SOLPS-ITER predictive simulations of the impact of ion-molecule elastic collisions on strongly detached MAST-U Super-X divertor conditions

Abstract

The role of ion-molecule ( D + − D 2 ) elastic collisions in strongly detached divertor conditions has been studied in the MAST-U Super-X configuration using SOLPS-ITER. Two strongly detached steady state solutions were compared, one obtained through a main-ion fuelling scan and the other through a nitrogen seeding scan at fixed fuelling rate. A significant difference in the electron–ion recombination (EIR) levels was observed; significant EIR in strongly detached conditions in the fuelling scan and negligible EIR throughout the seeding scan. This is partly because the fuelling scan achieves electron temperatures ( T e ) as low as 0.2 eV near the divertor target, compared to 0.8 eV in the seeding scan (EIR increases strongly below T e ≈ 1 eV), and partly due to higher divertor plasma densities achieved in fuelling scan. Features of the strongly detached seeded cases, i.e. higher temperatures and negligible EIR, are recovered in the fuelling scan by turning off D + − D 2 elastic collisions. Analysis suggests that dissipation mechanisms like line radiation and charge exchange (important for detachment initiation) become weak when T e falls below 1 eV, and that D + − D 2 elastic collisions are necessary for further heat dissipation and access to strongly recombining conditions in the fuelling scan. In the seeding scan, heat dissipation through D + − D 2 elastic collisions is weak. This could be because our nitrogen seeding simulations do not include interactions between nitrogen ions and neutrals, and the strongly detached cases contain high levels of N + in the divertor. As a result, the N + acts like a reservoir of energy and momentum which appears to weaken the impact of D + − D 2 elastic collisions on the divertor plasma energy and momentum balance, making it more difficult to access recombining conditions. This suggests that some of the differences between seeding and fuelling scans could be because energy and momentum exchange between impurities and neutrals is not sufficiently captured in our simulations.

The role of plasma–atom and molecule interactions on power & particle balance during detachment on the MAST Upgrade Super-X divertor

“…Ultimately, the ion flow into the divertor chamber becomes significant, increasing the flow velocity upstream, which decelerates towards the target due to the large ion sink (detachment phase III-figures 13(c) and (d)). Such ion flow profiles are qualitatively consistent with those obtained from MAST-U SOLPS-ITER modelling [33,34].…”

“…The numbers shown indicate the detachment of the following regions from the target: (1) the back-end of the ionisation region; (2) the back-end of the Molecular Activated Recombination (MAR) region; (3) the front-end of the Electron Ion Recombination (EIR) region; and (4) the back-end of the electron ion recombination / density region. The magnetic geometry in this illustration has been obtained from a SOLPS-ITER simulation (from[33,34]). Reproduced from[10].…”

Numerical study of divertor detachment in TCV H-mode scenarios

“…When target temperature T e,t falls from 3 eV to 1 eV, MAR is found to account for up to half of the reduction of target ion particle flux on TCV during density ramps [40]. When T e,t falls below 1 eV, the ion-molecule (D + − D 2 ) elastic collisions are necessary for further heat dissipation and access to strongly recombining conditions in the fuelling scan [41]. The mesh used in the simulation is shown in figure 2.…”

Initial Fulcher band observations from high resolution spectroscopy in the MAST-U divertor

“…Collisions between the plasma and the molecules result in energy and momentum transfer from the plasma to the molecules, which can be a critical ingredient to detachment according to SOLPS-ITER simulations [8][9][10]34]. Assuming that the rotational temperature is a proxy for the gas temperature of the molecules and thus a reasonable indicator for kinetic energy (see appendix), diagnosing the rotational distribution and inferring the rotational temperature can be utilised to observe signatures of energy transfer and may be used to diagnose such transfers as well as verify simulated predictions experimentally.…”

“…from the plasma to the neutral molecular cloud, resulting in power and momentum dissipation especially at the 'killer' flux tubes [8][9][10] (figure 1(a)). Vibrationally excited molecules can undergo plasma-chemistry reactions with the plasma, resulting in molecular ions such as D + 2 and, potentially, D − 2 → D − + D (figure 1(b)).…”