U. Neuner

Key results from the first plasma operation phase and outlook for future performance in Wendelstein 7-X

The first physics operation phase on the stellarator experiment Wendelstein 7-X was successfully completed in March 2016 after about 10 weeks of operation. Experiments in this phase were conducted with five graphite limiters as the primary plasma-facing components. Overall, the results were beyond the expectations published shortly before the start of operation [Sunn Pedersen et al., Nucl. Fusion 55, 126001 (2015)] both with respect to parameters reached and with respect to physics themes addressed. We report here on some of the most important plasma experiments that were conducted. The importance of electric fields on global confinement will be discussed, and the obtained results will be compared and contrasted with results from other devices, quantified in terms of the fusion triple product. Expected values for the triple product in future operation phases will also be described and put into a broader fusion perspective.

Design, capabilities, and first results of the new laser blow-off system on Wendelstein 7-X

We present a detailed overview and first results of the new laser blow-off system on the stellarator Wendelstein 7-X. The system allows impurity transport studies by the repetitive and controlled injection of different tracer ions into the plasma edge. A Nd:YAG laser is used to ablate a thin metal film, coated on a glass plate, with a repetition rate of up to 20 Hz. A remote-controlled adjustable optical system allows the variation of the laser spot diameter and enables the spot positioning to non-ablated areas on the target between laser pulses. During first experiments, clear spectral lines from higher ionization stages of the tracer ions have been observed in the X-ray to the extreme ultraviolet spectral range. The temporal behavior of the measured emission allows the estimate of transport properties, e.g., impurity transport times in the order of 100 ms. Although the strong injection of impurities is well detectable, the global plasma parameters are barely changed.

Sightline optimization of the multichannel laser interferometer for W7-X

A multichannel CO2 laser interferometer is planned for electron density profile measurements in the Wendelstein 7-X stellarator under construction. This article describes the mathematical methods used to find the optimum orientation of the various sightlines. The problem of reconstructing local densities from line integrated density measurements can be formulated as a matrix inversion. The quality of the reconstruction is largely determined by the condition number of the matrix describing the problem, which has to be minimized, i.e., the sightlines have to be chosen such that the information content of different channels is maximally distinct. For the W7-X interferometer four- and eight-sightline arrangements are investigated and their orientations are optimized for standard conditions of magnetic configuration and density profile. The optimized arrangements are tested by simulating the inversion of hypothetical reference density profiles in a number of different magnetic configurations. In the ideal case...

Plasma impurities observed by a pulse height analysis diagnostic during the divertor campaign of the Wendelstein 7-X stellarator

The paper reports on the optimization process of the soft X-ray pulse height analyzer installed on the Wendelstein 7-X (W7-X) stellarator. It is a 3-channel system that records X-ray spectra in the range from 0.6 to 19.6 keV. X-ray spectra, with a temporal and spatial resolution of 100 ms and 2.5 cm (depending on selected slit sizes), respectively, are line integrated along a line-of-sight that crosses near to the plasma center. In the second W7-X operation phase with a carbon test divertor unit, light impurities, e.g., carbon and oxygen, were observed as well as mid- to high-Z elements, e.g., sulfur, chlorine, chromium, manganese, iron, and nickel. In addition, X-ray lines from several tracer elements have been observed after the laser blow-off injection of different impurities, e.g., silicon, titanium, and iron, and during discharges with prefill or a gas puff of neon or argon. These measurements were achieved by optimizing light absorber-foil selection, which defines the detected energy range, and remotely controlled pinhole size, which defines photon flux. The identification of X-ray lines was confirmed by other spectroscopic diagnostics, e.g., by the High-Efficiency XUV Overview Spectrometer, HEXOS, and high-resolution X-ray imaging spectrometer, HR-XIS.

Impurity Transport Studies at Wendelstein 7-X by Means of X-ray Imaging Spectrometer Measurements

This paper reports on the effect of onand off-axis heating power deposition on the impurity confinement in purely electron cyclotron resonance heated He plasmas on the stellarator Wendelstein 7-X. Therefore, impurity transport times τI have been determined after Fe impurity injections by laser ablations and monitoring the temporal impurity emissivities by the x-ray imaging spectrometer HR-XIS. A significant increase of τI has been observed when changing the power deposition from onto off-axis heating with energy confinement times τE being mainly unaffected. In addition, the scaling of impurity transport properties with respect to a variation of heating power PECRH and electron density ne has been investigated by keeping the heating power deposition on-axis. The observed τI scaling compares well to known τI scaling laws observed in other machines. A comparison of τI and τE yields an averaged ratio of τE/τI=1.3 and transport times in the range of τI=40–130 ms and τE=40–190 ms. Comparing those absolute values to neoclassical predictions supports the recently observed nature of anomalous transport in Wendelstein 7-X, given within the up to now investigated operational parameters.