H mode discharges with feedback controlled radiative boundary in the ASDEX Upgrade tokamak

Abstract

Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to feedback control the total radiated power fraction and the divertor neutral particle density simultaneously in the ASDEX Upgrade tokamak. The variation of Psep=Pheat-Prad(core) by impurity radiation during H mode shows a similar effect on the ELM behaviour as that obtained by a change of the heating power. For radiated power fractions above 90%, the ELM amplitude becomes very small and detachment from the divertor plates occurs, whilst no degradation of the global energy confinement is observed (completely detached high confinement mode). Additional deuterium gas puffing is found to increase the radiated power per impurity ion in the plasma core owing to the combined effect of a higher particle recycling rate and a lower core penetration probability. The outer divertor chamber, which is closed for deuterium neutrals, builds up a high neutral pressure, the magnitude of which is determined by the balance of particle sources and pumping. For this particular situation, the effective pumping time of neon and argon is considerably reduced, to less than 0.3 s, mainly owing to an improved divertor retention capability. The radiation characteristics of discharges with a neon driven radiative mantle are modelled using a 1-D radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment

Overview on plasma operation with a full tungsten wall in ASDEX Upgrade

“…As it turned out, puffing N 2 is a very good substitute for intrinsic carbon in the full W AUG (see also below), differently to earlier experience in AUG, where the noble gases, Ar and Ne, were preferred for radiative scenarios [30] because there were easier to control in the presence of C PFCs. Since nitrogen shows some chemistry with W, laboratory investigations were performed which revealed the production of WN in the implantation region which saturates and is released at temperatures below 800 K [31].…”

A review of ELMs in divertor tokamaks

“…Fig. 17 from Kallenbach, et al [68] shows how the ELM behavior and confinement varied as the power flow across the separatrix was reduced from well above to below the H-mode threshold level. The Type-3 ELMS were so small and so rapid in the CDH mode discharge (Completely Detached H-mode) that no measurable heat pulse is observed on the divertor targets and the carbon sputtering is reduced significantly.…”

“…Plasma shaping has also been used to control ELMs [6, 191; however, many of these experiments were carried out in low power discharges where the ELMs were rather small. In higher power discharges, ASDEX-U has managed to use edge radiation to lower the power flow across the separatrix [20,21] so as to operate near the H-mode power threshold and obtain small amplitude, high frequency Type-3 ELMs, instead of Type-1 ELMs. It remains, however, to show that small amplitude ELMs can be produced when operating well above the H-mode power threshold.…”

Plasma surface interactions in impurity seeded plasmas

“…Originally, a major focus was the improvement of energy confinement, which was often observed in radiative regimes in combination with electron density profile peaking [2], [3], [4]. However, this topic calmed down after these improved confinement regimes could not be fully reproduced in larger devices under low collisionality conditions [5] [6] [7].…”