Spinning fusion plasma to stabilize it

A tokamak traps plasma inside a helical magnetic field that winds around, and delimits, a bagel-shaped confinement vessel. For a range of reasons—some fundamental, some practical—the magnetic field can’t be made strong enough to prohibit escape. Confinement is therefore imperfect: If the plasma becomes concentrated in eddies or other energetic instabilities, it can burst out of the confining field like a hernia. Now, a team from MIT’s Alcator tokamak (shown here) has demonstrated a method to stabilize tokamak plasma. Rotation is the key. When the whole plasma is made to rotate in its vessel, instabilities at the edge of the plasma are suppressed. And if the rotation has shear, turbulent eddies within the plasma are broken up. Modest rotation has been observed before at various tokamaks as an unintended consequence of the methods used to heat plasma. The MIT team, led by Yijun Lin and John Rice, has found a way to boost rotation twofold using VHF radio waves. The waves resonate with the cyclotron gyration of helium-3 ions, which are spread throughout the tokamak’s deuterium fuel as a minority component. It’s not clear how the momentum is transferred from the waves to the ³He²⁺ ions and then to the D⁺ ions. Still, by adjusting the proportion of ³He, the MIT team found they could maximize the rotation gain. In principle, the MIT method can be applied at ITER, whose design is being finalized at its site in Cadarache, France. (Y. Lin et al., Phys. Plasmas 16, 056102, 2009. ) — Charles Day

52.55.Fa, 52.30.-q, 52.35.Hr, 52.50.Qt