Analysis of shot-to-shot variability in post-disruption runaway electron currents for diverted DIII-D discharges

Abstract

In DIII-D experiments, rapid termination by Ar pellet injection sometimes produces a posttermination runaway electron (RE) current plateau, but this effect is highly non-reproducible on a shot-to-shot basis, particularly for diverted target plasmas. A set of DIII-D discharges is analyzed with two MHD codes to understand the relationship between the current profile of the target plasma and the amplitude of the RE current plateau. Using the linear stability code GATO, a correlation between the radial profile of the unstable mode just after Ar pellet injection and the observed appearance of an RE plateau is identified. Nonlinear NIMROD simulations with RE test-particle calculations directly predict RE confinement times during the disruption. With one exception, NIMROD predicts better RE confinement for shots in which higher RE currents were observed in DIII-D. But, the variation in confinement is primarily connected to the saturated mode amplitude and not its radial profile. Still, both sets of analyses support the hypothesis that RE deconfinement by MHD fluctuations is a major factor in the shot-to-shot variability of RE plateaus, though additional factors such as seed current amplitude can not be ruled out.

Runaway electrons and ITER

“…Increasing the plasma density by impurity injection is likely to produce a large scale breakup of magnetic surfaces when the impurities reach the q = 2 surface. This is seen in simulations of massive gas and shattered pellet injection [5][6][7] into plasmas in which the current is carried by near-thermal electrons. Because the growth rate of tearing modes is determined by near-thermal electrons [44], little difference is expected in the conditions or speed of surface breakup depending on whether the current carriers are near-thermal or relativistic electrons though a relativistic-electron current evolves towards regions of low density rather high temperature.…”

“…The large edge safety factor q 10 e makes these experiments far more stable to tearing and kink instabilities than plasmas with a lower q e . Runaway electron currents in ITER may arise that have a much lower safety factor; the presence of a q = 2 surface appears important [5][6][7]. In any case, the density must be raised at the plasma center to rapidly dissipate the current density of relativistic electrons there, whether the density increase is due to direct injection, diffusion, or mixing.…”

“…Non-intercepting flux tubes are seen in numerical simulations [5][6][7][8] of massive gas and shattered pellet injection-particularly near the magnetic axis and sometimes in the cores of magnetic islands. Nonintercepting flux tubes are also seen in models of the effects of non-axisymmetric magnetic fields [17,18] and are found to provide excellent confinement of relativistic electrons [17,19].…”

“…where J is the Jacobian of canonical coordinates, equation (8) for the electric field gives an evolution equation for the poloidal flux averaged over a surface of constant toroidal flux, equation (6), which is¯∥…”

Structure of the runaway electron loss during induced disruptions in TEXTOR

“…6 In DIII-D, the runaway current depends critically on MHD fluctuations, particularly the radial profile of the n ¼ 1 mode. 7 Disruptions with a substantial runaway population show at first a rapid decay of the plasma current followed by a plateau like formation with a reduced decay rate. During the first strong decay phase, the loop voltage is enhanced acting as the driving force for the REs.…”