The density limit as a consequence of the theory of sawtoothing and the drift wave transport code
At any beta and any q(0)<1, the jump of the derivative of the MHD solution across the singular q=1 layer is sensitive to a rapid variation of the slope of the current density occurring at some rho(2), if the distances ratio (rho(2)-rho(1))/rho(1) and the shear parameter s(rho(1))=s(1) are small. The result is that the instability growth rate can pass from small resistive to large MHD values, so that sawtooth relaxations can be triggered - if s(1) and/or (rho(2)-rho(1))/rho(1) is allowed to vary over a certain, correspondingly small range. Dissipative trapped electron transport in the `confining' zone beyond the q=1 surface is compatible with these requirements for the occurrence of sawteeth at sufficiently low densities but tends to prevent, for a fixed amplitude of the sawteeth, the MHD phase of the precursors from developing at higher densities. There are thus two possible scenarios for rising density: either the amplitude of the sawteeth increases, or the MHD instability is quenched. The sawteeth would then disappear, the current sweep into the core and the plasma would contract violently and disrupt. Dissipative trapped electron transport otherwise reproduces the Neo-Alcator scaling law in Ohmic discharges and the confinement degradation extremely well, when auxiliary heating is turned on.
Bibliographic Reference: Article: Proceedings of the Twelfth International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Vol. 2 (1989) pp. 233-241, International Atomic Energy Agency, Vienna (AT)
Record Number: 199010834 / Last updated on: 1994-12-01
Original language: en
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