The role of plasma-molecule interactions on power and particle balance during detachment on the TCV tokamak

A. Pshenov,F. Federici,B.P. Duval,D.S. Gahle,The Eurofusion Mst1 Team the EUROfusion MST1 Team,C. Bowman,M. Wensing,J.R. Harrison,The Tcv Team the TCV Team,K. Verhaegh,B. Lipschultz,A. Smolders,O. Myatra,D. Moulton,A. Kukushkin,C. Theiler,A. Fil,O. Février,A. Perek

doi:10.1088/1741-4326/ac1dc5

Abstract

This paper shows experimental results from the TCV tokamak that indicate plasma-molecule interactions involving and possibly D− play an important role as sinks of energy (through hydrogenic radiation as well as dissociation) and particles (ions) during divertor detachment if low target temperatures (<3 eV) are achieved. Both molecular activated recombination (MAR) and ion source reduction due to a power limitation effect are shown to be important in reducing the ion target flux during a density ramp. In contrast, the electron–ion recombination (EIR) ion sink is too small to play an important role in reducing the ion target flux. MAR or EIR do not occur during N2 seeding induced detachment as the target temperatures are not sufficiently low. The impact of is shown to be underestimated in present (vibrationally unresolved) SOLPS-ITER simulations, which could result from an underestimated rate. The converged SOLPS-ITER simulations are post-processed with alternative reaction rates, resulting in considerable contributions of to particle and power losses as well as dissociation below the D2 dissociation area. Those findings are in quantitative agreement with the experimental results.

Highlights

Power exhaust is a major challenge for future fusion devices [1, 2]
This paper shows experimental results from the TCV tokamak that indicate plasma-molecule interactions involving D+2 and possibly D− play an important role as sinks of energy and particles during divertor detachment if low target temperatures (< 3 eV) are achieved
Analysis of experiment and modelling of TCV tokamak discharges presented show that plasma-molecule interactions can result in additional significant ion sources/sinks and power losses compared to just atomic processes and can have a strong role during divertor detachment

Summary

Introduction

Power exhaust is a major challenge for future fusion devices [1, 2]. The material limits for the (plasma) heat flux to the target are in the order of 5–20 MW m−2, depending on how often the divertor should be replaced and what the expected non-plasma (e.g. neutrals and radiation) heat load is, which would be exceeded by an order of magnitude in a fusion reactor without mitigation [1,2,3,4,5]. Divertor heat flux loading (qt) as well as target temperature (Tt) can be tempered by increasing the density and/or inducing radiative losses through seeding extrinsic impurities, which will be required on reactors [1,2,3]. Minimising equation (1) with respect to Tt and comparing to a typical sheath-limited Tt of 100 eV, we see that the total (plasma) target heat flux load may only be reduced by a factor 4–5 without target pressure losses qt = ΓtγTt + Γt qkt in qpt ot Γt ∝ pt/Tt1/2 (1).

Results

Discussion

Conclusion