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Theory of hot particle stability

1987; AIP Publishing; Volume: 30; Issue: 9 Linguagem: Inglês

10.1063/1.866033

2163-4998

H. L. Berk, H. Vernon Wong, K. T. Tsang,

Solar and Space Plasma Dynamics

The investigation of stabilization of hot particle drift reversed systems to low frequency modes has been extended to arbitrary hot beta βH, for systems that have unfavorable field-line curvature. Steep profile equilibria are considered where the thickness of the pressure drop Δ, is less than plasma radius rp. The analysis describes layer modes which have mΔ/rp <1, where m is the mode number, and radial structure larger than Δ. Stabilization is classified as either being ‘‘robust,’’ where all excitations are positive energy, or ‘‘fragile,’’ where stability criteria exist to magnetohydrodynamic (MHD)-like and drift-compressional instabilities, but positive and negative energy waves are present (with the possibility that negative energy waves are destabilized by dissipative mechanisms). It is shown that in making a continuous transition from the fragile stability regime to the robust stability regime one must go through an unstable region. To bridge the unstable band in a physical manner one must either produce robust stability conditions very rapidly, or use transient stability techniques such as ponderomotive forces or transient minimum-B coils. The positive energy stabilization terms of the layer mode from wall stabilization terms and finite Larmor radius terms are explicitly exhibited. It is shown the robust stability can even be achieved with only wall stabilization for all possible m values of the layer modes if βH > (2)/(3) . When robust stability conditions are fulfilled, the hot particles will have their bounce frequency less than their grad-B drift frequency. This allows for a low bounce frequency expansion to describe the axial dependence of the magnetic compressional response. Such as expansion provides for another negative energy source in the theory, with the system then being susceptible to the drift compressional instability if dβH/dr >dβw/dr, where βw is the beta of the background plasma. If robust stabilization conditions are not fulfilled, the regions of fragile stability are extremely small if the MHD-like modes, the diamagnetic compressional and drift-compressional modes are to be simultaneously avoided.