Momentum transport in electron-dominated spherical torus plasmas

The Spherical Torus (ST), or low aspect ratio tokamak, operates at low toroidal fields, leading to high E x B rotational shearing rates. The National Spherical Torus Experiment (NSTX) operates between 0.35 and 0.55 T, which, when coupled to up to 7 MW of neutral beam injection, leads to central rotation velocities in excess of 300 km/s and E x B shearing rates up to 1 MHz. This level of E x B shear can be up to a factor of five greater than typical linear growth rates of long-wavelength ion (e.g., ITG) modes, at least partially suppressing these instabilities. Evidence for this turbulence suppression is that the inferred diffusive ion thermal flux in NSTX H-modes is often at the neoclassical level, and thus these plasmas operate in an electron-dominated transport regime. Recent studies indicate that in this regime the momentum transport properties can be different than those at higher aspect ratio, with the ion thermal diffusivity much greater than the momentum diffusivity. Despite this, the value of the momentum diffusivity as inferred from steady-state momentum balance is much larger than the neoclassical value. Analysis of perturbative experiments that used applied n = 3 magnetic fields to brake the plasma rotation indicated inward pinch velocities up to 40 m/s and perturbative momentum diffusivities larger by a factor of several than those values inferred from steady-state analysis with a zero pinch velocity assumed. The inferred pinch velocity values are consistent with values based on theories in which low-k turbulence drives the inward momentum pinch. Thus, in STs, the momentum transport can be a better probe of low-k turbulence than the energy transport. While the neoclassical ion energy transport effects can be relatively high and dominate the ion energy transport, the neoclassical momentum transport effects are near zero. This means that any residual low-k turbulence dominates the momentum transport. Understanding the source of the momentum transport, and how it scales to larger devices operating at lower collisionality, is critical to the performance of future ST-based Fusion Energy Development devices such as an ST-based Component Test Facility, as well as to conventional aspect ratio devices such as ITER, which is also expected to operate in electron-dominated transport regimes. (author)