Influence of pedestal radiation on the H–L transition using krypton seeded discharges at ASDEX Upgrade

Abstract

Krypton can be a tool to refine and analyse the H–L power threshold in detail, as a proxy for L–H threshold, taking into account radiative losses in the region where the H–L transition takes place and, thus, decouple the power flux over the pedestal top to the one over the separatrix. The commonly used ITPA 2008 L–H threshold scaling does not account for radiation but it is often assumed that it should be compared to the net power crossing the separatrix. Yet, dedicated analyses are still lacking. At ASDEX Upgrade, krypton radiates dominantly at the very edge of the confined plasma (ρ pol between 0.97 and 1.01). Recent results show that L–H and H–L transitions are initiated close to ρ pol = 0.98. As such, the bulk of krypton radiation is between the separatrix and a potential source location for the transition. Scans in krypton seeding level and heating power have been performed to study and compare the influence of the powers radiated from inside the separatrix and the pedestal top on the plasma confinement state. The heating power is compared to the usual L–H power scaling but corrected by either of these two radiations for seeded H–L transitions. The power scaling is shown to remain valid for higher input and radiated powers, up to 3–5 times the non-seeded L–H power threshold of AUG, when the radiation inside the separatrix is subtracted from the input power. Additionally, kinetic and radiation profiles are used to run TRANSP analyses to compare the edge ion heat to past studies from unseeded discharges. Using this physical parameter gives a reference point from a very different parameter range of input and radiated powers, densities and back, instead of forward, transition.