A systematic comparison of the drift surface properties of a number of non-circular magnetic coil geometries for EBT is presented. Drift surfaces have been calculated for both a vacuum field model and a model which includes an ambipolar potential (ϕ). The drift-surface information is used to estimate a confinement time, τ, for a range of mirror ratios at fixed midplane dimensions and with a fixed major radius. Improvement over simple circular coil confinement is expressed by an aspect-ratio enhancement factor, ARE (τ ∝ (ARE)2). For an inverse-dee-shaped coil, we find that ARE ∼ 1.4 in the vacuum field model, but ARE ∼ 1.1–1.2 when eϕ0 ∼ kT, where ϕ0 is the potential well depth. For both the vacuum model and the potential model, Andreoletti coils can provide high ARE values as well as large magnetic field scale lengths in the midplane. With eϕ0 = kT, we find ARE ∼ 1.6 for the 6:1 height-to-width ratio Andreoletti coils, while an ARE of ∼ 3.5 is obtained at a mirror ratio of 2.6 for a 2:1 Andreoletti system with radially shifted coils.
Passive deconfinement of runaway electrons using an in-vessel helical coil