Means of preventing low-frequency plasma instabilities possibly causing ``pumpout'' losses are discussed. The result is a prescription for determining experimentally a scaling law for stable confinement in toroidal magnetic wells. Based on measurement of fluctuations rather than instability thresholds, the method can be applied to existing unstable systems to help decide how best to modify them. Given an unstable stellarator or similar system, one measures 〈Ek2〉, the radial power spectrum of the electric field fluctuations averaged over other coordinates. Asymptotic behavior like 〈Ek2〉 ∝ k−3, confirmed in certain experiments, indicates instabilities susceptible to magnetic well stabilization and verifies applicability of the method. If applicable, magnetic well parameters sufficient for stability are given in terms of a measured quantity ∝ ∫ dk 〈Ek2〉. Preliminary application to the Model C stellarator indicates that an average well depth of only a few per cent would stabilize it if the length of locally unstable regions could be much reduced. Initial unsuccessful efforts to reduce pumpout by modifying the stellarator field did not achieve these goals, but the stable regime is perhaps not out of reach.
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T.K. Fowler (1966) studied this question.