Spring 2009

MST New Versatile Power Supply

Picture of Don Holly with the programmable power supply.
Don Holly with the programmable power supply.

PSL Physicist Don Holly has designed and built a versatile new Programmable Power Supply for the Madison Symmetric Torus (MST) magnetic confinement machine.

MST is a toroidal device which uses two magnetic fields (poloidal and toroidal) to confine and heat a hot dense plasma to help develop a fusion energy source and to help understand astronomical magnetic phenomena. Precise control of the magnetic fields is crucial to confining and heating the plasma and to understanding its behavior. The new power supply replaces several existing supplies which provide the MST toroidal field (the field which points in the long direction around the torus) and will provide improved, more flexible control of the magnetic field.

“The new power supply is basically a big audio amplifier, with a power output of about 25 MW,” says Holly, who notes this is about ten times the output of a diesel-electric locomotive. “The supply is not limited to a particular waveform, but can produce virtually any waveform that one can program in.” In contrast, the previous toroidal system consisted of several large supplies which could be connected in several ways to produce several waveforms, with much less flexibility.

 

Picture of programmable power supply
One of the 32 programmable power supply modules shown in a test rack during development.

The new supply consists of 32 modules, each of which contains a capacitor bank for energy storage, and a high-speed switch unit using four large Insulated Gate Bipolar Transistors (IGBTs). “The IGBTs switch every 100 microseconds,” says Holly, “and the circuitry varies how long they stay on each cycle, in order to make the output current waveform match the demand.” The supply can produce currents of about 25 kA at about 1 kV, and drives a transformer which steps the current up to 1 million amps and the voltage down to 25 V to create MST’s toroidal field. Holly notes that because the system is modular, more modules could be added to produce additional output power.

The system is presently in use to control the MST toroidal field although it is still being optimized for highest power operation. Meanwhile, Don is working on initial design of an even larger programmable power supply to control the MST poloidal magnetic field.

 

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