MHD modelling of density limit disruptions in tokamaks

Abstract

The magnetohydrodynamic (MHD) activity during density limit disruptions in tokamaks is modelled numerically by three-dimensional resistive reduced MHD simulations with a simple transport model including radiation losses. The simulations reproduce experimentally observed phenomena such as the destabilization of MHD modes near the plasma edge during the early profile contraction phase, followed by growth of the m = 2/n = 1 mode to large amplitude, a sequence of minor disruptions and the major disruption. A new theoretical model is given for the major disruption, which takes place in two phases: (1) an internal relaxation flattens the temperature in the central part of the discharge and (2) the current profile broadens. The internal instability of the first phase has a mainly m = 1/n = 1 convection pattern, but, because of non-linear coupling to the large m = 2/n = 1 mode, the magnetic perturbation has a strong m = 3/n = 2 component. During the internal relaxation, the large amplitude 2/1, 1/1 and 3/2 perturbations break up the magnetic surfaces isolating the q = 1 region from the stochastic region around q = 2, and the magnetic field becomes stochastic in the entire q <, 2 region. In the second phase of the major disruption, MHD turbulence first develops on the stochasticized fields, resulting in current filamentation, initially in the central region where q ^ 2. This leads to a broadening of the central current profile and a strong instability of the 2/1 mode. The disruption ends with rapid growth of the m > 2/n = 1 modes. The result is stochastic magnetic fields across the entire plasma and a large scale broadening of the current profile.

Quasicoherent Oscillations Induced by Nonthermal Electrons during Magnetic Reconnection in the T-10 Tokamak

“…While a variety of phenomenologically similar small-scale perturbations were observed during the abrupt growth of MHD modes in previous tokamak experiments [see, for example, ballooning modes (n 10) [8], microtearing modes (m 7-20) [9], or high-harmonic secondary oscillations induced due to nonlinear mode coupling [10]], present experiments suggest a new type of instability possibly connected with nonthermal electron beams localized around the magnetic islands.…”

“…[9], or high-harmonic secondary oscillations induced due to nonlinear mode coupling [10]], present experiments suggest a new type of instability possibly connected with nonthermal electron beams localized around the magnetic islands.…”

“…2(f) accordingly]. This effect as well as the shocklike magnetic perturbations with no poloidal phase shifts contradicts a possible connection of the oscillations with high-m; n tearing modes [9,10]. Furthermore, weak dependence of the repetition rate f osc on plasma parameters (n e , B t ) can also indicate no connection of the oscillations with an instability of fast ions (e.g., TAE modes [14]).…”

High resolution temperature and density profiles during the energy quench of density limit disruptions in Rijnhuizen tokamak project

“…This seems to be at variance with the complete stochastization of this region at this advanced phase of the profile erosion, as expected by some authors. 12 The corresponding pressure profile, p e2 , is more symmetric. It shows a small gradient at z/aϭ0.45, which is due to a corresponding gradient in the density, and at the position of the qϭ2 surface the pressure is equal to 2kPa, both on the top and the bottom.…”

“…Also, except for electron temperature and density, local plasma parameters are difficult to measure, or are not measured at all. Theoretically, [12][13][14][15][16][17] the description of the disruption is still not unique. Different models address different parts of the disruption but there is no continuous description of its time evolution.…”