Feedback stabilization of an ℓ = 0, 1, 2 high-beta stellarator

Feedback stabilization of the Scyllac 120° toroidal sector is reported. The confinement time was increased by 10–20 μs using feedback to a maximum time of 35–45 μs, which is over 10 growth times of the long-wavelength m = 1 instability. These results were obtained after circuits providing flexible waveforms had been used to drive auxiliary equilibrium windings. The resultant improved equilibrium agrees well with recent theory. It was observed that normally stable short-wavelength m = 1 modes could be driven unstable by feedback. This, instability, caused by local feedback control, increases the feedback system energy consumption. An instability involving direct coupling of the feedback ℓ = 2 field to the plasma ℓ = 1 motion was also observed. The plasma parameters were: temperature, Te ≅ Ti ≅ 100 eV; density, ne ≅ 2 × 1016 cm−3; radius, a≅ 1 cm; and β ≅ 0.7. Beta decreased significantly in 40 μs, which can be accounted for by classical resistivity and particle loss from the sector ends.