K. Steinmetz

Impurity Accumulation in Plasma Regimes with High Energy Confinement

Investigations of impurity accumulation phenomena in ASDEX are reviewed. There are four different operating regimes where pronounced accumulation is observed and these regimes are also characterized by improved energy confinement. In particular, medium-Z metallic ions are involved in accumulation processes whereas low-Z ions appear almost unaffected. The rapid accumulation observed in the case of metallic ions may be explained by neoclassical inward drifts if we assume that the anomalous diffusion is sufficiently suppressed, some indication of this being found from laser blow-off studies. The present results, however, can only be partly explained by neoclassical theory, according to which accumulation of low-Z impurities should also occur. The temporal behaviour of accumulation and the retarding effect of proton dilution for collision dominated transport are also discussed. Finally, we conclude that the full benefits of improved energy confinement can be achieved only if the impurity influxes are kept to a sufficiently low level. Expressed in terms of concentrations under low confinement conditions we have to postulate, for ASDEX, concentrations ≲ 10−4 for metals and ≲ 2% for all light impurities.

High power ICRF heating on the divertor tokamak ASDEX

First ICRF experiments on ASDEX have been performed at 67 MHz, corresponding to 2 Omega CH-heating of a hydrogen plasma at B0=2.2 T. Despite divertor operation ICRH is accompanied by a significant increase of impurity production which can drastically be reduced by means of wall carbonisation. RF power up to 2.3 MW is routinely coupled to the plasma for pulse lengths of up to 1 sec. The RF heating is found to depend strongly on plasma preheating. In combination with neutral beam injection the ICRF heating efficiency is even higher than the one of NI. Confinement degrades with ICRH to values in between NI-L-type and OH confinement.

Combined application of neutral injection, lower hybrid, and ion cyclotron resonance heating in ASDEX and synergetic effects

Confinement properties of ASDEX discharges are studied when NI and ICRH or NI and LH were applied simultaneously. ICRH leads to a larger change in plasma energy content when applied together with NI. At high power the beta p-increases of NI and ICRH are additive corresponding to the total heating power. In the combination of NI and LH the beta p-increase is the same when NI is applied to inductively or non-inductively driven plasma current despite the different current profiles. The effect of sawteeth on central confinement is described in detail in ohmically and beam heated plasmas as is the suppression of sawteeth by LH. The suppression of sawteeth by LH in low qa beam heated plasmas leads to a substantial increase of beta p. The H-mode can be realized with NI under current drive conditions. ICRH can cause the H-transition in combination with NI. The H-mode, however, can also be realized with ICRH alone.