Scrape-off Layer Of TEXTOR Research Articles

Spectroscopic determination of inverse photon efficiencies of W atoms in the scrape-off layer of TEXTOR

Optical emission spectroscopy can be applied to determine in situ tungsten particle fluxes from erosion processes at plasma-facing materials. Inverse photon efficiencies convert photon fluxes of WI and WII line transitions into W and particle fluxes, respectively, dependening on the local plasma conditions. Experiments in TEXTOR were carried out to determine effective conversion factors for different WI and WII transitions with the aid of WF6 injection into deuterium scrape-off layer plasmas in the electron temperature Te range between and . The inverse photon efficiencies or so-called effective -values have been determined for WI lines at , 429.5 nm, 488.7 nm, 498.3 nm, and 522.5 nm as well as for WII at and compared with theoretical calculations from the ADAS data base. Moreover, a multi-machine scaling for the -value in the range of Te between has been determined for the most prominent WI line at to considering experimental data from TEXTOR, ASDEX Upgrade, PSI and PISCES. Comparison with ADAS calculations for the same transition reveal a good qualitative agreement with the dependence on Te, but an underestimation of ADAS calculations of less than 25% over the full covered range of experimentally accessible Te in the multi-machine scaling. A good agreement within the experimental uncertainties is found between TEXTOR and ADAS -values for WI at and whereas an underestimation of up to a factor two of ADAS values for WI at and was measured. Potentially, reasons for the discrepancy are an overestimation of applied ionisation rate coefficients in ADAS for neutral W and a stronger electron dependence ne for these transitions.

Modelling of carbon deposition from CD4 injection in the far scrape-off layer of TEXTOR

To analyse the impurity transport in plasma-shadowed, remote areas, methane CD4 has been injected into the far scrape-off layer of TEXTOR through a cylinder equipped with a quartz micro balance (QMB). CD4 transport including break-up and resulting deposition on the QMB (shot-resolved) and on the cylinder top surface (shot-integrated) has been modelled with the codes ERO and 3D-GAPS. The modelling shows good agreement with the observations if reflection coefficients based on molecular dynamics simulations are used. In contrast to plasma-wetted areas, no enhanced erosion has to be applied.

Exposure of metal mirrors in the scrape-off layer of TEXTOR

Large molybdenum mirrors have been exposed in the SOL of TEXTOR in order to simulate conditions relevant for ITER optical components. Distortions of the reflectivity – increase as well as decrease – are found in the erosion and deposition dominated areas, respectively. The changes are most pronounced in the near UV and level off in the IR and can partly be attributed to observed surface changes. A novel periscope system was installed and mirrors exposed in a pilot experiment to simulate the transmission of light to distant sensors in ITER.

Modelling of the transport of methane and higher hydrocarbons in fusion devices

New Monte Carlo modelling with the ERO-TEXTOR code has been done for methane transport in the scrape off layer of TEXTOR using recently published rate coefficients of the methane family from Brooks/Janev. Compared to former used rate coefficients from Ehrhardt–Langer the new data result in significantly larger D/XB-values at plasma temperatures above ∼20 eV. The computed amount of re-deposition is at high plasma temperatures up to a factor of 100 smaller with the new rate coefficients. Parameter studies of the transport of locally injected methane and C 2D 4 in the divertor MkIIa of JET with ERO-JET (adapted version of ERO-TEXTOR) show a strong influence of the location of injection on D/XB. Calculated D/XB-values can increase up to a factor of 1000 if hydrocarbons are puffed at a location inside the SOL instead at the strike point.

A new technique for in situ measurements of erosion and redeposition in a single discharge

Based on the very different refractive constants of quartz substrates (∼1.5) and of amorphous deposits (>1.7) collected in the scrape-off layer of TEXTOR, in situ reflectometry has been developed in order to determine the growth rates. Laboratory experiments comparing the light intensities reflected from the rear side of the system glass/deposit to the theoretically calculated reflectivity demonstrate the feasibility of the technique. The optical constants needed are determined ex situ by ellipsometry. For the application to TEXTOR, the Stockholm TEXTOR collector probe system has been equipped with fibre optic light guides which end behind quartz substrates carried by a replaceable graphite head. This arrangement avoids disturbance due to vibrations or due to observation over long distances.

Oxygen gettering on graphite in the SOL of TEXTOR after wall conditioning with boron

Starting with carbonized walls as a reference condition, the TEXTOR vessel has first been flushed with reactive diborane gas (B 2H 6), later the inner plasma facing surface was boronized with a-C/B:H (∼ 100 nm). After the introduction of the boron into the machine a drastic decrease of the oxygen contamination of the plasma could be observed. Surprisingly, the amount of oxygen which has been collected on graphite targets in the scrape-off layer (SOL) showed just the opposite trend : it increased. This behaviour is explained with gettering of the oxygen ions by the simultaneously collected boron. The oxygen forms stable oxides with the boron due to energetic ion deposition keeping the plasma contamination low. The observations allow also the quantification of two other contributing terms : sticking of oxygen ions and the oxygen absorption by uptake from air. For the ex situ analysis, the graphite targets were studied by ion beam analysis (O, B, D, metals). XPS investigations showed the bonding state between oxygen and boron.

AES studies of redeposited impurities on samples exposed in the scrape-off layer of TEXTOR

Impurities transported in the scrape-off layer (SOL) of a tokamak TEXTOR were collected on Si samples exposed in SOL, and their amounts were measured quantitatively, using an in situ surface analysis station equipped with an Auger electron analyser and an ion gun. Tokamak discharges have been made in all carbon surroundings, i.e. graphite limiters and the carbonized liner, and also recentlly in boron-carbon coated (boronized) surroundings. The analysis shows that mainly carbon is collected accompanied by oxygen (less than ∼ 15%), and by boron (∼20%), after boronization. We present data concerning the amount of the redeposited carbon as a function of the toroidal distance between the sample and limiters, and of the electron density, in order to discuss the mechanism of its redeposition.

Effect of RF heating on DC electric fields in the SOL of TEXTOR

Radial profiles of the DC radial and poloidal fields in the scrape-off layer of TEXTOR in ohmic and ICRF heated discharges have been obtained by means of a fast scanning double probe. It is found that ICRF heating significantly changes both field components. Poloidal fields as high as 10 V/cm have been observed at 1 MW of ICRF power. The impact of these DC electric fields on the SOL will be discussed.

Metal impurity fluxes in the TEXTOR scrape-off layer measured by collector probes in the presence of ALT II toroidal belt limiter

Impurity fluxes of metals have been determined in the scrape-off layer of TEXTOR. Time-resolved measurements have been performed using graphite collector probes. Results were found when ALT II acted as the only limiter and were compared to earlier operational phases. Typical fluxes were (1–8)×10 18 atoms m −2 s −1. The metal density was determined to be in the range 10 15 atoms m −3, and the cross-field diffusion coefficient 6 m 2 s −1. A distinct reduction of deposited metals as well as deuterium and oxygen was found in comparison to earlier measurements performed in the presence of the poloidal and ALT I limiters. Increased fluxes were observed during auxiliary ICR heating.

Determination of the carbon redeposition rate in the scrape-off layer of TEXTOR by a new, time integrated collection technique

A method has been developed which allows the carbon redeposition in the SOL of TEXTOR to be deduced simply and rapidly from the interference colours of amorphous carbon films collected on targets during a number of discharges. Two sources of carbon — release from the wall and diffusion out of the plasma core — can be distinguished by appropriate arrangements of the poloidal and the toroidal limiters. The deposition rates can reach 2 × 10 16 C/cm 2 s two centimeters deep in the SOL, and the D/C ratio in the films is high (~ 0.4). This may become important for future devices when D-T operation is planned. Observations made after the boronization of the machine and in He-discharges are presented.

Influence of the pump limtter ALT-I on the heavy impurity fluxes in the scrape-off layer of textor

Time- and space-resolved measurements of the Fe, Cr, Ni, Ti and O impurity fluxes in the limiter shadow of TEXTOR with the Stockholm-TEXTOR probe were made during the test campaign for the pump limiter ALT-I. The wall components were metallic and routinely conditioned by RG-discharge cleaning. The influence of pump limiter head material (TiC, INC 600), geometry of the head, hydrogen gas inlet programs, neon puffing through and plasma shifting against the head have been studied. The metal fluxes in the shadow of the limiter are dominated by its material. They are higher when the pump limiter is used instead of the main limiters. The Fe + Cr + Ni signal increases by a factor of 2–3 and corresponds to a decrease in the electron density due to the pumping. About a 50-fold increase for Ti and a 30-fold for O is observed and suggests additional liberational processes are involved. Ti remained in the machine even after terminating the ALT-I activity. The impurity development is substantially independent of throat widths and hydrogen fueling variations, whereas neon puffing leads to a rapid decrease due to plasma edge cooling. Shifting the plasma against the pump limiter causes an outburst of Ti.

Probe measurements of impurities in the scrape-off layer of TEXTOR

By using carbon collector probes, deposited impurities in the TEXTOR plasma boundary have been measured. The cylindrical probes rotate behind a fixed slit in order to obtain time resolved information. Impurities on the ion side of the probe were measured. After exposures to TEXTOR discharges the probes were analysed by Rutherford backscattering.Three discharges were studied, two with hydrogen as the filling gas, and one with deuterium. For one of the hydrogen discharges a pumping limiter was used.The dominant impurities are oxygen, titanium, chromium, iron and nickel. The stainless steel components have a radial e-folding length of 30 mm before the breakpoint, while this length is only 5 mm for Ti. The origin of the steel components is the main limiter, while Ti has its origin in the pumping limiter ALT 1.The peak fluxes for the discharge 7024 at a distance of 10 mm from the main limiter were in units of atoms/cm2 s, for oxygen 6×1016, for chromium 3×1015, for iron + nickel 7.5×1015 and for titanium 1.5×1015. These fluxes vary in a systematic way with the filling gas, the use of the pumping limiter, and the distance between the main limiter and the probe.