H. J. Sun

Study of near scrape-off layer (SOL) temperature and density gradient lengths with Thomson scattering

Improvements to the Thomson scattering diagnostic have enabled the study of near scrape-off layer (SOL) decay lengths in the 2014 ASDEX Upgrade experimental campaign. A database of H-mode discharges has been studied using a two-line fit method for the core and log-linear fit for the near SOL region under both attached and detached divertor conditions. SOL electron temperature profiles have been found to have a radial exponential decay distribution which does not vary poloidally, consistent with the two-point model. In attached H-mode regimes, a log-linear regression shows that the SOL upstream dataset has the same main parametric dependencies as the scaling inferred from downstream Infrared camera measurements. A simple collisional relation from two-point model is found to best relate the upstream decay lengths and downstream divertor power widths. The SOL gradient length appears to be independent of pedestal parameters, but may correlate with the pedestal electron pressure parameters. Both the pedestal and SOL density and temperature scale lengths are linearly correlated with an almost constant gradient ratio, . The smaller gradient ratio and the fact that the Spitzer–Harm model is more valid, agrees with the studied plasma lying in the collisional regime. A transition to flat SOL ne profiles, previously reported for L-mode plasmas in many machines, has been observed in AUG detatched H-mode regimes. When the flattening of density profile happens in H-mode detached plasmas, the broadening of near SOL decay length also appears which may be good news for future machines.

Change of the scrape-off layer power width with the toroidal B-field direction in ASDEX upgrade

The change of the scrape-off layer power width in dependence of the toroidal magnetic field direction is investigated in L-mode discharges in the upper single null (USN) configuration in ASDEX Upgrade. The heat flux onto the outer and inner divertor plates is measured using a fast 2D infrared camera. The heat flux distribution is described by an exponential power fall-off length and a diffusive broadening in the divertor region . In this paper the parameters, and , for the inner and outer divertor target are compared for both toroidal magnetic field directions. For the divertor broadening no dependence on the toroidal magnetic field direction is observed. The comparison between the near scrape-off layer electron temperature fall-off length and the power fall-off length are in agreement with the 2-point model. It is concluded that electron conduction is the main contribution for the scrape-off layer parallel transport in these discharges. The ratio between inner, , and outer, , power fall-off length is dependent on the toroidal magnetic field direction. The numerical values are for favourable ion drift direction and / for non-favourable drift direction. The different ratios are explained by vertical drifts, which are dependent on the toroidal magnetic field direction.

Parameter dependences of the separatrix density in nitrogen seeded ASDEX Upgrade H-mode discharges

The upstream separatrix electron density is an important interface parameter for core performance and divertor power exhaust. It has been measured in ASDEX Upgrade H-mode discharges by means of Thomson scattering using a self-consistent estimate of the upstream electron temperature under the assumption of Spitzer-Härm electron conduction. Its dependence on various plasma parameters has been tested for different plasma conditions in H-mode. The leading parameter determining ne,sep was found to be the neutral divertor pressure, which can be considered as an engineering parameter since it is determined mainly by the gas puff rate and the pumping speed. The experimentally found parameter dependence of ne,sep, which is dominated by the divertor neutral pressure, could be approximately reconciled by 2-point modelling.