Simulations suggest that magnetic fields can calm plasma instabilities

August 15, 2016 by Raphael Rosen, Princeton Plasma Physics Laboratory
Magnetic perturbations in a fusion plasma. Credit: Gerrit Kramer

Physicists led by Gerrit Kramer at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have conducted simulations that suggest that applying magnetic fields to fusion plasmas can control instabilities known as Alfvén waves that can reduce the efficiency of fusion reactions. Such instabilities can cause quickly moving charged particles called "fast ions" to escape from the core of the plasma, which is corralled within machines known as tokamaks.

Controlling these instabilities leads to higher temperatures within tokamaks and thus more efficient fusion processes. The research was published in the August issue of Plasma Physics and Controlled Fusion and funded by the DOE Office of Science (Fusion Energy Sciences).

"Controlling and suppressing the instabilities helps improve the fast-ion confinement and performance," said Kramer, a research physicist at the Laboratory. "You want to suppress the Alfvén waves as much as possible so the fast ions stay in the plasma and help heat it."

The team gathered data from experiments conducted on the National Spherical Torus Experiment (NSTX) at PPPL before the tokamak was recently upgraded. Then they conducted plasma simulations on a PPPL computer cluster.

The simulations showed that externally applied magnetic perturbations can block the growth of Alfvén waves. The perturbations reduce the gradient, or difference in velocity, of the ions as they zoom around the tokamak. This process calms disturbances within the plasma. "If you reduce the velocity gradient, you can prevent the waves from getting excited," notes Kramer.

The simulations also showed that magnetic perturbations can calm Alfvén that have already formed. The perturbations alter the frequency of the plasma vibration so that it matches the frequency of the wave. "The plasma absorbs all the energy of the wave, and the wave stops vibrating," said Kramer.

In addition, the simulations indicated that when applied to tokamaks with relatively , the external magnetic perturbations could dislodge fast ions from the plasma directly, causing the plasma to cool.

Explore further: New imaging technique provides improved insight into controlling the plasma in fusion experiments

More information: G J Kramer et al, Mitigation of Alfvénic activity by 3D magnetic perturbations on NSTX, Plasma Physics and Controlled Fusion (2016). DOI: 10.1088/0741-3335/58/8/085003

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1 / 5 (1) Aug 16, 2016
They are clearly using the wrong perturbations if the plasma is cooling.
Something more anti-resonant to the Eigen tones of the Alfven waves and less prone to punching hot ion leakage holes in the magnetic bottle is called for.
Great graphic of the perturbations by the way, showing it's not an easy mathematical function of the leak to plug.
A bit less perfect toroidal symmetry might help. The bottle doesn't have to ring with Alfven waves like a finely tuned magnetic bell.
But hey, at least its real data on a Tokamak, part fail, part success, keep tweaking and trying..
1 / 5 (1) Aug 16, 2016
My comments were more aimed at their actual tests on NSTX than their simulations (also done by PPPL):-

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