Dissipation of plasma toroidal angular momentum is observed in the National Spherical Torus Experiment due to applied nonaxisymmetric magnetic fields and their plasma-induced increase by resonant field amplification and resistive wall mode destabilization. The measured decrease of the plasma toroidal angular momentum profile is compared to calculations of nonresonant drag torque based on the theory of neoclassical toroidal viscosity. Quantitative agreement between experiment and theory is found when the effect of toroidally trapped particles is included.

The National Spherical Torus Experiment (NSTX) has demonstrated the advantages of low aspect ratio geometry in accessing high toroidal and normalized plasma beta, and βN ≡ 108⟨βt⟩ aB0/Ip. Experiments have reached βt = 39% and βN = 7.2 through boundary and profile optimization. High βN plasmas can exceed the ideal no-wall stability limit, βNno−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, Ωcrit = ωA/(4q2), 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 βNno−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 βN. Equilibria are reconstructed including measured ion and electron pressure, toroidal rotation and flux isotherm constraint in plasmas with core rotation ωϕ/ωA up to 0.48. Peak pressure shifts of 18% of the minor radius from the magnetic axis have been reconstructed.

Research in the National Spherical Torus Experiment, NSTX, has been conducted to establish spherical torus plasmas to be used for high-, auxiliary heated experiments. The device has a major radius R 0 = 0.86 m, a midplane half-width of 0.7 m, and has been operated with toroidal magnetic field B 0 ≤ 0.3 T and I p ≤ 1.0 MA. The evolution of the plasma equilibrium is analyzed between shots with an automated version of the EFIT code. Limiter, double-null, and lower single-null diverted configurations have been sustained for several energy confinement times. Plasma stored energy has reached 92 kJ (t = 17.8 %) with neutral beam heating. Plasma elongation of 1.6 ≤ ≤ 2.0 and triangularity in the range 0.25 ≤ ≤ 0.45 have been sustained, with values of = 2.5 and = 0.6 being reached transiently. The reconstructed magnetic signals are fit to the corresponding measured values with low error. Aspects of the plasma boundary, pressure, and safety factor profiles are supported by measurements from non-magnetic diagnostics. Plasma densities have reached 0.8 and 1.2 times the Greenwald limit in deuterium and helium plasmas, respectively, with no clear limit encountered. Instabilities including sawteeth and reconnection events (REs), characterized by Mirnov oscillations, and perturbation of the I p , , and i evolution, have been observed. A low q limit was observed and is imposed by a low toroidal mode number kink instability.