[Show abstract][Hide abstract] ABSTRACT: Based on the analysis of data from the numerous dedicated experiments on
plasma disruptions in the TEXTOR tokamak mechanisms of the formation of runaway
electron beams and their losses are proposed. The plasma disruption is caused
by strong stochastic magnetic field formed due to nonlinearly excited low-mode
number MHD modes. It is hypothesized that the runaway electron beam is formed
in the central plasma region confined inside the intact magnetic surface
located between $q=1$ and the closest low--order rational [$q=4/3$ or $q=3/2$]
magnetic surfaces. The thermal quench time caused by the fast electron
transport in a stochastic magnetic field is calculated using the collisional
transport model. The current decay stage is due to the ambipolar particle
transport in a stochastic magnetic field. The runaway electron beam in the
confined plasma region is formed due to their acceleration the inductive
toroidal electric field. The runaway electron beam current is modeled as a sum
of toroidally symmetric part and a small amplitude helical current with a
predominant $m/n=1/1$ component. The runaway electrons are lost due to two
effects: ($i$) by outward drift of electrons in a toroidal electric field until
they touch wall and ($ii$)by the formation of stochastic layer of runaway
electrons at the beam edge. Such a stochastic layer for high--energy runaway
electrons is formed in the presence of the $m/n=1/1$ MHD mode. It has a mixed
topological structure with a stochastic region open to wall. The effect of
external resonant magnetic perturbations on runaway electron loss is discussed.
A possible cause of the sudden MHD signals accompanied by runaway electron
bursts is explained by the redistribution of runaway current during the
resonant interaction of high--energetic electron orbits with the $m/n=1/1$ MHD
[Show abstract][Hide abstract] ABSTRACT: Today's simulation of turbulence and its propagation in plasmas shows a complex 3-dimensional picture . However, experimental evidence for turbulence is often observed by 1-dimensional diagnostics , only. Integration along lines of sight complicate the analysis further. In addition turbulence diagnostics have a limited sensitivity with respect to the wave number spectrum which allows to diagnose only a small window of the variety of micro structures in plasmas. Therefore a comparison of simulations and experiments is difficult. However, it is indispensable for simulations to have 3-dimensional information on turbulence and structures to compare with. The paper discusses the 3-dim analysis of quasi coherent modes visible on a major part of the plasma radius and especially on the q = 1 surface. Figure 1: Coherence for LRC during flat top conditions showing the enhanced coherence in the range 80 kHz to 120 kHz At TEXTOR a multi location reflectometry sys-tem  with good radial resolution was used to study short and long range correlations of den-sity fluctuations in the range k ⊥ ≤ 3 cm −1 . The different locations of the antenna arrays yield in-formation on the toroidal extent of structures in the plasma. Therefore the plasma conditions are chosen in such a manner that a field line con-nects both antennae arrays. In the case under in-vestigation I p is in counter clock wise direction and magnetic field in clock wise direction con-necting the low field side (LFS) array with the top array. The toroidal separation is ∆φ = 247.5 • and the poloidal separation ∆θ is in the same range matching the q = 1 condition. To reach the q = 1 surface a low density high current ohmic plasma was used to extent the operation range of the reflectometer (26 GHz to 40 GHz) towards the plasma center. In all cases reported here, the reflectometer was operated in O-mode polar-ization to make sure that the reflection comes from the same iso density surface. With a second reflectometer the radial separation between top and LFS array as well as within the top array
41st EPS Conference on Plasma Physics, Berlin, Germany; 06/2014
[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. · 2.02 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. · 2.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The 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.
Journal of Nuclear Materials 07/2013; · 2.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Resonant 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.
Journal of Nuclear Materials 07/2013; · 2.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Laser-based methods are investigated for the development of an in situ diagnostic for spatially and temporally resolved characterization of the first wall in fusion devices. Here we report on the first systematic laser-induced ablation spectroscopy (LIAS) measurements carried out on various surface layers in the TEXTOR tokamak. These materials include a-C:D, mixed W/C/Al/D2, Oerlikon Balzers ‘Balinit’ diamond-like carbon layers and EK98 fine-grain graphite. In LIAS, the bulk or deposited material is evaporated during the plasma discharge by intense laser radiation. The light emitted by particles entering the edge of the ionizing tokamak plasma is then observed by optical spectroscopy. In the measurements taken, it was found that the studied layers can be identified by their characteristic line emission. A good correlation between the observed line intensity and layer thickness is found. The observed plumes show target material dependence. To analyze layers formed during tokamak operation, further investigation of the ablation process and reference materials for cross calibration is required.
[Show abstract][Hide abstract] ABSTRACT: Experiments have been carried out in the TEXTOR, ASDEX Upgrade (AUG) and Alcator C-Mod (C-Mod) tokamaks to study melt-layer motion, macroscopic W-erosion from the melt as well as the changes of material properties such as grain-size and voids. In addition the effect of multiple exposures is studied to judge the potential amelioration of inflicted melt damage. The parallel heat flux at the radial position of the PFCs in the plasma ranges from around q∥ ∼ 45 MW/m2 at TEXTOR up to q∥ ∼ 500 MW/m2 at C-Mod which covers scenarios close to ITER parameters, allowing samples to be exposed and molten even at shallow divertor angles. Melt-layer motion perpendicular to the magnetic field is observed consistent with a Lorentz-force originating from thermoelectric emission of the hot sample. While melting in the limiter geometry at TEXTOR is rather quiescent causing no severe impact on plasma operation, exposure in the divertors of AUG and C-Mod shows significant impact on operation, leading to subsequent disruptions. The power-handling capabilities are severely degraded by forming exposed hill structures and changing the material structure by re-solidifying and re-crystallizing the original material. Melting of W seems highly unfavorable and needs to be avoided especially in light of uncontrolled transients and misaligned PFCs.
Journal of Nuclear Materials 01/2013; · 2.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ion cyclotron wall conditioning (ICWC) discharges, in pulsed-mode operation, were
carried out in the limiter tokamak TEXTOR to explore safe operational regimes for the experimental
parameters for possible ICWC-discharge cleaning in International Thermonuclear Experimental
Reactor (ITER) at half field. Antenna coupling properties obtained during the ion cyclotron range
of frequencies (ICRF) wall conditioning experiments performed in helium–hydrogen mixture in
TEXTOR were analysed in relation to the obtained ICWC-plasma characterization results. Satisfactory
antenna coupling in the mode conversion scenario along with reproducible generation of
ICRF plasmas for wall conditioning, were achieved by coupling radio frequency (RF) power from
one or two ICRF antennas. The plasma breakdown results obtained in the TEXTOR tokamak
have been compared with the predictions of a zero-dimensional RF plasma production model. The
present study of ICWC emphasizes the beneficial effect of application of an additional (along with
toroidal magnetic field) stationary vertical (BV � BT) or oscillating poloidal magnetic field (BP �
BT) on antenna coupling and relevant plasma parameters.
[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: Volatile tungsten hexa-fluoride was locally injected into the TEXTOR tokamak as a marker for material migration studies. The injection was accompanied by puffing N-15 rare isotope as a nitrogen tracer in discharges with edge cooling by impurity seeding. The objective was to assess material balance by qualitative and quantitative determination of a global and local deposition pattern, material mixing effects and fluorine residence in plasma-facing components. Spectroscopy and ex situ ion beam analysis techniques were used. Tungsten was detected on all types of limiter tiles and short-term probes retrieved from the vessel. Over 80% of the injected W was identified. The largest tungsten concentration, 1 × 1018 cm−2, was in the vicinity of the gas inlet. Co-deposits contained tungsten and a mix of light isotopes: H, D, He-4, B-10, B-11, C-12, C-13, N-14, N-15, O-16 and small quantities of F-19 thus showing that both He and nitrogen are trapped following wall conditioning (He glow) and edge cooling.
Journal of Nuclear Materials 01/2012; · 2.02 Impact Factor
[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 12/2011; T145. · 1.03 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The characteristics of W plasma-facing components under high heat fluxes were investigated in view of the material choice for the divertor in future fusion devices. Thin W sheets were exposed in the TEXTOR tokamak to multiple steady-state loads (20 MW at 36°), resulting in substantial W melting. One sample was subjected to post-exposure edge localized mode (ELM) simulation in the electron beam facility JUDITH 1 to determine the modified behavior of the pre-damaged components. The material structure after exposure and the cracking thresholds and patterns under repetitive transient thermal loads were investigated. Grain sizes from the original μm up to the mm range were observed. Void-like structures from the low μm up to the mm range were found at different regions of the resolidified sample. Exposure of the molten and recrystallized specimens to ELM-like thermal loads revealed a shift of the cracking threshold towards 300 °C, indicating an unfavorable increase in the ductile-to-brittle transition temperature.
Physica Scripta 12/2011; T145. · 1.03 Impact Factor