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Questions of tokamak equilibria with "canonical" profiles of current density and pressure obtained by minimisation of the total plasma energy under the constraint of preserving the plasma current are considered. Equilibria with canonical profiles can have an infinite number of solutions when there is a vacuum region between a free plasma boundary and a conducting wall. It is natural in this case to impose a new constraint by fixing the shape of the plasma boundary. This can be achieved in most tokamaks by systems of shape and position feedback control. For a fixed boundary it is shown that a bifurcation of solutions still exists. For small deviations of the plasma boundary from the circular shape, the number of solutions does not exceed two. The problem of finding "canonical" profiles under the new constraint of having zero net current density at the edge is considered. This is a more realistic constraint, both from the point of view of stability of low order MHD modes and from experimental evidence. The qualitative effect of perturbations of the edge current density related to cooling due to radiation losses or to local heating by heat pulses is investigated.

Additional information

Authors: LAZZARO E, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB);SHCHEPETOV S V, Institute of General Physics, Department of Plasma Physics, Vasilova U 38, Moscow 117942 (USSR)
Bibliographic Reference: Report: JET-P(89)17 EN (1989)
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
Record Number: 198911047 / Last updated on: 1994-12-01
Original language: en
Available languages: en