Ph. Mertens

Overview of the ITER-like wall project

Work is in progress to completely replace, in 2008/9, the existing JET CFC tiles with a configuration of plasma facing materials consistent with the ITER design. The ITER-like wall (ILW) will be cr ...

Densities and velocity distributions of atomic hydrogen and carbon, measured by laser-induced fluorescence with frequency tripling into the vacuum UV

By focussing a commercial dye laser pumped with a XeCl excimer laser into phase matched Xe-A or Kr-A gas mixtures, radiation at the third-harmonic frequency has been generated in the wavelength regions: 1142–1165, 1178–1186, 1203–1224, and 1268–1290 Å. VUV powers up to about 200 W have been detected by a calibrated Au-photodiode. The vacuum uv radiation has been used for the fluorescence excitation of H and C atoms produced by thermal dissociation or by a gas discharge. Absolute densities have been derived by a comparison of fluorescence intensities with intensities from Rayleigh scattering in argon. As a further application, velocity distributions of C atoms sputtered from a graphite target by 1.5 keV argon ions have been measured. These data are in good agreement with a Thompson distribution corresponding to a surface energy of 8.2 eV.

High confinement and high density with stationary plasma energy and strong edge radiation cooling in the upgraded Torus experiment for technology oriented research (TEXTOR-94)

An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of the 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high β (up to a normalized beta, βn=2) with low edge q values (down to qa=2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges are described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power remains. No deleterious effects of the neon seeding on fusion reactivity and plasma stability have been observed.An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of the 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high β (up to a normalized beta, βn=2) with low edge q values (down to qa=2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges are described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power ...

Development and testing of a bulk tungsten tile for the JET divertor

The ITER-like wall project has been launched to design, manufacture and test all the necessary components in view of their installation in a dedicated shutdown 2008?09. One of the RD (ii) W lamellae design, which is a mechanical assembly of several stacks of W blades. Both concepts were realized as test tiles and tested under high heat flux loading. The lamellae test tile showed a high performance fulfilling the requirements. Finally, the lamellae concept was chosen to be the primary solution and comprehensive numerical analyses were performed for the further optimization of the concept.

Measurement of velocity distributions of sputtered carbon atoms using laser-induced fluorescence in the vacuum UV

Abstract Materials containing carbon such as graphites or carbides are often used as wall or limiter materials in fusion devices. For these materials, a knowledge of the velocity distribution of the sputtered particles is very important. Laser-induced fluorescence is one method to obtain these data. Since the resonance lines of the C-atom are in the vacuum UV wavelength range,tunable radiation has to be provided in this region. The transitions 2p23P → 2p3s3P0 at 1657 A have been excited by shifting dye laser radiation using stimulated Raman scattering in H2 (8th anti-Stokes line). Sputtering experiments have been performed by bombarding graphite and TiC probes with 1.5 keV argon ions at a current of about 30 mA. We have measured velocity distributions which can be well approximated by a Thompson distribution with a surface energy of 8.3 eV.

Tungsten and beryllium armour development for the JET ITER-like wall project

For the ITER-like wall project at JET the present main chamber CFC tiles will be exchanged with Be tiles and in parallel a fully tungsten-clad divertor will be prepared. Therefore three R&D programmes were initiated: Be coatings on Inconel as well as Be erosion markers were developed for the first wall of the main chamber. High heat flux screening and cyclic loading tests carried out on the Be coatings on Inconel showed excellent performance, above the required power and energy density. For the divertor a conceptual design for a bulk W horizontal target plate was investigated, with the emphasis on minimizing electromagnetic forces. The design consisted of stacks of W lamellae of 6 mm width that were insulated in the toroidal direction. High heat flux tests of a test module were performed with an electron beam at an absorbed power density up to 9 MW m −2 for more than 150 pulses and finally with increasing power loads leading to surface temperatures in excess of 3000 ◦ C. No macroscopic failure occurred during the test while SEM showed the development of micro-cracks on the loaded surface. For all other divertor parts R&D was performed to provide the technology to coat the 2-directional CFC material used at JET with thin tungsten coatings. The W-coated CFC tiles were subjected to heat loads with power densities ranging up to 23.5 MW m −2 and exposed to cyclic heat loading for 200 pulses at 10.5 MW m −2 . All coatings developed cracks perpendicular to the CFC fibres due to the stronger contraction of the coating upon cool-down after the heat pulses.

Spectroscopic measurements of the ion temperature profile in front of a limiter in TEXTOR-94

Ion temperatures have been measured in front of a limiter in TEXTOR-94 under various plasma conditions by means of the Doppler broadening of the C5+ line (λ = 529 nm), which is excited by charge exchange processes with hydrogen (deuterium) atoms recycled at the limiter surface. The ion temperatures have been compared with electron temperatures measured by atomic beam techniques. It has been found that the ion temperatures are considerably larger than those of the electrons. In the scrape-off layer (SOL) the ratio between the ion and electron temperatures (Ti/Te) can reach values of up to three at low plasma densities, but this ratio decreases with increasing density. The SOL e-folding length of the ion temperature is much larger than that of the electron temperature. The influence of the test limiter position rL on the ion temperature measurements has been investigated but found to be of no significant importance.

Velocity distribution of hydrogen atoms sputtered from metal hydrides

Abstract Owing to the interaction of the limiters and walls of a tokamak with the surrounding plasma, the material of these components is often saturated with hydrogen. In order to learn about its behavior under ion bombardment, the sputtering of metal deuterides by argon ions has been investigated. Targets made of niobium or vanadium saturated with deuterium are bombarded by ion currents of about 25 mA at 1 keV. The released D atoms have been detected in front of the target by laser-induced fluorescence at Lα (121.5 nm). By scanning the absorption profile, the velocity and energy distributions of the D atoms have been determined from the Doppler shift. The typical width of the energy distribution function was about 1 eV. By comparison of the fluorescence signal with the Rayleigh scattering signal in argon, absolute sputtering yields have been estimated. Values over 40% have been found.

Comparing scrape-off layer and divertor physics in JET pure He and D discharges

Though helium plasmas are one option for the low activation phase of ITER, little effort has thus far been devoted to studying them in a large, diverted tokamak. A recent campaign on JET has therefore sought to address some of the important questions related to helium operation (He concentrations near 90%) in single null configuruations, particularly with regard to edge and divertor physics. This contribution compiles a selection of results from these experiments, in which, in each case, discharges have been chosen to match as closely as possible previous, well characterised D plasmas in both L and ELMing H-modes. These matched pulses are used to draw conclusions regarding the principle source and location of carbon production in D plasmas, to compare and contrast the mechanisms of the density limit and the detachment process in D and He, to investigate the nature of cross-field power transport in the SOL and to gain insight into the process by which ELM energy is transported to the divertor targets

On corrections to spectroscopically measured Franck-Condon energies arising from motion of the parent molecules and from ion collisions in fusion plasmas : relevant time scales for atomic velocity distributions

We show that while corrections to spectroscopically measured Franck?Condon energies, arising from motion of the parent molecules at the edge of fusion plasmas, are insignificant in many practical cases, ion collisions play an important role in randomizing and redistributing the velocities of the atomic products of molecular dissociation. Formulae are obtained from which the time scales for randomization and equilibration of the atomic velocity distribution may be estimated, and hence compared with characteristic times for other important atomic processes. Examples of the latter are: spontaneous emission, production of atoms by electron impact-induced dissociation and ionization of molecules, electron collisions of the first and second kinds, charge-exchange recombination and electron impact ionization. We show that the time scales of randomization and equilibration can be comparable with those for some of the other processes listed above, but are much longer than typical times for spontaneous emission. The ion?atom collisional processes considered here therefore affect the atomic velocity distribution mainly after the atoms produced directly by molecular dissociation have arrived in their ground states. The formulae derived for these collisional time scales may well be useful in modelling hydrogen transport and recycling in fusion plasmas.