The data acquisition system of the RFX tomographic diagnostic
The VME data acquisition system is described for a new tomographic diagnostic currently under development at the RFX nuclear fusion experiment. The system architecture, and its integration into the existing Control and Data Acquisition System of RFX is presented. A modelling approach in the definition and analysis of the distributed software architecture is then discussed. Finally the user interface requirements of the system and the implemented solutions are presented. I. INTRODUCTION RFX is a large magnetic confinement, nuclear fusion experiment currently in operation in Padova, Italy at the Istituto Gas Ionizzati of CNR [ll. Measurement of the intensity of soft X-ray and bolometric emission from the plasma in a fusion device can be related to properties such as plasma position, shape, impurity distribution, and magnetohydrodynamic phenomena. The use of tomographic methods, which provide multiple views of plasma emissivity along chords through a plasma crosssection, allows for the reconstruction of the spatial distribution of the plasma emissivity. In RFX, plasma emission is measured along up to 78 chords during the plasma pulse which can last up to 250 ms and is repeated about every 10 minutes. Due to the limited number of plasma emission views, the tomographic procedures implemented for plasma emissivity reconstructions do not use finite element techniques as used in medical tomography, rather a least square approach of analytic solutions of the Radon transform constructed using base function expansions: Fourier series for the angular components of the emissivity, various base function expansions for its radial components depending on the kind of inversion implemented [2]. The result of a tomographic inversion is therefore a set of parameters to be used in the chosen angular and radial expansions of the emissivity function. Due to the limitation in the number of angular harmonics and in the maximum order of the radial expansion functions, which are related to geomeVic factors in the chord displacements, the number of such parameters is usually much less than the number of acquired views, and the results of the tomographic inversion form a more compact representation of the information produced by the diagnostic. The interpretation of such results requires however much care because of the finite order of the harmonic expansion which causes aliasing of any unresolvable features and produce unavoidable artefacts. For this reason, multiple tomographic inversions, based on different function expansions, are required in order to obtain a 0018-9499/96$05