Resistive wall stabilized operation in rotating high beta NSTX plasmas

The National Spherical Torus Experiment (NSTX) has demonstrated the advantages of low aspect ratio geometry in accessing high toroidal and normalized plasma beta, and β N ≡ 10 8 ⟨β t ⟩ aB 0 / I p . Experiments have reached β t = 39% and β N = 7.2 through boundary and profile optimization. High β N plasmas can exceed the ideal no-wall stability limit, β Nno− wall , for periods much greater than the wall eddy current decay time. Resistive wall mode (RWM) physics is studied to understand mode stabilization in these plasmas. The toroidal mode spectrum of unstable RWMs has been measured with mode number n up to 3. The critical rotation frequency of Bondeson–Chu, Ω crit = ω A /(4 q 2 ), describes well the RWM stability of NSTX plasmas when applied over the entire rotation profile and in conjunction with the ideal stability criterion. Rotation damping and global rotation collapse observed in plasmas exceeding β Nno− wall differs from the damping observed during tearing mode activity and can be described qualitatively by drag due to neoclassical toroidal viscosity in the helically perturbed field of an ideal displacement. Resonant field amplification of an applied n = 1 field perturbation has been measured and increases with increasing β N . Equilibria are reconstructed including measured ion and electron pressure, toroidal rotation and flux isotherm constraint in plasmas with core rotation ω ϕ /ω A up to 0.48. Peak pressure shifts of 18% of the minor radius from the magnetic axis have been reconstructed.