Digital controller improves power management for nuclear fusion

March 09, 2016 // By PAUL BUCKLEY
Engineers at the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) have developed a digital version of a key analog electronic component that helps regulate the current that powers the coils in PPPL's National Spherical Torus Experiment-Upgrade (NSTX-U).

The device, known as a digital firing generator, replaces an analog device in the previous machine that was less accurate and harder to maintain.  The state-of-the-art controller controls the AC to DC converter that precisely manages the plasma in fuels experiments in doughnut-shaped tokamaks for the upgraded nuclear fusion machine.

The upgrade brings NSTX-U in line with other tokamaks around the world that employ the same kind of device.

"The digital firing generator is very important for ensuring that NSTX-U operates effectively and reliably," said Neumeyer. “These new generators extend the life of the power supplies that form the backbone of PPPL's electrical power system, and provide the precise control necessary to drive currents in the NSTX-U magnet coil up to 140,000 amps — higher than any previous experiment at PPPL."

The ability to better manage the electric current flowing into NSTX-U, the world's most advanced spherical tokamak, will provide insights into how to control plasma, the soup of electrons and charged atomic nuclei that swirl within fusion facilities. With better control, scientists will be able to perform experiments on NSTX-U to advance the design of a working fusion reactor.

The new generator links the computer that controls NSTX-U and a device called a 'thyristor rectifier' that adjusts the voltage, and thus the current, for NSTX-U experiments. Through a computer command sent via fiber optic cables, the digital firing generator causes the AC (alternating current) that flows into PPPL to convert to DC (direct current) and deliver the amount requested for an experiment. The team also built the fiber optic links that make the conversion possible.

"A single thyristor rectifier can generate up to 2,000 volts of DC current at 24,000 amps, for about three seconds," said Mozulay. "That amount of voltage corresponds to 48 megawatts of power, which, during the three-second pulse, could power approximately 8,000 average-sized New Jersey homes."

NSTX-U has 32 pairs of thyristor rectifiers, each controlled by its own digital firing generator. These rectifiers help to double the heating power and magnetic field strength that the upgrade has made possible. "All of the firing generators were designed, built, and tested here at PPPL," explained Mozulay.