Impurity enrichment studies with induced scrape-off layer flow on DIII-D

Abstract

A series of controlled experiments has been carried out in DIII-D to induce a bulk ion flow in the SOL and evaluate its effect on the localization of impurities in the divertor. This induced SOL flow was created by simultaneous deuterium puffing and divertor exhaust using a divertor cryopump, and the impurity enrichment was measured directly. The experiments were designed to compare enrichment in discharges with and without induced flow having otherwise similiar divertor parameters. Significant increases in impurity compression and enrichment are observed when flow is induced, and the degree of impurity enrichment in the divertor is found to be dependent on the impurity of interest. Detailed particle measurements made possible by the direct measurement of impurity densities in several reservoirs indicate reasonable particle balance for helium throughout the duration of the discharge. Conversely, while the total input of neon is balanced by the total exhaust by the end of a discharge, particle balance is not observed during the course of the discharge. A significant wall inventory with a short release time (∼10 ms) is surmised.

Chapter 4: Power and particle control

“…Better closure of divertor mechanical leaks enhances compression of deuterium, helium [232,240] and argon [241] but appears to have no effect on helium or argon enrichment. Induced scrapeoff layer flow by the 'puff and pump' technique did not significantly alter the enrichment of helium [242,212,240] but enhanced the neon and argon enrichment in DIII-D [242] and JT-60U [243], although not in ASDEX-U [212] or JET [240]. The reasons for this discrepancy…”

“…Neon and argon, are ionised much closer to the divertor target and, therefore, experience the frictional drag force as ions, driving them back toward the target. As a consequence, the enrichment (and compression) increases with increasing noble gas mass and it is usually larger than unity for the heavier species in all divertor conditions [242,244,240]. The enrichment of neon and argon increases with increasing density up to detachment [242,244,240] after which it drops.…”

“…As a consequence, the enrichment (and compression) increases with increasing noble gas mass and it is usually larger than unity for the heavier species in all divertor conditions [242,244,240]. The enrichment of neon and argon increases with increasing density up to detachment [242,244,240] after which it drops. However, such an effect is less likely to occur in next step devices because the ratio of λ IONIS to divertor size will be much smaller than in present experiments for all regimes (including detached divertor plasmas).…”

Compatibility of the radiating divertor with high performance plasmas in DIII-D

“…For this "radiating divertor" concept to be practical, however, the confinement and stability of the plasma cannot be compromised by excessive leakage of the seeded impurities into the core plasma. One proposed way of reducing impurity influx is to enhance the directed scrape-off layer (SOL) flow of deuterium ions toward the divertor [1][2][3][4][5].…”

Pedestal Characteristics of H-Mode Plasmas in JT-60U and ASDEX Upgrade

“…ELMs allow sustainment of steadystate phases with exhaust of impurities such as helium ash [5][6][7][8]. ELMy H-mode experiments performed in many divertor tokamaks have also demonstrated favorable energy confinement.…”