Plasma generation has been achieved in the stellarator W7-AS using neutral beam injection only. Plasma evolution is analysed using a zero-dimensional model. The model gives guidelines for this method to be successful. Both a low neutral gas starting pressure and a small fraction of wall recycling are necessary to get `ignition' by beams. The predictions of the model are compared to experiments on W7-AS and LHD.

The xenon spectrum, excited in ECR- and NBI-heated plasmas with central electron densities of around 10(20) m(-3) and central electron temperatures from 0.7 to 2.5keV, has been studied photoelectrically with a multichannel grazing-incidence spectrometer. Besides numerous well-known lines of Zn- and Cu-like Xenon, more than 50 additional lines which have not yet been published in the literature have been found and partly identified,.

During the last shutdown the Stellarator W7-AS underwent two major modifications: First, the limiters were replaced by ten divertor modules, and the diagnostic set associated with the plasma boundary and target plate regions was greatly expanded. Secondly, the previously counter tangential neutral beam injector box was shifted to a co-position. Thus, the heating efficiency should be considerably increased at low magnetic fields and high densities. After resuming experiments these improvements will be used to test the boundary island divertor concept and further expand operational boundaries during the remaining experimental time until permanent shutdown in 2002. The present operational boundaries are reviewed with respect to the stability of high β and density limit discharges. Discharges with good confinement properties will be discussed where further progress was achieved after installing control coils to modify the size and properties of vacuum field islands. In contrast to the usual net-current free mode, W7-AS also allows operation at large toroidal currents. In this way disruption-like events in the presence of rather large external poloidal fields can be produced.

The development in Europe of negative ion sources and accelerators is carried out principally at Cadarache, France and at Garching, Germany. Other work is in progress at the Ecolc Polytechnique, France, and CIEMAT, Spain. The main thrust of the programme so far has been driven by the requirements for the 1 MeV D0 injection on ITER. It is now being proposed to develop a new negative ion source and accelerator for use on present and future European fusion experiments. This paper summarises the present and proposed future programmes in Europe.

Neutral beam injection experiments on W7-AS started at the end of 1988 with four ion sources mounted on two beamlines. The total neutral beam power was 1.5 MW, raising the central beta to β0 = 2.7% — far from the limit expected to be ≈ 4.5%. Aiming at testing β limits in W7-AS, the power has recently been doubled by adding two additional sources to each of the two beamlines. With 3 MW of hydrogen beam in the torus vessel, β0 = 4% was obtained without any sign of a limit, such as instabilities or enhanced mode activities [1]. One of the two available calorimeters was reinstalled in the W7-AS torus. This way it was demonstrated that the full neutral beam power of 1.5 MW per injector has indeed been injected into the torus. Beam blocking in the port was not observed, but there was a loss of neutral beam power in the front chamber of the injector which was obviously due to insufficient pumping. It was tried to measure the relative power of the eight different beams by comparing the diamagnetic energy achieved with plasma injection shots. The results, however, were not very promising. By regapping the extraction grids of the ion sources and raising the extraction voltage from 45 kV to 50 kV for hydrogen, the power is now being increased additionally by ≈ 10%.

The upgraded NB heating power on W7-AS allowed extension of the accessible parameter space. In first experiments, a remarkable increase of the ion temperatures was obtained. The results are discussed on the basis of a detailed transport analysis which confirms the neoclassical nature of the ion transport up to the 1 mfp regime. The maximum NB heating power was applied to investigate beta limits. Although beta values close to the predicted stability limit could be reached, no clear indication of a limit has been found so far. Several experimental observations on W7-AS point to the necessity of a divertor. The feasibility of a divertor making use of the natural islands in an optimized stellarator was investigated, and a possible solution for an island divertor on W7-AS is outlined.