Spectroscopic diagnostics for ablation cloud of tracer-encapsulated solid pellet in LHD

Abstract

In the Large Helical Device ͑LHD͒, various spectroscopic diagnostics have been applied to study the ablation process of an advanced impurity pellet, tracer-encapsulated solid pellet ͑TESPEL͒. The total light emission from the ablation cloud of TESPEL is measured by photomultipliers equipped with individual interference filters, which provide information about the TESPEL penetration depth. The spectra emitted from the TESPEL ablation cloud are measured with a 250 mm Czerny-Turner spectrometer equipped with an intensified charge coupled device detector, which is operated in the fast kinetic mode. This diagnostic allows us to evaluate the temporal evolution of the electron density in the TESPEL ablation cloud. In order to gain information about the spatial distribution of the cloud parameters, a nine image optical system that can simultaneously acquire nine images of the TESPEL ablation cloud has recently been developed. Several images of the TESPEL ablation cloud in different spectral domains will give us the spatial distribution of the TESPEL cloud density and temperature.

Imaging polychromator for density measurements of polystyrene pellet cloud on the Large Helical Device

“…5 Figure 1 shows the scheme of NIOS. The polychromator consists of nine identical lenses (10 mm diameter, 190 mm focal length), each being equipped with both individual aperture and interference filter, field lens (53 mm diameter, 75 mm focal length), and a 12-bit compact digital camera (PCO "Pixelfly") with a fast electronic shutter.…”

“…In Ref. 5, approximately linear dependence of the averaged cloud electron density on the pellet ablation rate was reported. In Ref.…”

“…TESPEL (Tracer Encapsulated Solid PELlet) injection system and diagnostic setup for pellet ablation measurements are described in Ref. 5. The density of target LHD plasma was maintained by the injection of fast neutral particle beams (NBI) in all discharges.…”

“…Resulting values were 10 17 -10 18 cm −3 for hydrogen pellets 6 and 10 16 -10 17 cm −3 for polystyrene pellets. 5 Cloud density evaluations were based on the hydrogen Balmer-beta line width measurements over the whole pellet cloud since Stark broadening dominates all other broadening mechanisms. In Ref.…”

Tracer-encapsulated solid pellet injection system

“…So, to get the two-dimensional density profile in the ablated cloud, the new system, called NIOS system, with 9 narrow bandwidth filters and fast CCD camera has been installed. 12 With this system, we have obtained the density and the electron temperature 2D profiles in the pellet vicinity. 13 The cloud electron temperature is estimated by the ratio of the full H β intensity to continuum spectra intensity.…”