Radial and axial losses in a multipole-mirror experiment

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Abstract
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 (1013 < n < 1015 cm−3) dominated by radial loss through the multiple-mirror regime (1011 < n < 1013cm−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 ≤ Bo ≤ 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 ≈ 1012cm−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.