Identification of Resistive Drift Waves in the Spherator, and their Stabilization by Shear

Abstract

The observation of resistive drift waves in plasmas produced in the sheared magnetic field of the spherator is reported. The plasmas were produced by electron cyclotron heating, at a density of 1010−1011/cm3 and electron temperatures ≤1 eV. An investigation of oscillation amplitude demonstrates the existence of a stable regime (relative amplitude ≤0.5%) in toroidal fields near the condition of maximum shear (poloidal field and toroidal field approximately equal). For the purpose of further identification of the modes, experiments were carried out in a marginally unstable regime, resulting in nonturbulent single modes. An approximate theory of nonlocal normal modes using an average radial width is derived, and is confirmed by comparison in a special case with a more accurate calculation taking into account the radial dependence. This theory predicts the expected wave parameters for the most unstable modes. The wave parameters of the single modes are measured for the case of argon, and are found to be in excellent agreement with the frequencies and wavelengths predicted for the most unstable modes. Finally, the theory predicts that plasmas of low electron temperature, low ion mass, sharp density gradient, and high density should be the most unstable to resistive drift waves. A series of experiments in which these parameters were varied agrees with these predictions.