Pellet injection with improved confinement in ASDEX

Abstract

Tokamak discharges with repetitive pellet fuelling were investigated in the ASDEX divertor device. The importance of sufficiently high divertor recycling for a high density at the separatrix and for successful density buildup in the bulk plasma was demonstrated. In contrast to low recycling discharges where no permanent improvement of the energy confinement time was achieved, in OH-heated discharges with high recycling an energy confinement time of 160 ms was reached, the normal value being 80 ms in the rollover region. The peaked density profiles in this case were accompanied by reduced or suppressed sawtooth activity and finally ended in a phase of strong central impurity accumulation. The particle transport was characterized by strong, non-classical inward drift, while the improved energy transport can be explained by the following alternatives: (la) a local model which assumes neo-Alcator χe for the electrons and χi= 3χneocl for the ions in the gas puff cases, reducing to χi= χneocl for the optimum pellet cases; (1b) the assumption χi= χneocl under all conditions and an electron energy confinement worse than neo-Alcator in the rollover region in gas-puff-discharges; (2) a profile consistency picture where Te(a) determines the energy confinement. Low power, NI heated discharges with pellet fuelling behave like Ohmic discharges, while for high power in the L-mode no successful density buildup was possible, and τE was not improved. The H-regime was extended from a density maximum n̄e = 0.8 × 1020 m-3 without pellets to n̄e = 1.2 × 1020 m-3 by the injection of pellets. In this case a density buildup takes place, but further density profile peaking could not be observed.

Observation of Impurity Accumulation After Hydrogen Multi-Pellet Injection in Large Helical Device

“…A possible result is finally obtained at D=0.2 m 2 /s and V =−1 m/s, of which the values indicate the same ones as commonly used in the LHD [15] . This result reveals that the accumulation of light impurities (such as carbon) does not occur in the LHD too, similar to the tokamak result [20] . Finally, the uncertainties of the impurity density and Z eff values in the present analysis are estimated.…”

Core impurity transport in Alcator C-Mod L-, I- and H-mode plasmas

“…Understanding impurity transport is important in its own right, since impurity accumulation can lead to ion dilution and radiative collapse. In the enhanced confinement operational regimes (H-mode and ITB plasmas) envisioned for future devices, long impurity confinement times are widely seen, with impurity transport approaching neo-classical levels in the barrier region [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. Since the neo-classical impurity density profile peaking factor increases strongly with atomic charge, H-mode and ITB plasmas in devices with tungsten (Z = 74) walls will potentially have a serious problem with tungsten accumulation and subsequent radiation.…”

Neoclassical analysis of impurity transport following transition to improved particle confinement

“…However, discharges in the Alcator C tokamak with frozen fuel pellets injected into the plasma showed enhanced energy confinement [1] and strongly peaked, nearly neoclassical impurity density profiles [2]. More recent experiments on ASDEX [3][4][5], TEXT [6][7][8], JET [9] and TFTR [10] also demonstrated impurity peaking on axis following the injection of frozen hydrogen pellets or a transition to other improved confinement regimes. Similarly, Z-dependent impurity accumulation, in general agreement with estimates of neoclassical transport, has been measured during neutral beam heated H-mode discharges in PBX [11,12].…”