[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. · 1.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The energy resolved efficiency of two bolometer detector prototypes for ITER with 4 channels each and absorber thicknesses of 4.5 μm and 12.5 μm, respectively, has been calibrated in a broad spectral range from 1.46 eV up to 25 keV. The calibration in the energy range above 3 eV was performed against previously calibrated silicon photodiodes using monochromatized synchrotron radiation provided by five different beamlines of Physikalische Technische Bundesanstalt at the electron storage rings BESSY II and Metrology Light Source in Berlin. For the measurements in the visible range, a setup was realised using monochromatized halogen lamp radiation and a calibrated laser power meter as reference. The measurements clearly demonstrate that the efficiency of the bolometer prototype detectors in the range from 50 eV up to ≈6 keV is close to unity; at a photon energy of 20 keV the bolometer with the thick absorber detects 80% of the photons, the one with the thin absorber about 50%. This indicates that the detectors will be well capable of measuring the plasma radiation expected from the standard ITER scenario. However, a minimum absorber thickness will be required for the high temperatures in the central plasma. At 11.56 keV, the sharp Pt-L3 absorption edge allowed to cross-check the absorber thickness by fitting the measured efficiency to the theoretically expected absorption of X-rays in a homogeneous Pt-layer. Furthermore, below 50 eV the efficiency first follows the losses due to reflectance expected for Pt, but below 10 eV it is reduced further by a factor of 2 for the thick absorber and a factor of 4 for the thin absorber. Most probably, the different histories in production, storage, and operation led to varying surface conditions and additional loss channels.
The Review of scientific instruments 12/2013; 84(12):123501. · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microwave stray radiation resulting from unabsorbed multiple reflected ECRH / ECCD beams may cause severe heating of microwave absorbing in-vessel components such as gaskets, bellows, windows, ceramics and cable insulations. In view of long-pulse operation of WENDELSTEIN-7X the MIcrowave STray RAdiation Launch facility, MISTRAL, allows to test in-vessel components in the environment of isotropic 140 GHz microwave radiation at power load of up to 50 kW/m 2 over 30 min. The results show that both, sufficient microwave shielding measures and cooling of all components are mandatory. If shielding/cooling measures of in-vessel diagnostic components are not efficient enough, the level of stray radiation may be (locally) reduced by dedicated absorbing ceramic coatings on cooled structures.
INTERNATIONAL CONFERENCE ON FUSION REACTOR DIAGNOSTICS, Villa Monastero, Varenna, Italy; 09/2013
[Show abstract][Hide abstract] ABSTRACT: The objective of Wendelstein 7-X is to demonstrate steady-state operation at -values of up to 5%, at ion temperatures of several keV and plasma densities of up to 2 1020 m 3. The second operational phase foresees a fully steady-state high heat flux (HHF) divertor. Preparations are underway to cope with residual bootstrap currents, either by electron cyclotron current drive or by HHF protection elements. The main steady-state heating system is an electron cyclotron resonance heating facility. Various technical improvements of the gyrotrons have been implemented recently. They enable a reliable operation at the 1MW power level. Some of the technical issues preparing plasma diagnostics for steady-state operation are exemplified. This includes the protection against non-absorbed microwave radiation.
[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. · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: For determining the thermal diffusivity in WEGA the electron cyclotron resonance heating power from a 28 GHz gyrotron is square-wave modulated to provide a periodically varying energy source at the plasma centre causing a modulation mainly of the electron temperature Te. A fast Si-diode bolometer system, whose 16 channel view the entire plasma cross-section, is employed to monitor the heat propagation process. A fast Fourier transform analysis shows clear coherency of the line-integrated signals among different channels and monotonic radial increment of the phase delays of the fundamental components in the central channel signals, allowing the determination of the electron thermal diffusivity coefficient D. A purely diffusive heat transport is assumed and is simulated using a Monte Carlo method. The thermal diffusivity is determined by matching the simulated results to the measured ones. A typical value of D = 1.9 m2 s−1 is obtained. This value is then compared with the result based on a local power balance analysis. Within the error bars the local power balance calculation yields a similar diffusivity value. The limitations and conditions of using bolometric diagnostic for this purpose are discussed.
[Show abstract][Hide abstract] ABSTRACT: This paper reports on detailed investigation of a fully 28 GHz EBW
(electron Bernstein wave) heated plasma in the WEGA stellarator. The
plasma shows a fast transition into the "OXB-state" when the threshold
density is reached. The profiles become peaked. The EBW emission
diagnostic measures a radiation temperature of several keV, which
origins from a supra-thermal electron population. The angular dependence
of the mode conversion could be confirmed with a movable launching
mirror. The toroidal current and the plasma conductivity were measured
for different microwave launch positions.
[Show abstract][Hide abstract] ABSTRACT: A bolometric diagnostic system with features necessary for steady-state operation in the superconducting stellarator W7-X was designed. During a pulse length of 1800 s with an ECRH (electron cyclotron resonance heating) power of 10 MW, the components suffer not only from a large thermal load but also from stray radiation of the nonabsorbed isotropic microwaves. This paper gives an overview of the technical problems encountered during the design work and the solutions to individual problems to meet the special requirements in W7-X, e.g., component thermal protection, detector offset thermal drift suppression, as well as a microwave shielding technique.
The Review of scientific instruments 10/2010; 81(10):10E134. · 1.52 Impact Factor
[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. · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: On WEGA stellarator electron Bernstein waves have been applied for plasma heating. The excitation of the electrostatic Bernstein waves was possible due to an efficient two-step OXB mode conversion process from electromagnetic waves. Overdense plasma operation was reached in argon and helium and, rather surprisingly, a supra-thermal electron component could be detected. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Contributions to Plasma Physics 01/2010; 50(8):785-789. · 0.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fundamental investigations on EBW excitation, propagation and absorption are presented. In particular the OXB mode conversion and EBW current drive with 2.45 GHz EBWs are investigated and compared with modeling. High density operation with 28 GHz EBWs could be achieved at 0.5 T (2nd harmonic). This regime is characterized by a additional strongly supra-thermal electron distribution with energies >10 keV. These electrons were detected by measurement of the EBW emission and by X-ray detection.
Mateev, M.; Benova, E.: 36th European Physical Society Conference on Plasma Physics. Contributed Papers, European Physical Society (2009). 01/2009;
[Show abstract][Hide abstract] ABSTRACT: Over-dense plasma heating with electron Bernstein waves (EBWs) is of large importance for fusion, since ultra high density operation is a promising fusion scenario for stellarators and heliotrons as well as for spherical tokamaks. EBWs are generated by the OXB-mode conversion process, which necessitates a threshold density (O-mode cut-off density) and a steep density gradient. This paper reports the achievement of a plasma, which is sustained by 28 GHz EBW-heating only. It was demonstrated that once the critical density for the OXB-mode conversion process is reached, the EBW-heating takes over and changes the density profile into a strongly peaked profile, which generates density gradients required for efficient mode conversion. In addition the experiment shows that central resonant power deposition can be achieved by the control of the N || -component as predicted by ray tracing calculations. A total absorption of 87% was estimated from the microwave stray radiation level.
[Show abstract][Hide abstract] ABSTRACT: form only given. The thin detector foil of the widely used metal resistive-type bolometer is composed of an absorber, a substrate and two meander resistors. The two resistors on the detector foil together with other two identical ones coated on a reference foil form a Wheatstone bridge. Absorption of plasma radiation causes a temperature-induced resistivity increase of the detector foil, driving an unbalance in the Wheatstone bridge. Since the bolometers sometimes lie in an environment of a significant neutral gas background, the bridge can be affected by the neutral gas pressure. To minimize this effect, the bolometer heads are constructed with a pressure compensation mechanism. In this work, the metal resistive bolometers, including the types being used on ASDEX-upgrade and JET, as well as those planned for W7-X and ITER, are investigated under different neutral gas pressure conditions. The foil either possesses a Kapton or mica substrate, carrying a gold absorber, or a silicon nitride substrate, carrying a platinum absorber. The gas pressure is increased up to 20Pa, covering the divertor conditions expected for W7-X and ITER. An evident pressure effect has been observed for all the kinds of the investigated bolometers. The induced bridge signals are converted to an equivalent plasma-radiated power flux based on the bolometer equation and the detector parameters, which are calibrated for each measurement condition by ohmic heating. An error factor, defined as signal equivalent power flux per unit pressure change of about 2.0-8.0Wm-2Pa-1 is obtained. For the W7-X and ITER divertor bolometers, presumably suffering from a pressure change up to 0.5Pa and 10Pa, respectively, non-negligible measurement errors were derived. The study results indicate that the compensation still needs to be improved further for cases with high background pressures, as in the divertor region. This effect usually is negligible for the bulk plasma bolometers lo- ated outside the divertor where the neutral gas pressure is usually much lower. The work presents also an analysis of the reasons for the effect described above. It is concluded that the observed neutral gas pressure effect arises from strain and gas-cooling induced meander resistance deviations between the detector and the reference foil. Cooling rates of the foils at different gas pressures have been calculated according to the realistic geometry of the bolometer heads. Concepts for remedying the effect, based on modification of the bolometer head structure as well as reducing the gas-cooling contribution to the total cooling rates of the foils, are proposed.