Plans for the first plasma operation of Wendelstein 7-X

Abstract

Wendelstein 7-X (W7-X) is currently under commissioning in preparation for its initial plasma operation phase, operation phase 1.1 (OP1.1). This first phase serves primarily to provide an integral commissioning of all major systems needed for plasma operation, as well as systems, such as diagnostics, that need plasma operation to verify their foreseen functions. In OP1.1, W7-X will have a reduced set of in-vessel components. In particular five graphite limiter stripes replace the later foreseen divertor. This paper describes the expected machine capabilities in OP1.1, as well as a selection of physics topics that can be addressed in OP1.1, despite the simplified configuration and the reduced machine capabilities. Physics topics include verification and adjustment of the magnetic topology, testing of the foreseen plasma start-up scenarios and feed-forward control of plasma density and temperature evolution, as well as more advanced topics such as scrape-off layer (SOL) studies at short connection lengths, and transport studies. Plasma operation in OP1.1 will primarily be performed in helium, with a hydrogen plasma phase at the end.

Advanced electron cyclotron heating and current drive experiments on the stellarator Wendelstein 7-X

“…The resulting flattening of the central profile would also short-wire the induced central current peaks, so that the confinement can be recovered leading to a rise of the profiles until the next crash. These fast internal current changes are not visible by the rogowski coils, because of slowly varying shielding currents with a L/R-time of several seconds [3].…”

“…The machine was not fully equipped with wall protecting graphite tiles leaving a larger surface of the CuCrZr-cooling structure unprotected. Furthermore, instead of the foreseen 10 divertor elements, a graphite limiter was installed on the high field side (HFS) in each bean plane of the five modules preventing a direct contact of the plasma with the unprotected regions of the plasma vessel [3]. Therefore, the plasma volume was limited to an effective minor radius of a = 0.49 m. In parallel, the flexible coil system of W7-X was used to operate a magnetic field configuration at a lower rotational transform developing …”

Error field measurement, correction and heat flux balancing on Wendelstein 7-X a

“…The limiter connection lengths in the absence of error fields are characterized by three regions [13]. These regions each have a unique connection length to the neighboring limiter or from one side of the limiter to the other (all making at least one full toroidal transit.…”

Verification of the SPEC code in stellarator geometries

“…(7) with increasing accuracy as numerical resolution is increased and with an error approaching machine precision for sufficiently high resolution. Two stellarator configurations are considered: a classical l = 2 stellarator with 5 field periods [13] and a W7-X limiter configuration of the first experimental campaign [7]. The latter includes a large n = 5, m = 6, magnetic island chain located inside the last closed flux surface, where the vacuum rotational transform is ι = 5/6.…”

“…SPEC has been verified in axisymmetry and for slightly perturbed configurations [3][4][5][6]. Here we present the first SPEC calculations of equilibria in stellarator geometries, including a Wendelstein 7-X (W7-X) vacuum field limiter configuration of the first experimental campaign [7].…”