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When there are fluctuations, the usual argument for automatic ambipolarity no longer applies, because the fluctuations contribute to the mean momentum balance. The ambipolar electric field E(r) is now determined by ambipolarity of the neoclassical and anomalous particle fluxes. For electrostatic fluctuations the fluxes are only weakly non-ambipolar, because of the ion drift driven by their mean inertia and pressure tensor. E(r)-B(theta)U(parallel) is therefore close to the neoclassical prediction. However, the flow along a stochastic magnetic field is strongly non-ambipolar, and can change the direction of E(r) from inwards to outwards. Impurity neoclassical fluxes are most sensitive to the electric field, because of their high Z, and the abrupt change in E(r) at the onset of stochasticity can cause a rapid pump-out of impurities. The observed reduction in impurity density during periods of MHD activity, such as sawteeth or ELMs, agrees qualitatively with analytic prediction.

Additional information

Authors: STRINGER T E, JET Joint Undertaking, Abingdon, Oxon. (GB)
Bibliographic Reference: Report: JET-P(92)59 EN (1992) 14 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
Record Number: 199211280 / Last updated on: 1994-11-29
Original language: en
Available languages: en