29th Symposium on Fusion Technology (SOFT 2016)

P4.053 Optimization of optical dumps for H-alpha spectroscopy in ITER

8 Sep 2016, 14:20

1h 40m

Foyer 2A (2nd floor), 3A (3rd floor) (Prague Congress Centre)

Board: 53

Poster D. Diagnostics, Data Acquisition and Remote Participation P4 Poster session

Speaker

Evgeny Andreenko (NRC “Kurchatov institute”)

Description

The performance of ITER Main Chamber H-alpha & Visible Spectroscopy is challenged by the problem of separating the contribution of visible light emitted in the scrape-off-layer (SOL) from the background of much higher intensity, produced by the divertor stray light (DSL) reflected by the all-metal first wall (S.Kajita, et al., PPCF, 2013). A differential (bifurcated-line-of-sight) measurement scheme was proposed to solve this problem (24th IAEA FEC, 2012, ITR/P5-44). It assumes simultaneous observation of two neighboring fields on the first wall with noticeable local difference in the wall reflection Rw, e.g. ensured by using the optical dumps (OD) with reduced reflectivity of incident light. However, the differential scheme could be effective, provided the spectral intensity of SOL emission and the normalized line shape of DSL spectrum are not perturbed by the OD. Thus, the proper OD design should minimize the perturbation of DSL line shape, caused by 1) spectral non-uniformity of DSL directivity at the OD entrance pupil, and 2) the difference of bidirectional reflectance distribution functions (BRDF) of the OD and the first wall. The effect of the OD design on the BRDF and the accuracy of the SOL emission measurements are evaluated for the few lines of sight directed from ITER equatorial port to the ODs located in blanket modules ##1-8 by simulation with the use of Zemax OpticStudio software. The data of predictive modeling of the flat-top stage of inductive mode of ITER operation, derived by the SOLPS4.3 (B2-EIRENE) code are used to calculate the spectral and angular profiles of the incident light on the ODs. A number of OD models are considered, and the respective BRDFs and measurement errors are evaluated for the certain ITER plasma discharge scenario (low-density far-SOL L-mode) in order to optimize the OD design.