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The early time evolution of helical plasma perturbations can be described by expanding the velocity field V(x,t) in a time series, the coefficients of which can be calculated by using the ideal MHD equations as an algorithm. In the present article the stabilising effect that a longitudinal magnetic field has on such perturbations is studied and a short-time criterion, valid for both Z-pinches and cylindrical tokamaks, is derived. It is found that even if the divergence of the perturbation is equal to zero, the system is not marginally stable. In particular, if the helicity of the unperturbed magnetic field is opposite to the helicity of the plasma perturbation, the conditions q less than 1 approximately unstable and q greater than 1 approximately stable are obtained, whereas, if the two helicities are of the same direction, the system is stable. The modification of the above criterion due to para- or diamagnetic poloidal currents is also discussed.

Additional information

Authors: PSIMOPOULOS M, Imperial College of Science and Technology, Physics Department, London (GB);LI D, Academia Sinica, Hefei (CN)
Bibliographic Reference: Article: Plasma Physics Controlled Fusion, Vol. 35 (1993) pp. 467-475
Record Number: 199310666 / Last updated on: 1994-11-29
Original language: en
Available languages: en