Particle confinement scaling experiments in the Culham Levitron

Experiments are described which investigate the parametric variation of particle confinement in heated and unheated decaying plasmas in the Culham Superconducting Levitron. The parameter range investigated, expressed in terms of a collisionality parameter, is similar to that found in the current generation of reverse-field pinch experiments and in the edge regime of present-day tokamak experiments. The variation of the particle confinement time with electron temperature, electron-density and magnetic field strength was found to be approximately of the form τp ∝ T½ B2/n which is that expected for classical diffusion . The measured confinement times were, however, shorter than those predicted by classical diffusion by a factor of 3–10, depending on the magnetic shear. The diffusion rate was found to be approximately proportional to where s is the mean shear length. Low-frequency spectra of floating-potential fluctuations were measured, and it was found that the amplitude of these fluctuations was reduced by increasing magnetic shear. The resistive-g mode has been suggested as a possible explanation for the low-frequency fluctuations, and the diffusion coefficient predicted for this mode (D = γ/k2) has a scaling similar to that observed.