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(EX3/6) Operational Limits of High Density H-Modes in ASDEX Upgrade

   
V. Mertens , K. Borrass , M. Kaufmann , P. T. Lang , R. Lang , H. W. Müller , J. Neuhauser , R. Schneider , J. Schweinzer , W. Suttrop , ASDEX Upgrade Team 
 
Max-Planck-Institut für Plasmaphysik, EURATOM-IPP Association, Garching und Berlin, Germany

Abstract
Systematic investigations of H-mode  density limit (H$\to$L-mode back transition) plasmas with gas fuelling and alternatively with additional pellet injection  from the magnetic high-field-side HFS are being performed in the new closed divertor configuration DV-II. The resulting database covering a wide range of the externally controllable plasma parameters $I_p$, $B_t$ and $P_{heat}$ confirms that the H-mode threshold power exceeds the generally accepted prediction $P^{L\to H}_{heat} \propto \overline B_t$ dramatically when one approaches Greenwald densities. Additionally, in contrast to the Greenwald scaling a moderate $B_t$-dependence of the H-mode density limit is found. The limit is observed to coincide with divertor detachment  and a strong increase of the edge thermal transport, which has, however, no detrimental effect on global $\tau_E$.

The pellet injection scheme from the magnetic high-field-side HFS, developed recently on ASDEX Upgrade , leads to fast particle drifts which are, contrary to the standard injection from the low-field-side, directed into the plasma core. This improves markedly the pellet particle fuelling effciency. The responsible physical mechanism, the diamagnetic particle drift of the pellet ablatant was successfully verified recently. Other increased particle losses on respectively different time scales after the ablation process, however, still persist. Generally, a clear gain in achievable density and plasma stored energy is achieved with stationary HFS pellet injection compared to gas-puffing .

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