Z eff profile diagnostics using visible bremsstrahlung continuum for nonaxisymmetric plasmas with finite β in large helical device

Abstract

An astigmatism-corrected Czerny–Turner-type visible spectrometer coupled with a charge-coupled device has been installed in large helical device (LHD) to measure visible bremsstrahlung continuum. A full vertical profile has been observed from the elliptical plasmas at horizontally elongated plasma cross section through a 44 fiber parallel array with vertical observation length of ∼1 m. Line emissions can be easily eliminated by use of the visible spectrometer instead of an interference filter. A nonmonotonic bremsstrahlung emission profile, which is originated in the thick ergodic layer surrounding the main plasma, has been observed for normal discharges in all the magnetic configurations of LHD. After analyzing the detailed structure, the lower half of the vertical bremsstrahlung emission profile is found to be free of the strong edge bremsstrahlung emission in inwardly shifted magnetic configurations (Rax≤3.60 m). The nonmonotonic bremsstrahlung emission disappeared in extremely high-density discharges (ne≥1014 cm−3) because of the reduction of the plasma outside boundary. When the local emissivity is calculated using Abel inversion, the solution is very sensitive to the distortion of the magnetic surface structure. The influence of the magnetic surface distortion based on the finite β effect is examined as error estimation in addition to unclear edge plasma boundary due to the presence of the ergodic layer. The result indicates that the determination of the normalized minor radius for each observation chord gives a larger influence on the Abel inversion rather than the determination of the chord length. When the observed chord-integrated bremsstrahlung intensity profile is flat, the resultant uncertainty seen in the bremsstrahlung emissivity profile becomes large, in particular, at the plasma center. The Zeff profile calculated with consideration of density and temperature profiles is verified in neutral-beam-heated discharges with H2 and C pellet injections, where the Zeff values should be close to 1 and 6, respectively. Analysis on the flat and hollow electron density profiles results in a fairly flat Zeff profile.

Observation of Impurity Accumulation After Hydrogen Multi-Pellet Injection in Large Helical Device

“…Finally, the uncertainties of the impurity density and Z eff values in the present analysis are estimated. The errors mainly originate from the Thomson scattering measurement, the atomic model, the EUV spectrometer system, the Abel inversion process [21] , and the absolute intensity calibration [10] . The error is estimated to be 30% at most in the present study.…”

Extension of Wavelength Range in Absolute Intensity Calibration of Space-Resolved EUV Spectrometer for LHD Diagnostics

“…One of the uncertainties is originated in the Abel inversion process. The uncertainty is estimated to be less than 5% [9]. The electron temperature measured by Thomson scattering system also brings the uncertainty, which is estimated to be less than 3%.…”

Simultaneous Observation of Tungsten Spectra of W0 to W46+ Ions in Visible, VUV and EUV Wavelength Ranges in the Large Helical Device

“…The coil system consists of a set of two continuous superconducting helical coils with a poloidal pitch number of two and a toroidal pitch number of 10 and three pairs of superconducting poloidal coils. Figure 1a illustrates the top view of the shape of the plasma in the LHD device together with schematic drawings of the neutral beam injection (NBI) for heating, the impurity pellet injection, and the spectroscopic diagnostics consisting of two flat-field grazing incidence EUV spectrometers (denoted as "EUV Short" [24] and "EUV Long" [25]), a normal incidence 20 cm VUV spectrometer (denoted as "VUV 109L" [26]), and an astigmatism-corrected Czerny-Turner-type 30 cm visible spectrometer (denoted as "MK300" [27]). Neutral hydrogen atoms are used as beam particles in the experiments presented in this paper.…”