1,184 research outputs found
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
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 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
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
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
Observation of geodesic acoustic modes (GAMs) and their radial propagation at the edge of TEXTOR tokamak
No full textThe electrostatic potential and density fluctuations have been measured at the edge of the TEXTOR tokamak by two toroidally distant Langmuir probe arrays. The geodesic acoustic mode (GAM) zonal flows (ZFs) are observed in potential fluctuations with a toroidal and poloidal symmetric structure. The GAM frequency, f(GAM), changes monotonically with the local temperature and is close to the frequency-dispersion predicted by theories. Bispectral analysis shows clear nonlinear coupling between the GAM and broadband ambient turbulence. The GAM packet has a narrow radial extent with k(r) similar or equal to 0.5-0.7 cm(-1) and exhibits explicitly a radially outward propagation. Furthermore, the radial correlation structure of GAMs and their radial propagation have been investigated in a wide range of parameters by varying plasma density ((n) over bar (e0) = (1.5-3.0) x 10(19) m(-3)) and edge safety factor (5.0 <= q(a) <= 5.9). It is found that the magnitude of the GAM correlations reduces remarkably with the increase in the plasma density approaching the density limit, while the radial wavelength of GAMs only decreases slightly in higher density and larger q(a) discharges. With increasing plasma density, the radial propagating phase speed of GAMs is strongly reduced along with the drop in the local temperature. The results provide new evidence on the propagation properties of GAM ZFs
Electrode biasing on TEXTOR: a tool for fundamental physics studies
No full textThe application in TEXTOR of an externally controlled radial electric field E-r imposed by means of an electrode, has allowed to ascertain many aspects of the physics of the creation of E-r and of its effect on radial transport. Radial conductivity was shown to depend on parallel viscosity with the latter's nonlinear response to E-r providing the basic ingredient for E-r bifurcation, typical for L- to H-mode transitions. Simultaneous time and space resolved measurements of E-r and of the plasma flows in the edge by means of a newly developed inclined Mach probe have allowed to further substantiate the role of parallel viscosity and of neutral collisions in the damping of rotation. The causal role of grad E-r in bringing about the transport changes has been proven by showing that the field shear is spatially correlated with and temporally leads the density gradient, as well as by comparison with theoretical modeling
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
Tritium loading study of tungsten pre-exposed to TEXTOR plasmas
No full textThe uptake of tritium at 573 K by polycrystalline tungsten limiter material pre-exposed to TEXTOR plasmas was investigated by the imaging plate technique (IP) and found to be mostly non-homogeneous on the plasma facing surface. Particularly high concentrations of tritium were apparent in areas attributed to carbon deposition. The surface density of tritium outside of deposition zones was essentially comparable on both sides of the examined tungsten plate. Under a stream of argon at ambient temperature tritium was predominantly released as tritiated water. While tritium is initially liberated with rates in the (MBq/h) range, after a few days the rate drops to about 100 Bq/h, decreasing even further thereafter. Under atmospheric conditions the concentration of tritium on the surface remained virtually unchanged over a rather extended period of time, i.e. more than 500 d. Tritium in surface zones other than of “deposition” was also firmly trapped at ambient temperature
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