Application of beam emission spectroscopy to NBI plasmas of Heliotron J

Abstract

This paper describes the application of the beam emission spectroscopy (BES) to Heliotron J, having the nonsymmetrical helical-magnetic-axis configuration. The spectral and spatial profile of the beam emission has been estimated by the numerical calculation taking the collisional excitation processes between plasmas (electrons/ions) and beam atoms. Two sets of the sightlines with good spatial resolution are presented. One is the optimized viewing chords which have 20 sightlines and observe the whole plasma region with the spatial resolution Δρ less than ±0.055 using the newly designed viewing port. The other is 15 sightlines from the present viewing port of Heliotron J for the preliminary measurement to discuss the feasibility of the density fluctuation measurement by BES. The beam emission has been measured by a monochromator with a CCD camera. A good consistency has been obtained between the spectral profiles of the beam emission measured by the monochromator and the beam emission spectrum deduced by the model calculation. An avalanche photodiode with an interference filter system was also used to evaluate the signal-to-noise (S/N) ratio of the beam emission in the present experimental setup. The modification of the optical system is being planned to improve the S/N ratio, which will enable us to estimate the density fluctuation in Heliotron J.