Stability of plasmas penetrated by neutral gas

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Abstract
For a plasma body of density n and characteristic macroscopic dimension Lc there exists a critical ion density nc = 1/σcmLc where σcm ≃ 3 × 10−19 m2 for hydrogen. When n ⪅ nc the plasma is permeable to neutral gas, whereas it becomes impermeable to the same gas when n ≫ nc. Permeable and impermeable plasmas have different equilibrium and stability properties. So far, tokamak experiments have been performed in the range of permeable plasmas, whereas full-scale steadystate fusion reactors are likely to operate in the impermeable range. The effects of plasma-neutral-gas interaction, finite resistivity, and viscosity on the gravitational instability modes of a magnetized plasma have been investigated: There are both direct and indirect neutral-gas effects on the ballooning and flute modes. The direct effects are due to frictional forces from collisions, and the indirect ones to the influence of the plasma-neutral-gas balance on the unperturbed pressure and density distributions. In an impermeable plasma, the driving mechanism of the ballooning and flute instabilities can be localized to a narrow, dense and cool, partially ionized boundary region, in which plasma-neutral-gas collisions and viscosity strongly reduce the instability growth rates. Further reductions of these rates are expected to arise in more advanced and non-linear approaches. In tokamaks, resistive ballooning modes should be practically unaffected by neutral-gas collisions. However, when n approaches nc from below, a collisionless ballooning mode is expected to arise from an increase of the gradients in the boundary region. This behaviour is consistent with observations, including adiabatic-compression experiments.

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