Search

A computer simulation technique is applied to the modelling of Balmer line shapes in dense divertor conditions. The spectral profile of lines with a high principal quantum number n is sensitive to Stark broadening and can be used as a density diagnostic. In contrast, an analysis of the shape of low or moderate n lines such as Dα (n = 3), Dβ (n = 4), and Dγ (n = 5) is more intricate because the Stark effect is weaker and can compete with thermal Doppler broadening. We examine this issue and address the relative contribution of the Stark and Doppler effects on the first Balmer lines. Analyses of experimental spectra are performed. [ABSTRACT FROM AUTHOR]

In plasmas coupled to an external energy source like a beam of energetic electrons, one can observe the non‐linear coupling of Langmuir waves with ion sound and electromagnetic waves. This non‐linear interaction of waves changes the structural and radiative properties of plasmas. Coherent wave packets are trapped in regions of high intensity, and are subjected to a wave collapse cycle. We show that the electric field of the wave packets can modify the spectral lines emitted by hydrogen atoms. Our model for this field is a sequence of envelope solitons oscillating at the plasma frequency or the upper hybrid frequency. The calculations presented concern the Hα and Hβ Balmer lines of hydrogen and show the possible modifications of the line shapes if wave collapse conditions affect the edge plasma. [ABSTRACT FROM AUTHOR]

Kinetic Monte Carlo simulations of coupled atom-radiation transport in optically thick divertor plasmas can be computationally very demanding, in particular in ITER relevant conditions or even larger devices, e.g. for power plant divertor studies. At high (∼ 1015 cm-3) atomic densities, it can be shown that sufficiently large divertors behave in certain areas like a black body near the first resonance line of hydrogen (Lyman α). This suggests that, at least in part, the use of continuum model (radiation hydrodynamics) can be sufficiently accurate, while being less time consuming. In this work, we report on the development of a hybrid model devoted to switch automatically between a kinetic and a continuum description according to the plasma conditions. Calculations of the photo-excitation rate in a homogeneous slab are performed as an illustration. The outlined hybrid concept might be also applicable to neutral atom transport, due to mathematical analogy of transport equations for neutrals and radiation. [ABSTRACT FROM AUTHOR]

Power exhaust is one of the major challenges that future devices such as ITER and DEMO will face. Because of the lack of identified scaling parameters, predictions for divertor plasma conditions in these devices have to rely on detailed modelling. Most plasma edge simulations carried out so far rely on transport codes, which most of the times consist of a fluid code for the plasma coupled to a kinetic Monte Carlo (MC) code for neutral particles. One of the main difficulties in interpreting code results is the statistical noise from the MC procedure, which makes it difficult to define a convergence criterion for the simulations. In this work, we elaborate on similarities between noisy transport code simulations and turbulence simulations, and argue that the time averaged solution is a well defined stationary solution for the system. We illustrate these ideas with a simple slab test case with fluid neutrals, to which we add synthetic noise. In this case, the effects of noise are found to be significant only at high noise levels and for large enough correlations times. [ABSTRACT FROM AUTHOR]

A line shape model that accounts for low-frequency fluctuations is applied to the Balmer series of the hydrogen isotopes (atomic transitions n → 2). The plasma fluid parameters (density, temperature, etc.) are described as random variables characterized by a prescribed joint probability density function. A special emphasis is devoted to Stark broadened lines, which are sensitive to the plasma density. It is shown that fluctuations can lead to a significant increase of the line width if the fluctuation level Δ N e/〈 N e〉 (with 〈 N e〉 and Δ N e being respectively the density averaged along the line-of-sight and its RMS value) is large. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The effects of Scrape-off layer (SOL) turbulent fluctuations on the ionization/recombination source terms in the continuity equations for impurities are investigated. These effects come in addition to those related to turbulent transport, and could be important in the SOL owing to the large relative fluctuation levels measured there. Given the time scales involved, the ionization balance does not follow adiabatically fluctuations. In order to investigate these effects, we have developed a time dependent solver for the population kinetics of the full atomic system. A first calculation is performed for beryllium, using turbulent fields from the TOKAM2D non isothermal code. Turbulent fluctuations are shown to shift substantially the abundances of the different ionization stages via source averaging effects. A stochastic model, which could potentially be used to generate fluctuation averaged ionization/recombination rate coefficients, is then used to model the output of TOKAM2D. The results for abundances are compared to those of the population kinetics code. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Stochastic processes may be used as an efficient model for the line shapes affected by Stark effect in edge plasma conditions. We check the accuracy of this approach on the Lyman γ line of hydrogen perturbed by ions only. We also use a stochastic process to investigate the effect of plasma oscillations on a line shape. Our model assumes that Langmuir waves are a sequence of oscillating electric fields changing their amplitude and phase according to an exponential waiting time distribution. The possible effects of Langmuir waves on the Lyman α Stark profile are discussed in edge plasma conditions. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

In this paper we present some investigations of the broadening mechanisms affecting the emission line profiles of impurities present in the divertor regions of tokamaks. For that purpose, we have considered two visible lines (C IV n = 5-6 and n = 6-7) emitted by lithium-like carbon ions for which the contributions of the Zeeman, Stark and Doppler effects are examined. The paper is focused on the impact of differing electron and ion temperatures on the broadening and shapes of the C IV n = 6-7 line for conditions relevant to a detached tokamak divertor plasma. It is shown, that the C IV n = 6-7 line profile calculated with differing electron and ion temperatures may be significantly different from the one calculated with equal temperatures but the same density. This suggests that the electron density determination from the fit of the measured spectra of such a line may be inaccurate when using equal ion and electron temperatures instead of differing ones. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The radiation due to atomic lines in the tokamak edge and divertor can be strongly coupled to the plasma dynamics. Self-consistent (charged particle - atom - photon) simulations carried out for ITER conditions with the transport code B2-EIRENE have indicated modifications of the effective ionization rate induced by photon absorption effects. Up to now, all of the investigations done so far were made assuming a plasma background whose typical spatial variation scales are much larger than the radiation transport scale. In this paper, we examine the validity of this approximation within an effective transport model, relying on a statistical parameterization of the turbulent fluctuations. The model is inspired from the quasilinear theory used for plasma turbulence and assumes a small ratio lturb/ lmfp between the turbulence correlation length and the photon mean free path. We apply the model to calculations of the penetration of a radiation pencil in an optically thick turbulent plasma. The validity of this model for intermediate lturb/ lmfp is next discussed through comparison to a 'model-coefficient' approach, widely used in problems involving linear stochastic differential equations and suitable for arbitrary turbulence correlation length (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Improving the description of the far scrape-off layer (SOL) in current edge code suites such as SOLPS is the subject of current efforts, given the importance of main chamber plasma wall interactions (first wall recycling and sputtering) in the design and operation of tokamaks. One of the specific features of the far SOL is the large amplitude of turbulent fluctuations. Neutral particles (atom and molecules) are affected by these fluctuations through the rates of atomic physics processes. In accordance with experimentally measured PDF (probability density function) for both the plasma density and temperature fluctuations, n e and T e are modeled using a Beta PDF. Space and time correlations are accounted for by using an autogressive process. The effects of fluctuations on the neutral atoms penetration depth, as well as on the beryllium sputtering yield are discussed (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

High resolution spectra emitted by C3+ ions and measured along vertical chords in the divertor of the JT-60U tokamak in presence of an X-point MARFE are compared to synthetic profiles calculated taking into account Doppler and Stark effects but not the Zeeman effect because the latter was suppressed experimentally thanks to a linear polarizer transmitting only the component parallel to the toroidal magnetic field. For each viewing chord, the electron density is obtained using equal ion and electron temperatures determined from C IV line intensities observed with a large band visible spectrometer. In complement to the electron temperature distribution obtained previously from line intensities and collisional-radiative modeling, a spatial distribution of the electron density is obtained here for the JT-60U detached divertor with an X-point MARFE. For spectra measured along the most central viewing chords with respect to the MARFE (intensity peak of the strong radiation), it is suggested from the spectral fitting that the emission comes from two plasma layers whose parameters have been obtained (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]