398 research outputs found
Hydrogen Ion Cyclotron Wall Conditioning for Fuel Removal on TEXTOR and ASDEX Upgrade
No full textIon Cyclotron Wall Conditioning (ICWC), applicable in presence of the toroidal magnetic field, is envisaged in ITER to recover from disruptions, leaks and torus vents, for recycling control and for fuel removal. Various experiments on different devices as well as modeling efforts are advancing to consolidate this technique. This contribution focuses on a selection of recent hydrogen ICWC experiments on ASDEX Upgrade and TEXTOR. The ASDEX Upgrade experiment aimed at comparing isotopic exchange efficiencies previously obtained on Carbon devices to the ITER relevant Tungsten wall. The experiment on TEXTOR aimed at assessing the performance of H2-ICWC for codeposited layer removal. The latter being a particular important fuel removal aspect since it is predicted that a major part of tritium in-vessel inventory build-up on ITER will be due to the formation of tritium rich codeposited layers
Long-term carbon transport and fuel retention in gaps of the main toroidal limiter in TEXTOR
No full textThe 1.1–1.5 mm wide gaps between tiles of the main toroidal belt limiter in TEXTOR were utilized to study the long-term impurity deposition and fuel retention in gaps. The tiles were exposed during a full tokamak campaign of 9365 s of plasma to various discharge conditions and wall conditioning, accumulating of up to 30 μm thick layers at the gap entrance. It was found that (i) gaps trap impurities twice as efficient as the top surface, (ii) the deposition in the toroidal gaps is twice as high as in the poloidal, (iii) carbon deposition decays with a fall-off length of about 0.7 mm towards the gap bottom, (iv) deposition on the bottom is significantly higher than on the adjacent side walls of gaps, and (v) the amount of deuterium scales with the amount of carbon with D/C varying from 3% to 30% depending on the surface temperature
Status of electron temperature and density measurement with beam emission spectroscopy on thermal helium at TEXTOR
No full textBeam emission spectroscopy on thermal helium is used at the TEXTOR tokamak as a reliable method to obtain radial profiles of electron temperature T-e(r, t) and electron density ne(r, t). In this paper the experimental realization of this method at TEXTOR and the status of the atomic physics employed as well as the major factors for the measurement\u27s accuracy are evaluated. On the experimental side, the hardware specifications are described and the impact of the beam atoms on the local plasma parameters is shown to be negligible. On the modeling side the collisional-radiative model (CRM) applied to infer ne and Te from the measured He line intensities is evaluated. The role of proton and deuteron collisions and of charge exchange processes is studied with a new CRM and the impact of these so far neglected processes appears to be of minor importance. Direct comparison to Thomson scattering and fast triple probe data showed that for high densities ne > 3.5 x 10(19) m(-3) the T-e values deduced with the established CRM are too low. However, the new atomic data set implemented in the new CRM leads in general to higher Te values. This allows us to specify the range of reliable application of BES on thermal helium to a range of 2.0 x 10(18) < n(e) < 2.0 x 10(19) m(-3) and 10eV < T-e < 250eV which can be extended by routine application of the new CRM
Fuelling efficiency of massive gas injection in TEXTOR: mass scaling and importance of gas flow dynamics
No full textFuelling efficiency is an important parameter in designing a massive gas injection system for suppression of runaway electrons in ITER. In this work Z-dependence of fuelling efficiency is measured for TEXTOR. The dependence covers the following gases: He, Ne, Ar, Kr, Xe and a 10% Ar-D2 mixture. It is shown that the fuelling efficiency significantly decreases with the gas mass, from above 0.5 for He to below 0.03 for Xe.To explain the variation of the efficiency with the gas mass and pressure a simple model of gas flow from the valve to the plasma edge is developed. The flow model is validated using available laboratory flow measurements of a TEXTOR-like injection system. An unsteady gas flow and a premature plasma disruption are shown to explain the mass dependence of the efficiency
Long-term erosion and deposition studies of the main graphite limiter in TEXTOR
No full textErosion and deposition behaviour of the TEXTOR main toroidal belt limiter ALT-II has been studied for two generations of limiter tiles with different surface shapes. Two specially prepared tiles with marker holes were subsequently exposed during experimental campaigns with a total plasma duration of >7000 s and a surface averaged background ion fluence exceeding 2 x 10(25) m(-2). The tile surface relief was measured by optical profilometry and SIMS before and after the exposure. The highest surface erosion of >10 mu m on both tiles was observed in regions with an angle of incidence of the magnetic field alpha of greater than or similar to 1 degrees i, whereas in the regions with alpha similar to 0.1 degrees both net-erosion and net-deposition were observed depending on the tile shaping. The marker holes were always deposition dominated. Modelling by the ERO code explains qualitatively the surface erosion and deposition behaviour in terms of background flux dilution for shallow angles of incidence
Spatially and temporally resolved measurements of runaway electrons in the TEXTOR tokamak
No full textSpatially and temporally resolved measurements of runaway electrons have been performed in the TEXTOR tokamak. The main diagnostic for the investigations is a newly developed runaway probe which is mounted at a reciprocating mechanism. In addition, the runaway electrons are studied in the plasma by synchrotron emission. The scanning probes consists of fluorescing crystals covered by a graphite housing and is located at different radial positions. In this study, two generations of the probes are presented, a simple one for proving the principle and a more advanced one which allows the measurement of the energy distribution of the runaways. A strong decay of the runaway electrons at the plasma edge was observed. Runaway electrons have been measured at different plasma densities by synchrotron radiation and the probe. The reduction of the number of runaways have been measured with increasing plasma densities during low density plasma discharges by both diagnostics. The spectrum of runaway electrons produced during low density plasma discharges was measured. Finally, measurements of runaways with high temporal resolution of 0.05 ms have been carried out during a disruption discharge
Comparison of 13C2H4 and 13Ch4 injection through graphite and tungsten limiters in TEXTOR
No full textExperiments in the plasma-wall interaction facility at TEXTOR have been carried out to compare (i) the local transport of (CH4)-C-13 and (C2H4)-C-13 injected through a gas aperture implemented in spherical limiters made of polycrystalline tungsten and fine-grain graphite and (ii) the properties of local deposited carbon layers on the polished limiter surface. Each limiter was positioned in the scrape-off layer and exposed to ten identical ohmic deuterium discharges of 4s length-accumulating 40 plasma seconds. The injection rates were normalized to the amount of injected C-13. The analysis is twofold: spectroscopy was applied to follow up in situ the observable hydrocarbon break-up products (C-2, CD, C+, and C2+), and post mortem analysis was used to determine the C-13 deposition efficiency. The penetration depths of hydrocarbon break-up products in the plasma were determined. Significant CH emission was observed in front of W limiter prior to injection. The C-13 deposition efficiency shows a dependence on both the limiter material and on the injected hydrocarbon species
Transport of argon and iron during a resonant magnetic perturbation at TEXTOR-DED
No full textSince ergodic divertors are proposed to control the impurity concentration in tokamak plasmas the influence of a resonant magnetic perturbation (RMP) from the dynamic ergodic divertor (DED) in the m/n = 3/1 mode on argon impurity transport is studied at the tokamak TEXTOR. Although the application of a DED current below the threshold for m/n = 3/1 tearing mode excitation in an NBI-heated L-mode plasma does not change the argon transport coefficients in r/a < 0.8, an additional transport mechanism within an error of 34% cannot be excluded. Observed changes of the iron ion density distribution for r/a < 0.6 can only be explained by a change of the transport. In addition the argon injection experiments show that an RMP reduces the inflow of particles into r/a < 0.8 and that a tearing mode seems to increase the local transport although at first view the experimental data indicate a reduction of the transport
Exposure of tungsten nano-structure to TEXTOR edge plasma
No full textW nano-structures (fuzz), produced in the linear high plasma device, NAGDIS, were exposed to TEXTOR edge plasmas (ohmic He/D mixed plasma and pure D plasma) to study formation, erosion and C deposition on W fuzz in tokamak plasmas for the first time. Fuzz layers were either completely eroded or covered by C deposit. There was no clear indication of W fuzz growth under the present conditions. There was no significant difference of C deposition between 'thick' fuzz (500-600 nm in thickness) and 'thin' fuzz (300-400 nm) in the He/D plasma. On the W fuzz surface. C deposition was enhanced probably due to reduction of effective sputtering yield and effective reflection coefficient of carbon ions, similar to roughness effects. Formation and erosion of W fuzz in tokamak devices and role of impurities are discussed. (C) 2010 Elsevier B.V. All rights reserved
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