A. Pospieszczyk

Recent developments for plasma edge diagnostics using atomic beams

Abstract Li-atom beams with velocities of 1.5 × 105 cm/s and 1× 106 cm/s have been used to measure ne profiles in the density range 1011–1013 cm−3 from the Li line emission with a spatial resolution of about 1 mm. Injecting in addition a different type of atoms, the ionization rate of which shows a strong dependance on the electron temperature in the interesting range of 5–100 eV (e.g. carbon and helium), also radial Te profiles are obtained. A high intensity 30 keV Li beam is employed to measure radial profiles of impurity ion concentrations (e.g. He, C, 0) by charge exchange recombination spectroscopy. For the different purposes several types of injectors have been developed: thermal beams using ovens, suprathermal beams using laser ablation techniques, and a high energy beam using a plasma ion source in combination with a charge exchange cell. They are located at different poloidal and toroidal positions. The spatial line emission profiles of the beams are recorded either by Si diode array cameras or by a photomultiplier in combination with a scanning mirror so that a spatial resolution better than 2 mm is achieved. The combination of all these systems delivers information about the structure of the TEXTOR boundary layer in poloidal and toroidal direction.

Re-cycling experiments in the DITE tokamak

Re-cycling is studied in the DITE tokamak by changing the working gas from hydrogen to deuterium and back to hydrogen. The change in the composition of the plasma is followed both during the discharge and from shot to shot by using a scanning Fabry-Perot interferometer, a mass spectrometer and a carbon thermal desorption probe. The results of the three techniques are consistent and show that the gas trapped and desorbed from the wall plays a major part in the re-cycling process. A detailed analytical modelof the principal re-cycling mechanisms is proposed.

Plasma edge physics in the textor tokamak with poloidal and toroidal limiters

Abstract Investigations of the plasma edge in TEXTOR are presented on the one hand by comparing results obtained with poloidal and toroidal limiters and on the other hand by discussing general problems of plasma edge physics which are independent of the limiter configuration. The characteristic properties of plasma flow to the different limiters are analyzed and show e.g. that the fraction of total ion flow to the limiter is much larger in the case of a toroidal limiter (80%). Density and heat flux profiles are presented which demonstrate that for both types of limiters a significant steepening of the scrape-off layer (SOL) occurs close to the limiter, leading to a small heat load e-folding length of 5–8 mm. The velocity distribution of recycled neutral hydrogen at a main limiter has been determined from the Doppler broadening of the H α line. The data clearly show that a large fraction of particles (30–50%) is reflected at the limiter surface having energies of about the sheath potential. Significant isotopic effects (H/D) concerning the plasma edge properties and the plasma core are presented and their relation to enhanced particle and energy transport in hydrogen compared to deuterium is discussed. A decrease of the cross field diffusion coefficient with increasing density can be deduced from density profile measurements in the SOL and a comparison with density fluctuations is given. The role of oxygen for impurity release is demonstrated. A new type of wall conditioning —boronization — is described, with two major improvements for quasi stationary conditions: reduction of oxygen and better density control. Best results with ICRH have been obtained under these conditions.

Plasma edge research on TEXTOR

The relations between plasma wall interaction and plasma core properties are studied by spectroscopic methods applied to the emission of neutral particles at the plasma boundary. The interdependence between the structure of the scrape-off layer (SOL), particle-fluxes and impurity release are analyzed. In particular, the importance of molecule formation for impurity release and particle recycling in a machine with an all-carbon surrounding is shown. These mechanisms can lead to a stationary discharge with a cold plasma mantle (detached plasma), the properties of which are discussed. Local heating inside the SOL with ICRH is evident and some consequences for gas release and impurity generation are drawn.

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 ...

Properties of the TEXTOR boundary layer

Abstract First measurements on the TEXTOR boundary layer are reported. The hydrogen recycling in front of the four limiter segments has been studied by means of a CCD-camera, which proved to be a good instrument to center the discharge for symmetric plasma-limiter contact. The composition of the neutral fluxes from the limiter have been measured: oxygen fluxes are about a factor of ten higher than the metal fluxes; within the error limits the composition does not change with varying limiter radius. Electron densities in the scrape-off layer away from the limiter have been determined by injecting an Li-atom beam from a thermal source and by observing its emission as a function of radius. Similar measurements have been made in front of the limiter with sputtered Cr and O atoms. Both methods gave for the magnetic surface of the limiter radius n c ≈ 1 × 10 12 /cm 3 . Infrared observations of a test limiter with a CCD-camera and a PbSe-detector have been performed to record the thermal loads. About 10% of the input power flows to the limiter.

Investigation of radiation enhanced sublimation of graphite test-limiters in TEXTOR

Abstract Graphite test-limiters have been used as single main limiters in TEXTOR and exposed to convective heat loads of typically 10–20 MW/m 2 . Local carbon release has been studied by means of local spectroscopy of CI-and CII-emission lines. Surface temperature distributions were determined by CCD camera measurements in the IR spectral region. It was found that the increase of the local carbon flux was below 15% when the surface temperature increased locally up to about 1900°C demonstrating that radiation enhanced carbon emission (RES) is negligible under these conditions. The suppression of RES-emission is discussed in view of present RES models.

Plasma-wall interaction and plasma performance in textor — A review

Abstract TEXTOR, a tokamak primarily devoted to plasma-wall interaction studies, has been operated during the last 2–3 years under a wide variety of discharge conditions. Discharges with Ohmic heating only and with additional ICR heating in the MW-range were produced over an extended range of plasma parameters. Different limiter materials were used — Inconel, stainless steel, graphite, TiC coated graphite. The surface of the first wall (liner) consisted either of Inconel or of a carbon layer deposited upon the Inconel. Characteristic features of the discharge have been modified by making use of the modular pump limiter ALT-I and by applying a localized perturbation coil for “ergodization” of the magnetic field structure in the boundary. The characteristics of plasma-wall interaction under these conditions and their influence on the global performance of the TEXTOR plasma are described and discussed. Use has been made of novel diagnostics specifically developed for edge plasma investigation.

A study of impurity transport in the plasma boundary of TEXTOR using gas puffing

The transport of carbon and oxygen impurities has been studied in TEXTOR by introducing the gases CH, and CO through a small hole in a test limiter. The toroidal distributions of different charge states of the impurities have been measured using a CCD camera with optical filters. Local impurity ion temperatures have been calculated from the Doppler broadening of line emission measured with a high resolution spectrometer. The spatial distributions and the ion temperatures have been modelled using the LIM Monte Carlo impurity code, with experimentally measured plasma profiles. Good agreement is obtained for both sets of measurements. The comparison shows the breakup energies of the atomic fragments to be < 1 eV. The fuelling efficiency of different gas species is discussed.

Observations of low frequency density fluctuations in a tokamak edge plasma

Density fluctuations in the outer edge of the plasma, behind the limiter, have been investigated in the TEXTOR tokamak by a new technique using a probing beam of thermal neutral lithium. With this method, radial profiles of the electron density and its fluctuations are obtained and thus also the relationship between turbulence and plasma parameters, especially the density gradient scale lengths. The turbulences are found to consist of coherent fluctuations in the frequency range of sawtooth oscillations (<100 Hz) and fluctuations having a broadband spectrum of frequencies up to ~1 MHz. The frequency spectra vary, depending on radial position and the density gradient scale length. It is also shown that the fluctuation level is well correlated with the density gradient scale length.