Spectroscopic Investigation of Neutral and Impurity Dynamics in the Edge Region of ADITYA-U Tokamak

Abstract

he “edge” region of tokamak plasma separates the hot core plasma from the material boundary. The proper understanding of this plasma region is very crucial for maintaining the hot fusion grade core plasma and to provide the adequate gas fuelling. Spectroscopic diagnostics remain the most reliable diagnostic method for diagnosing the edge plasmas. This thesis presents the detailed characterization of ADITYA-U tokamak's edge plasma using various spectroscopic techniques by carrying out experiments, simulations, and modelling. ADITYA-U tokamak has a major/minor radius of 0.75 m/0.25 m respectively with a toroidal magnetic field of ~0.75 − 1.5 T and plasma currents of ~80 − 250 kA. The passive spectral emissions from fuel neutrals and impurity-ions are measured using spectrometers and filter photomultiplier-tube combinations. The collection optics are designed according to the experimental requirements. The ion/neutral temperatures are estimated from the line shapes of the measured spectra. Estimation of ion/neutral temperatures by supposing the spectral lines are only broadened by the Doppler broadening leads to large temperature values which cannot be explained reasonably. This anomaly in ion/neutral temperature estimation is removed by introducing the broadening by the Zeeman splitting as the emissions are subjected to magnetic fields in tokamaks. This has led to several interesting results such as the existence of a poloidal asymmetry in fuel-neutral temperature obtained from the Hα spectra. The fuel neutrals acquire relatively higher temperatures (~ 4 eV) on the high-field side as compared to the low-field side (~ 2 eV). Analysis of Hα-spectra also revealed the presence of warm and hot fuel neutrals, originating from different atomic and molecular processes. Using the Zeeman-corrected temperatures, the radial profile of C+ and O4+ ion temperatures reveal higher ion temperatures at the locations of MHD islands which may be related to the transport of ions inside the magnetic islands. Furthermore, Li density and radiative power losses are estimated from the opacity-influenced Li spectral emission in the Li2TiO3 particle injection experiments. The thesis also contributes to the understanding of edge plasma's interaction with the graphite-limiter and the stainless-steel vessel-wall. The spectral intensities of Hα,O+ and C2+ emissions are measured from the vessel-wall and limiter. Interestingly, it has been observed that the graphite-limiter and the vessel-wall contribute almost equally to carbon recycling. It has been concluded that the unexposed vessel-wall acts as a sink and becomes a source after several plasma discharges.Furthermore, it has been observed using the DEGAS2 code that the molecular processes cannot be ignored in estimating hydrogen influx using the ionization per photon (S/XB) factor for Te  7 eV in the edge region. Using the impurity transport code STRAHL, the diffusivities of C, O,Ne, Ar, and Fe impurity species illustrate a clear dependency of impurity-mass on impurity transport. For establishing the measurements techniques of vibrational temperatures of N2 molecule in tokamak far-edge/divertor regions, experiments are carried out in radio-frequency produced nitrogen-contaminated hydrogen plasmas. The vibrational temperatures of N2 molecules are estimated using the Boltzmann method by recording the different N2 molecular bands. The contribution of hydrogen Fulcher-band contaminating the N2 1PS bands are identified and successfully removed using line-shape simulations.