Researchers at the DIII-D National Fusion Facility in San Diego report the first experimental evidence of helicon current drive, a method that uses radio waves to push current through the plasma inside a tokamak. The result appeared in Physical Review Letters, with General Atomics physicists Jeff Lestz, Bob Pinsker and Bart Van Compernolle as co-lead authors.A tokamak is a doughnut-shaped machine that holds plasma, a superheated, electrically charged gas, in place with magnetic fields. Most tokamaks induce the plasma’s current with a central coil called a solenoid, which must be reset, ending each pulse. Pinsker told Gizmodo that DIII-D pulses last about 10 seconds. Lestz said commercial designs generally need steady operation.Researchers tried radio waves of this kind in the 1980s. The paper says the current they saw was explained by an unintended switch to a different wave type, so helicon current drive was never confirmed.Isolating the currentThe team launched helicon waves at 476 megahertz from a 30-module antenna about 5 feet (1.5 meters) long, according to the paper. In the current drive test, 370 kilowatts were injected, and the team compared three kinds of shots: helicon power, no added power, and microwave heating of similar strength. Heat alone can reshape a plasma’s current, so the comparison separates the two effects.With helicon power, electron temperature near the center rose by about 1 kiloelectronvolt, a unit of particle energy that corresponds to roughly 21 million degrees Fahrenheit (11.6 million Celsius). The safety factor, a measure of the magnetic field’s twist, flattened faster than in the heating-only shots. Sawteeth, repeating temperature drops in the core, began at about 1,500 to 1,600 milliseconds, versus 1,900 to 2,000 milliseconds or not at all without helicon power. The authors calculated a driven current of 20 kiloamperes, plus or minus 10, near the center. Simulations predicted 21 kiloamperes in that region. The paper says the plasma was dense enough to exclude the other wave type.Limits and next stepsThe test plasmas had far lower electron pressure than reactors are expected to have, the paper said. In these plasmas, simulations predicted only about 40% to 47% of the power would be absorbed on the first pass. Only one side of the antenna worked at the time, so the team couldn’t compare current driven in opposite directions. The paper says it can’t yet tell whether the 20 kiloamperes reflects incomplete absorption or lower efficiency. Its claim that the method scales to reactors is a theoretical extrapolation.Pinsker told the American Nuclear Society it is “too early to tell that definitively” whether helicon current drive will be the preferred option. He said combining several current drive types would add complexity. Gizmodo reported that he sees raising helicon power as the most direct way to improve the results. The KSTAR tokamak has also installed a helicon antenna and begun commissioning, the paper said.The work was funded by the U.S. Department of Energy’s Office of Science. The study was published in Physical Review Letters.