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

Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to simultaneously feedback-control the total radiated power fraction and the divertor neutral particle density in the ASDEX Upgrade tokamak. The variation of P_s_e_p=P_h_e_a_t-P_r_a_d(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 due 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 accelerated to less than 0.3 s mainly due to an improved divertor retention capability. The radiation characteristic of discharges with a neon-driven radiative mantle are modelled using a 1-d radial impurity transport code which has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment. (orig.)Available from TIB Hannover: RA 71(1/284) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman