Density limits in helium plasmas at JET

Abstract

Recently at JET a pure helium campaign has been performed. The exploration of the density limit in L-mode limiter as well as L-mode and H-mode diverted plasmas was one of the main objectives. In L-mode plasmas the density can be increased until the total radiated power equals the heating power, hence the density limit is determined by a radiative collapse. When compared to deuterium plasmas the L-mode density limit in helium is twice as high. In H-mode no significant difference in the density limit process has been observed. The pedestal density just before the H to L-mode back transition is the same in deuterium and helium plasmas. The H to L-mode back transition is followed by a development of a MARFE [J. Nucl. Mater. 145-147 (1987) 15] which eventually leads to a density limit disruption.

The role of plasma–wall interactions in thermal instabilities at the tokamak edge

“…15 Finally, we demonstrate that the model proposed here for MARFE threshold explains also recent findings in JET. 14 The HFS ''wall'' MARFE determined the density limit in L-mode deuterium JET plasmas, however, in helium discharges the ''wall'' MARFE did not occur and much higher plasma densities were achieved. Remarkably, this happened even though the impurity concentration in helium plasmas is slightly higher compared to that in deuterium plasmas of the same density due to a higher physical sputtering of carbon limiters by helium ions.…”

“…11 Recently, similar results have been obtained on JET. 14 By increasing the plasma wall clearance, the outflow of charged particles and energy to the inner wall is reduced and, consequently, the influx of recycling neutrals of the working gas and eroded impurities into the plasma is decreased. The properties of the recycling particles, e.g., their ionization rate, control their penetration into the plasma and thus should be taken into account in a MARFE analysis.…”

“…This could, in particular, explain observations on JET that no wall MARFE occurred in helium plasmas and that a much higher density limit was achieved compared to deuterium plasmas. 14 The influx of impurities is determined both by the plasma outflow to the wall and by the erosion rate of the wall material. Therefore, the erosion mechanisms should be also included in a realistic model of the MARFE.…”

Origin of Tokamak Density Limit Scalings

“…3) the ability to exceed the density limit by profile peaking, which would not affect the density at the 2/1 surface [21] 4) the doubling of the density limit in helium observed in Reference [22] likely a direct result of the collisionality scaling of Equation 3 assuming fixed impurity densities…”

Influence of the Dynamic Ergodic Divertor on the Density Limit in TEXTOR

“…Over the years, results from both experimental observations and theoretical models show that the interaction between plasma, wall, and recycling neutrals of the working gas plays an important role in the thermal instabilities leading to MARFE formation [3][4][5][6]. In the tokamak TEXTOR, the density limit has been significantly extended with fresh boronization or siliconization [3] and exceeded by a factor of 1.7 the Greenwald-density limit, n GW e m ÿ3 1 10 17 kA ÿ1 m ÿ1 I p =a 2 , with the total plasma current I p and minor radius a measured in kA and m, respectively [7].…”