Y. Marandet

Deuterium inventory in Tore Supra: reconciling particle balance and post-mortem analysis

Fuel retention, a crucial issue for next step devices, is assessed in present-day tokamaks using two methods: particle balance performed during shots and post-mortem analysis carried out during shutdowns between experimental campaigns. Post-mortem analysis generally gives lower estimates of fuel retention than integrated particle balance. In order to understand the discrepancy between these two methods, a dedicated experimental campaign has been performed in Tore Supra to load the vessel walls with deuterium (D) and monitor the trapped D inventory through particle balance. The campaign was followed by an extensive post-mortem analysis phase of the Tore Supra limiter. This paper presents the status of the analysis phase, including the assessment of the D content in the castellated tile structure of the limiter. Indeed, using combined surface analysis techniques, it was possible to derive the relative contributions of different zones of interest on the limiter (erosion, thick deposits, thin deposits), showing that the post-mortem inventory is mainly due to codeposition (90% of the total), in particular due to gap deposits. However, deuterium was also evidenced deep into the material in erosion zones (10% of the total). At the present stage of the analysis, 50% of the inventory deduced from particle balance has been found through post-mortem analysis, a significant progress with respect to previous studies (factor 8–10 discrepancy). This shows that post-mortem analysis can be consistent with particle balance provided specific procedures are implemented (dedicated campaign followed by extensive post-mortem analysis). Both techniques are needed for a reliable assessment of fuel retention in tokamaks, giving complementary information on how much and where fuel is retained in the vessel walls.

Transport of neutral particles in turbulent scrape-off layer plasmas

The effect of turbulence on the transport of neutral species (atom, molecules) in plasmas is investigated. A stochastic model relying on a multivariate gamma distribution is introduced to describe turbulent fluctuations, and implemented in EIRENE. The effects of fluctuations on the neutral density and ionization source radial profiles are investigated. The role of temperature fluctuations is discussed in detail. Calculations with ITER scrape-off layer parameters are presented, and two distinct regimes with respect to the effects of temperature fluctuations are identified, depending on the far SOL mean temperature. Finally, the influence of fluctuations on impurity contamination is discussed.

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.

An analytical model for the Ly α redistribution function in conditions of tokamak edge plasmas

Radiation redistribution is investigated for applications to magnetic fusion studies. An analytical model, suitable for Monte Carlo simulations of radiative transfer, is developed for the redistribution function of the resonance line of hydrogen isotopes. The model retains Zeeman and Stark effects in the atoms rest frame and in the impact approximation. The Zeeman effect is shown to play a major role on redistribution. Discussions are given on consequences of the Doppler effect in the laboratory frame.

Modelling of Spectral Lines Emitted by Hydrogen Isotopes for Ionising and Recombining Plasma Conditions of Tokamak Edges

The plasma in the periphery or in a divertor chamber of a magnetic fusion device plays an important role in the spread of particle and heat power. Optimising the role of such a plasma requires its characterization and the understanding of all the mutual interactions of its constituents (ions, electrons, and neutrals), and also their interactions with the device materials. Among the various diagnostic methods, high-resolution passive spectroscopy is well suitable for such a purpose since it is non-intrusive. However, diagnostics based on this method requires an accurate modelling of the observed spectra. A fitting routine has been developed for the interpretation of Da spectra measured in front of a neutraliser plate of the Tore-Supra Ergodic Divertor. It takes into account both Zeeman and Doppler effects, the instrumental function, and the relaxation of the neutrals induced by elastic collisions with the plasma ions. On the other side, in order to study high-n Balmer lines and the continuum emission of deuterium in detached divertor plasmas, a more complex lineshape code developed for Stark broadening has been recently coupled to a code accounting for line-merging into the continuum. The code allows the convolution with a Gaussian (or a Lorentzian) to account for the Doppler broadening and the instrumental function. The Stark lineshape code has also been updated to include Zeeman effect and can be used for conditions where both Stark and Zeeman effects are important like in ITER-divertor plasmas.

Spectroscopic analysis of the plasma-neutral relaxation near to the plasma boundary

Abstract In the vicinity of the plasma edge of a magnetic fusion device, several populations of neutral emitters and ionic particles coexist. An elastic collision mechanism between ions and neutrals has been recently identified as a significant relaxation process between these populations. We have studied the relaxation of the neutral particle velocity distribution function (VDF) with a random walk model in velocity space. This VDF is shown to obey a Fokker–Planck equation in the diffusive case. For cases where a non diffusive regime in the velocity space is assumed, our approach leads to a VDF with an asymptotic power law. The model is applied to the observation of the Deuterium Balmer 3–2 transition, D α , spectra observed in the edge of the Tore-Supra Tokamak. The fit of observed asymmetric spectra requires the use of the relaxation model for three different populations of neutrals. A possible explanation for the wing decay of symmetric D α is the presence of a superdiffusive transport in velocity space.

Fuel recovery experiments with isotopic plasma wall changeover during long discharges in Tore Supra

Tritium (T) retention constitutes an outstanding constraint for ITER. It has been proposed that the end of the discharge could be used for reducing the amount of tritium trapped in the device by switching to He or H2 injection during the ∼200xa0s of plasmas following the burning phase (power and plasma current ramp down). Thanks to the long discharge capabilities of Tore Supra, long pulse experiments (>xa0mn) have been carried out to evaluate the effectiveness of such a scenario in reducing the tritium inventory during plasma operations. Starting with the device operated only in D2, series of changeover experiments from D2 to He and from D2 to H2 have been carried out in Tore Supra. The results demonstrate that with He the amount of D recovered after 130xa0s is limited to 0.8xa0×xa01022xa0D whilst no further gain is foreseen. From these experiments, it is demonstrated that He injection will not contribute to the drop of the tritium inventory in the vessel. In contrast, with H2 injection the amount of D recovered after 250xa0s is ∼4.2xa0×xa01022xa0D with no limitation observed in the amount that could be removed from the vessel. The higher efficiency in removing D from the vessel by H2 injection compared to He is attributed to the H charge-exchange (CX) flux (four to six times larger than the He CX flux) allowing for a significantly stronger plasma wall interaction with carbon deposition and layer areas. In Tore Supra, since most of the D retention through co-deposition with eroded material (C) takes place in these areas, H plasmas result in a better removal efficiency of D(T) from these regions. These experimental observations are supported by the results obtained using the EIRENE code for evaluating both the ion and CX fluxes for He and H plasmas. Finally, the consequences of removing D(T) from the vessel for the next discharges are unfavourable for both the He and H2 removal methods. Indeed, in both cases, twice the amount of D(T) removed through the isotope exchange has to be re-injected since co-deposition of the re-injected D(T) will also take place in addition to the plasma wall isotope exchange. In these conditions, the low efficiency of the H2 gas injection for controlling the plasma isotopic ratio inhibits a recovery of the initial plasma isotopic ratio over a time scale in the range of 200xa0s.

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 1s2pu20093Po–1sndu20093D 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 1s2pu20093Po–1sndu20093D with n = 8–10. Applications to spectroscopic diagnostics in existing and future magnetic fusion-oriented devices like ITER are discussed.