Experimental studies of plasma confined in a toroidal heliac

Abstract

This thesis describes in detail the properties of and experimental results obtained on the first toroidal heliac (helical axis stellarator) SHEILA. The machine construction, operation and the Langmuir probe diagnostics used in these experiments are described. Computational results for the wide range of vacuum configurations made accessible by the addition of a helical winding to the basic heliac coil configuration are presented and discussed. The main part of this thesis is dedicated to the study of a low beta argon plasma (ß ~ 10-4) confined in various heliac configurations. The existence of plasma equilibrium surfaces is verified experimentally. Observations of phenomena associated with magnetic surface resonances, e.g. the deterioration of plasma confinement when the rotational transform profile includes certain low-order rational surfaces, are described. Some features associated with the surface destruction and the magnetic island formation appear in both plasma pressure and floating potential profiles. A known symmetry-breaking error in coil position is shown to have significant effect on the vacuum magnetic configurations. Low frequency coherent fluctuations in plasma density, floating potential and magnetic field are studied for three typical heliac configurations. Spatial mode structures are analyzed in a magnetic coordinate system. The measured frequencies and wavenumbers of the fluctuations, together with their parametric dependences on the magnetic field and the electron collision frequency, are compared with a collisional drift wave model. The agreement is excellent. The model is derived from a linearized two-fluid theory and related through an appropriate magnetic coordinate transformation to the heliac geometry. The influence of these fluctuations on the observed particle confinement is briefly discussed. Show more