A. D. Whiteford

Modelling of measured tungsten spectra from ASDEX Upgrade and predictions for ITER

Tungsten (W) has moved into the focus of fusion research being a main candidate for the plasma facing components (PFCs) of ITER and a future fusion reactor. A main ingredient for understanding the influence of W as a plasma impurity and its impact on the plasma is the spatially resolved spectroscopic diagnosis of W. The focus of the experimental investigations at ASDEX Upgrade is on the most intense emissions of W ions (about I-like W21+ to Mn-like W49+) in the VUV to the soft x-ray region covering the electron temperature range from about 0.5?5.0?keV. The relative shape of the fractional abundances of the ionization stages Se-like W40+ to Ni-like W46+ and of the bundle of ionization stages between Sn-like W24+ and Y-like W35+ was determined. Calculated fractional abundances using published ionization and recombination rates do not accurately describe the experimental temperature dependence. Adjustments to the recombination rates were calculated to reconcile with the measurements. The spectral features of W at 0.4?0.8?nm, around 5?nm, between 12 and 14?nm and between 10 and 30?nm have been recorded and compared with modelling results. The quality of agreement is best for highly charged ionization stages and short wavelengths and decreases for lower charged ionization stages and longer wavelengths. However, in the latter case the predictions manage to reproduce the total emissivity in each considered spectral range and also the rough distribution of emissions versus wavelengths within these spectral ranges. The modelling of the SXR range at 0.4?0.8?nm looks very similar to the measurement. Further observations of weaker spectral features between 0.6 and 0.7?nm, between 1.8 and 3.5?nm and at 8?nm could be attributed to certain ionization stages. The modelling of W spectra for ITER predicts emissions of Cr-like W50+ to about C-like W68+ at 0.1?0.15?nm, 1.8?4.0?nm and around 8?nm.

Tungsten: an option for divertor and main chamber plasma facing components in future fusion devices

The tungsten programme in ASDEX Upgrade is pursued towards a full high-Z device. The spectroscopic diagnostic of W has been extended and refined and the cooling factor of W has been re-evaluated. The W coated surfaces now represent a fraction of 65% of all plasma facing components (24.8 m(2)). The only two major components that are not yet coated are the strikepoint region of the lower divertor as well as the limiters at the low field side. While extending the W surfaces, the W concentration and the discharge behaviour have changed gradually pointing to critical issues when operating with a W wall: anomalous transport in the plasma centre should not be too low, otherwise neoclassical accumulation can occur. One very successful remedy is the addition of central RF heating at the 20-30% level. Regimes with low ELM activity show increased impurity concentration over the whole plasma radius. These discharges can be cured by increasing the ELM frequency through pellet ELM pacemaking or by higher heating power. Moderate gas puffing also mitigates the impurity influx and penetration, however, at the expense of lower confinement. The erosion yield at the low field side guard limiter can be as high as 10(-3) and fast particle losses from NBI were identified to contribute a significant part to the W sputtering. Discharges run in the upper W coated divertor do not show higher W concentrations than comparable discharges in the lower C based divertor. According to impurity transport calculations no strong high-Z accumulation is expected for the ITER standard scenario as long as the anomalous transport is at least as high as the neoclassical one.

Calculation and experimental test of the cooling factor of tungsten

The cooling factor of W is evaluated using state of the art data for line radiation and an ionization balance which has been benchmarked with experiment. For the calculation of line radiation, level-resolved calculations were performed with the Cowan code to obtain the electronic structure and excitation cross sections ( plane-wave Born approximation). The data were processed by a collisional radiative model to obtain electron density dependent emissions. These data were then combined with the radiative power derived from recombination rates and bremsstrahlung to obtain the total cooling factor. The effect of uncertainties in the recombination rates on the cooling factor was studied and was identified to be of secondary importance. The new cooling factor is benchmarked, by comparisons of the line radiation with spectral measurements as well as with a direct measurement of the cooling factor. Additionally, a less detailed calculation using a configuration averaged model was performed. It was used to benchmark the level-resolved calculations and to improve the prediction on radiation power from line radiation for ionization stages which are computationally challenging. The obtained values for the cooling factor validate older predictions from the literature. Its ingredients and the absolute value are consistent with the existing experimental results regarding the value itself, the spectral distribution of emissions and the ionization equilibrium. A table of the cooling factor versus electron temperature is provided. Finally, the cooling factor is used to investigate the operational window of a fusion reactor with W as intrinsic impurity. The minimum value of nT tau(E), for which a thermonuclear burn is possible, is increased by 20% for a W concentration of 3.0 x 10(-5) compared with a plasma without any impurities, except for the He ash which is considered in both cases.

Ionization state, excited populations and emission of impurities in dynamic finite density plasmas: I. The generalized collisional–radiative model for light elements

The paper presents an integrated view of the population structure and its role in establishing the ionization state of light elements in dynamic, finite density, laboratory and astrophysical plasmas. There are four main issues, the generalized collisional-radiative picture for metastables in dynamic plasmas with Maxwellian free electrons and its particularizing to light elements, the methods of bundling and projection for manipulating the population equations, the systematic production/use of state selective fundamental collision data in the metastable resolved picture to all levels for collisonal-radiative modelling and the delivery of appropriate derived coefficients for experiment analysis. The ions of carbon, oxygen and neon are used in illustration. The practical implementation of the methods described here is part of the ADAS Project.

Dielectronic recombination data for dynamic finite-density plasmas III. The beryllium isoelectronic sequence

Dielectronic recombination data for the beryllium isoelectronic sequence has been calculated as part of the assembly of a dielectronic recombination database necessary for modelling of dynamic finite-density plasmas (Badnell et al. 2003, A&A, 406, 1151). Dielectronic recombination coecients for a selection of ions from this sequence are presented and the results discussed.

Tritium transport experiments on the JET tokamak

An overview is given of the experimental method, the analysis technique and the results for trace tritium experiments conducted on the JET tokamak in 2003. Observations associated with events such as sawtooth collapses, neo-classical tearing modes and edge localized modes are described. Tritium transport is seen to approach neo-classical levels in the plasma core at high density and low q(95), and in the transport barrier region of internal transport barrier (ITB) discharges. Tritium transport remains well above neo-classical levels in all other cases. The correlation of the measured tritium diffusion coefficient and convection velocity for normalized minor radii r/a = [0.65, 0.80] with the controllable parameters q95 and plasma density are found to be consistent for all operational regimes (ELMy H-mode discharges with or without ion cyclotron frequency resonance heating, hybrid scenario and ITB discharges). Scaling with local physics parameters is best described by gyro-Bohm scaling with an additional inverse beta dependence.

R-matrix electron-impact excitation calculations along the F-like iso-electronic sequence

We present results for the electron-impact excitation of F-like ions from Ne+ to Kr27+ using an R-matrix approach where the intermediate-coupling frame transformation method has been used to obtain level-resolved collision strengths. For each calculation, the target has been expanded using 87 terms (195 levels) with the following configuration basis: 2s2 2p5, 2s 2p6, 2s2 2p4 3l, 2s 2p5 3l and 2s2 2p4 4l. Comparisons are made with previous R-matrix calculations for the fine-structure transition over the whole sequence and other, more extensive, calculations on Ne+ and Fe17+. We examine iso-electronic trends of both low- and high-temperature effective collision strengths. This complete, self-consistent set of radiative and collision data is archived in multiple databases.

A radiation-damped R -matrix approach to the electron-impact excitation of helium-like ions for diagnostic application to fusion and astrophysical plasmas

Electron-impact excitation collision strengths for transitions between all singly excited levels up to the n = 4 shell of helium-like argon and the n = 4 and 5 shells of helium-like iron have been calculated using a radiation-damped R-matrix approach. The theoretical collision strengths have been examined and associated with their infinite-energy limit values to allow the preparation of Maxwell-averaged effective collision strengths. These are conservatively considered to be accurate to within 20% at all temperatures, 3×105-3×108 K for Ar16+ and 106-109 K for Fe24+. They have been compared with the results of previous studies, where possible, and we find a broad accord. The corresponding rate coefficients are required for use in the calculation of derived, collisional-radiative, effective emission coefficients for helium-like lines for diagnostic application to fusion and astrophysical plasmas. The uncertainties in the fundamental collision data have been used to provide a critical assessment of the expected resultant uncertainties in such derived data, including redistributive and cascade collisional-radiative effects. The consequential uncertainties in the parts of the effective emission coefficients driven by excitation from the ground levels for the key w, x, y and z lines vary between 5% and 10%. Our results remove an uncertainty in the reaction rates of a key class of atomic processes governing the spectral emission of helium-like ions in plasmas.

Atomic data for modelling fusion and astrophysical plasmas

Trends and focii of interest in atomic modelling and data are identified in connection with recent observations and experiments in fusion and astrophysics. In the fusion domain, spectral observations are included of core, beam penetrated and divertor plasma. The helium beam experiments at JET and the studies with very heavy species at ASDEX and JET are noted. In the astrophysics domain, illustrations are given from the SOHO and CHANDRA spacecraft which span from the solar upper atmosphere, through soft x-rays from comets to supernovae remnants. It is shown that non-Maxwellian, dynamic and possibly optically thick regimes must be considered. The generalized collisional-radiative model properly describes the collisional regime of most astrophysical and laboratory fusion plasmas and yields self-consistent derived data for spectral emission, power balance and ionization state studies. The tuning of this method to routine analysis of the spectral observations is described. A forward look is taken as to how such atomic modelling, and the atomic data which underpin it, ought to evolve to deal with the extended conditions and novel environments of the illustrations. It is noted that atomic physics influences most aspects of fusion and astrophysical plasma behaviour but the effectiveness of analysis depends on the quality of the bi-directional pathway from fundamental data production through atomic/plasma model development to the confrontation with experiment. The principal atomic data capability at JET, and other fusion and astrophysical laboratories, is supplied via the Atomic Data and Analysis Structure (ADAS) Project. The close ties between the various experiments and ADAS have helped in this path of communication.

Overview of transport, fast particle and heating and current drive physics using tritium in JET plasmas

Results are presented from the JET Trace Tritium Experimental (TTE) campaign using minority tritium (T) plasmas (n(T)/n(D) 2 MA) and monotonic q-profiles. In CH discharges the gamma-ray emission decay times are much lower than classical (tau(Ts) + tau(alpha s)), indicating alpha confinement degradation, due to the orbit losses and particle orbit drift predicted by a 3-D Fokker-Planck numerical code and modelled using TRANSP.