Performance improvement of two-dimensional EUV spectroscopy based on high-frame-rate CCD and signal normalization method in Large Helical Device

Abstract

In the Large Helical Device (LHD), two-dimensional (2D) distribution of edge impurity line emissions from the ergodic layer with a three-dimensional (3D) magnetic field structure has been observed in the wavelength range of 30–650 Å by horizontally scanning a space-resolved extreme ultraviolet (EUV) spectrometer during a steady phase in electron cyclotron heating (ECH) and ion cyclotron range of heating (ICRF) discharges with long pulse duration (τd ≥ 10 s). The 2D distribution of impurity line emissions can be measured at the upper or lower half in the full vertical range of LHD plasmas by a single horizontal scan. The performance of 2D EUV spectroscopy, however, was not sufficient for discussing the impurity transport in the ergodic layer from the viewpoints of the quality of 2D images and the requirement of long pulse discharges. The 2D EUV spectrometer is therefore modified by installing a high-frame-rate charge-coupled detector (CCD) and increasing its horizontal scanning speed. As a result, the quality of 2D images is significantly improved with increased horizontal spatial resolution, and then the 2D measurement is also possible for neutral beam injection (NBI) discharges with short pulse duration (τd ∼ 3 s). In addition, temporal and shot-by-shot intensity variations in the edge EUV emission are significantly reduced by applying a signal normalization method using another EUV spectrometer with high time resolution. A 0.5 µm poly(ethylene terephthalate) (PET) filter is also installed in front of an entrance slit of the space-resolved EUV spectrometer. The spike noise originating from high-energy neutral particles, which are enhanced in low-density NBI discharges, has been successfully reduced. Typical examples of the 2D distribution of impurity line emissions with improved quality are presented for several impurity species.

Estimation of Partial Carbon Radiation at Each Ionization Stage of C²⁺to C⁵⁺Ions in Large Helical Device

“…CIII (977.02 Å, 2s2p-2s 2 ), CIV (1548.2 Å, 2p-2s), CV (40.27 Å, 1s2p-1s 2 ) and CVI (33.73 Å, 2p-1s) [3,4]. Two space-resolved EUV spectrometers named EUV_Short2 and EUV_Long2 are used for simultaneous vertical profile measurements of CIII (386.2 Å), CIV (384.17 Å), CV (40.27 Å) and CVI (33.73 Å) in the range of −0.6 ≤ Z ≤ 0.6 m with a time resolution of 100 ms [5][6][7]. The EUV_Long, VUV_109L and VUV_106R have a similar observation range, as shown in Fig.…”

“…Although no magnetic surface exists in the ergodic layer, virtual magnetic surfaces are assumed at ρ > 1 by extrapolating the magnetic flux surface at ρ = 1. This assumption is generally used in LHD when the analysis is carried out for the ergodic layer [6], as shown in Fig. 4 (b).…”

EUV and VUV Spectra of NeIII-NeX Line Emissions Observed in the Impurity Gas-Puffing Experiments of the Large Helical Device

“…Some of the lines are blended with each other, thus, wavelengths of blended lines are enclosed in parentheses in the table. The lines listed in this table will be used for further spectroscopic researches using space-resolved EUV and VUV spectrometers which have been developed in LHD [23][24][25][26][27]. Comparison between the spectroscopic results and the impurity transport simulation [28] is also planned to reproduce the enhancement of the Ne line emissions coincident with the reduction of the divertor heat load.…”

Divertor Detachment with Multi-Species Impurity Seeding in LHD

“…However, the detachment disappeared 0.4 s after the Ne seeding. Figure 4 shows the time evolution of the NeVIII emission radial profile measured by EUV spectroscopy [10]. NeVIII or lower charge states can be considered as the dominant radiator since T e at 0.8 < |r/r LCFS | < 1.0 where plasma radiation mainly occurred is lower than the NeIX ionization energy of 1195.3 eV.…”