author’s e-mail: sbanerje@ipr.res.in Observations of H α emission profiles in Aditya tokamak Santanu Banerjee, J. Ghosh, R. Manchanda, R. Dey*, N. Ramasubramanian, M.B. Chowdhuri, Ketan M. Patel, Vinay Kumar, P. Vasu, P. K. Chattopadhyay, P.K. Atrey and Aditya Team Institute for Plasma Research, Bhat, Ghandhinagar 382 428, India * Max-Planck-Institut für Plasmaphysik, Boltzmannstraße 2, D-85748, Garching, Germany Emissions from the hydrogen Balmer alpha (λ = 656.28 nm) has been recorded for a large number of plasma discharges in the Aditya tokamak using a 1m Czerny-Turner spectrometer equipped with an 1800 grooves/mm reflection grating [5]. Eight simultaneous vertically collimated line-of-sights, using individual lens – fiber combination from a top port of the tokamak view a poloidal cross-section of the plasma. The line-of-sights can be moved along the major radius to obtain emissions from different major-radial positions on a shot–to–shot basis. Abel-like matrix inversion has been performed to obtain radial profile of volume emissivities from these chord-integrated intensities. Considerable H α emission is observed in the bulk plasma indicating a considerable neutral penetration. Further, a second peak in the H α radial profile has been observed at ρ(r/a) ~ +/- 0.3 -- +/- 0.5 in majority of discharges irrespective of the plasma column position. This observation suggests a considerable accumulation of neutrals in the region of ρ(r/a) ~ +/- 0.3 -- +/- 0.5. CV to CIII line ratio variations at the same location also suggest a substantial presence of neutrals explained by the charge-exchange, involving collisions between H-like carbon ions and neutral hydrogen atoms. Keywords: tokamak, Abel-like matrix inversion, Balmer alpha, neutral transport. 1. Introduction Neutral atom transport properties play an important role in tokamak plasma confinement [1]. Therefore, it is necessary to measure the neutral atom density profile in a tokamak. Direct measurements of the neutral (hydrogen) density n 0 , in tokamak plasmas are almost lacking. Except for few attempts with LIF [2], often indirect information via charge exchange spectroscopy (CX) of sensible intensity ratios of impurity (carbon) ions is used [3,4]. Several investigations of spectral line ratios in both VUV and X-ray regions of the spectra often required substantial presence of neutral atoms in the hotter central regions of the tokamak to resolve the observed anomalies. In this paper we measured the neutral atom density profile by monitoring the hydrogen Balmer alpha (λ = 656.28 nm) emission from the Aditya tokamak. This radiation results from the collisional excitation of a neutral hydrogen atom from the device wall or limiter surface enters into the main plasma. It is generally assumed that the neutral hydrogen atoms can penetrate to the hot central plasma by diffusion from the edges through successive charge transfers with the working gas ions [1]. Section 2 briefly describes the experimental setup. In section 3 an overview of the calibration of the spectroscopic instruments has been given. Finally, in section 4 the obtained results are discussed. 2. Experimental setup Up to eight simultaneous chords or lines-of-sight (LOS) can be set up from a top port window on Aditya along a poloidal diameter from the inboard to outboard edge. The 1 mm core diameter of fibers and collimating lenses (diameter = 11 mm, focal length = 19 mm) yield a spatial resolution of 25 mm at the mid-plane (increasing to 42 mm at the bottom edge of the plasma). Figure 1A shows a schematic of the viewing geometry. The back ends of these (∼20 m long) fibers couple to eight 400 m core diameter input fibers on the spectrograph entrance slit with a center-to-center separation of 700 m. The slit height can accommodate up to nine of these 400 m core fibers. The ninth fiber is used as a reference track for recording spectrum from a neon lamp during discharges [5]. The Multi-track Spetrometer system consists of a 1.0 m long Czerny–Turner spectrometer fitted with a 2D CCD (charge-coupled device) camera with 1024 × 256 pixels (1 pixel = 26 × 26 m). A normal incidence spectrometer was used to record the spectrum in the wavelength region 110 – 300 nm, which views plasma from a radial port. The view chord 29 J. Plasma Fusion Res. SERIES, Vol. 9 (2010) ©2010 by The Japan Society of Plasma Science and Nuclear Fusion Research (Received: 30 October 2009 / Accepted: 15 January 2010)