor Series Pass Power Supplies
• Off-line Switch-mode Power Supplies
• Bipolar Power Amplifier Type Power Supplies
• New Booster Vertical Steering Coil Power Supplies
• New Booster Horizontal Steering Coil Power Supplies

Ramp Magnet Power Supplies                                                                                                    [ Go to Top ]

In booster dipole and quadrupole power supply, trapezoidal current passes through the main coils with a repetition rate of 1 Hz. Current is ramped up from 20 A to 1000 A, in 200 ms, held there for 50 ms and forced to decrease faster thereafter so that injection for the next cycle is possible within given time. In booster dipole and quadrupole magnet power supplies, four SCR bridges are put in series with 2 bridges having bypass SCRs across them. Turn-off of bypass SCRs in conjunction with impressing of pre-charged filter capacitor voltage across the highly inductive load brings about transition from ramp start to flat top beginning.

Booster dipole magnet power supply		Booster quadrupole magnet power supply

Thyristor Controlled Power Supplies                                                                                                    [ Go to Top ]

This scheme consists of input transformer followed by a SCR bridge and L-C filter. The SCR bridge output is either 6 pulsed or 12 pulsed depending on the stability requirement and load time constant. Most of the power supplies with current above 80 A, load power 2 kW or above and load time constant 0.2 s to 1 s follow this scheme with required stability ranging from ± 0.01% to ± 0.1%. Current ripple is attenuated partly through reducing voltage ripple with L-C filter, and partly by the inductance present in the load. The attenuation in the field ripple is further achieved with the vacuum chamber. Booster dipole power supply follows essentially this scheme with an additional active filter at the output to bring about further ripple reduction and some distinguishing features to be mentioned earlier. The cathode power supply for microtron uses back to back SCR for AC voltage control on the primary side. The power supplies developed for TL2 quadrupoles, TL2 dipoles, Storage ring quadrupole and dipole follow this scheme.

TL2 quadrupole magnet power supply		SRS quadrupole magnet power supply

Transistor Series Pass Power Supplies                                                                                                    [ Go to Top ]

In this scheme single phase mains voltage is stepped down, rectified, filtered and control is achieved with a following transistor series pass element. These power supplies are rated at low current (less than 10 A) and low load power (less than 200 W) and have stability of ±0.01-0.1%, feeding load with time constant 60-68 ms. This scheme is used for power supplies for TL1 dipole, extraction magnet coil, vertical steering coil and horizontal steering coil in TL1, TL2, booster quadrupoles correction coils and correction coils in storage ring. These power supplies are bipolar in nature and polarity reversal is brought about by MOSFET switches or relays following the series pass stage.

TL-1 quadrupole magnet power supply		TL-1 steering coil power supply

Pre-regulator Followed by Transistor Series Pass Power Supplies                                        [ Go to Top ]

In this scheme the input transformer is followed by pre-regulator and transistor series pass element. Required stability for these power supplies ranges from ± 0.05% to ± 0.1% with load time constant of 15 ms to 200 ms. The pre-regulator is mostly a half-controlled or full-controlled SCR bridge. Booster dipole correction coil power supplies have a DC to DC chopper as pre-regulator. Booster quadrupole tune adjustment secondary coil power supply follow this scheme in ramp mode with pre-regulator SCR bridge, which switches over to inversion mode in the ramp down regime of current cycle. TL1 quadrupole power supplies also follow this scheme

Off-line Switch-mode Power Supplies                                                                                                    [ Go to Top ]

These power supplies operate in half bridge configuration and are used for TL2 steering coil and Storage ring sextupole steering coils. Bipolar operation is possible with polarity change over MOSFET switches.

Bipolar Power Amplifier Type Power Supplies                                                                                          [ Go to Top ]

Bipolar operation is provided by the power amplifier working in class AB mode. No additional stage for polarity reversal is required. These are used to feed vertical steering coils of Indus-1 SRS sextupoles.

New Booster Vertical Steering Coil Power Supplies:                                                                                                            [ Go to Top ]

These upgraded systems replace earlier power supplies which were not true bipolar and could not operate near zero current. These new power supplies feed five independent vertical steering coils with Booster Ramp Current Profile (peak current rating of 20A), are true bipolar in nature and can operate at very close to zero current. These power supplies were designed, fabricated, tested, and handed over for regular accelerator operation.

Photographs of New Booster Vertical Steering Coil Power Supplies

New Booster Horizontal Steering Coil Power Supplies:                                                                                     [ Go to Top ]

These power supplies feed ramped bipolar current to horizontal steering coils which are wound on the booster dipole magnets along with the main coil. They drive the specified ramp current through these coils against the voltage induced by the main dipole coil. There are six such power supplies. The major specifications of the power supplies are as under :

The power circuit of the power supplies consists of a 3-phase transformer, diode bridge, L-C filter, and BJT based series pass element feeding the magnet load. The control circuit consists of outer current loop along with an inner load-voltage loop. A precise feed forward of the load voltage has been provided to achieve better tracking performance. The voltage loop has a control bandwidth of about 15 kHz. The control bandwidth of the current loop is about 150 Hz.

The need to develop true bipolar power supplies to replace the old power supplies was felt because the old power supplies were not able to provide precise control near zero current. Also, they could either give +ve current or -ve current but not both in a cycle. A significant improvement and consistency was observed in the booster beam current after the new power supplies were deployed.

Photograph of New Booster Horizontal Steering Coil Power Supplies