Ballistic transport phenomena in TJ-II

Abstract

Perturbative transport experiments have been performed at the stellarator TJ-II. Both the inward propagation of edge cooling pulses induced by the injection of nitrogen, and the outward propagation of heat pulses due to spontaneous spikes of the central temperature have been analysed. It has been found that the observed propagation is incompatible with diffusive transport models. Simultaneous inward and outward propagation eliminates an explanation in terms of a pinch. A numerical simulation with a resistive interchange turbulence model suggests that the observed propagation is related to the successive destabilizations of pressure gradient driven modes associated with rational surfaces.

Neoclassical physics in full distribution function gyrokinetics

“…This physics is an essential part of neoclassical theory and is intrinsically embedded in all model operators described here in Eqs. (3), (9), (17), or (18) which allow to exactly recover the neoclassical viscous stress tensor in the banana and plateau regimes-see e.g., details in Ref. 21.…”

“…These facts have been emphasised both theoretically: through the description of avalanching and spreading, [3][4][5][6][7][8][9] through the characterisation of nonlocal, nondiffusive behaviour 1,[10][11][12] and experimentally through some yet-to-be-understood experimental jigsaws: deep inconsistencies with a (fixed gradient) local and diffusive modeling have indeed been reported in perturbative (either hot or cold pulse) experiments, [13][14][15][16][17] off-axis heating experiments, 19,20 or whilst reporting Bohm-like scalings of the energy confinement time. 18 An accurate description of such dynamics requires the simultaneous and self-consistent treatment of the full gyrokinetic distribution function (full-f modeling), in full-torus (global) tokamak geometry and for a prescribed distribution of sources and sinks (flux-driven description).…”

Transport studies using laser blow-off injection of low-Z trace impurities injected into the TJ-II stellarator

“…The study of cold pulse propagation, using either laser blow-off or pellet injection, has shown that electron energy transport has a localized character in the W7-AS stellarator [6] and the RFP reversed-pinch [7], which is in contrast to the LHD stellarator [8] and to tokamaks such as TEXT [9] and JET [10] which exhibit non-local behaviours. In the TJ-II cold pulse propagation was previously studied using nitrogen puffing [11]. Here, a method is described to determine the delay and decay times of the perturbation detected by an Electron Cyclotron Emission (ECE) radiometer [12] for a range of plasma densities.…”

Alfvénic Propagation: A Key to Nonlocal Effects in Magnetized Plasmas

“…(II) The sign of the perturbation reverses, i.e., a cooling at the edge results in heating of the core, and vice versa. Such evidence has accumulated through dedicated experiments, including edge cooling by laser blowoff (LBO) of impurities, pellets, radio frequency (RF) heating, and plasma current ramping, performed in several devices, including tokamaks and stellarators [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The ubiquity of these phenomena calls naturally for some common fundamental physics basis.…”