Overview of diagnostic performance and results for the first operation phase in Wendelstein 7-X (invited)

Abstract

Wendelstein 7-X, a superconducting optimized stellarator built in Greifswald/Germany, started its first plasmas with the last closed flux surface (LCFS) defined by 5 uncooled graphite limiters in December 2015. At the end of the 10 weeks long experimental campaign (OP1.1) more than 20 independent diagnostic systems were in operation, allowing detailed studies of many interesting plasma phenomena. For example, fast neutral gas manometers supported by video cameras (including one fast-frame camera with frame rates of tens of kHz) as well as visible cameras with different interference filters, with field of views covering all ten half-modules of the stellarator, discovered a MARFE-like radiation zone on the inboard side of machine module 4. This structure is presumably triggered by an inadvertent plasma-wall interaction in module 4 resulting in a high impurity influx that terminates some discharges by radiation cooling. The main plasma parameters achieved in OP1.1 exceeded predicted values in discharges of a length reaching 6 s. Although OP1.1 is characterized by short pulses, many of the diagnostics are already designed for quasi-steady state operation of 30 min discharges heated at 10 MW of ECRH. An overview of diagnostic performance for OP1.1 is given, including some highlights from the physics campaigns.

Feasibility of line-ratio spectroscopy on helium and neon as edge diagnostic tool for Wendelstein 7-X

“…It has already been showed in TEXTOR that this new model gives better agreement with other diagnostics than the old model. 5 Comparisons with other edge diagnostics at W7-X are now being conducted, in particular with the Langmuir probes in the limiter 6,7 and at the multi-purpose fast manipulator 3,7 installed during the first operation phase.…”

Limiter observations during W7-X first plasmas

“…In addition to a set of low resolution near-IR cameras [7], we had a 3-5 µm band infrared (FLIR SC8303HD) camera (125 Hz full-frame rate, 1344 × 768 pixels) and a 400-800 nm AVT Prosilica GX-1050C (100 Hz full-frame rate, 1024 × 1024 pixels) color visible camera co-located on the same line of sight in Module 3 [8]. Additionally, an 8-14 µm DIAS IR camera (50 Hz, 640 × 480 pixels) viewed one side of Limiter 5 [9].…”

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

“…Using a combination of measurements from diamagnetic loops, TS, XICS, and other profile diagnostics, the energy confinement time is estimated at ${\tau }_E\approx 0.10$ s. At our present understanding of the consistency and calibration accuracy of the diagnostics, the absolute value of τ E is believed to be accurately 20%, whereas relative changes in τ E can be detected down to 5%–10%, in particular, with the diamagnetic loops. An overview of the various diagnostics in operation in OP1.1 is presented elsewhere 20 . The triple product achieved was $0.8\times {10}^{19}$ m –3  keV s for this 1.3 s discharge.…”