Showing papers by "A. J. H. Donné published in 1997"
TL;DR: In this paper, it was shown that electron heat transport is governed by alternating layers of good and bad thermal conduction, and the formation of sharp off-axis maxima on the profile was attributed to heat deposition precisely 'on top of' a transport barrier.
Abstract: Experiments with strong localized electron cyclotron heating (ECH) in the RTP tokamak show that electron heat transport is governed by alternating layers of good and bad thermal conduction. For central deposition hot filaments are observed inside the q = 1 radius. Moving the ECH resonance from the centre to the edge of the plasma results in discrete steps of the central electron temperature. The transitions occur when the minimum q value crosses q = 1,2,5/2 or 3, and correspond to the loss of a transport barrier situated close to the rational q value. Close to the transitions a new type of sawtooth activity is observed, characterized by the formation of sharp off-axis maxima on the profile, which collapse abruptly. The formation of the off-axis maxima is attributed to heat deposition precisely `on top of' a transport barrier.
116 citations
TL;DR: In this article, a high resolution multiposition Thomson scattering setup is described to measure the electron temperature (Te) and density (ne) of a hot plasma, which is operational at the Rijnhuizen Tokamak Project.
Abstract: A high resolution multiposition Thomson scattering setup to measure the electron temperature (Te) and density (ne) of a hot plasma is described. The system is operational at the Rijnhuizen Tokamak Project. Light from a high power pulsed ruby laser is scattered by the free plasma electrons and relayed to a Littrow polychromator for spectral analysis. The spectrally and spatially resolved light is detected by a GaAsP photocathode. The two-dimensional image is intensified and recorded with a charge-coupled device camera. Te in the range of 50 eV–6 keV can be measured at 115 spatial elements of 2.6 mm length along the laser beam. The observation error is <6% at ne=2×1019 m−3 and smaller for higher ne. The high resolution and accuracy enabled the observation of small scale structures in Te and ne.
45 citations
TL;DR: In this article, an experimental investigation into electron temperature profile perturbations due to MHD tearing modes is performed on the TEXTOR tokamak, by means of an ECE diagnostic with a good temporal and radial resolution, m = 1 and m = 2 precursors to sawtooth crashes and major disruptions, respectively, are studied.
Abstract: An experimental investigation into electron temperature profile perturbations due to MHD tearing modes is performed on the TEXTOR tokamak. By means of an ECE diagnostic with a good temporal and radial resolution, m = 1 and m = 2 precursors to sawtooth crashes and major disruptions, respectively, can be studied. The specific temperature perturbations due to a rotating m = 2 magnetic island prior to a disruption have been studied. The phase of the m = 2 MHD oscillation reverses at the rational q surface. The amplitude distribution of the perturbation has been determined. A frequency analysis of the MHD oscillations on different radial positions close to the rational surface shows an increase of higher harmonics in the frequency spectrum due to the difference in magnetic field topology. Temperature profiles over magnetic islands could be measured. A secondary temperature maximum in the centre of the m = 2 magnetic island has been found. Because of the good radial resolution of the ECE diagnostic an estimate of the island width and the position of the q = 2 surface could be obtained. In one case a movement of the q = 2 surface, due to peaking of the current profile, of more than 7 cm, i.e. 15% of the minor radius, was found. Island widths of have been observed.
32 citations
TL;DR: In this paper, a four channel pulsed radar reflectometer in TEXTOR-94 was used to study density profile perturbations due to large rotating m/n=2/1 magnetic islands.
Abstract: Density profile perturbations due to large rotating m/n=2/1 magnetic islands have been studied by means of a four channel pulsed radar reflectometer in TEXTOR-94. The time of flight of the microwave pulses was strongly affected by the shallow density gradients over the magnetic islands. A detailed analysis showed the existence of secondary reflection layers inside the magnetic island, which is an indication of a strong density peaking. This may be explained by assuming that the particle transport is reduced inside the magnetic island
25 citations
TL;DR: In this paper, the authors used reflectometry on ITER to measure the density profile in the main plasma and divertor regions, and to estimate the plasma position and shape in order to provide a standby reference for the magnetic diagnostics in long pulse discharges.
Abstract: Reflectometry will be used on ITER to measure the density profile in the main plasma and divertor regions, and to measure the plasma position and shape in order to provide a standby reference for the magnetic diagnostics in long pulse discharges. The high temperatures of the ITER core and the resultant significant relativistic downshift of the second-harmonic electron cyclotron absorption imply that both low-field side O-mode and high-field side lower cut-off (X−l mode) systems are required to access the full plasma profile. A low-field side upper cut-off (X−u mode) system will also be required for measurements of the scrape-off layer. For measurements of the plasma position and shape, an O-mode system is optimum due to the large range of magnetic field along the plasma periphery and the wide range of possible plasma configurations achievable on ITER. A robust real-time calibration technique of the whole transmission line is required. It is likely that an accurate estimate of the position of the plasma will require the simultaneous use of signals from the profile reflectometer. For the divertor, profiles with peak densities in the range 1019–1022/m3 are to be measured with a target resolution of 3 mm. The large density range will necessitate the use of more than one system. Installing these reflectometers on ITER incurs additional difficulties such as the routing of the millimetre wave radiation around the complicated first wall and divertor structures and design of antennas able to operate through the first wall and blanket.
23 citations
TL;DR: A four-channel pulsed radar reflectometer has been developed for the Rijnhuizen Tokamak Project as mentioned in this paper, which combines a high temporal resolution (2 μs) and a good spatial resolution (1 cm) with limited access demands.
14 citations
TL;DR: In this article, a double pulse multiposition Thomson scattering diagnostic, under construction at RTP, is discussed, where light from a double pulsed ruby laser (pulse separation: 10−800 μs, max. 2×12.5 J) is scattered by the free electrons of the tokamak plasma and relayed to a Littrow polychromator for spectral analysis.
Abstract: In this article a double pulse multiposition Thomson scattering diagnostic, under construction at RTP, is discussed. Light from a double pulsed ruby laser (pulse separation: 10–800 μs, max. 2×12.5 J) is scattered by the free electrons of the tokamak plasma and relayed to a Littrow polychromator for spectral analysis. The spectrally resolved light is recorded by two ICCD detectors. Simulations show that the system sensitivity will be such that electron temperatures in the range of 100 eV–7 keV can be determined with an accuracy as good as 2%–3% for electron densities of 1020 m−3, with a spatial resolution down to 2.6 mm. With this diagnostic the dynamics of small scale structures in the electron temperature profile will be studied.
6 citations
TL;DR: In this article, a 20-channel electron cyclotron absorption diagnostic has been developed at the Rijnhuizen tokamak project, where microwave signals are injected from the high field side and detected at the low field side in the equatorial midplane.
Abstract: A new 20-channel electron cyclotron absorption diagnostic has been developed at the Rijnhuizen tokamak project. It is the first time the electron pressure profile in a tokamak plasma can be measured directly with a time resolution of 1 ms. The diagnostic measures simultaneously the emission and absorption of the second harmonic electron cyclotron frequencies. Microwaves are injected from the high field side and detected at the low field side in the equatorial midplane. The transmitted power is measured after a single pass through the plasma column. The absorption measurements are complicated by nonresonant losses: refraction of the injected microwaves (losses up to 100%), and scattering of microwaves by density fluctuations (losses 2%–3%). A fast algorithm has been developed to obtain a quantitative measure for these nonresonant losses. This calculation method is based on a parametrization of the experimental data. Combining the electron cyclotron absorption (ECA) measurements and the parametrization prov...
5 citations
TL;DR: In this article, the effect of mode scrambling on the operation of pulsed radar reflectometers working in the ordinary polarization mode on devices with a high magnetic shear is studied, where the magnetic field changes considerably on length and/or time scales which are similar to or smaller than those of the probing wave.
Abstract: In this article the effect of mode scrambling on the operation of pulsed radar reflectometers working in the ordinary polarization mode on devices with a high magnetic shear is studied. Mode scrambling occurs when the magnetic field changes considerably on length and/or time scales which are similar to or smaller than those of the probing wave. In this case the polarization of the mode is not conserved during passage through the plasma, and power is transferred from ordinary to extraordinary mode and vice versa. Calculations with a full-wave code are performed for two different high-shear devices: the RFX reversed field pinch and the LHD torsatron. The conclusion of the work is that the density profiles in these devices can excellently be measured by a pulsed radar reflectometer operating in the ordinary mode.
4 citations
TL;DR: In this paper, the authors considered a high performance, low loss, broadband line for reflectometry on ITER and proposed a solution for the coupling of several microwave emitter receivers, working in a fundamental waveguide, into a single broadband line.
Abstract: Corrugated circular transmission lines are considered as high performance, low loss, broadband lines for reflectometry on ITER. These lines are proposed as a possible solution for the equatorial low-field-side system and as a part (receiver-vacuum vessel) of the transmission lines for the high-field side and the out-of-midplane reflectometer system, because of their restricted access. Theoretical and experimental studies show the good properties of these lines: low losses, a smooth characteristic transmission, and a very low level of parasitic reflections. Quasioptical systems are also proposed as a solution for the coupling of several microwave emitter receivers, working in a fundamental waveguide, into a single broadband corrugated line.
3 citations