Abstract. The Large Helical Device (LHD) and Wendelstein 7-X (W7-X, under construction) are experiments specially designed to demonstrate long pulse (quasi steady-state) operation, which is an intrinsic property of Stellarators and Heliotrons. Significant progress had been made in establishing high performance plasmas. A crucial point is the increasing impurity confinement at high density observed at several machines (TJ-II, W7-AS, LHD) which can lead to impurity accumulation and early pulse termination by radiation collapse. In addition, theoretical predictions for non-axisymmetric configurations predict the absence of impurity screening by ion temperature gradients in standard ion-root plasmas. Nevertheless, scenarios were found where impurity accumulation was successfully avoided in LHD and W7-AS due to the onset of friction forces in the (high density and low temperature) scrape-off-layer, the generation of magnetic islands at the plasma boundary and to a certain degree also by ELMs, flushing out impurities and reducing the net-impurity influx into the core. In both the W7-AS High Density H-mode (HDH) regime and in the case of application of sufficient ECRH heating power a reduction of impurity core confinement was observed. The exploration of such purification mechanisms is a demanding task for successful steady-state operation. Impurity transport at the plasma edge/SOL was identified to play a major role for the global impurity behaviour in addition to the core confinement.

Ion temperatures have been measured in the TJ-II stellarator using passive emission spectroscopy in parallel and perpendicular directions. A highenergy component was observed from the tails of the H α line emission in both directions, suggesting a non-negligible population of suprathermal ions. The role of this fast ion energy component, which has been observed in both stellarator and tokamak plasmas when heated by electron cyclotron resonance heating in the additional heating of the plasma bulk is investigated by means of a simple model. The origin of such a fast component is discussed in terms of different mechanisms.

We study the effect of low-order rational surfaces on electron heat transport in plasmas confined in the TJ-II stellarator (Alejaldre et al 1990 Fusion Technol. 17 131) and heated by electron cyclotron waves. Enhancement of core electron heat confinement is observed when the rational surface is placed in the vicinity of the power deposition zone, either by performing a magnetic configuration scan or by inducing Ohmic current in a single discharge. The key to improving heat confinement seems to be a locally strong positive radial electric field, which is made possible by a synergistic effect between enhanced electron heat fluxes through radial positions around low-order rationals and pump out mechanisms in the heat deposition zone.

This paper presents the latest results on confinement studies in the TJ-II stellarator. The inherently strong plasma–wall interaction of TJ-II has been successfully reduced after lithium coating by vacuum evaporation. Besides H retention and low Z, Li was chosen because there exists a reactor-oriented interest in this element, thus giving special relevance to the investigation of its properties. The Li-coating has led to important changes in plasma performance. Particularly, the effective density limit in NBI plasmas has been extended reaching central values of 8 × 1019 m−3 and T e ≈ 250–300 eV, with peaked density, rather flat T e profiles and higher ion temperatures. Due to the achieved density control, a second type of transition has been added to the low density ones previously observed in ECRH plasmas: higher density transitions characterized by the fall in Hα emission, the onset of steep density gradient and the reduction in the turbulence; which are characteristics of transition to the H mode. Confinement studies in ECH plasmas indicate that lowest order magnetic resonances, even in a low shear environment, locally reduce the effective electron heat diffusivities, while Alfven eigenmodes destabilized in NBI plasmas can influence fast ion confinement.

The ISX-A (Impurity Study Experiment) tokamak operated with major radius R =92 cm, minor radius a =26 cm, and relatively low toroidal magnetic field B T < 15 kG. 1 * 2 Only Ohmic heating was appliedo Studies of plasma confinement in this device yielded unusually favorable results in comparison with empirical scaling formulas., For example, the gross-energy-confinement times, r E = !&[/(n e T e +W|Ti)dv]/Po m E. 9 exceeded the values expected from the scaling of Jassby et at? by factors of 1-3 (lo6 average) and were larger than the values predicted by the Hugill-Sheffield formula 4 [with scaling l-l] by factors of 1.5-4.5 (3.1 average). At line-average densities (n e ) above 10 13 cm" 3 , the ISX-.A data are closest to the scaling proposed by Mirnov, 5 r E = (3 x 1(T 9 )a(cm) x/(A)« e l72 sec (n e is given in units of 10 13 cm" 3 ), although they still exceed the expectations by an average value of 1.2. Also, the maximum value of n e achieved before a major disruption occurred was 7xl0 13 cm" 3 , a factor almost 4.5 times larger than that anticipated by B T /R 0 scaling. 6 The largest values of toroidal beta, P T (0) equal to No. GA-A14133, 1976 (to be published); see also Ref" 5, above. 7 G. R. Hopkins and John M. Rawls, Nucl. Technol. 36, 171 (1977), and references contained therein. 8 P