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

Abstract

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.

Characterization of the W7-X scrape-off layer using reciprocating probes

“…OP1.2b 18 samples for material exposition IPP-LBO1 [21] OP1.2a/b Holds four coated glass targets for laser ablation PPPL-PMPI1 [22] OP1.2b Horizontal powder flinger for boron impurity injection As an interesting note, the various patches of slightly different hues in the island region in Figure 2 b) are all intersected by either the upper divertor in module 1 or the lower divertor in module 3. However, the number of toroidal orbits around the entire machine (major radius R ∼ 5.5 m) until the field line hits the target varies, resulting the in the connection length distribution being in Figure 2 b) being more complex than the divertor target map in Figure 2 c).…”

Prospects of X-ray imaging spectrometers for impurity transport: Recent results from the stellarator Wendelstein 7-X (invited)

“…Figure 3 shows an overview of observed He-and H-like spectra, measured using different crystals as listed in Table I. The Si, Ti, and Fe impurities were injected using a laser blowoff (LBO) system 36 and Ar was injected by a gas puff. All spectra are measured along the central lines of sight of the spectrometers, 12 and the given count rates have been averaged for 200 ms after an impurity injection.…”

“…This time constant has been denoted as the impurity transport time τ i throughout this paper, representing a measure that is related to the impurity confinement properties of the plasma. 36…”

Operation of probe heads on the Multi-Purpose-Manipulator at W7-X in OP 1.2a

“…The fluctuation probe features a four pin mach probe and 22 Langmuir pins poloidally arrayed in two lines, those pins were selected to be operated as floating potential pins and ion saturation pins, with one on a swept biasing for electron temperature and density measurements. From the poloidal array the poloidal propagation velocity of turbulent fluctuations can be estimated by correlation analysis, allowing to deduce the radial electric field under the assumption the poloidal Probe for a Laser Blow Off (LBO) system [6] and a material sample holder were deployed in OP 1.2a.…”