[show abstract][hide abstract] ABSTRACT: This paper focuses on a study of the principal operation aspects of standard ICRF heating antennas in the
ion cyclotron wall conditioning (ICWC) mode: (i) ability of the antenna to ignite the cleaning discharge
safely and reliably in different gases including those most likely to be used in ITER – He, H2, D2 and their
mixtures, (ii) the antenna capacity to couple a large fraction of the RF generator power (>50%) to low density (�1016–1018 m-�3) plasmas and (iii) the RF power absorption schemes aimed at improved RF plasma homogeneity and enhanced conditioning effect. The ICWC discharge optimization in terms of RF plasma wave excitation/absorption resulted in successful simulation of the conditioning scenarios for ITER operation at full field (JET) and half-field (TEXTOR, TORE SUPRA, ASDEX Upgrade).
Journal of Nuclear Materials 08/2013; 419:S1029–S1032. · 1.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper reports on the recent assessment of the Ion Cyclotron Wall Conditioning (ICWC) technique for isotopic ratio control, fuel removal and recovery after disruptions, which has been performed on TORE SUPRA, TEXTOR, ASDEX Upgrade and JET. ICWC discharges were produced using the standard ICRF heating antennas of each device, at different frequencies and toroidal fields, either in continuous or pulsed mode. Intrinsic ICWC discharge inhomogeneities could be partly compensated by applying a small vertical magnetic field, resulting in the vertical extension of the discharge in JET and TEXTOR. The conditioning efficiency was assessed from the flux of desorbed and retained species, measured by means of mass spectrometry. In Helium ICWC discharges, fuel removal rates between 1016D.m-2.s-1 to 3.1017D.m-2.s-1 were measured, with a linear dependence on the coupled RF power and on the He +
density. ICWC scenarios have been developed in D or H plasmas for isotopic exchange. The H (or D) outgassing was found to increase with the D (resp. H) partial pressure. In continuous mode, wall retention is on the average two to ten times higher than desorption
, due to the high reionization probability of desorbed species in ICWC discharges, where the electron density is about 1018m-3. Retention can be minimized in pulsed ICWC discharges without severely reducing outpumping. Pulsed He-ICWC discharges have been successfully used on TORE SUPRA to recover normal operation after disruptions,
when subsequent plasma initiation would not have been possible without conditioning.
Journal of Nuclear Materials 08/2013; 415:S1021–S1028. · 1.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Specimens of dust particles collected in the TEXTOR and ASDEX-Upgrade (AUG) tokamaks were examined in order to determine the composition, size and surface and internal structure. The study was performed by means of SEM, TEM, FIB and EDX. The size of particles from both devices varies from hundreds of nananometers to hundreds of micrometers. Dust from TEXTOR contains mainly carbon, boron, silicon, whereas samples from AUG contain also tungsten eroded by plasma from the machine wall
Materials Challenges and Testing for Supply of Energy and Resources, Springer. 01/2012; 3:3-11.
[show abstract][hide abstract] ABSTRACT: Fuelling 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.
[show abstract][hide abstract] ABSTRACT: a b s t r a c t In future fusion devices like ITER deposition of impurities will likely occur in areas, remote from plasma and on the sensitive components of optical diagnostics, like mirrors and windows. Deposition in remote areas may lead to the tritium retention and therefore represent a safety issue. Deposition on optical com-ponents will severely deteriorate their optical properties potentially leading to the shutdown of the respective diagnostic systems. An active control over deposition is therefore highly desirable. The paper contains an overview of experiments on active control over carbon deposition undertaken in TEXTOR tokamak. A prototype of diagnostic duct was exposed in the scrape-off layer plasmas of TEXTOR where several techniques were applied to mitigate carbon deposition on diagnostic mirrors located inside this prototype. The complete suppression of carbon deposition on the surface of diagnostic mirror was achieved by feeding the deuterium gas inside the diagnostic duct.
Journal of Nuclear Materials 01/2011; · 1.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Advanced Limiter Test-II (ALT-II) is a large area toroidal pump limiter in the TEXTOR tokamak. Discrete target plates located in channels at eight toroidal positions behind the main limiter surface neutralize a portion of the plasma efflux from the core. The resulting gas is exhausted by eight external pumps. The primary experimental goals of ALT-II are aimed at power loading studies and plasma density control during long pulse (4 s), high power (6 MW) tokamak discharges. It is found that both the power and the plasma flow to the limiter are asymmetric and depend on line density. Peak neutral pressures of 0.8 m torr and removal rates of up to 0.15 torrL/s per pump station are achieved in the Ohmic phase. The projected exhaust efficiency of ALT-II with full pumping is 5-10%. During ion cyclotron resonance frequency heating, the particle removal rate exceeds 0.4 torrL/s per blade, and the exhaust efficiency is 4-5% for power levels up to 2.6 MW.
[show abstract][hide abstract] ABSTRACT: Tungsten is foreseen as the plasma-facing component material for baffles, the dome and strike-point area in the ITER divertor. Quantification of the W source, which is connected with the components lifetime and W plasma concentration, is one of the outstanding issues in the qualification process. A dedicated experiment in TEXTOR with the exposure of a W/C twin limiter to the near scrape-off layer plasma has been carried out in order to address the W sputtering and local material mixing in the electron temperature range between Te=30 and 85 eV, achieved with deuterium fueling in four steps. The Te range is comparable to the baffle region and the strike-point area during non-detached transient phases of the ITER divertor plasma. Quantification of the W sputtering yield and the impinging impurity fluxes was performed with the aid of optical spectroscopy, in particular by observation of WI and WII lines. As no inverse photon efficiencies in the plasma parameter range of the twin limiter experiment exist, we performed in a second experiment for the first time a calibration of WI and WII photon efficiencies with local injection of WF6 through a gas inlet into the TEXTOR edge plasma. The in situ determined effective inverse photon efficiency of about 85 for the most prominent WI line at 400.9 nm, which is in good agreement with GKU modelling for the covered Te range and 650 for the WII line at 434.8 nm, has been applied to the corresponding photon fluxes in the twin limiter experiment. The W sputtering yield decreases from 5.2 to 0.5%, thus by about one order of magnitude, with a reduction of Te from 85 eV down to 30 eV and a simultaneous increase of the impinging deuterium ion flux by 50% occurs. A lower limit for the prompt redeposition has been estimated at 50% by analyzing the WI to WII flux ratio. Local measurement of OII (441.6 nm) and CII (426.7 nm) provided impurity flux ratios of 0.6% for O and 5.2% for C related to the deuterium recycling, respectively ion flux. Both flux ratios remain constant for all phases of the discharge with plasma edge cooling. W erosion is predominantly caused by sputtering of higher ionization stages of O and C impinging on the W limiter half and not by the fuel species itself. Plasma cooling below the physical sputtering threshold could not be achieved without impurity seeding.
Physica Scripta 01/2011; T145. · 1.03 Impact Factor
[show abstract][hide abstract] ABSTRACT: Measurements of the plasma edge electron density n e and temperature T e fields during application of a fast rotating, resonant magnetic perturbation RMP field show a characteristic modulation of both, ne and Te coherent to the rotation frequency of the RMP field. A phase delay between the ne(t) and Te(t) waveforms is observed and it is demonstrated that this phase delay is a function of the radius with r depending on the relative rotation of the RMP field and the toroidal plasma rotation. This provides for the first time direct experimental evidence for a rotation dependent damping of the external RMP field in the edge layer of a resistive high-temperature plasma which breaks down at low rotation and high resonant field amplitudes.
Physics of Plasmas 01/2010; 17(6):060702. · 2.38 Impact Factor
[show abstract][hide abstract] ABSTRACT: The main challenge for the Thomson scattering (TS) diagnostic on the TEXTOR tokamak is the detailed study of fast plasma events at a high spatial resolution and a high repetition rate of the measurements. The diagnostic uses intra-cavity probing of the plasma with a repetitively pulsed ruby laser and a fast CMOS camera as detectors. Since 2004, the TS system on TEXTOR has been gradually and systematically enhanced for the measurements of fast plasma events. For that it has recently been upgraded to obtain a multi-pass configuration. Two spherical mirrors have been installed that force the laser beam to probe the plasma a specified number of times before it is directed back into the laser medium. The diagnostics with the upgraded probing system have achieved the measurement accuracy of 3% for the electron temperature and 1.5% for the electron density at <1 cm spatial resolution and 3 × 1019 m−3 plasma density and can measure at 5 kHz during an interval up to 8 ms. This makes it possible to detect, amongst others, fine structures of magnetic islands and variations of the edge pedestal in the ELMy limiter H-mode.
[show abstract][hide abstract] ABSTRACT: a b s t r a c t Silicon samples were exposed in the scrape-off layer of the TEXTOR plasma using a fast reciprocating probe, with the aim of studying carbon deposition and deuterium retention during Dynamic Ergodic Divertor (DED) operation. Separate samples were exposed for 300 ms at the flat-top phase of neutral beam heated discharges. The exposure conditions were varied on a shot-to-shot basis by external mag-netic perturbations generated by the DED in the m/n = 3/1, DC regime, base configuration. Nuclear Reac-tion Analysis (NRA) was used to characterise collector sample surfaces after their exposure. Enhanced concentrations of both carbon and deuterium (C 3–10 Â 10 16 at./cm 2 , D 8–60 Â 10 15 at./cm 2) were found. The D/C ratio was less than unity which indicates that most of the carbon and deuterium were co-depos-ited. Carbon e-folding lengths of about 2 cm were found on both toroidal sides of the probe independent of DED perturbations.
Journal of Nuclear Materials - J NUCL MATER. 01/2009; 390.
[show abstract][hide abstract] ABSTRACT: Castellation will be used in divertor and first wall components to provide thermo-mechanical stability of ITER. Radioactive fuel may be stored in the gaps of castellated structures representing a safety issue for ITER. Tungsten castellated structures with different shapes were exposed in TEXTOR to investigate the impact of cell shaping on impurity transport and fuel deposition in the gaps. After exposure a significant intermixing of tungsten was detected in carbon deposits in the gaps reaching 70at.% of W in the deposited layer. This will provide difficulties in cleaning the gaps in ITER. Poloidal gaps of shaped cells contained a factor or 3 less deuterium than those of rectangular cells, the carbon deposition exhibited only marginal advantages of a new geometry. Poloidal and toroidal gaps contained comparable amount of C and D. Significant deposition at the bottom of gaps was measured which could only partly be reproduced by modeling.
Journal of Nuclear Materials, v.390-391, 556-559 (2009). 01/2009;
[show abstract][hide abstract] ABSTRACT: New results are shown on carbon deposition and mixed layer formation on tungsten surfaces in TEXTOR test limiter experiments by varying surface conditions such as surface roughness (Ra: 10nm–1μm), temperature (300–900°C), and initial surface carbon concentration (0–60%). It is found that surface roughness significantly affected C deposition for both W and graphite substrates such as increase in the C deposition and extension of the C deposition area. Not only the surface roughness parameter Ra but also detailed surface morphology closely relate to C deposition on tungsten. Carbon deposition hardly occurred at least above ∼520°C on tungsten under TEXTOR edge plasma conditions. Carbon behavior on tungsten at 770–930°C depends on the incident carbon ion energy. Although tungsten and carbon mixing layers affected C deposition, their effect is less than the roughness effect.
Journal of Nuclear Materials, v.390-391, 44-48 (2009). 01/2009;
[show abstract][hide abstract] ABSTRACT: During the static 6/2 Dynamic Ergodic Divertor experiments in TEXTOR, a significant influence of the edge resonant magnetic perturbation (RMP) on the turbulent blob transport in the scrape-off layer (SOL) has been observed. In ohmic discharges without the RMP, the blobs extend 4-5 cm deep into the SOL with a radially outward moving speed of about 1 km s-1 and hence constitute a strong outflow of mass. With the application of the RMP, the blob amplitudes and their radially moving velocity are both reduced, resulting in a significant reduction of the blob transport in the SOL. The reduction effect of the RMP on blobs is found to be robust to changes in the operational regime and to phasing variations of the RMP as well. The blob dynamics appears to be consistent with the paradigm of the radial motions of the blob structures driven by the interchange instability.
[show abstract][hide abstract] ABSTRACT: Castellation is foreseen for the first wall and divertor area in ITER. The concern of the fuel accumulation and impurity deposition in the gaps of castellated structures calls for dedicated studies. Recently, a tungsten castellated limiter with rectangular and roof-like shaped cells was exposed to the SOL plasmas in TEXTOR. After exposure, roughly two times less fuel was found in the gaps between the shaped cells whereas the difference in carbon deposition was less pronounced. Up to 70at.% of tungsten was found intermixed in the deposited layers in the gaps. The metal fraction in the deposit decreases rapidly with a depth of the gap. Modeling of carbon deposition in poloidal gaps has provided a qualitative agreement with an experiment. The significant anisotropy of C and D distributions in the toroidal gaps was measured.
Journal of Nuclear Materials 01/2009; · 1.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Measurements of plasma rotation and electric field are crucial for the study of plasma confinement and transport. The present
paper is devoted to experimental observations of poloidal asymmetry in perpendicular plasma rotation with correlation reflectometry
[show abstract][hide abstract] ABSTRACT: To shed some light on the development of the fast m/n = 1/1 precursor to the sawtooth crash and its influence on plasma transport properties in the vicinity of the q = 1 surface, series of dedicated experiments have been conducted on the Tore Supra and TEXTOR tokamaks. It has been concluded that, before a crash, the hot core gets displaced with respect to the magnetic axis, drifts outwards by as much as 8–10 cm and may change its shape. Observation of the magnetic reconnection process has been made by means of electron cyclotron emission diagnostics. The heat pulse is seen far outside the inversion radius. The colder plasma develops a magnetic island on the former magnetic axis, after the hot core expulsion. Different kinds of behaviour of the m = 1 precursor before the crash, with respect to the displacement of the hot core and the duration of the oscillating phase, have been observed. An ideal kink model alone cannot be used for explanation; therefore, resistive effects play an important role in the mode development. Possible mechanisms that lead an m = 1 mode to such behaviour, and their links to the change in the central q-profile, are discussed. Results have been discussed in the light of various theoretical models of the sawtooth.
[show abstract][hide abstract] ABSTRACT: It has been observed in the T-10 tokamak that immediately after off-axis electron cyclotron resonance heating (ECRH) switch-off, the core electron temperature stays constant for some time, which can be as long as several tens of milliseconds, i.e. several energy confinement times (τE), before it starts to decrease. Whether or not the effect is observed depends critically on the local magnetic shear in the vicinity of the q = 1 rational surface, which should be close to zero. It is hypothesized that a small shear can induce the formation of an internal transport barrier. Measurements of density fluctuations in the transport barrier with a correlation reflectometer show immediately after the ECRH switch-off a clear reduction in the fluctuation level, corroborating the above results. The delayed temperature decrease has also been observed in similar discharges in the TEXTOR tokamak; however, the delay is restricted to ~ 1 × τE.