1,418 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 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
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
The influence of resonant magnetic perturbations on edge transport in limiter h-mode plasmas in TEXTOR
No full textIn this contribution, we report on experimental results on edge transport in limiter H-mode plasmas in TEXTOR under the influence of the Dynamic Ergodic Divertor (DED). These plasmas are characterized by a pedestal structure mainly visible in the electron density, resulting in increased electron pressure gradients of up to 30 kPa/m over a pedestal width of 25 mm at high pedestal collisionalities (nu(e.) = 1 - 10), and with high frequency ELMs in the range of 300-1500 Hz. Under the influence of DED the pedestal pressure is gradually reduced and completely collapses to L-mode when the laminar zone extends all the way across the pedestal width. Toroidal plasma rotation is maintained at H-mode levels by the torque introduced by DED in the stochastic region. The perturbed magnetic topology has been optimized to access conditions with a density pump-out which are strongly governed by wall pumping capabilities in TEXTOR. (C) 2009 Elsevier B.V. All rights reserved
Particle confinement control with external resonant magnetic perturbations at TEXTOR
No full textTwo very contrary particle confinement stages were obtained at TEXTOR-DED by application of resonant magnetic perturbations. On the one hand a spontaneous build up of the total number of particles N-tot with correlated increase in the particle confinement time tau(p) was observed and on the other hand a controlled decrease of N-tot and tau(p) - the so called stochastic particle pump out is seen. Numerical analysis of the perturbed magnetic field topology shows that both domains can be distinguished by the ratio of short connection length field lines touching a specific resonant flux surface (here the q = 5/2 surface) to the complete perturbed layer width. During improved particle confinement, the hyperbolic fixed points (X-points) of the pitch resonant islands are directly connected to the DED target followed by an less than or similar to 40% increase in tau(p). The subsequent increase in the E x B shear rate Omega(ExB) at the q = 5/2 surface and a steepening of del n(e)(r) suggests a reduction of the radial particle transport. On the opposite, complete stochastisation of this island chain, i.e. a predominant diffusive field line characteristics, causes a less than or similar to 30% decrease of tau(p) with a reduction in Omega(ExB) at the q = 5/2 surface and del n(e)(r) indicating enhanced effective outward particle transport. (C) 2009 Elsevier B.V. All rights reserved
New Scenarios of ICRF wall conditioning in TEXTOR and ASDEX Upgrade
No full textInter-machine studies of wall conditioning with the ICRF discharges have been performed in the limiter (TEXTOR) and divertor (ASDEX Upgrade (AUG)) tokamaks in the presence of a toroidal magnetic field (>= 2 T) using the conventional ICRF antennas without modifications in hardware. The vessel oxidation treatment by pulsed ICRF discharges in (He + O-2)-mixture (TEXTOR) is analyzed in terms of ratios of the RF pulse length to the O-2-puff duration. A successive set of deuterium and helium ICRF discharges was developed for post-oxidation wall cleaning and analyzed in the light of TEXTOR recovery to the normal plasma operation. A new scenario of ICRF wall conditioning in (He + H-2)-mixture at two frequencies was applied in AUG and compared with the standard glow discharge in terms of outgassing efficiency. Modeling of the absorbed RF power was done to clear up a role of the H-2 concentration in the homogeneity of ICRF plasmas and the generation of high-energy ions. (c) 2007 Elsevier B.V. All rights reserved
Overview of material migration and mixing, fuel retention and cleaning of ITER-like castellated structures in TEXTOR
No full textPlasma-facing components (PFCs) in ITER will be castellated by splitting them into small-size blocks to maintain the thermo-mechanical stability. However, there are concerns in particular on retention of codeposited radioactive fuel in the gaps. An R&D program is underway in TEXTOR addressing this acute issue of castellation. Material migration and fuel inventory are investigated using long- and short-term discharge-resolved experiments with castellated structures in TEXTOR. Significant impurity transport to the gaps was detected and results were in part quantitatively reproduced with 3D-GAPS code.Deposits containing up to 70 at.% of tungsten on the gap areas closest to the plasma were detected in recent experiments. Deposition in the gaps accompanied by metal mixing demand for development of effective cleaning techniques. In experiments with ITER-like castellation, the gaps were cleaned from carbonaceous deposits using oxygen plasmas at 350 degrees C. This contribution contains an overview of experimental and modeling results along with recommendations for PFCs in ITER. (C) 2010 Elsevier B.V. All rights reserved
Material deposition and migration processes with resonant magnetic perturbation fields at TEXTOR
No full textResonant Magnetic Perturbations (RMPs) are applied with the Dynamic Ergodic Divertor (DED) at TEXTOR to control the plasma edge transport and the plasma surface interaction. This leads to the formation of a three-dimensional (3D) topology of the scrape-off layer (SOL). To quantify the erosion/deposition balance and the material migration in this 3D boundary, spherical test limiters were exposed to plasmas with and without RMP fields applied. Methane doped with 13C as tracer element was injected through a gas inlet in the test limiter. The local gas source was monitored by spatially resolving spectroscopy and the resulting deposition patterns on the limiters were analysed with colourimetry and nuclear reaction analysis. These measurements were compared to simulations of the magnetic field topology simulations. The data provide evidence of a particle migration dominated by an ExB drift within stochastic zones of the 3D plasma boundary
Resonant features of energy and particle transport during application of resonant magnetic perturbation fields at TEXTOR and DIII-D
No full textIn this paper, results of a direct comparison of TEXTOR and DIII-D experiments with resonant magnetic perturbation (RMP) fields are presented. This comparison of resistive L-mode plasmas at TEXTOR with highly conductive H-mode plasmas at DIII-D is useful to identify generic physics mechanisms during application of RMP fields with a strong field line pitch angle alignment in the plasma edge. A reduction in the pedestal electron pressure p e with increasing extension of the vacuum modelled stochastic layer and p e recovery with decreasing layer width is found caused by a q 95 resonant reduction in the edge (0.8< N<0.95) electron temperature T e(q 95) on both devices. For RMP edge-localized mode (ELM) suppressed H-mode plasmas at DIII-D, the gradients T e and nominal values of T e are reduced in this edge region while increasing in the pedestal (0.95< N<1.0) with RMP field applied and both are highly dependent on q 95. In contrast, an increase in the central ion temperature with strong steepening of the ion temperature profile at mid-radius is foundagain being highly dependent on q 95. However, these resonant thermal transport effects are only seen in high triangularity plasmas revealing a strong shape dependence of the thermal transport. In contrast to the highly q 95 dependent thermal transport features, the reduction of n eknown as density pump outshows a much weaker dependence on q 95. We show the potential to reduce the RMP induced particle pump out by fine tuning of the RMP spectral properties. At low resonant field amplitudes enhanced particle confinement is seen in high-field side limited L-mode discharges on both devices while higher resonant field amplitudes yield particle pumps out. © 2012 IAEA, Vienna.SP
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