Density limit investigations on ASDEX

Abstract

Density limit investigations on ASDEX have been performed under a variety of conditions: ohmically heated and neutral injection heated plasmas in H2, D2 and He have been studied in different divertor configurations, after various wall coating procedures, with gas puff and pellet fuelling, and in different confinement regimes with their characteristically different density profiles. A detailed description of the parametric dependence of the density limit, which in all cases is a disruptive limit, is given. This limit is shown to be a limit to the density at the plasma edge. Therefore, the highest densities corresponding to neRqa/Bt>30*1019 m-2.T-1 are obtained with centrally peaked ne profiles. Radiation from the main plasma at the density limit is always significantly below the total input power. The plasma disruption is due to an m=2 instability which for medium and high qa is preceded by one or more minor disruptions. In this range of qa, the disruptive instability is initiated by the occurrence of a Marfe on the high field side as a consequence of strong plasma cooling in this region. The duration of the Marfe increases with increasing distance between the plasma edge and the q=2 surface. After penetrating onto closed flux surfaces the Marfe leads to a current contraction and a subsequent destabilization of the m = 2 mode. In helium plasmas a strongly radiating, poloidally symmetric shell is observed before the density limit instead of a Marfe. An instantaneous destabilization of this mode is observed at low qa. Detailed measurements of plasma edge and divertor parameters close to the density limit indicate the development of a cold, dense divertor plasma before the disruption. Models describing the scrape-off layer and the divertor region predict an upper limit to the edge density at low divertor temperatures according to power balance considerations. Their relations to the experimental findings, especially the low field side cooling, ar

Theory of tokamak disruptions

“…Disruptions caused by violating the density limit on ASDEX have a delay of about 20 ms between the stopping of the rotation of magnetic perturbations-mode locking-and the current quench. 27 The time delay between mode locking and the current quench can vary over about two orders of magnitude on DIII-D, from roughly 10 ms to seconds. 28 The empirical difference between plasmas that exhibit a short versus a long delay is not known, but could provide useful guidance for theory.…”

Ultrahigh Densities and Volume Recombination inside the Separatrix of the Alcator C-Mod Tokamak

“…This radiation strongly affects the core by modifying fueling and impurity sources. It has been generally found that MARFEs typically occur at a fraction, 50 -90%, of the density limit [2,3]. The poloidal location of the MARFE at the high-field edge and its radial extent reaching to closed flux surfaces has been seen as evidence for poloidally asymmetric transport of power across flux surfaces in the core plasma [1,4].…”

“…The poloidal location of the MARFE at the high-field edge and its radial extent reaching to closed flux surfaces has been seen as evidence for poloidally asymmetric transport of power across flux surfaces in the core plasma [1,4]. All of the above characteristics have led to the study of MARFEs both experimentally [1][2][3][5][6][7][8][9] and theoretically [10][11][12][13][14][15][16][17][18]. The modelling of the MARFE as an impurity radiation-condensation instability is fairly well accepted.…”

Thermal Equilibrium and Density Limit in Tokamak-Reactor

“…Here I is the total toroidal current in plasma. As it is shown in experiments an auxiliary heating increases the value of n c by factor 1.5 and more [1,[6][7][8]. One may expect that the fusion power also can increase the critical plasma density.…”