[Show abstract][Hide abstract] ABSTRACT: An overview of the diagnostics which are essential for the first operational phase of Wendelstein
7-X and the set of diagnostics expected to be ready for operation at this time are presented. The
ongoing investigations of how to cope with high levels of stray Electron Cyclotron Resonance Heating
(ECRH) radiation in the ultraviolet (UV)/visible/infrared (IR) optical diagnostics are described
Review of Scientific Instruments 11/2014; 85:11D818. · 1.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microscopic turbulence properties in the edge of toroidally confined fusion plasmas are studied by comparative analysis of experimental data from seven devices, collected in an international edge turbulence database. The database contains Langmuir probe measurements of fluctuations in the floating potential and ion saturation current across the last closed flux surface. They are used to address statistical properties and particle transport. Universal features of plasma edge turbulence such as an increase in skewness across the scrape-off layer (SOL) as footprints of density blobs are recovered in all devices. Analysis of the correlation lengths and times reveals power law scaling relations with macroscopic drift-wave parameters, albeit weaker than would be expected for drift-wave turbulence. As a result, the turbulent diffusivity scales with the inverse of the magnetic field strength, which is closer to Bohm-like scaling than to gyro-Bohm scaling. Nearly identical scaling relations are determined in the confined plasma edge and the SOL, pointing to a strong connection between drift-wave turbulence in the edge and blobs in the SOL. The contributions of blobs and holes (negative density spikes) to the radial particle transport are analyzed qualitatively with a conditional averaging approach. Blobs are connected to outward transport in the SOL of all devices whereas holes exhibit no uniform propagation pattern.
[Show abstract][Hide abstract] ABSTRACT: The critical issues in the development of diagnostics, which need to work robust and reliable under quasi-steady state conditions for the discharge durations of 30 min and which cannot be maintained throughout the one week duration of each operation phase of the Wendelstein 7-X stellarator, are being discussed.
The Review of scientific instruments 10/2012; 83(10):10D730. · 1.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Highlights
► In long pulse fusion devices diagnostics are subject to severe heating. ► Several concepts to reduce the heat influx were applied. ► Heat conduction was improved to keep the temperature within acceptable limits.
[Show abstract][Hide abstract] ABSTRACT: To measure fluctuations of plasma density, electron temperature and electric plasma potential in the edge of fusion experiments simultaneously and with high spatial resolution, the best approach is generally agreed to be the fast sweeping of the Langmuir probe characteristic, a technique which has been applied on several experiments in the past few years. “Fast” in this context means that the sweep frequency should be well above typical fluctuation frequencies which are known to range up to a few 100 kHz. A new experimental arrangement for such measurements has been used on the Wendelstein 7-AS stellarator, making use of amplifiers in the reciprocating probe head, only few cm from the probe tips. It is thus possible to sweep the probes in the MHz range and to obtain good quality probe characteristics without having to correct for cable resonances and capacitance.Four poloidally displaced probe tips were swept with 1–4 MHz and the voltage and current signals were recorded with a sampling frequency of 50–200 MHz. Ion saturation current and floating potential fluctuations obtained from the swept signals are compared with static measurements of these two quantities measured simultaneously by further tips on the same probe head.If we use “standard” probe theory to analyse these measurements, the temperature fluctuations contribute significantly when calculating the plasma potential from the floating potential. In standard probe theory some basic assumptions are made, e.g., on the ratio of electron to ion temperature or the secondary electron emission coefficient of the probe tips. The influence of changes in these assumptions on the calculated phase angle between density, electron temperature and electric potential fluctuations and on the resulting radial particle and thermal transport is discussed. The fluctuation-induced radial particle transport is compared with the value obtained from ion saturation current and floating potential neglecting temperature fluctuations.
Beiträge aus der Plasmaphysik 11/2010; 38(S1):134 - 144.
[Show abstract][Hide abstract] ABSTRACT: This paper presents an overview of the advance in probe design and measurements made at JET during the MkIIa divertor experimental campaign. In addition to the fixed probe system, new probes have been implemented specifically to investigate the effect of plasma resistivity on probe derived electron temperatures and to measure turbulence in the SOL.
Beiträge aus der Plasmaphysik 11/2010; 38(S1):61 - 67.
[Show abstract][Hide abstract] ABSTRACT: The status of the diagnostic developments for the quasistationary operable stellarator Wendelstein 7-X (maximum pulse length of 30 min at 10 MW ECRH heating at 140 GHz) will be reported on. Significant emphasis is being given to the issue of ECRH stray radiation shielding of in-vessel diagnostic components, which will be critical at high density operation requiring O2 and OXB heating.
The Review of scientific instruments 10/2010; 81(10):10E133. · 1.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The three-dimensional structure of turbulence in the edge (inside the last closed flux surface) of the WEGA stellarator is studied focusing on the parallel dynamics. WEGA as a small stellarator with moderate plasma parameters offers the opportunity to study turbulence with Langmuir probes providing high spatial and temporal resolution. Multiple probes with radial, poloidal and toroidal resolution are used to measure density fluctuations. Correlation analysis is used to reconstruct a 3D picture of turbulent structures. We find that these structures originate predominantly on the low field side and have a three-dimensional character with a finite averaged parallel wavenumber. The ratio between the parallel and perpendicular wavenumber component is in the order of 10-2. The parallel dynamics are compared at magnetic inductions of 57 and 500 mT. At 500 mT, the parallel wavelength is in the order of the field line connection length 2\pi R/\iotabar . A frequency resolved measure of kpar/ktheta shows a constant ratio in this case. At 57 mT the observed kpar is much smaller than at 500 mT. However, the observed small average value is due to an averaging over positive and negative components pointing parallel and antiparallel to the magnetic field vector.
Plasma Physics and Controlled Fusion 08/2009; · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Wendelstein 7-X, currently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany, is a modular advanced stellarator, combining the modular coil concept with optimised properties of the plasma. The magnet system of the machine consists of 50 non-planar and 20 planar superconducting coils which are arranged in five identical modules, forming a toroidal five-fold symmetric system. The majority of operational magnetic configurations will have rotational transform ι/2π = 1 at the boundary. Such configurations are very sensitive to symmetry breaking perturbations, which are the consequence of imprecisely manufactured coils or assembly errors. To date, all 70 coils have been fabricated, and the first two half-modules of the machine have been assembled. The comparative analysis of manufactured winding packs and estimates of the corresponding level of magnetic field perturbation are presented. The dependency of the error fields on the coil assembly sequence is considered, as well as the impact of the first assembly errors. The influence of different construction uncertainties is discussed, and measures to minimise the magnetic field perturbation are suggested.
Fusion Engineering and Design 06/2009; 84:408-412. · 1.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stellerators can be operated without the need of significant plasma current for generation of poloidal magnetic field components. Wendelstein 7-X (W7-X) is optimized for stiff magnetic configurations by minimizing pressure driven currents, such as bootstrap and Pfirsch–Schlüter currents. In Addition to the largely reduced plasma currents, the HELIAS-type device is a low magnetic shear configuration required for magnetic boundary islands. However, the diverting magnetic field structure is already influenced by small net plasma currents. In particular, the W7-AS divertor results revealed a significant sensitivity of the X -point location with respect to favorable edge plasma states. Thus, a precise X -point control for W7-X is envisaged and, although expected net plasma currents range in the order of 10–100 kA only, stabilization by adaptive controllers is foreseen to an accuracy of about 1 kA . This stabilization should be reliable even after 2000 s of plasma operation. A precondition for achieving this is a reliable current detection. For this purpose, W7-X will be equipped with Rogowski of which the first has now been assembled onto one of the plasma vessel sectors. The integral performance and sensitivity inhomogeneities have been tested by employing the digital integrator of W7-X [A. Werner, Rev. Sci. Instrum. 77, 10E307 (2006)] and a cable energized with 200 A , representing the plasma current.
Review of Scientific Instruments 11/2008; · 1.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Wendelstein 7-AS was the first modular stellarator device to test some basic elements of stellarator optimization: a reduced Shafranov shift and improved stability properties resulted in β-values up to 3.4% (at 0.9 T). This operational limit was determined by power balance and impurity radiation without noticeable degradation of stability or a violent collapse. The partial reduction of neoclassical transport could be verified in agreement with calculations indicating the feasibility of the concept of drift optimization. A full neoclassical optimization, in particular a minimization of the bootstrap current was beyond the scope of this project. A variety of non-ohmic heating and current drive scenarios by ICRH, NBI and in particular, ECRH were tested and compared successfully with their theoretical predictions. Besides, new heating schemes of overdense plasmas were developed such as RF mode conversion heating—Ordinary mode, Extraordinary mode, Bernstein-wave (OXB) heating—or 2nd harmonic O-mode (O2) heating. The energy confinement was about a factor of 2 above ISS95 without degradation near operational boundaries. A number of improved confinement regimes such as core electron-root confinement with central Te ≤ 7 keV and regimes with strongly sheared radial electric field at the plasma edge resulting in Ti ≤ 1.7 keV were obtained. As the first non-tokamak device, W7-AS achieved the H-mode and moreover developed a high density H-mode regime (HDH) with strongly reduced impurity confinement that allowed quasi-steady-state operation (τ ≈ 65 τE) at densities (at 2.5 T). The first island divertor was tested successfully and operated with stable partial detachment in agreement with numerical simulations. With these results W7-AS laid the physics background for operation of an optimized low-shear steady-state stellarator.
[Show abstract][Hide abstract] ABSTRACT: Stationary Langmuir probe measurements of ion saturation current and floating potential in a plasma cannot give direct information on density and plasma potential fluctuations in the presence of temperature fluctuations. This problem can be avoided if the probe bias voltage is continuously swept faster than the fluctuation time scale, recording the current-voltage characteristic. This article reports the development of a spatiotemporal highly resolving Langmuir probe array with 15 fast swept tips, operating in the strongly magnetized, collisionless edge plasma of the Wendelstein 7-AS stellarator [Plasma Phys. Controlled Fusion 31, 1579 (1989)]. The probe tips are aligned in the poloidal direction, the tip spacing is 2 mm, and the sweeping frequency is 1.4 MHz. Current and voltage data are sampled with 50 MHz. The high bandwidth of the measurement is achieved by placing miniaturized differential amplifiers close to the probe tips in order to do an impedance transform. The surface-mounting technology and an additional inverse feedback module are utilized, allowing for an input voltage range of +/-100 V, and a common mode rejection rate of 55 dB at 4 MHz, which is sufficient to resolve the nonlinear probe characteristic. For the evaluation of the data, a fit model for stationary probes is employed and found adequate. Changes of the plasma parameters during one voltage sweep are taken into account by a linear interpolation of the fit parameters. Spatio-temporal fluctuation data gained by a fast swept Langmuir probe array, which can be relevant for the turbulent radial transport of particles and energy, are presented.
Review of Scientific Instruments 06/2007; 78(5):053505. · 1.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Electrostatic fluctuations have been measured in the scrape-off layer of ASDEX and W7-AS by Langmuir probes and by observation of Hα light and light emitted from a fast Li atom beam with high spatial and temporal resolution. It was demonstrated that these fluctuations contribute a significant, if not dominant, fraction of the anomalous radial particle transport. The basic properties of the fluctuations are the same in both experiments. A model for an instability mechanism specific to the SOL is presented including density, temperature and electric potential fluctuations. From this model mixing length estimates for the radial transport and resulting density and pressure gradients in the SOL are derived and compared to measurements in the mid-plane and in the divertor of ASDEX. In spite of several simplifications in the model a quantitative agreement up to factors of 1-3 and a qualitative agreement for variations of discharge parameters is achieved between the model predictions and the measurements. Data from poloidal multi-pin probe arrays are decomposed into a sum of spatial-temporal "events" by means of a fitting procedure. Centres of selected events serve as reference points for the analysis of the dynamics in their surroundings. Averaging shows that positive and negative potential events appear mostly in pairs with the E x B drift in between directed radially outward.
Physica Scripta 07/2006; 51(5):610. · 1.03 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Edge-localized modes (ELMs) have been observed by diagnostic systems with high spatial and temporal resolution in the ASDEX and JET tokamaks. During the ELMs, substructures exist in the scrape-off layer (SOL) with sizes of 2–5 cm and lifetimes of the order of 20–50 µs. Their poloidal–temporal evolution is directly observed, and the radial transport due to turbulent E × B drift in these substructures is estimated. A comparison is made between 'normal' fluctuations between ELMs and these substructures in terms of poloidal size and velocity and related radial transport. The increased radial transport during ELMs is shown to be mainly due to these substructures. The observed fluctuation amplitudes and velocities of the substructures are found to be compatible with the model of radial transport by plasma 'blobs' and plasma potential gradients generated by the sheath boundary conditions. The transport due to turbulent radial E × B flows is compared during and between ELMs, and the observation of ELM substructures in the main plasma SOL is put into context with the observation of substructures on the target plates.
[Show abstract][Hide abstract] ABSTRACT: The fluctuations in the edge plasma of magnetic fusion experiments play an important role in terms of anomalous energy and particle transport. Experiments on Wendelstein 7-AS (Renner H et al 1989 Plasma Phys. Control. Fusion 31 1579) were conducted to investigate the propagation of actively fed signals driven by electrical probes. The perspective of these investigations is an improved understanding of the turbulence and potentially a method to tailor the edge plasma turbulence. Two moveable poloidal probe arrays were used for the experiments, one located on the inboard side of the vessel and the other on the outboard side. A subset of probe tips was used for actively driving the plasma by different control signals, the remaining probes collected fluctuation data in the plasma boundary. With this set-up it was possible to investigate the signal propagation in the poloidal direction and also parallel to the magnetic field. Poloidally, we find a significant cross-correlation between active and passive probes. From analysis of the cross-coherency and phases of the signal with passive probe tips, a dominant influence of the background plasma rotation on the applied signals is observed. In the case of externally driven waves by several phase-locked active probes, the direction of the wave propagation with respect to the plasma rotation (co- or counter-rotating) is essential for an effective coupling to the turbulence. Also, the signal propagation parallel to the magnetic field depends on co- or counter-rotation with respect to the background plasma rotation. The parallel phase velocity was found to be compatible with the electron thermal velocity.