Tokamak Experiment For Technology Oriented Research Research Articles

Extraction of quasi-coherent modes based on reflectometry data

The identification of turbulence sources would drive to a deeper understanding of confinement dynamics in tokamak plasmas. Turbulence results from a mixture of instabilities corresponding to sources at different timescales and spatial scales. Using poloidal correlation reflectometry and multi-pin Langmuir probe, it was shown in the T-10 and the Tokamak Experiment for Technology Oriented Research (TEXTOR) tokamaks that the reflectometry frequency spectrum is the superposition of several components: broadband component, quasi-coherent (QC) modes and low-frequency components. The relevance of QC modes is associated with their link with the trapped electron mode instability. This link was exhibited in the transition from the linear ohmic confinement (LOC) to the saturated ohmic confinement (SOC) regime. A method is presented in this paper to extract the QC mode component from the reflectometry data, enabling its separation from the broadband component and the study of its time evolution. It is a first step toward the discrimination of turbulence sources. The central idea explores a way to combine the approach of signal processing and machine learning. The continuous wavelet transform on the basis of complex Morlet wavelet has proved to be efficient in providing a decomposition of a signal at different scales over time for fluctuation tackling; clustering techniques, such as the mini-batch K-means, are able to tackle clusters at different scales. The method was applied to Tore Supra and TEXTOR reflectometry data. In Tore Supra, the amplitude of the extracted QC mode component decreases during the LOC–SOC transition. In TEXTOR, the amplitude of the coherent spectra of the extracted QC mode component is similar to the experimental coherent spectra obtained through correlation reflectometry. The developed method permits the extraction of components, preserving their physical and statistical properties.

Runaway electron generation as possible trigger for enhancement of magnetohydrodynamic plasma activity and fast changes in runaway beam behavior

Peculiar phenomena were observed during experiments with runaway electrons: rapid changes in the synchrotron spot and its intensity that coincided with stepwise increases in the electron cyclotron emission (ECE) signal (cyclotron radiation of suprathermal electrons). These phenomena were initially observed in TEXTOR (Tokamak Experiment for Technology Oriented Research), where these events only occurred in the current decay phase or in discharges with thin stable runaway beams at a q = 1 drift surface. These rapid changes in the synchrotron spot were interpreted by the TEXTOR team as a fast pitch angle scattering event. Recently, similar rapid changes in the synchrotron spot and its intensity that coincided with stepwise increases in the non-thermal ECE signal were observed in the EAST (Experimental Advanced Superconducting Tokamak) runaway discharge. Runaway electrons were located around the q = 2 rational magnetic surface (ring-like runaway electron beam). During the EAST runaway discharge, stepwise ECE signal increases coincided with enhanced magnetohydrodynamic (MHD) activity. This behavior was peculiar to this shot. In this paper, we show that these non-thermal ECE step-like jumps were related to the abrupt growth of suprathermal electrons induced by bursting electric fields at reconnection events during this MHD plasma activity. Enhancement of the secondary runaway electron generation also occurred simultaneously. Local changes in the current-density gradient appeared because of local enhancement of the runaway electron generation process. These current-density gradient changes are considered to be a possible trigger for enhancement of the MHD plasma activity and the rapid changes in runaway beam behavior.

On the effects of magnetic perturbations on fast ion losses studied at TEXTOR

One criterion for the ignition of a fusion plasma is sufficient fast ion confinement. A key aspect in that context is the maintainability of good fast ion confinement in the presence of non-axisymmetric fields, such as those found in stellarators and during the application of resonant magnetic perturbations (RMPs) in tokamaks. This paper focuses on the influence of RMPs on the fast ion losses, studied at the Tokamak experiment for technology-oriented research (TEXTOR), a medium-sized device. TEXTOR is equipped with a flexible perturbation coil system, the dynamic ergodic divertor, allowing for the application of static or rotating perturbation fields. A rotating directional probe is used for the radially resolved fast ion loss measurements. The results achieved are presented and discussed for the poloidal/toroidal perturbation modes m/n = 3/1 and 6/2, with static and rotating fields.

The implementation of the Wendelstein 7-X control a data acquisition concepts at VUV/XUV overview spectrometers HEXOS

HEXOS (high efficiency extreme ultraviolet overview spectrometer) is an optimized set of four efficient VUV/XUV spectrometers. It is suitable for a complete coverage of the wavelength range of interest with sufficient spectral resolution. The spectrometers cover the entire wavelength range of 2.5–160nm with high performance (up to 9999 spectra at spectra rate of 1000s−1). To operate according to the Wendelstein 7-X (W7-X) control and data acquisition guidelines all necessary concepts for safety, autonomous and subordinated operation, and segment program controlled experiment operation will be implemented at HEXOS. The design of the HEXOS control and data acquisition system and the implementation of the main W7-X control and data acquisition concepts are described. An outlook on the test phase at the TEXTOR (Tokamak Experiment for Technology Oriented Research) device and the commissioning phase at W7-X is given.

Electron cyclotron emission imaging in tokamak plasmas

We discuss the recent history and latest developments of the electron cyclotron emission imaging diagnostic technique, wherein electron temperature is measured in magnetically confined plasmas with two-dimensional spatial resolution. The key enabling technologies for this technique are the large-aperture optical systems and the linear detector arrays sensitive to millimeter-wavelength radiation. We present the status and recent progress on existing instruments as well as new systems under development for future experiments. We also discuss data analysis techniques relevant to plasma imaging diagnostics and present recent temperature fluctuation results from the tokamak experiment for technology oriented research (TEXTOR).

Absolute intensity calibration of the Wendelstein 7-X high efficiency extreme ultraviolet overview spectrometer system

The new high effiency extreme ultraviolet overview spectrometer (HEXOS) system for the stellarator Wendelstein 7-X is now mounted for testing and adjustment at the tokamak experiment for technology oriented research (TEXTOR). One part of the testing phase was the intensity calibration of the two double spectrometers which in total cover a spectral range from 2.5 to 160.0 nm with overlap. This work presents the current intensity calibration curves for HEXOS and describes the method of calibration. The calibration was implemented with calibrated lines of a hollow cathode light source and the branching ratio technique. The hollow cathode light source provides calibrated lines from 16 up to 147 nm. We could extend the calibrated region in the spectrometers down to 2.8 nm by using the branching line pairs emitted by an uncalibrated pinch extreme ultraviolet light source as well as emission lines from boron and carbon in TEXTOR plasmas. In total HEXOS is calibrated from 2.8 up to 147 nm, which covers most of the observable wavelength region. The approximate density of carbon in the range of the minor radius from 18 to 35 cm in a TEXTOR plasma determined by simulating calibrated vacuum ultraviolet emission lines with a transport code was 5.5x10(17) m(-3) which corresponds to a local carbon concentration of 2%.

A hypothesis of inductive drive to explain the sawtooth measurements of tokamak experiment for technology oriented research (TEXTOR)

A hypothesis, based on the current density profile determined from the principle of minimum dissipation of magnetic energy, is applied to explain the measurement of q(0) and current variation in a sawtooth cycle in tokomak experiment for technology oriented research (TEXTOR) [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1985), Vol. I, p. 193]. A sawtooth oscillation is triggered when the on-axis current density in a configuration with m=0 and n=0 symmetry is driven inductively to a limit.

Overview of Current Density Measurements and Sawtooth Studies on TEXTOR

The analysis of Faraday rotation measurements shows that the safety factor on the magnetic axis stays well below one during the full sawtooth cycle.The current density profile has a unique shape during the sawtoothing plateau phase of ohmic discharges, independent of plasma current and magnetic field, when it is plotted in suitable normalized coordinates.The observation of short lasting spikes on the Faraday rotation signals can be interpreted as the formation of a transient current sheet during the sawtooth crash.Direct measurements of the magnetic amplitude of the internal kink mode with a fast polarimeter channel show that the displacement of the core because of the growing precursor mode stays much smaller than the q = 1 radius just prior to the sawtooth collapse.The versatile neutral beam heating system on Tokamak EXperiment for Technology Oriented Research (TEXTOR) allows the study of the influence of the plasma rotation on the sawtooth period, which has a minimum when the magnetohydrodynamic rotation in the tokamak frame is stopped.

Improved two-point model for limiter scrape-off layer

An analytical model for a limiter scrape-off layer (SOL) is proposed, which takes self-consistently into account both conductive and convective contributions to the heat transport in SOL. The particle flows in the SOL main part are determined by considering the recycling of neutrals. The model allows us to interpret the results of numerical simulation by the code EMC3-EIRENE [Y. Feng, F. Sardei, P. Grigull, K. McCormick, J. Kisslinger, D. Reiter, and Y. Igitkhanov, Plasma Phys. Controlled Fusion 44, 611 (2002)] for the edge region of Tokamak Experiment for Technology Oriented Research (TEXTOR) [Proceedings of the 16th IEEE Symposium on Fusion Engineering, 1995 (Institute for Electrical and Electronics Engineers, Piscataway, NJ, 1995), p. 470].

Parametric dependence of density limits in the Tokamak Experiment for Technology Oriented Research (TEXTOR): Comparison of thermal instability theory with experiment

The observed dependence of the TEXTOR [Tokamak Experiment for Technology Oriented Research: E. Hintz, P. Bogen, H. A. Claassen et al., Contributions to High Temperature Plasma Physics, edited by K. H. Spatschek and J. Uhlenbusch (Akademie Verlag, Berlin, 1994), p. 373] density limit on global parameters (I, B, P, etc.) and wall conditioning is compared with the predicted density limit parametric scaling of thermal instability theory. It is necessary first to relate the edge parameters of the thermal instability theory to n̄ and the other global parameters. The observed parametric dependence of the density limit in TEXTOR is generally consistent with the predicted density limit scaling of thermal instability theory. The observed wall conditioning dependence of the density limit can be reconciled with the theory in terms of the radiative emissivity temperature dependence of different impurities in the plasma edge. The thermal instability theory also provides an explanation of why symmetric detachment precedes radiative collapse for most low power shots, while a multifaceted asymmetric radiation from the edge MARFE precedes detachment for most high power shots.

Thermal instability theory analysis of multifaceted asymmetric radiation from the edge (MARFE) in Tokamak Experiment for Technology Oriented Research (TEXTOR)

The density limits for a series of shots in TEXTOR [Tokamak Experiment for Technology Oriented Research, E. Hintz, P. Bogen, H. A. Claaßen et al., in Contributions to High-Temperature Plasma Physics, edited by K. H. Spatschek and J. Uhlenbusch (Akademie Verlag, Berlin, 1994, p. 373)], over a range of heating powers, that ended in multifaceted asymmetric radiation from the edge (MARFE) have been analyzed within the context of thermal instability theory. The prediction of MARFE onset agrees with observation to within the experimental uncertainty.

Effect of edge turbulence on threshold of multifaceted asymmetric radiation from the edge instability in tokamaks

The radiative condensation instability believed to be responsible for the occurrence of multifaceted asymmetric radiation from the edge (MARFE) in tokamaks has been investigated in the presence of a background turbulence of saturated drift resistive ballooning modes (DRBM). These modes are a natural source for edge fluctuations in tokamaks. For the m=1 MARFE, it has been shown that a modest level of DRBM turbulence can significantly lower the threshold radiation rate required for excitation of MARFEs, enhance its growth rate, and introduce a real part to the mode frequency. Some of these results are consistent with observations of onset of MARFEs in recent experiments on Joint European Torus (JET) [Proceedings of the 17th European Conference on Controlled Fusion and Plasma Heating, Amsterdam (European Physical Society, Petit-Lancy, 1990), Vol. 14b-1, p. 339] and Tokamak Experiment for Technology Oriented Research (TEXTOR) [Phys. Rev. Lett. 71, 1549 (1993)].

Manufacturing and Testing of Actively Cooled Test Limiters for TEXTOR Made of an LPPS Tungsten-Coated TZM Heat Sink

To investigate the erosion and redeposition phenomena of fusion-related materials under stationary conditions, actively cooled test limiters were developed for the Tokamak Experiment for Technology Oriented Research (TEXTOR). The test limiters allow experiments under stationary conditions within a plasma pulse length of 10 s. Heat loads of typically 10 MW/m2 are removed by pressurized water; the volume flow is 10 m3/h, the pressure is 15 bar, and the minimum coefficient of heat transfer is nearly 70 000 W/m2·K. The limiters were manufactured as low-pressure plasma-spraying thermally sprayed tungsten-coated heat sinks made of the molybdenum alloy TZM. The required properties of the tungsten coating were developed by the use of a statistically based optimization routine. Optimized, actively cooled limiters were successfully tested in Forschungszentrum Jülich’s Material Research Ion Beam Test Facility (MARION) with hydrogen beams. Maximum heat loads of up to ~17 MW/m2 were applied without any failure of either the heat sink or the cooling system. The steady state of the surface temperature was measured within 2 s. Analytical and numerical models describing the effects of heat load distribution and spatial temperatures were found to be in excellent agreement with numerical predictions. In an additional experiment, loss of coolant was simulated. Transition boiling was generated, and after repeated heat loads higher than 10 MW/m2, cavitational damage of the heat sink occurred. Concerning the material selection for heat sinks of hypervapotrons and other cooling systems based on enhanced boiling of the cooling liquid, this result might be of special interest.

Sawtooth Oscillations in Reacting Plasmas

An important issue infusion power plant design involves the effect of sawteeth on power transients and ash removal. A model was previously presented for sawtooth oscillations in tokamaks based on a thermal instability mechanism mediated by turbulence in the q < 1 core. This model has been validated against current machines such as the Joint European Torus (JET) and the Tokamak Experiment for Technol.ogy-Oriented Research (TEXTOR). Here, it is adapted to include effects of alpha particle heating in a reacting plasma. The goal is to study the interaction between fusion reactions and the sawtooth turbulence dynamics. The alpha particle heating rate function is sensitive to temperature variations and could possibly result in a thermal instability mechanism for amplifying sawtooth temperature fluctuations under typical conditions. Taking International Thermonuclear Experimental Reactor (ITER)-like conditions as an example, calculations are performed to illustrate this generic behavior. These conditions suggest that while the sawteeth are effective in helium ash removal from the q < 1 region, they can produce significant spikes in the alpha power.

Gyrosheath near the tokamak edge

A new model for the structure of the radial electric-field profile in the edge during the high confinement (H mode) is proposed. Charge separation caused by the difference between electron and ion gyromotion, or more importantly in the tokamak, the banana motion (halo effect) can self-consistently produce an electric dipole moment that causes the sheared radial electric field. The calculated results based on the model are consistent with DIII-D [Fusion Technol. 8, 441 (1985)] and Tokamak Experiment for Technology Oriented Research (TEXTOR) [J. Nucl. Mater. 122, 123, 1124 (1984)] experimental results.

Hydrocarbon Formation on Carbon Surfaces Facing a Hydrogen Plasma

The hydrocarbon formation of carbon materials exposed to hydrogen atoms and ions is reviewed. It turns out that surface conditions and modification play a major role in hydrocarbon formation. Graphite exposed to hydrogen plasma exhibits surface properties similar to those of amorphous hydrogenated carbon films and redeposited carbon films. The total chemical erosion yield of these carbon materials is similar for thermal atomic hydrogen and for energetic hydrogen ions, reaching maximum values of ∼0.1 eroded carbon per incoming hydrogen. However, the spectrum of formed hydrocarbons is determined by the energy of the impinging hydrogen. By a thermal H0 exposure, CH3 is formed together with equal amounts of C2Hx and C3Hx. With an energetic H+ bombardment, the main reaction product is CH4 with minor contributions of C2Hx and C3Hx. The amount of C2Hx and C3Hx formation decreases with increasing H+ energy. Hydrocarbon formation at low energies and high flux densities, as in the scrape-off layer (SOL) of fusion devices, is characterized by a broadening of the temperature dependence together with a slight decrease of the absolute erosion yield. Similar results have been obtained by an in situ study of the hydrocarbon formation in the SOL of the Tokamak Experiment for Technology Oriented Research (TEXTOR) plasma using a sniffer probe system.

Development of a High Vacuum, High Temperature Movable Limiter Support

The Advanced Limiter Test-II is a full toroidal belt pump limiter for the Tokamak Experiment for Technology Oriented Research (TEXTOR), consisting of eight curved blade segments that extend around the full circumference of the machine. The blades are supported at their ends from TEXTOR's Inconel liner by 16 worm gear screw jacks, which allow the positions of the limiter blades to be adjusted relative to the plasma centerline. Mechanical linkages and rotary feedthroughs link each jack to a drive motor outside the vacuum vessel. This allows the TEXTOR operators to adjust the positions of the blades remotely without opening the machine to the atmosphere. The lubricants and materials normally used in mechanical drive components are unacceptable in the tokamak environment. A prototype jack mechanism was designed and built with a Type 316L stainless steel housing and two sets of moving parts. The first set consisted of Nitronic 60 parts running against parts of aluminum bronze (CDA 642). The second set was made entirely of Nitronic 60. All running surfaces were coated with Dicronite, a WS/sub 2/ solid film lubricant. High vacuum, high temperature tests of these are reported in this paper.