Radial and axial losses in a multipole-mirror experiment

Plasma confinement is studied experimentally in the Berkeley Ten Meter Multiple-Mirror Device. Stability is achieved by a set of linked quadrupoles which produce an average minimum-B field configuration. The density decay is measured from the high-density regime (10 13 < n < 10 15 cm −3 ) dominated by radial loss through the multiple-mirror regime (10 11 < n < 10 13 cm −3 )for which either axial or radial loss may be more important. The effect of mirror ratio (2 ≤ M ≤ 4) and magnetic field strength (1.1 ≤ B o ≤ 2.1 KG) on plasma loss is investigated. The observed confinement times are predicted reasonably well by an analytical estimate and by a numerical simulation. Each calculation accounts for axial and radial loss due to classical processes only, with the radial diffusion enhanced by the highly eccentric elliptical flux surfaces. A multiple-mirror normal mode is achieved with centre-cell density in the range n ≈ 10 12 cm −3 and with axial profiles in qualitative agreement with those predicted by the numerical simulation. The results indicate that the confinement in a stabilized multiple-mirror can be governed by classical collisions alone.