First demonstration of radiative power exhaust with impurity seeding in the island divertor at Wendelstein 7-X

Abstract

Radiative power exhaust by impurity seeding was demonstrated for the first time in island divertor configurations at the stellarator Wendelstein 7-X. Feasibility of stable plasma operation was shown during seeding with both neon (Ne) and nitrogen (N 2 ). High radiative power losses (≥80%) were found to reduce the divertor heat loads globally by 2/3 with both seeding gases injected at a single toroidal location into one of five magnetic islands. Heat flux detachment was achieved for the price of a loss of (≥-15%) in the stored energy. Ne seeding allows for sustained enhancement of edge radiation with a very slow decay of line emission of several tens of seconds after the end of the injection indicating a high recycling of this noble gas at the carbon main plasma facing components. In N 2 seeded discharges it is shown that a response of line emission and plasma parameters is in correlation to the puff duration which indicates a higher level of absorption of this seeding gas in the wall. Continuous N 2 seeding results in global cooling of the scrape-off layer (SOL) and decay of radiation over several seconds after the injection. Damping of counter-streaming SOL flows, and divertor particle fluxes induced by Ne and N 2 seeding have been measured and provide evidence for a reduction of the convective part of the divertor heat fluxes. Losses in density in response to seeding can be compensated by feedback controlled divertor fueling. The controlled reduction of heat fluxes within this complex 3D edge island geometry is a very promising finding concerning detachment control in a future all-metal divertor.

Heat and particle flux detachment with stable plasma conditions in the Wendelstein 7-X stellarator fusion experiment

“…Plasma radiation was a dominant mechanism for dissipation of energy. As the radiation characteristics (L z ) of carbon is similar as for nitrogen results obtained here should to a large extent be valid for detachment aided with nitrogen seeding [13]. The results presented in this work form a very promising outlook on the overall steady state compatibility of the detached island divertor concept in future experiments and a stellarator-based reactor.…”

Measurements of visible bremsstrahlung and automatic Bayesian inference of the effective plasma charge Zeff at W7-X

“…Z eff can be inferred by comparing predicted and measured bremsstrahlung emission signals. Two example cases from experiments 20180920.042 and 20181016.023 are depicted in figure 3a and 3b, showing discharges in which the plasma was seeded by Ne and N 2 , respectively [12]. Ne was injected at 5, 7, 9 and 11 s for 200 ms, whereas N 2 was injected at 2 s for 50 ms and continuously at 3-7 s; a second valve was open at 5-7 s, increasing the gas flow rate by a factor of ≈ 2.…”

First attempt to quantify W7-X island divertor plasma by local experiment-model comparison

“…Therefore, it is expected that a combination of the code and local diagnostics is able to provide a more complete and quantitative physical picture of the 3D island divertor plasma. In fact, the EMC3-Eirene code has been routinely applied to W7-X to interpret experimental results, covering a variety of physical topics [1,[5][6][7][8][9][10][11][12][13]. This work is less physicsoriented.…”