1,721,169 research outputs found
Calculation of 3D magnetic fields produced by MHD active control systems in fusion devices
No full textAn accurate control of the magnetic boundary of a thermonuclear plasma is an important issue in magnetic confinement research. The development of methods for the active control of magnetohydrodynamic instabilities and correction of error fields is mandatory in view of fusion reactors or experimental test reactors under design. Recently, a very effective control scheme, named clean mode control (CMC), has been proposed in a reversed field pinch experiment (RFX-mod). The CMC is based on the real-time correction (cleaning), under simplifying hypothesis, of the sideband harmonics in the magnetic field produced by the discrete local active coils. In this paper, we focus on the problem of carefully computing the penetration of sidebands through a realistic load assembly, but still with a simplified cylindrical geometry, to allow for a detailed comparison with the crude CMC algorithm without the additional complication of the coupling between the poloidal harmonics due to the toroidal geometry
Development and tests of a simple multifoil spectrometer for highly time-resolved line intensity measurements in the RFX experiment
No full textA simple and cost-effective spectrometer dedicated to line-intensity monitoring in the soft X-ray range has been realized, installed and tested in the RFX reversed field pinch experiment for the study of magnetically confined fusion plasmas. The technique of photon energy selection by means of multilayer material filters has been applied to the instrument described here, which, together with the associated electronics, has been designed in an original compact version for RFX. This present version allows on-line monitoring with good time resolution (10 kHz bandwidth) of line emission from carbon and oxygen in their H- and He-line states (which fall within the soft X-ray energy range), but, with modifications of the set of material filters, different wavelength ranges can be chosen and then other lines can be monitored. A detailed description of the instrument is given here and the data analysis technique is presented. Particular attention is dedicated to a critical analysis of the technique and to the discussion of the experimental errors which may be associated with this measurement; new results on the estimate of systematic errors and experimental improvements to eliminate the problem of continuum radiation are presented. As a demonstration of the proper operation of the instrument, data taken under a wide range of conditions in 0.5-0.6 MA RFX plasmas are shown and discussed. Good agreement of the measurements with other spectroscopic diagnostics already operating on the machine is found; by exploiting the high time resolution of the instrument, interesting results concerning the RFP configuration`s dynamics are obtained. Based on this prototype, a multichord diagnostic has been designed for imaging the impurity emission radial profiles
Electron Temperature Measurements with High Time Resolution in RFX
No full textCore electron temperature measurements have been performed in the large, high-current RFX reversed field pinch experiment (a = 0.46 m, R = 2 m, target plasma current = 2 MA) with the double filter technique. With this technique the soft x-ray emission is measured by two Si detectors, covered by different Be-filters, and viewing the same region of the plasma. By choosing the filters in such a way that only the emission from the continuum part of the spectrum is measured, the ratio of the two signals is a function of the highest electron temperature along the line of sight of the detectors.
With this diagnostic, the electron temperature has been measured in RFX with a bandwidth of approximately 2 kHz. This allows the direct measurement, for the first time in a RFP on a single shot basis, of coherent oscillations, associated with relaxation events and showing under some conditions a sawtooth-like character.
The measurements of electron temperature obtained with this method are in good agreement with those performed with other diagnostics
Total radiation losses and emissivity profiles in RFX
Full text linkIn the framework of adding new information to the reversed field pinch (RFP) confinement database, the radiation emitted by RFX plasmas has been investigated using an eight chord bolometric camera, whose detectors have been calibrated absolutely. From the experimental data the emissivity profiles have been reconstructed by means of a generalized tomography reconstruction algorithm. This analysis confirms that the radiation emitted in RFX is systematically concentrated at the edge. The dependence of the emitted power on the plasma density shows that the radiation increases approaching the high density regime, but it rarely goes beyond 30% of the input power for stationary discharges. This behaviour is strongly dependent on the concentration of impurities but, in any case, in RFX there is no evidence of disruptions. A simple local energy balance allows a preliminary evaluation of the radial heat flux profile to be obtained. These measurements indicate that an active impurity screening mechanism is acting in the edge and that transport is the major energy loss mechanism in RFX
Edge Localized asymmetric radiative phenomena in RFX
No full textMeasurements performed with a high resolution tomography system have shown that the total radiation patterns in the RFX plasmas are characterised by remarkable structures, localised in the edge and asymmetric, both in the toroidal and in the poloidal directions. The main source of symmetry breaking in the toroidal direction is mode locking i.e. unstable MHD modes lock in phase and to the wall during the setting-up of the magnetic con®guration and produce a toroidally localised macroscopic distortion of the plasma surface with enhanced plasma wall interaction. In this paper the toroidal and poloidal structure of the stationary emissivity pro®les in the locked mode region will be presented and its correlation with the local position and shape of the magnetic perturbation will be highlighted, thus giving a more complete picture of the locked mode structure in particular at the plasma edge. Ó 1999 Elsevier Science B.V. All rights reserved
Imaging of helical coherent structures in the Reversed Field Pinch
No full textImages obtained with soft X-ray tomography in the RFX reversed field pinch device are presented. They show that the plasma can be in two different regimes, characterized by a different degree of magnetic chaos. Evidence of a coherent helical structure emerging from the chaotic plasma core is shown. These images indicate the power of tomography, which allows the study of the dynamics of a Hamiltonian system like the magnetic field line
Advanced feedback control of magnetohydrodynamic instabilities: comparison of compensation techniques for radial sensors
No full textSoft x-ray tomographic imaging in the RFX reversed field pinch
No full textSXR cross-sectional distributions measured in the European reversed field pinch (RFP) experiment RFX by means of a tomographic diagnostic are described. Various reconstruction techniques have been compared. A correction technique to minimize the systematic effect due to the non-identical thicknesses of the material filters has been developed. A number of experimental situations have been explored, and a study of the correlation between the MHD and SXR properties of the plasma has been made. The analysis concentrates both on standard discharges and on the recently discovered quasi-single-helicity states, whose signature is the growth of a sizeable m = 1 structure in the emissivity distributions. Detailed imaging of the plasma in these conditions is presented
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