Researchers at General Atomics have captured the first experimental evidence of helicon current drive in the DIII-D tokamak, utilizing radio-frequency waves to generate electrical current inside a high-temperature plasma without relying on conventional transformer induction.
Confining plasma hotter than 100 million degrees inside a tokamak fusion reactor requires a massive electrical current flowing through the confined gas. Traditionally, this current has been generated by transformer action inside a large central solenoid. That method operates on a strict timer and forces reactor discharges to eventually end.
General Atomics Team Tests Helicon Waves
A research team led by physicist Jeff Lestz at General Atomics in San Diego turned to a technique known as a helicon current drive.
It utilizes radio waves to transfer energy and momentum directly to electrons within the plasma.
The DIII-D team utilized a wave launching structure called a “comb line” and a radio-frequency source. The equipment has a rated output of 1.2 MW. The power actually coupled to the plasma during the experiment measured 370 kW. Using magnetic field measurements from the Motional Stark Effect and the equilibrium reconstruction code EFIT, the researchers analyzed the safety factor profile. They observed that safety factor profiles near the magnetic axis flattened faster in helicon-injected discharges than in reference or electron cyclotron heating control discharges.
Researchers Quantify Driven Current
The team reported a driven current estimated at 20±10 kA concentrated near the magnetic axis.
Bob Pinsker, a scientist at General Atomics and one of the authors of the paper published in Physical Review Letters, noted the rigor applied to the data.
Alternative Approaches Trace Back to the 1980s
Researchers explored fast wave current drive in the 1980s without success. They anticipated very weak effects under the operational regimes available at the time. Everybody who tried it in the ’80s was kind of hoping for a miracle.
We were hoping that that part of the model was wrong,
Pinsker said.
Interest revived when Victor Vdovin at the Kurchatov Institute in Moscow campaigned for the approach globally. When pitched to DIII-D, researchers recognized that technological advancements in plasma measurement and comb-line antenna design offered a new path forward.
- EAST (China) and WEST (France): Employ lower hybrid current drive, though it carries a density limit restricting its utility primarily to plasma outer edges.
- STEP (United Kingdom): Designed to use nearly 200 gyrotrons injecting power via electron cyclotron waves through vacuum vessel openings.
- ITER: Continues to plan around a pulsed solenoid approach while other facilities target steady-state noninductive solutions.
Future Reactors Face Scaling Hurdles
The recent experiments confirm that helicon waves can penetrate deep into high-density plasma and deposit energy as predicted. The setup did not achieve continuous fusion power or net energy generation.
The estimated 20 kA driven near the core remains small compared to the multi-million-ampere currents required by full-scale power plants.
Future development will require scaling up radio-frequency power significantly and verifying whether the technique operates effectively in high-performance regimes like H-mode with high bootstrap current fractions. It’s possible,
Pinsker said regarding whether helicon current drive will become the standard for future machines, adding, I would say it’s too early to tell that definitively.