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The diffusion driven current in tokamaks and stellarators is investigated. In stellarators neoclassical bootstrap currents are serious obstacles to achieving optimum confinement since the profile of the rotational transform is modified during the rise phase of the discharge. In general stellarator geometry the magnitude of the bootstrap current depends on the localization of trapped particles and the kinetic equation has to be solved in order to compute the parallel viscosity which drives the bootstrap current. It can be shown that in configurations with symmetry - axisymmetry, helical symmetry, quasi-helical symmetry - bootstrap current and radial particle loss are proportional to each other. The bootstrap current in helical invariant stellarators can have the opposite sign compared with tokamaks. If the symmetry of the configuration is destroyed, it is possible to minimize the bootstrap current by a suitable choice of the magnetic field geometry. The parallel and the perpendicular viscosities have to be calculated for the various regimes of collisionality starting from the kinetic equation. The viscous tensor of a collisional plasma allows these forces to be calculated for an arbitrary stellarator geometry. A geometrical factor can be defined which characterizes the dependence of the bootstrap current on the topology of the magnetic surface.

Additional information

Authors: WOBIG H, Max-Planck-Institut fur Plasmaphysik, Garching bei München (DE)
Bibliographic Reference: Report: IPP 2/297 EN (1988)
Availability: Available from Max-Planck-Institut für Plasmaphysik, 8046 Garching bei München (DE)
Record Number: 198910343 / Last updated on: 1994-12-01
Original language: en
Available languages: en