Scientists make some outstanding progress through a latest Lower Hybrid Current Drive experiment.

In a breakthrough at MIT’s Plasma Science and Fusion Center (PSFC), researchers demonstrated how to overcome barriers to steady-state tokamak operation using microwaves. Seung Gyou Baek led a team of researchers which experimented with a Lower Hybrid Current Drive (LHCD) on MIT’s Alcator C-Mod tokamak before it ended operation in September 2016.  Plasma current is generated by launching microwaves into the tokamak, which push electrons in one direction achieving steady-state operation. In addition, Alcator’s strong magnets have allowed investigation of LHCD at a plasma density high enough to be relevant for a fusion reactor. Referring to the magnetic coil filling the center of the torus, Baek said:

“The conventional way of running a tokamak uses a central solenoid to drive the current inductively. But that inherently restricts the duration of the tokamak pulse, which in turn limits the ability to scale the tokamak into a steady-state power reactor.”

Baek and his colleagues believed that LHCD is the solution to this problem but they discovered that the plasma current generation vanished as soon as plasma density was increased to higher levels. He referred to the research of the scientist Gregory Wallace and explained that he measured the fall-off to be much faster than expected, which was not predicted by theory. The last decade people have been trying to understand this, because unless this problem is solved you can’t really use this in a reactor.

To boost effectiveness and overcome the LHCD density limit, researchers closely examined how lower hybrid (LH) waves respond to the tokamak environment. They found that lower hybrid waves drive plasma current by transferring their momentum and energy to electrons in the plasma. Paul Bonoli, the Head of the PSFC’s Physics Theory and Computation Division who is also a Senior Research Scientist, compared it surfing by saying,

“You are on a surfboard and you have a wave come by. If you just sit there the wave will kind of go by you. But if you start paddling, and you get near the same speed as the wave, the wave picks you up and starts transferring energy to the surfboard. Well, if you inject radio waves, like LH waves, that are moving at velocities near the speed of the particles in the plasma, the waves start to give up their energy to these particles.”

In order to transfer wave momentum to plasma particles in tokamaks including C-Mod, we need high enough temperatures to provide good matching on the first pass from the antenna, launching them to the core plasma. Consequently, researchers observed that injected microwaves travel through and beyond the plasma core, interact multiple times with the edge, and dissipate. Higher plasma density activates this process even more. In addition to Wallace, Baek gave credit to a PSFC research scientist, Syun’ichi Shiraiwa, for his extensive simulations indicating the scrape-off layer as the most likely location for LH wave power depletion in the following words:

“The scrape-off layer is a very thin region. In the past, RF scientists didn’t really pay attention to it. Our experiments have shown in the last several years that interaction there can be really important in understanding the problem, and by controlling it properly you can overcome the density limit problem.”

Using Alcator C-Mod in the last two decades, comprehensive research conducted on the edge and scrape-off-layer demonstrated that increasing the total electrical current in the plasma narrows the width of the scrape-off-layer reducing turbulence. This suggests a reduction or elimination of its negative effects on the microwaves. Motivated by this, PSFC researchers devised an LHCD experiment to push the total current from 500,000 Amps to 1,400,000 Amps, enabled by C-Mod’s high-field tokamak operation. They found that the effectiveness of the LHCD to generate plasma current at high density reappeared, which pushed the LHCD density limit.

Results suggest steady-state fusion reactors are plausible. Baek believes that the outcomes of the experiment support PSFC’s proposals to place the LHCD antenna at the high-field (inboard) side of a tokamak, near the central solenoid. Placing it in this quiet area, as opposed to the turbulent outer mid-plane, would minimize destructive wave interactions in the plasma edge while protecting the antenna and increasing its effectiveness. Baek believes the relationship between the current drive and the scrape-off layer could be investigated on any tokamak, regardless of high-density operation, He said,

“I hope our recipe for improving LHCD performance will be explored on other machines, and that these results invigorate further research toward steady-state tokamak operation,”