Investigation of oxygen impurity transport using the O⁴⁺visible spectral line in the Aditya tokamak

Abstract

Intense visible lines from Be-like oxygen impurity are routinely observed in the Aditya tokamak. The spatial profile of brightness of a Be-like oxygen spectral line (2p3p 3D3–2p3d 3F4) at 650.024 nm is used to investigate oxygen impurity transport in typical discharges of the Aditya tokamak. A 1.0 m multi-track spectrometer (Czerny–Turner) capable of simultaneous measurements from eight lines of sight is used to obtain the radial profile of brightness of O4+ spectral emission. The emissivity profile of O4+ spectral emission is obtained from the spatial profile of brightness using an Abel-like matrix inversion. The oxygen transport coefficients are determined by reproducing the experimentally measured emissivity profiles of O4+, using a one-dimensional empirical impurity transport code, STRAHL. Much higher values of the diffusion coefficient compared with the neo-classical values are observed in both the high magnetic field edge region and the low magnetic field edge region of typical Aditya ohmic plasmas, which seems to be due to fluctuation-induced transport. The diffusion coefficient at the limiter radius in the low-field (outboard) region is typically ∼ twice as high as that at the limiter radius in the high-field (inboard) region.

Evaluation of an Oxygen Transport Coefficient in the Aditya Tokamak Using the Radial Profile of O4+ Emissivity and the Importance of Atomic Data Used Therein

“…The radial emissivity profiles were obtained using an Abel-like matrix inversion technique [19] from the chord integrated brightness of O 4+ emissions. The details of this conversion have been described in the references [15,20]. The error estimation was done by incorporating the statistical error present in the absolute calibration and the one due to the spectral line fitting using a Gaussian profile to obtain the brightness, which is the area under the curve of the spectral line profile.…”

“…This higher value of D compared to the calculated one by neo-classical transport theory is explained using the fluctuation induced transport. It was found that the transport driven by the ion temperature gradient (ITG) and dissipative trapped electron (DTE) modes was good enough to explain the diffusivity in the high field side, and the further higher value of diffusivity in the low field side was explained by using the combination of ITG and resistive ballooning modes (RB) [15]. The error bars in D represent the modification of the required diffusion coefficients to get the best-fit between experimental and calculated emissivity profiles due to the charge in the T e profile peaking factor.…”

“…In the Aditya tokamak, the experimental spatial profile of brightness of this transition was modeled to obtain the oxygen diffusion coefficient using STRAHL impurity transport code [14] and the required PEC from the ADAS database [10]. The obtained higher value of the diffusion coefficient was explained by fluctuation-induced transport [15]. The radial profile of this emission was also employed to validate the impurity transport code developed using a semi-implicit numerical technique for the study of impurity transport in tokamak plasmas [16].…”

Modeling of the Hα Emission from ADITYA Tokamak Plasmas

“…The spatial profile of H α emission is regularly measured by a multi-track spectrometer. This diagnostic consists of a 1-m visible spectrometer having a grating with 1800 grooves/mm [17]. The detector, coupled on the focal plane of the exit port of the spectrometer, is a charge-coupled device (CCD) having a dimension of 1024 × 256 pixels and each pixel size is 26 × 26 µm 2 .…”

Space-resolved visible spectroscopy for two-dimensional measurement of hydrogen and impurity emission spectra and of plasma flow in the edge stochastic layer of LHD

“…Therefore, the transport of fuel and impurities is largely affected by the magnetic field structure. In tokamaks, edge impurity and hydrogen emissions have been investigated using spectroscopic systems, [2][3][4][5] where interesting plasma transport properties have been revealed. Helical devices, however, usually have a complex magnetic field structure due to the intrinsic non-axisymmetric configuration as compared to tokamaks, and we need special care to conduct the spectroscopic measurements.…”