Physicists Achieve First Clear Demonstration of "Helicon" Current Drive in a Fusion Reactor

Physicists at General Atomics say they have achieved the first clear demonstration of "helicon current drive" in a working fusion experiment, a result published Tuesday that could help future fusion power plants sustain their reactions continuously rather than in short pulses. The findings, reported in Physical Review Letters by lead author Jeff Lestz and colleagues, describe experiments on the company's DIII-D tokamak, a donut-shaped magnetic confinement device operated for the US Department of Energy in San Diego. A preprint version of the study is also available on arXiv.

Tokamaks like DIII-D confine superheated plasma with magnetic fields generated partly by a current flowing through the plasma itself, and that current is traditionally induced by a central transformer-like coil called a solenoid. The catch, as General Atomics physicist Bob Pinsker explained in comments carried by Phys.org, is that a solenoid can only drive current while its magnetic field is still changing, which limits how long a pulse can run; DIII-D's own pulses are capped at around 10 seconds, and even the much larger ITER reactor under construction in France is expected to be limited to roughly 1,000 seconds in normal operation.

Helicon waves, a type of radio-frequency wave, offer one potential way around that limit because they are absorbed more gradually by the plasma's electrons than conventional microwave current-drive methods, allowing them to penetrate deeper into a reactor-scale plasma's hot, dense core. In the General Atomics experiment, the waves were launched from a "comb-line" antenna, a row of radiating elements arranged along the tokamak's inner wall and phased so that the waves travel in a single preferred direction.

Operating at megawatt-level power, the system produced clear electron heating in the plasma core and changes to the plasma's internal magnetic structure that evolved noticeably faster than in comparable experiments using conventional microwave heating, a pattern the authors treat as the signature of genuine current driven by the helicon waves rather than ordinary heating alone. Pinsker called it "the first clear success in this effort" after years of work on the concept.

Still a long way from power plants

The research team cautioned that the result is a proof of concept rather than a demonstration of a reactor-ready technology; the antenna hardware used reached coupled power levels above half a megawatt for pulses longer than two seconds, well short of what a continuously operating plant would require. Fusion researchers have pursued steady-state, solenoid-free operation for decades because pulsed tokamaks face repeated thermal and mechanical stress every time a pulse starts and stops, a problem that would be magnified in a commercial plant expected to run for years with minimal interruption.