R. Doerner

Bulk-boronized graphites for plasma-facing components in ITER

Abstract Newly developed bulk-boronized graphites and boronized C-C composites with a total boron concentration ranging from 1 wt% to 30 wt% have been evaluated as plasma-facing component materials for the International Thermonuclear Experimental Reactor (ITER). Bulk-boronized graphites have been bombarded with high-flux deuterium plasmas at temperatures between 200 and 1600 °C. Plasma interaction induced erosion of bulk-boronized graphites is observed to be a factor of 2–3 smaller than that of pyrolytic graphite, in regimes of physical sputtering, chemical sputtering and radiation enhanced sublimation. Postbombardment thermal desorption spectroscopy indicates that bulk-boronized graphites enhance recombinative desorption of deuterium, which leads to a suppression of the formation of deuterocarbon due to chemical sputtering. The tritium inventory in graphite has been found to decrease by an order of magnitude due to 10 wt% bulk-boronization at temperatures above 1000 °C. The critical heat flux to induce cracking for bulk-boronized graphites has been found to be essentially the same as that for non-boronized graphites. Also, 10 wt% bulk-boronization of graphite hinders air oxidation nearly completely at 800° C and reduces the steam oxidation rate by a factor of 2–3 at around 1100 and 1350 °C.

A fast scanning probe for DIII–D

A fast reciprocating probe has been developed for DIII–D which can penetrate the separatrix during H mode with up to 5 MW of NBI heating. The probe has been designed to carry various sensor tips into the scrape‐off layer at a velocity of 3 m/s and dwell motionless for a programmed period of time. The driving force is provided by a pneumatic cylinder charged with helium to facilitate greater mass flow. The first series of experiments have been done using a Langmuir probe head with five graphite tips to measure radial profiles of ne, Te, φf, ne, and φf. The amplitude and phase of the fluctuating quantities are measured by using specially constructed vacuum compatible 5‐kV coaxial transmission lines which allow us to extend the measurements into the MHz range. TTZ ceramic bearings and fast stroke bellows were also specially designed for the DIII–D probe. Initial measurements will be presented.

Performance of boron/carbon first wall materials under fusion relevant conditions

Abstract The conditioning of the plasma facing wall in thermonuclear confinement experiments has been performed very successfully by the application of amorphous boron containing hydrogenated carbon films. Boronization leads to tokamak discharges with significantly reduced oxygen and carbon contaminations. For high heat flux components (especially in future quasi-stationary confinement experiments) new boron/carbon materials have to be developed: monolithic tiles of boronated graphites which can be brazed to watercooled substrates or thick B 4 C-coatings on graphite or high-Z coolant tubes. A variety of bulk materials (boronated graphites with boron contents in the range from 3 to 30%, so-called coat mix material on the basis of B 4 C) and coatings (amorphous B/C films, thick B 4 C layers applied by LPPS or CVD methods) were characterized systematically. In addition the behaviour of these materials was investigated under thermal loads; erosion and disruption simulation experiments were performed in electron and ion beam high heat flux test facilities. Physical and chemical sputtering of the coat-mix-material was studied in the PISCES-B facility in dependence on the hydrogen ions fluence.

Experimental simulation of the gaseous divertor concept in PISCES-A

Abstract The concept of the reentrant divertor (or gaseous divertor) has been suggested as a possible solution to the divertor heat load problem in next generation tokamaks. The idea of the reentrant divertor is to redistribute the divertor heat flux over a large surface area by radiation and/or elastic and inelastic collisions with neutral particles. Simulation experiments are performed in the PISCES-A linear plasma device to test the basic concept and to evaluate the axial and radial particle and heat transport. To date, data have been obtained in steady state hydrogen and argon plasmas at densities of up to 2 × 10 13 cm −3 and electron temperatures of 5–30 eV. With moderate gas feed (10 mTorr) to a simulated divertor slot (length 90 cm) we have observed the electron temperature to decrease axially from 25 eV to 3 eV. At higher neutral pressure (> 25 mTorr) a neutralizer regime is found, where the plasma density at the simulated divertor target can be reduced by more than two orders of magnitude. Radial plasma loss is proportional to the neutral pressure and greatly enhanced as compared to the Bohm rate and the classical diffusion rate. The axial plasma heat flux to the divertor target is reduced by a factor of up to 2 × 10 3 .

E×B transport in the DIII-D boundary plasma

We have measured the electrostatic turbulence and associated particle transport in the DIII-D boundary plasma using a fast reciprocating Langmuir probe array located on the outboard midplane. Both the normalized rms fluctuation levels (density and floating potential) and the fluctuation-driven particle transport are altered by the L-H transition in the SOL. At the separatrix, the density fluctuation level is reduced a factor of 2, consistent with reflectometry results. There is a corresponding decrease in the turbulent particle flux. Deeper in the SOL, the turbulent particle transport in H-mode exceeds the L-mode value. The perpendicular diffusion coefficient D ⊥ and particle confinement time τ p have been estimated, assuming that the transport is purely turbulent and uniform on a flux surface. We find D ⊥ =0.7 D B (L) and 0.04 D B (ELM-free H), and τ p =54 ms (L) and 480 ms (ELM-free H).

Scrape-off layer measurements in DIII-D

In this paper, scrape-off layer measurements in DIII-D are presented as a function of the main discharge plasma parameters. A systematic study is under way to understand and predict the behavior of the edge and divertor plasma in DIII-D and this scaling behavior will be crucial for the design of ITER. To facilitate the studies, a fast reciprocating Langmuir probe incorporating five graphite tips was installed at the midplane of DIII-D which has the capability of performing multiple plunges 1 cm inside the separatrix during 5 MW of NBI. Density and temperature profiles in the midplane (reciprocating probe), near the top (Thomson scattering) and at the lower divertor plate (fixed Langmuir probe array) are compared by mapping the measurements into magnetic flux coordinates. Local pressure measurements are compared on different parts of a flux surface. The three different local measurements also indicate the spatial evolution of plasma conditions as plasma approaches the divertor plate. Ohmic and L-mode discharges exhibit similar (exponential) density and temperature decay in the scrape-off layer. H-mode discharges, however, display a faster spatial decay reflecting at least a factor of 3 decrease in the perpendicular diffusion coefficient. Consistency of the magnitude and scaling behavior of the edge profile parameters with models of the scrape-off layer is examined.

Helium line emission measurements in PISCES-B as a tool for Te-profile determinations in tokamak boundary plasmas

Abstract The potential of helium spectroscopy for quasicontinuous electron temperature profile measurements in the boundary layer of a tokamak plasma is tested. Particularly thermal atomic helium beams are easy to produce and can penetrate relatively far into typical tokamak boundary plasmas because of their high ionization energy. However, in the case of helium and for the range of densities and temperatures characteristic for tokamak boundaries the validity of the corona model cannot be assumed. Therefore it is vital to measure Hel-line intensities in well diagnosed plasmas with tokamak boundary like conditions, which may be compared with calculated ones. For the production of a plasma with parameters close to that of a tokamak boundary layer the PISCES-B facility was used, where plasmas with up to T e ≈ 30 eV simultaneously with n e ≈ 5 × 10 12 /cm 3 could be obtained. T e - and n e -profiles were determined both by fixed and reciprocating Langmuir probes. The helium was injected into the central part of an argon plasma, and its spectral line emission in the visible was measured by means of an OMA attached to an optical spectrometer. Six triplet and six singlet emission lines of HeI were measured and the ratio of these — which is assumed to be a function of T e — evaluated for a range of temperatures and densities. From their absolute intensities also the individual line excitation rates could be derived as a function of electron temperature. It was found that the majority of the experimental values both for the line excitation rate and the line intensity ratio do not agree with the calculated values, which take only excitation from the ground state into account. On the other hand, experimental intensity ratios were found, which do not show a density dependence, and are recommended to be used for an experimental technique, which would allow a quasicontinuous T e -profile measurement in the plasma boundary region of tokamaks.

Helium transport in enhanced confinement regimes on the TEXTOR and DIII-D tokamaks

Comparisons of helium (He) transport and exhaust in L-mode and in an enhanced confinement regime (H-mode), which is induced by a polarizing electrode, have been made for the TEXTOR tokamak. The results show an increased tendency for He accumulation when bulk plasma energy and particle confinement are improved during the polarization induced H-mode. Since these results imply that a high He pumping efficiency may be necessary for H-mode burning plasmas, we have begun exploring He transport in a divertor H-mode, similar to that proposed for International Thermonuclear Experimental Reactor (ITER). A collaborative program has been initiated to measure He transport and scaling on DIII-D during L-mode, H-mode, and ELMing H-mode plasma conditions. To simulate the presence of He ash in DIII-D, a 25 ms He puff is injected into a DIII-D plasma resulting in a He concentration of {approx}5%. The time dependence of the He{sup 2+} density profiles in the plasma core is measured by charge-exchange recombination spectroscopy at 11 radial locations.

Plasma diagnostics for the DIII-D divertor upgrade

The DIII‐D tokamak is being upgraded to allow for divertor biasing, baffling, and pumping experiments. This paper gives an overview of the new diagnostics added to DIII‐D as part of this advanced divertor program. They include tile current monitors, fast reciprocating Langmuir probes, a fixed probe array in the divertor, fast neutral pressure gauges, and Hα measurements with TV cameras and fiber optics coupled to a high‐resolution spectrometer.

Low energy data on radiation enhanced sublimation of graphite

Abstract Erosion of POCO graphite by helium in PISCES-A was measured by carbon spectroscopy at temperatures from 550°-2000°C, energies of 30–250 eV and fluxes of 1−2×10 22 m −2 s −1 , Yields at low energies were higher than predicted in current models. The effect of redeposition is discussed.