Physics of radiation-driven islands near the tokamak density limit

Abstract

In previous work [1], the onset criterion for radiation driven islands [2] in combination with a simple cylindrical model of tokamak current channel behavior was shown to be consistent with the empirical scaling of the tokamak density limit [3]. A number of the unexplained phenomena at the density limit are consistent with this novel physics mechanism. In this work, a more formal theoretical underpinning, consistent with cylindrical tearing mode theory, is developed for the onset criteria of these modes. The appropriate derivation of the radiation-driven addition to the modified Rutherford equation is discussed.Additionally, the ordering of the terms in the MRE is examined in a regime near the density limit. It is hoped that given the apparent success of this simple model in explaining the observed global scalings will lead to a more comprehensive analysis of the possibility that radiation driven islands are the physics mechanism responsible for the density limit.In particular, with modern diagnostic capabilities detailed measurements of current densities, electron densities and impurity concentrations at rational surfaces should be possible, enabling verification of the concepts described above.

Radiation-drivenm = 2 island formation and dynamics near density limit in experimental advanced superconducting tokamak ohmic plasma

“…In this paper, we report the occurrence of a radiation-driven m = 2 island for the first time in the experimental advanced superconducting tokamak (EAST) ohmic plasma near the density limit. The mode onset criterion agrees well with that of the thermo-resistive disruptions model [3,[6][7][8]. An m = 1 mode that was driven by a large m = 2 island was found during the partial disruption.…”

“…The criterion for the onset of radiation-driven islands can be used in combination with tokamak current channel behavior to explain the empirical scaling of the well-known tokamak density limit [1][2][3]. Specifically, the onset criterion for the radiation-driven mode near the tokamak density is [4]: 2 (1)…”

Edge shear flows and particle transport near the density limit of the HL-2A tokamak

“…The current shrinkage then leads to an increased current density gradient and the onset of resistive MHD instabilities. In particular, a thermo-resistive tearing mode model [12][13][14], in which radiative cooling is balanced with the ohmic heating inside magnetic islands, has been invoked to explain the dependence of the current density in the Greenwald limit scaling.…”

Helical temperature perturbations associated with radially asymmetric magnetic island chains in tokamak plasmas

“…17 is of particular importance because this term plays a crucial role in a recently developed theory of density limit disruptions in tokamak plasmas. [17][18][19][20][21] The main conclusion of this paper is that, in the limit jD 0 wj ( 1 and 0 jdj < 1, the term in question is nonexistent in both the narrow-island and the wide-island limits. The calculation in Ref.…”

“…17 is of particular importance because it plays a crucial role in a recently developed theory of density limit disruptions in tokamak plasmas. [17][18][19][20][21] The aim of this paper is to incorporate radial island asymmetry into the simple theory of nonlinear tearing mode stability in tokamak plasmas due to Rutherford, 7 in a (not entirely successful) attempt to resolve the aforementioned disagreement between the results of Refs. 10 and 17.…”