Population modelling of the He II energy levels in tokamak plasmas: I. Collisional excitation model

Helium is widely used as a fuel or minority gas in laboratory fusion experiments, and will be present as ash in DT thermonuclear plasmas. It is therefore essential to have a good understanding of its atomic physics. To this end He II population modelling has been undertaken for the spectroscopic levels arising from shells with principal quantum number n = 1 to 5. This paper focuses on a collisional excitation model; ionization and recombination will be considered in a subsequent article. Heavy particle collisional excitation rate coefficients have been generated to supplement the currently-available atomic data for He II, and are presented for proton, deuteron, triton and α-particle projectiles. The widely-used criterion for levels within an n shell being populated in proportion to their statistical weights is reassessed with the most recent atomic data, and found not to apply to the He II levels at tokamak densities (1018–1021 m-3). Consequences of this and other likely sources of errors are quantified, as is the effect of differing electron and ion temperatures. Line intensity ratios, including the so-called ‘branching ratios’ and the fine-structure β1, β2, β3 and γ ratios, are discussed, the latter with regard to their possible use as diagnostics.EURATOM 633053Research Councils UK EP/P012450/

Studies of K and L shell spectra of impurity ions in tokamak plasmas

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Diagnostic exploitation of complex heavy elements in tokamak plasmas

In future burning plasma experiments, a promising method for the diagnosis of first wall erosion is to detect the onset of emission from nonintrinsic materials embedded in the plasma facing components as they are revealed and enter the confined plasma. Likely high Z species were introduced into the Joint European Torus plasmas by laser ablation. Results from targets of single species (W and Hf) and dual species (W/Hf composite) are presented. The composite target experiment has shown that it is possible to distinguish reliably between emission from near neighbor heavy elements, even with instrument resolutions of /~0.001. Forward prediction to power plant conditions necessitates a model of the complex spectra and a system for generating the required atomic data and producing spectral feature emissivity coefficients is described. An important consideration is to enable exploration of different instrumentation so the spectral resolution of the compound features are tuned to the diagnostic capability

Developments in soft X-ray spectral modelling for fusion : areas of overlap with astrophysics

This paper focuses on the diagnostic analysis in magnetically confined fusion plasmas with attention drawn both to the methodology in common with that in astrophysics and to points of departure - such as species, transport, high density and the role of neutral hydrogen. Comment is also made on the state of high quality theoretical collision data from a fusion perspective and how it can support rigorous error analysis of better spectroscopic measurement

Carbon charge exchange analysis in the ITER-like wall environment

Charge exchange spectroscopy has long been a key diagnostic tool for fusion plasmas and is well developed in devices with Carbon Plasma-Facing Components. Operation with the ITER-like wall at JET has resulted in changes to the spectrum in the region of the Carbon charge exchange line at 529.06 nm and demonstrates the need to revise the core charge exchange analysis for this line. An investigation has been made of this spectral region in different plasma conditions and the revised description of the spectral lines to be included in the analysis is presented

Atomic modeling and instrumentation for measurement and analysis of emission in preparation for the ITER-like wall in JET

In the near future the interior materials of JET will be changed to a beryllium wall and tungsten divertor so as to resemble the ITER design. Initially there will be no carbon strike plates in the divertor but there is the possibility of adding these later for an exact match. Such a change requires upgraded spectroscopic diagnostics in order to measure the spatially dependent influx of these impurities and their concentrations in the confined plasma. With 74 ionization stages (many of them very complex) the high Z of tungsten also places new demands on the atomic physics required to model its behavior which in turn informs the design of potential diagnostic systems. Modeled emission patterns from the upgraded JET are used as part of the diagnostic specification. We describe a suite of spectroscopic diagnostics in the visible, extreme ultraviolet, vacuum ultraviolet, and x-ray spectral regions suitable for these tasks. (c) 2006 American Institute of Physics

Transient impurity events in JET with the new ITER-like wall

Transient impurity events leading to an unexpected increase in radiated power have been studied in JET from the installation of the ITER-like wall. A total of 1800 events over 2800 plasma discharges have been detected. None have led to permanent changes in the plasma conditions. Of all the events 60% show traces of W and 25% of either Ni, Fe or Cr from either Inconel or steel structures. They occur mainly in diverted magnetic configuration, independently of strike-point position. The effect of disruptions on dust redistribution has been investigated using the Thomson scattering diagnostic and correlated with transient impurity event occurrence. The number of dust events detected increases with disruption force and, in comparison to the full-C wall, the amount of dust mobilized is found to be about an order of magnitude lower. Their time evolution correlates well with that of the transient impurity events

Method for experimental determination of Z dependence of impurity transport on JET

The prediction of impurity peaking in future fusion devices such as ITER necessitates the study of the dependence on Z of the impurity transport in present devices. In this paper we describe a novel technique to determine the transport of impurities with different atomic numbers independently. A technique has been developed that allows simultaneously the measurement of the transport of Ne and Ar in the same discharge while minimizing the systematic errors in the spectroscopic measurements. The reproduction of the charge-exchange measured densities, absolute vaccum ultra-violet line intensities and absolute soft x-ray intensity is achieved in an impurity transport simulation. The method used to estimate the errors on the transport coefficients of neon (Ne) and argon (Ar) is presented. In the plasma region where the diffusion and convection coefficients are determined for hybrid discharges, the transport of Ne and Ar is observed to exceed neoclassical predictions. In the same regions, the diffusion coefficients of both impurities are similar. The convection coefficients are also comparable for Ne and Ar. The peaking of Ne and Ar density profiles are comparable during the period where the intermittent slow reconnecting n = 1 mode is stable in these hybrid discharges