Confinement of a plasma in a trap formed by a combined magnetic field

Abstract

The confining properties of an adiabatic trap formed by a magnetic field which increases in both the axial and radial directions have been investigated. The field was produced by the addition of a field having an ordinary mirror-type configuration (the main field H0||) and a field produced by a system of conductors arranged parallel to the axis of the trap (the stabilizing field H⊥). The conductors were distributed uniformly with respect to azimuth around the side wall of the vacuum vessel. The trap was filled with a plasma with density n ≈ 109-1010 cm-3 and proton energy Ti ≈ 5 keV (Tc ≈ 20 eV), and the dependence of the plasma lifetime on the magnitude of H⊥ and on the neutral gas pressure was measured. The results obtained show that a combined field of this kind ensures the stable confinement of a plasma free of disturbances caused by magnetohydrodynamic instability (at least for β = nI/(H2/8π) ≈ 10-4). The stabilization of the instability was verified by carrying out an analysis of the plasma oscillations for different values of H⊥. The decay of the plasma was determined by charge-exchange between the fast ions and the residual gas; the maximum confinement time was 0.06 sec with p = 7 × 10-9 mm Hg. A qualitative picture of the radial distribution of plasma density in the trap was obtained.