Oscillating-Field Current-Drive Schemes for Tokamaks

Author: Schalit, Mark Alan

Year: 1989

Degree: Dissertation (Ph.D.)

Advisor: Bellan, Paul Murray

Committee Members: Bellan, Paul Murray; Cowan, Eugene W.; Keller, Herbert Bishop; Corngold, Noel Robert; Saffman, Philip G.; Gould, Roy Walter

Option: Applied Physics

DOI: 10.7907/xjrw-pq56

Abstract

A novel current-drive scheme for steady-state tokamak operation is investigated in which external coils are applied to induce time-periodic fluid-type, fluctuations within the plasma; a nonlinear interaction between these fluctuations results in a time-averaged EMF, which maintains the large-scale magnetic field against Ohmic dissipation. Analytical and numerical modeling of this current-drive scheme is presented for low-frequency schemes (where the nonlinear < u⃗ x b⃗ > EMF is dominant) and for higher-frequency schemes (where the < j⃗ x b⃗ > Hall EMF is dominant). The Hall EMF is dominant at frequencies well above the ion-cyclotron frequency (referred to the strength of the static axial field) - except in the case of the rotamak, where the oscillating electric field is in the same direction as the static axial field.

A figure-of-merit for these current-drive schemes is the ratio of the strength of the static axial current to the strength of the oscillating current. This ratio is always much less than unity in all standard MHD calculations. As the electronion collision frequency vanishes, the ratio approaches infinity for the case of the rotamak. The ratio also approaches infinity for the m = 1 analogue of the rotamak - but only in the restrictive case where the static axial field becomes vanishingly small and where the DC magnetic fields are a small fraction of the AC magnetic fields. For the m = 1 analogue, the currents are confined to a skin layer as the axial field becomes very large, with the ratio of DC current strength to the oscillating current strength approaching unity.

The analysis presented here is compared and contrasted with existing theories and to a number of recent experiments.

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