One obstacle to the attainment of stable fusion plasmas is a phenomenon that can stop fusion reactions and damage reactor walls. The global Alfvén eigenmode (GAE) is a common wave-like disturbance that interferes with fusion reactions.

A 2015 upgrade makes the National Spherical Torus Experiment the most powerful experimental fusion facility, or tokamak, of its type in the world. (Source: PPPL)The same neutral beam particles that heat plasma are responsible for GAEs. The particles are ionized into atomic nuclei inside the gas and trigger the GAEs, which drive out these fast ions, cooling the plasma and halting fusion reactions.

Researchers at the Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), N.J., successfully suppressed this instability with a second neutral beam injector recently installed as part of an upgrade to the National Spherical Torus Experiment-Upgrade. Beams from the second injector flowed through the plasma at a higher pitch-angle, in a direction roughly parallel to the magnetic field that confines the hot gas. These “outboard” beams are distinguished from “inboard” beams that the original injector produces, which flow through the plasma and the magnetic field in a more perpendicular fashion.

GAEs were suppressed in milliseconds as fast ions from the outboard beam combined with those from the inboard beam to increase the density of the ions and alter their distribution in the plasma. The sudden alteration reduced the gradient, or slope, of the ion density, without which GAEs were unable to form and ripple through the plasma.

The result validated predictions of the HYM computer code developed at PPPL and could prove useful to ITER, the international fusion facility under construction in France. ITER is expected to demonstrate the ability to confine a burning plasma and produce 10 times more power than it consumes.