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

Abstract

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.

FIG. 1.

Representative flux surfaces, field lines, and part of the superconducting coil set of W7-X are shown in this CAD drawing.

FIG. 2.

Profiles of ${ϵ}_{\text{eff}}$ for the standard OP1.1 configuration and the alternative configuration.

FIG. 3.

Time traces (left) and profiles (right) of electron and ion temperatures and electron density for a 1.3 s discharge, 20160308.008, heated continuously with 2.7 MW of ERCH. The profile data are taken at t = 1.1 s.

FIG. 4.

Time traces (left) and profiles (right) of electron and ion temperatures and electron density for a 6 s discharge, 20160309.006. The time trace for density is noisy due to suboptimal performance of the interferometer laser on that shot, not due to any unusually large density fluctuations. The profile data are taken at t = 5.5 s where the heating power was 0.6 MW.

FIG. 5.

Time traces (left) and profiles (right) of electron and ion temperatures and electron density for a 6 s discharge, 20160310.007, in the alternative magnetic configuration. The profile data are also taken at t = 5.5 s where the heating power was 0.6 MW.

FIG. 6.

The nominally identical ECR heating programming (top) and the evolving bootstrap currents (bottom) for three different configurations, the standard OP1.1 configuration (blue) 20160309.010, the alternative OP1.1 configuration (black, 20160309.029), and an in-between configuration (red, 20160309.018). A clear tendency for lower bootstrap current in the alternative configuration is seen.

FIG. 7.

Simulation results from a high density discharge heated with 10 MW of on-axis O2 ECRH are shown here.

FIG. 8.

Simulation results from a high density discharge heated with 20 MW is shown, 10 MW of on-axis O2 ECRH and 10 MW of neutral beam heating.

FIG. 9.

The triple product is plotted versus pulse length in this graph, putting the OP1.1 results in perspective and showing that the combinations of triple product and pulse lengths for OP2 in W7-X will be rather unique. The figure is based on data from Ref. and is an augmented version of one supplied to the authors by T. Morisaki. The pink color indicates regions that LHD can access, and the blue shows the additional parameter space spanned by existing tokamaks.