Ambipolar magnetic fluctuation-induced heat transport in toroidal devices

Abstract

The total magnetic fluctuation-induced electron thermal flux has been determined in the Madison Symmetric Torus ͑MST͒ reversed-field pinch ͓Fusion Technol. 19, 131 ͑1991͔͒ from the measured correlation of the heat flux along perturbed fields with the radial component of the perturbed field. In the edge region the total flux is convective and intrinsically ambipolar constrained, as evidenced by the magnitude of the thermal diffusivity, which is well approximated by the product of ion thermal velocity and the magnetic diffusivity. A self-consistent theory is formulated and shown to reproduce the experimental results, provided nonlinear charge aggregation in streaming electrons is accounted for in the theory. For general toroidal configurations, it is shown that ambipolar constrained transport applies when remote magnetic fluctuations ͑i.e., global modes resonant at distant rational surfaces͒ dominate the flux. Near locations where the dominant modes are resonant, the transport is nonambipolar. This agrees with the radial variation of diffusivity in MST. Expectations for the tokamak are also discussed.

Anomalous ion heating from ambipolar-constrained magnetic fluctuation-induced transport

“…We extend here the prior calculations of ambipolar constraints in magnetic fluctuation-induced transport [14][15][16] to calculate the heating of ions intrinsic to these constraints. Although the origin of these constraints and the underlying physics that introduces them has been detailed in the literature, 13,[18][19][20][21][22] we present here a brief introduction.…”

“…13 The role of turbulent granularity in magnetic fluctuation-induced electron transport has been documented in a series of papers examining successively higher transport moments. [14][15][16] For the particle and field-aligned momentum fluxes, it was found that the transport from mag-netic turbulence nearly vanishes relative to the electrostatic fluxes and the flux of the non self-consistent quasilinear approximation. 14,15 The residual transport is smaller than the quasilinear or electrostatic fluxes by the ratio of ion to electron current contributions in Ampére's Law, and therefore resembles an ambipolar-constrained transport flux.…”

“…In prior calculations it was also found that for electron thermal transport, the flux of perpendicular energy breaks into ambipolar-and nonambipolar-constrained components. 16 The former dominates when the magnetic fluctuations responsible for transport are resonant at distant rational surfaces. This is the situation in the MST edge, where the fluctuations driving electron thermal transport are the coreresonant unstable tearing modes.…”

“…The measured flux reflects an ambipolar constraint because it can be expressed as a Rechester-Rosenbluth diffusivity, but with the ion thermal velocity in place of the electron thermal velocity. 16 The flux is also convective, despite a robust temperature gradient. Near the rational surfaces of transport-causing fluctuations, located in the core region in the MST, the nonambipolarconstrained component dominates.…”

“…Subsequent to the predictions of Ref. 16, core transport in MST was inferred through profile measurements and power balance analysis. The thermal diffusivity increases by a factor of 40 in going from the edge to the core.…”