Physics of the density limit in the W7-AS stellarator

Abstract

Density-limit discharges in the W7-AS stellarator, with constant line-integrated density and a duration of up to 2 s, were studied at three values of the toroidal magnetic field (B = 0.8, 1.25 and 2.5 T). The central factor governing the physics of the density limit in stellarators was demonstrated to be the decreasing net power to the plasma when the centrally peaked radiated power density profile exceeds that of the deposited power density. The process was further accelerated by the peaking of electron density under these conditions. In discharges with B = 2.5 T, simulations of the centrally peaked radiation power density profiles could be shown to be due to peaked impurity density profiles. Laser blow off measurements clearly inferred an inward pinch of the injected aluminium. These discharges had the electron density profile form found in the improved confinement H-NBI mode on W7-AS.The aim of producing steady-state discharges at the highest possible density in stellarators is naturally of special interest for reactor operation. Such a scenario has been best achieved in H-mode discharges, in which ELMs restricted the impurity influx to the plasma and an equilibrium in the plasma parameters with suitably low radiation power levels was possible. A density scan in ECRH discharges highlights the need to control impurity sources and choose electron densities well below the density limit in order that steady-state operation can be attempted in discharges without ELMs.A simple model of bulk radiation predicted that the limiting density should depend on the square root of heating power and this was experimentally confirmed. The magnetic field scaling of the limiting density found experimentally in this simple model will partly depend on the term concerning the radial profile of the impurity density, which in turn is a function of the diffusion coefficient and inward pinch of the impurity ions. Theoretical studies have shown that an assumption about the B dependence of the thermal conductivity leads to density limit scaling laws with an explicit B dependence.

Multifaceted asymmetric radiation from the edge-like asymmetric radiative collapse of density limited plasmas in the Large Helical Device

“…3 These scaling laws are in basic agreement with theoretical models. 17,21 In order to compare our experimental data with the scaling laws for other helical machines, a plot of n e as a function of the volume-averaged absorbed power and magnetic field, ( P abs B/V) 0.5 , is shown in Fig.…”

“…[1][2][3] In helical devices, the radiation-induced density limit has particular importance, since it is a main cause leading to collapse of the plasma when the radiated power increased with density exceeds the deposited power. 3,4 This type of density limit is thought to be related to the onset of an edge thermal instability arising from the increase of impurity radiation with reduced temperature. [5][6][7] In tokamaks, radiative collapse may result in a cooling of the plasma boundary resulting in a contraction of the plasma.…”

“…[2][3][4][17][18][19][20][21][22] However, the experimental results of the radiation structure have rarely been reported in helical devices. In this Letter, we make the first report of asymmetric radiative collapse in the Large Helical Device ͑LHD͒ and relate asymmetric radiative collapse to the MARFE phenomenon commonly observed in tokamaks and to the density limit in LHD.…”

Density limits in toroidal plasmas

“…More recent data from W7-AS also show the collapse occurring when total radiation and input power are balanced [191]. In these cases, the plasma density and radiation profiles were peaked, with core radiation from higher Z elements predominating.…”

“…Typical thermal quench behavior can be seen in traces from the W7-AS device in figure 17. If gas puffing is reduced, it is possible for a stellarator to recover from the quench [8,191].…”