A new current drive option for tokamak-based power plants

Researchers at the DIII-D National Fusion Facility published a Physical Review Letters (PRL) paper describing the first experimental confirmation of helicon current drive, a method that could be used to operate a steady state tokamak in regimes that would be used for fusion power.

Powering a tokamak: The main way plasma is confined in a tokamak is with a large central solenoid to inductively drive a current. However, this approach is inherently limited: It relies on flux through the transformer changing with time, resulting in a pulse length limit that is dependent on the solenoid cycle.

While it's broadly agreed that a steady-state tokamak would be preferred-in part because it could make the machine more economical by reducing damage from cyclic stresses- a pulsed approach isn't necessarily a dealbreaker, and some projects such as ITER continue to plan with that approach in mind.

But projects chasing a nonpulsed approach also must use a noninductive form of current drive.

Noninductive current drive schemes: A naturally occurring toroidal current called the bootstrap current can provide a significant fraction of the total plasma current, especially under reactor-like conditions, but supplemental current drive is needed to achieve steady-state conditions.

Due to the profile shape of the bootstrap current, this additional current drive is particularly needed in the plasma's mid-radius region.

Multiple approaches exist.

Both the Experimental Advanced Superconducting Tokamak (EAST) in China and the Tungsten Environment in Steady-State Tokamak (WEST) in France have successfully used a version of radiofrequency (RF) current drive called lower hybrid current drive (LHCD) to achieve long pulses. But LHCD has a density limit above which it can't drive current, making it much less useful for fusion power, where it is expected to only be usable at the plasma's outer edge.

The Spherical Tokamak for Energy Production (STEP) in the United Kingdom is intended to be a prototype power plant aiming for steady-state operations, requiring a different approach. STEP will be using nearly 200 gyrotrons to inject power using electron cyclotron waves. This approach requires only an opening in the vessel to let in the beam, which can propagate through the vacuum. This offers a level of simplicity, but the efficiency is currently low, which could become a key consideration in trying to develop an economical reactor.

A second form of RF current drive called fast wave current drive-now known as helicon current drive-was explored in the 1980s without success. Bob Pinsker, a scientist at General Atomics and one of the authors of the PRL paper, was one of the researchers pursuing this option.

"There was a whole wave of experiments that attempted to establish helicon current drive in the '80s, and none of them, including my thesis experiment, was entirely successful," he said.

But this actually was not surprising. Calculations using the standard model indicated that in the regimes that tokamaks at that time were able to explore, helicon current drive was expected to be "very, very weak."

"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," said Pinsker. "I used to always say that the great thing about fast waves in lower hybrid range is that they are just the thing that you would want in a reactor plasma. The problem always was that we didn't have a reactor around to test it in."

Helicon current drive has been considered for many modern tokamaks despite never having been proven experimentally, until now.

Key developments: Experiments to demonstrate helicon current drive were shelved for several decades. As Pinsker described it, Victor Vdovin at the Kurchatov Institute in Moscow campaigned for the return of studies exploring the approach, pitching the scheme-which he rebranded to "helicon current drive"-to several projects around the world when his home institution would not let him study it unless the idea had been proven elsewhere.

When he approached DIII-D, the team gave it consideration. Pinsker said once they started taking a closer look, they realized that several key developments had been made since the 1980s that they believed gave them a shot at success.

DIII-D can approach reactor-like plasma parameters, eliminating a primary limitation of earlier studies. Two additional technological developments were expected to help: General Atomics designed a wave launching structure called a "comb line" that was expected to launch the wave more efficiently, and plasma measurement techniques had developed to allow for the observation of currents driven within the plasma.

The team designed an experiment and published the results in a PRL paper. Pinsker is a joint first author on the paper with Jeff Lestz and Bart Van Compernolle, who are also scientists at General Atomics, which operates DIII-D.

A clear view: Pinsker said the key physics result was that they were able to clearly see heating in the plasma that resulted from absorption of the helicon wave as predicted.

"It demonstrates that we're right to think that we could get sufficiently reactor-like parameters that we could actually get into the strong absorption regime, and so that in itself is something that is seemingly a first," he said.

Pinsker also said it was technologically important that the comb line antenna worked as intended and that they were able to conduct measurements of the current in the plasma that clearly demonstrated that they were driving current in the way they thought they were.

"We've tried to be as rigorous about this as we can and have done proper statistical studies, and that was, I would say, a hallmark of this experiment," he said. "We really went to extremes to make sure that we couldn't be accused of cherry-picking and showing one fluke shot that had a big effect."

Next steps: Experimental confirmation of helicon current drive is no small step, but further studies are needed to demonstrate how it behaves in different regimes. The experiment described in the PRL paper only looked at driving helicon current at the plasma core and only looked at low confinement modes.

"What we really need to extend this is to increase the power quite a bit, and, of course, that's not so easy," Pinsker said, adding that it's hard for him to imagine tokamaks using multiple types of current drive because of how much it would increase complexity, and it's still an open question as to which approach will be most effective and efficient.

Could helicon current drive be the one?

"It's possible," said Pinsker. "I would say it's too early to tell that definitively."