An Alfv n eigenmode similarity experiment

The major radius dependence of Alfvén mode stability is studied by creating plasmas with similar minor radius, shape, magnetic field (0.5 T), density ( n e ≃3×10 19 m −3 ), electron temperature (1.0 keV) and beam ion population (near-tangential 80 keV deuterium injection) on both NSTX and DIII-D. The major radius of NSTX is half the major radius of DIII-D. The super-Alfvénic beam ions that drive the modes have overlapping values of v f / v A in the two devices. Observed beam-driven instabilities include toroidicity-induced Alfvén eigenmodes (TAE). The stability threshold for the TAE is similar in the two devices. As expected theoretically, the most unstable toroidal mode number n is larger in DIII-D.