M. Koubiti

Narrowing of Doppler spectral line shapes by correlated ion and electron temperature fluctuations

The influence of low-frequency temperature fluctuations on Doppler line shapes is investigated. It is shown that the line profile can be strongly narrowed if the fluctuations of ion and electron temperatures are correlated. This result shows that the widely used model retaining only velocity fluctuations in the analysis of Doppler lines might lead to misleading conclusions about turbulence level.

Analysis of asymmetric Dα spectra emitted in front of a neutralizer plate of the Tore-Supra ergodic divertor

A spectroscopic analysis of the deuterium D? line (? = 6561.0??) emitted in the edge of Tore-Supra just in front of the equatorial neutralizer plate of the ergodic divertor is presented. This analysis, which relies on a line shape modelling, is focused on asymmetric D? spectra which are obtained in some diverted discharges. It will be shown that three populations of neutrals are?necessary to interpret such spectra. A cold atoms population results from the dissociation of molecules desorbed from the surface, and a warm one from charge-exchange reactions with the plasma ions. The last population of neutrals is produced by normal reflection of plasma ions on the neutralizer V-shaped notches \mbox{(V-points)} and has an anisotropic velocity distribution function. The anisotropy of this velocity distribution function is due to its initial form and to its partial relaxation by neutral-ion elastic collisions. The analysis based on a line shape fitting program including a relaxation model allows one to estimate the relative contributions of the neutral populations of recycled deuterium to the?D? emission and their Doppler temperatures. Finally, the dependence of the ion and neutral temperatures deduced from the fits and the dominant molecular dissociation channels are discussed as a function of the edge electron density and temperature.

A spectroscopic investigation of turbulence in magnetized plasmas

Abstract We present a spectroscopic investigation of turbulence in the Tore-Supra edge plasma, where deuterium spectral lines are found to exhibit a power-law behavior in their wings. Such a feature is not predicted by the equilibrium line broadening theory in the conditions of the edge plasma, where the thermal Stark effect is negligible. Therefore, the possible role of turbulence is investigated along two separate paths. Indeed, both the Stark and the Doppler profiles may differ significantly from the equilibrium profiles.

Spectroscopic study of a carbon pellet ablation cloud

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, ne = 6.5 × 1022 m−3, and plasma volume, V = 5.3 × 10−6 m3, are simultaneously derived from a fit of the CII λ 723 nm ([1s22s2]3p 2Po–3d 2D) 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 m3 is derived from an analysis of the reabsorption effect appearing in fine structure lines of the CIII λ 117 nm ([1s2]2s2p 3Po–2p2 3P) transition. The assumption of electric charge neutrality in the ablation cloud yields ne = 4.7 × 1020 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.

Stark broadening of high-members of the helium diffuse series in divertor plasmas

Stark profiles of high-members (n ≥ 7) of the helium triplet diffuse series 1s2p 3Po–1snd 3D are calculated for tokamak divertor conditions using dipole reduced matrix elements obtained with a hydrogenic approximation. It is shown that the so-called standard model of Stark broadening is particularly suited to the description of the isolated lines 1s2p 3Po–1snd 3D with n = 8–10. Applications to spectroscopic diagnostics in existing and future magnetic fusion-oriented devices like ITER are discussed.

Revisiting the Stark Broadening by fluctuating electric fields using the Continuous Time Random Walk Theory

Stark broadening of atomic lines in plasmas is calculated by modelling the plasma stochastic electric field using the CTRW approach [1,2]. This allows retaining non Markovian terms in the Schrodinger equation averaged over the electric field fluctuations. As an application we consider a special case of a non separable CTRW process, the so called Kangaroo process [3]. An analytic expression for the line profile is presented for arbitrary waiting time distribution functions. A preliminary application to the hydrogen Lyman α line is discussed.

Effects of Turbulent Temperature Fluctuations on the Core of Doppler Line Shapes

In this work, the influence of the turbulent plasma temperature fluctuations brought on the Hα Doppler line shape is investigated. We show that the core of the line profile is modified by the plasma turbulence. The consequences on the ion temperature determination from Doppler line profile analysis are discussed.

Effect Of Fine Structure On The Accuracy Of A Spectroscopic Plasma Diagnostic

In this work we analyze the role played by fine structure in the modeling of spectral line shape. In the case of non hydrogenic impurities, fine structure obviously cannot be neglected since the corresponding components are often resolved. This is generally not the case for hydrogenic lines, which have larger Doppler widths. Nevertheless, we show that in some tokamak plasmas conditions fine structure must be taken into account so as to ensure accurate line width measurements.We present a numerical code developed by our group, which is able to calculate line shapes emitted by any kind of atoms perturbed by electric and magnetic fields. Magnetic field effects on fine structure spectra of both impurities and deuterium are investigated for tokamak edge conditions, i.e. for emitter temperature varying from a fraction of eV to a few tens of eV, and for magnetic field in the range of 0–5 teslas.

Investigation of non thermal effects from the Dα line wings in edge plasmas

The far wings of intense Dα lines measured at the edge of the Tore Supra Tokamak are found to exhibit a power‐law behavior. The characteristic exponent is not far from two. Since the low density rules out thermal Stark broadening, we discuss non thermal effects which may arise from the edge plasma drift‐wave turbulence. We suggest that both the Stark and the Doppler profile could be affected by the turbulence.

Diagnostics Of Detached Plasmas Using High‐n Lines And Continuum Spectra Of D and He

A Stark line shape code has been coupled to a collisional‐radiative model and to an analytical model for the merging into the continuum of the Balmer lines. The coupled codes have been used for temperature and density diagnostics of detached plasmas. In contrast with the occupation probability and the lowering of the continuum edge approaches, the analytical line merging model used here consists in the use of Lorentzian profiles for highly excited Balmer transitions of deuterium. In addition, high‐n helium lines (1s2p‐1snl) up to n=12 observed in the JET divertor have been preliminary analyzed. The intensities of these experimental helium lines decrease more rapidly with the upper state nl than the intensities calculated with state populations at local thermodynamic equilibrium (LTE).