Spectroscopic study of a carbon pellet ablation cloud

Abstract

Carbon pellets were injected into high-temperature plasmas produced in the Large Helical Device (LHD), a heliotron-type fusion experimental device. Radiation from a high-density plasma formed around the pellet core, the so-called ablation cloud, was observed and its spectrum in the UV-visible wavelength range was obtained. The observed spectrum is found to be dominated by emission lines of CII and CIII ions, and their level populations are determined from the measured line intensities. The result suggests that LTE (local thermodynamic equilibrium) is established over the measured excited levels for both ions, and the electron temperature is determined to be 2.5 eV and 3.0 eV for the CII and CIII ions, respectively, on the assumption that level populations follow a Boltzmann distribution. For the plasma dominated by the CII lines, the electron density, n e = 6.5 × 10 22 m −3 , and plasma volume, V = 5.3 × 10 −6 m 3 , are simultaneously derived from a fit of the CII λ 723 nm ([1s 2 2s 2 ]3p 2 P o -3d 2 D) line profile which is subject to various broadening effects such as Stark broadening. For the plasma dominated by CIII lines, V = 4.5 × 10 −2 m 3 is derived from an analysis of the reabsorption effect appearing in fine structure lines of the CIII λ 117 nm ([1s 2 ]2s2p 3 P o -2p 2 3 P) transition. The assumption of electric charge neutrality in the ablation cloud yields n e = 4.7 × 10 20 m −3 . These results suggest such a structure in the ablation cloud that a dense plasma dominated by the CII lines is surrounded by a larger and lower density plasma which is responsible for the CIII lines. The assumption of complete LTE for CIII levels, which is used in the analysis, is shown to be approximately true with the help of collisional-radiative model calculations, for which the reabsorption effect is taken into account. Thus, in this study various plasma parameters in the ablation cloud are determined and information about the cloud structure is obtained.

Estimate of the Deposition Profile of Carbon Pellets Using a High-Speed VUV Imaging System in the LHD

“…[13]. Experimentally, the electron density in the ablation cloud is several orders of magnitude higher than that of the background plasma [14] . The temporal variation of the intensity of an emission line from the ablation cloud, for instance the Hα line, can be regarded as a measure of the spatial profile of the particle deposition by assuming that the intensity is proportional to the ablation rate.…”

Line-Shape Code Comparison through Modeling and Fitting of Experimental Spectra of the C ii 723-nm Line Emitted by the Ablation Cloud of a Carbon Pellet

“…Equation (2) indicates that the electron interactions with the emitter result in a homogeneous broadening represented by a Lorentzian function whose full width at half maximum (FWHM) is proportional to the plasma electron density n e and inversely proportional to the square root of the electron temperature T e . It should be noted that the dipole reduced matrix elements are involved through the operator R.R present in relation Equation (2). As the line considered here is of the same type as the Li-like (Li I, B III, N V) 2s-2p, i.e., ∆n = 0 transitions, we will not discuss the validity of impact electronic collision operators such the above one.…”

“…To have a better understanding of why different approaches might end up with different best-fit plasma parameters, it was recommended to contributors willing to analyze the proposed experimental data cases to calculate the relevant line profiles for a small prescribed grid of parameters. The first experimental case concerned the C II 723-nm 1s 2 2s 2 3p 2 P°-1s 2 2s 2 3d 2 D line emitted by the ablation cloud of a carbon pellet injected in the Large Helical Device (LHD) [2]. The data for this case consisted of two spectra, both measured along a line-of-sight that is nearly perpendicular to the magnetic field line but with a polarizer rotated, either nearly parallel "horizontal", or nearly perpendicular "vertical", to the magnetic field (see Figure 1).…”

A study of tungsten spectra using large helical device and compact electron beam ion trap in NIFS

“…The parameters of the ablation cloud of the cylindrical carbon pellet (1.2mm L u 1.2mm I ) measured with spectroscopic method are n e =5x10 16 cm -3 and T e =2.5eV for CII and n e =5x10 14 cm -3 and T e =3.0eV for CIII [26,27]. A typical example of visible spectra measured from the ablation cloud is plotted in Fig.11.…”