Invariance of Divertor Retention on External Particle Flow in Detached ASDEX Upgrade Discharges

Abstract

Divertor plasmas with strong external gas puffing in ASDEX Upgrade have shown very efficient impurity retention, increasing with the divertor neutral gas density. The experiments presented here use feedback-controlled gas puffs in discharges with different pumping speed to keep the divertor neutral gas flux density the same. This allows for the first time a decoupling of the divertor neutral gas flux density and the external gas flow. The resulting plasmas are almost identical and show identical impurity retention, clearly demonstrating the importance of the divertor neutral gas density over the externally induced flow. PACS numbers: 52.55.Fa, 52.40.Hf Strong external gas puffing into the edge of a fusion plasma strongly affects its properties. Generally, in Ohmically heated and L-mode plasmas, the line averaged density is increased, eventually leading to a density limit disruption. The energy and particle confinement times decrease with external gas puffing. More directly, however, gas puffing modifies the scrape-off layer and divertor plasma in a divertor tokamak, and increasing the neutral gas density in the divertor has been proposed for a long time as a means of increasing power losses via radiation or collisions with the neutral gas [1,2]. Experiments with strong puffing of D 2 in the DIII-D tokamak [3,4] and in JET [5] resulted in a power load reduction at the divertor target plates and in a detached plasma at the outer target plate [5]. In all these cases, however, the energy confinement time was degraded by the gas puffing.Strong gas puffing has also often been proposed as a means of inducing particle flows in the scrape-off layer. Such flows should entrain impurities by friction, thereby flushing them into the divertor [6]. Recently the flow of impurities in the scrape-off layer of DIII-D was investigated using the "puff and pump" technique, where Ar was puffed either at the top of the vacuum vessel or in the private flux region [7]. Additional puffing of deuterium reduced the Ar content in the plasma strongly, and this effect was attributed to the streaming of Ar with the D 2 into the divertor chamber. Similar experiments on ASDEX Upgrade have also shown a strong increase of the exhaust rate for Ne and Ar [8] and for He [9] with external D 2 gas puffing.These ASDEX Upgrade results were interpreted, however, rather as an effect of the associated increase of the neutral gas and plasma densities in the divertor, which would tend to shift the ionization zone for the recycling noble gas atoms closer to the target plates. In this region, the strong background plasma flow towards the target plates would increasingly dominate over the thermal force trying to push impurity ions out of the divertor chamber, back towards the bulk plasma. This explanation of the observed variation of divertor retention with neutral density is supported by earlier, 1D scrape-off layer modeling calculations for ASDEX [10], but requires obvious confirmation by the 2D modeling effort currently under way.Experiments carried out at f...

Plasma Edge Physics with B2‐Eirene

“…In ASDEX Upgrade no strong effect on impurity compression and overall divertor performance was seen by puff and pump experiments, neither in Div-I nor in Div-II [129,138], in agreement with B2-Eirene predictions and in contradiction to results of DIII-D [130].…”

“…The externally applied fluxes (gas puffs) compete with intrinsic recycling fluxes, which are usually several orders of magnitude larger. Consistent with this picture, it was proven experimentally [129] at ASDEX Upgrade that the internal recycling fluxes are responsible for the divertor retention of impurities, not the external flux. This was demonstrated by doing shots with identical divertor neutral densities, but different pumping speed (switching off turbo-pumps) and therefore different external flux.…”

Taming the plasma–material interface with the ‘snowflake’ divertor in NSTX

“…A comparison of the inferred electron pressure in the SP region p e = n e T e ∼ 25-80 Pa to the measured MP pressure p e ∼ 50-120 Pa confirmed a pressure decrease from midplane to target in the SFD. In a partially detached divertor SP region with T e 1-2 eV, a significant reduction of divertor physical and chemical sputtering rates, as well as an improved impurity entrainment in a hydrogenic plasma flow are expected [9,10,27,30,31]. While the physical mechanism is yet to be confirmed, this is consistent with the observed reduction in Z eff and impurity concentration in the SFD discharges.…”

Understanding the SOL flow in L-mode plasma on divertor tokamaks, and its influence on the plasma transport

“…Previous measurements showed that neither ion flux nor M k was increased during gas puffing into the main chamber during the divertor pumping, i.e 'puff and pump' (PP), in AUG [40] and JT-60U [41] although the impurity concentration in the main plasma was reduced. Measurements of the SOL flow at the HFS (above baffle) and LFS (midplane and X-point) SOLs during intense gas puffing at the plasma top (1-3 · 10 22 s À1 ) were recently performed in JT-60U L-mode with the normal B t [42], where only the HFS divertor plasma was partly detached.…”