Propagation of cold pulses and heat pulses in ASDEX Upgrade

Abstract

Experiments on electron heat transport were performed in the tokamak ASDEX Upgrade, mainly in ohmically heated plasmas, applying either edge cooling by impurity injection or edge heat pulses with ECH. Repetitive pulses within one plasma discharge were made allowing Fourier transformation of the temperature perturbation. This yields a good signal to noise ratio up to high harmonics and allows a detailed investigation of the pulse propagation. For densities lower than 1:8 10 19 m ,3 , an increase of the central electron temperature was found as the response to the edge cooling via impurity injection similar to observations made in other tokamaks. The inversion does not appear instantaneously, but with a time delay roughly compatible with diffusion. Modeling of the propagation of the cold pulses in the framework of the IFS-PPPL model yields qualitative agreement. However the predicted increase of the ion temperature is not observed experimentally on the fast time scale. The response to ECH heat pulses is not perfectly symmetrical to cold pulse experiments, but the similarities suggest a common underlying physical mechanism. No inversion of the heat pulse is found, instead the initial pulse from the edge is associated with a second, much slower heat pulse in the centre which is similar (and not symmetrical) to that of the cold pulses. It is found that the central increase is related to the arrival of the pulse close to the inversion radius and not to the initial pulse.

Non-local heat transport, rotation reversals and up/down impurity density asymmetries in Alcator C-Mod ohmic L-mode plasmas

“…Following the edge cooling, there was a rapid increase of the core electron temperature, on a time scale (∼5 ms) faster than the diffusive time. This effect is suggestive of a short-lived internal transport barrier (ITB), triggered by the sudden increase of the edge temperature gradient, which can be modeled with an abrupt drop in the core electron thermal conductivity [19,20,21,23,24,25,27], and which persists for the duration of the edge cooling (∼30 ms). A model for this transient increase in electron heat transport (a drop in thermal conductivity) is shown in Fig.3, with a 25% drop in χ e near r/a = 0.3 (solid to dash-dot lines).…”

Neoclassical physics in full distribution function gyrokinetics

“…and the Chandrasekhar function G GðvÞ ¼ UðvÞ À vU 0 ðvÞ 2v 2 (16) Relaxing the constraints Eqs. (4) and (5) practically means when running a gyrokinetic code that V k and T need not to be calculated at each time step and updated back into the collision scheme.…”

“…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).…”

Observation of core electron temperature rise in response to an edge cooling in toroidal helical plasmas

“…In order to promote a better understanding of the electron heat transport, the electron heat transport analysis for both transient and steady state has been carried out diligently in many tokamaks 1-3 and helical systems. [4][5][6] One of the significant issues found in these studies is a "nonlocal transport phenomenon" observed in perturbation experiments on many tokamaks [7][8][9][10][11][12] and a few helical systems. 4 In particular, a rise of the core electron temperature T e invoked by the rapid cooling of the edge plasma has been observed in various tokamaks with both ohmically heated plasmas and plasmas with an auxiliary heating, such as electron cyclotron heating ͑ECH͒, at a sufficiently low density ͑e.g., Ref.…”