Conference paper

SOLPS5 modelling of ELMing H-mode on TCV

Although ohmic H-modes have long been produced on the TCV tokamak and the effects of ELMs at the divertor targets studied in some detail [1], no attempt has yet been made to model the scrape-off layer (SOL) in these plasmas. This contribution describes details of the first such efforts to do so. Simulations with the coupled fluid-Monte Carlo SOLPS5 code are constrained by careful upstream Thomson scattering and fast reciprocating Langmuir probe profiles and the results compared w ith measurements at the divertor targets. Recent experiments with high power ECRH at the third harmonic have produced large, possibly Type I ELMs on TCV for the first time, but in standard ohmic H-modes operating close to the L-H transition threshold power, only Type III ELMs are obtained. Typical single null lower H-mode discharges have I p = 400 kA, n/n GW   0.3 and steady ELMing phases with f ELM   100 Hz, where each ELM exhausts only a few 100 J of plasma stored energy. As such, these ELMs cannot be compared with larger events typical elsewhere regarding the magnitude of target power fluxes etc., but their behaviou r with respect to transport in the SOL and interaction with the targets is no different. In fact, their benign nature makes the fluid plasma simulation in some ways more appropriate since the ELMs are insu fficiently large to require a true kinetic simulation and are likely to be less perturbing in the sense of parallel heat flux limits and variations in sheath heat transmission coefficients. The modelling attempts described here broadly follow the approach in [2], seeking first the closest match to upstream experimental profiles during inter-ELM phases using a step-like ansatz for the perpendicular particle and heat diffusivities (D and χ i,e ) in the edge and SOL regions, but also introducing a poloidal variation of the transport coefficients both in the main chamber and dive rtor. This is extremely important in TCV, where the unconventional divertor geometry means that care must be taken in the presence of steep H-mode edge barriers to tailor differently the transport in this region compared with the core. Similar reasoning applies even more to the ELM itself, which is known to burst into the SOL in the outboard, unfavourable curvature region and is thus extremely poloidally localised. This has also been accounted for in the simulations which, as in the earlier SOLPS5 attempts to simulate Type ELMs in JET [3], m odel the ELM as an instantaneous local increase in the transport coefficients and simulate the subsequent SOL transport in a time dependent way. Good agreement is found between code and experime nt in the inter-ELM phase. Experimentally, these Type III ELMs are too rapid for comparison on an individual ELM basis. Instead, many similar events are coherently averaged and simulation comp ared with power and par ticle flux measurements at the divertor targets and line integrated observations of recycling in the divertor volume obtained using a new fast AXUV diode camera system filtered for Lyman-alpha emission.

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