The Thomson scattering diagnostic at Wendelstein 7-X and its performance in the first operation phase

Abstract

The optimized stellarator Wendelstein 7-X started operation in December 2015 with a 10 week limiter campaign. Divertor experiments will begin in the second half of 2017. The W7-X Thomson scattering system is an essential diagnostic for electron density and temperature profiles. In this paper the Thomson scattering diagnostic is described in detail, including its design, calibration, data evaluation and first experimental results. Plans for further development are also presented. The W7-X Thomson system is a Nd:YAG setup with up to five lasers, two sets of light collection lenses viewing the entire plasma cross-section, fiber bundles and filter based polychromators. To reduce hardware costs, two or three scattering volumes are measured with a single polychromator. The relative spectral calibration is carried out with the aid of a broadband supercontinuum light source. The absolute calibration is performed by observing Raman scattering in nitrogen. The electron temperatures and densities are recovered by Bayesian modelling. In the first campaign, the diagnostic was equipped for 10 scattering volumes. It provided temperature profiles comparable to those measured using an electron cyclotron emission diagnostic and line integrated densities within 10% of those from a dispersion interferometer.

Dual-laser wavelength Thomson scattering at Wendelstein 7-X

“…The wavelength (λ =1064 nm) is in the near infrared region were plasma background light is low and sensitive detectors 5 (silicon avalanche diodes) for the detection of the scattered light are available. The well-known λ =1064.14 (1064) nm transition provide the lowest threshold of laser lines in Nd:YAG crystals 4 and is therefore widely used; the W7-X TS diagnostic is now equipped with three such lasers 3 . In our dual-laser wavelength application the λ=1318.8 (1319) nm Nd:YAG transition was chosen.…”

“…At the exit the beam combiner is used as beam splitter to separate the two wavelength in order to allow the measurement of the laser energy and alignment monitoring for each laser. The TS light is collected using two separate collection optics 3,4 viewing the inner and outer half-profiles of Te and ne. The light is led through multiple optical fibers to a battery of polychromators with 5 detection channels each.…”

“…For RFX-mod, reverse field pinch plasmas, the combination of a Nd:YAG (λ=1064 nm) and a Nd:YLF (λ =1053 nm) laser was chosen, to measure the sensitivities of the spectral channels. At W7-X the basic TS system 3,4 includes a pulsed, high power Nd:YAG laser with λ=1064 nm, together with five interference filter polychromators for spectral analysis of the scattered light in the near infrared region between λ=750-1061 nm. The system is able to measure Te up to approximately 10 keV within an error of ~10%, depending on ne and background light.…”

“…This paper will discuss the layout of the double laser set-up, its benefits, and demonstration for its use as an in-situ calibration method. A detailed description of the TS system layout and TS data analysis in W7-X can be found in the literature 3,4,5 .…”

Acceleration of Bayesian model based data analysis

“…In the first case the results of both diagnostics agreed, in the second they show different density estimations. Both belong to a set of scenarios that were previously compared after the experimental campaign [59].…”