Lionello Marrelli

Soft x-ray and bolometric tomography in RFX

The article presents the design and the first results of a new eight camera, soft x-ray (SXR) and bolometric tomography which has been installed in RFX, a large (R=2 m, a=0.46 m), high current reverse field pinch experiment operating in Padova. The main features of the diagnostic, which has been designed with standards compatible with the requirements of large fusion experiments, are: (a) it allows simultaneous reconstruction of both SXR and total radiation emissivity structures with high spatial and time resolution (bandwidth up to 200 kHz for SXR and 10 kHz for bolometry). Moreover, computer controlled remote actions which interchange up to five material filters in front of the detectors allow the exploration of different energy ranges; (b) it can be easily decoupled from the machine without breaking the main vacuum and can be operated during first wall baking up to 300 °C thanks to a cooling circuit; (c) the detection chain is designed to measure very low signals [in the order of 1 nA for SXR diodes an...

Ion and electron local transport inside single helicity islands in the reversed field pinch

The ion and electron diffusion coefficients inside the helical magnetic core in a single helicity reversed field pinch (RFP) toroidal plasma have been numerical determined by a Monte Carlo test particle approach. A new algorithm aimed at approximating with polyhedra the magnetic surfaces obtained with Poincare cross sections has been developed. Stationary simulations of test ion and electron transport have been performed: they allow the determination of the average diffusion coefficients inside the helical core both for ions and electrons, which are much lower than those found in standard multiple helicity plasmas. Results are discussed and compared with those found in previous RFP simulations. The technique may be relevant also for stellarator and tokamak studies.

Electron temperature measurements with high time resolution in RFX

Core 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.

Development and tests of a simple multifoil spectrometer for highly time-resolved line intensity measurements in the RFX experiment

A 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.

IMPROVED PERFORMANCE OF THE THOMSON SCATTERING SYSTEM IN RFX

A Thomson scattering diagnostic is used in the RFX reversed field pinch experiment to measure single pulse, multipoint electron temperature (Te) profiles. Recent significant improvements have provided reliable and routine operation of the diagnostic. Custom developed CAMAC charge analog-to-digital converters have been introduced, which sample the plasma light 170 ns after the laser light and have greater sensitivity. The spectrometer configuration and the calibration procedure have been optimized. These upgrades have finally led to better fitted scattering spectra and more regular estimates of Te profiles, allowing the measurement of temperatures as low as 20 eV. The introduction of a second spectrometer has recently added a further ten positions to the original ten point profile, resolving the often steep Te edge gradient. On axis values are in good agreement with other Te measurements from SXR diagnostics (either SiLi and double filter method). Te profiles are typically rather flat, but some dependence ...

Error analysis for thermonuclear plasma bolometric measurements performed with widely used miniaturized metal resistor detectors

A general qualitative and quantitative analysis of the experimental and numerical errors which affect the plasma total radiated power measurements performed in thermonuclear fusion experiments with widely used miniaturized low-noise metal resistor bolometers is presented in this paper. This analysis, which generalizes the results of previous work on the detection limit, is motivated by the fact that, by means of bolometric tomography, detailed reconstruction of the radiated power emissivity distributions, in particular during fast phenomena, requires a high level of precision in the raw measurement and then a thorough control of the error sources. Since these bolometers are widely used in the largest tokamak, RFP and Stellarator experiments and are also considered for the reference design of the bolometric diagnostics in the International Thermonuclear Experimental Reactor (ITER), the study presented here has been accomplished in a machine-independent way. Practical formulae and methods to estimate the experimental errors that do not depend on the magnetic configuration are presented.

Signal processing and general purpose data acquisition system for on-line tomographic measurements

New analog signal conditioning electronics and data acquisition systems have been developed for the soft x-ray and bolometric tomography diagnostic in the reverse field pinch experiment (RFX). For the soft x-ray detectors the analog signal processing includes a fully differential current to voltage conversion, with up to a 200 kHz bandwidth. For the bolometers, a 50 kHz carrier frequency amplifier allows a maximum bandwidth of 10 kHz. In both cases the analog signals are digitized with a 1 MHz sampling rate close to the diagnostic and are transmitted via a transparent asynchronous xmitter/receiver interface (TAXI) link to purpose built Versa Module Europa (VME) modules which perform data acquisition. A software library has been developed for data preprocessing and tomographic reconstruction. It has been written in C language and is self-contained, i.e., no additional mathematical library is required. The package is therefore platform-free: in particular it can perform online analysis in a real-time applic...

Bolometric and SXR Tomography in RFX

Among the various diagnostic systems which equip the RFX Reversed Field Pinch experiment (Rostagni, 1995; Bagatin et al., 1995) the new integrated bolometric and soft x-ray tomography is one of the most complex. It has been designed and to operate in a harsh environment following standards compatible with the requirements of large fusion experiments. The bolometric detectors which have been used are the same adopted in the major tokamaks (Mast et al., 1991)

Three-dimensional simulations of plasma turbulence in the RFX-mod scrape-off layer and comparison with experimental measurements

The tokamak scrape-off layer (SOL) plasma dynamics is investigated in a circular limiter configuration with a low edge safety factor. Focusing on the experimental parameters of two ohmic tokamak inner-wall limited plasma discharges in RFX-mod [Sonato et al., Fusion Eng. Des. 74, 97 (2005)], nonlinear SOL plasma simulations are performed with the GBS code [Ricci et al., Plasma Phys. Controlled Fusion 54, 124047 (2012)]. The numerical results are compared with the experimental measurements, assessing the reliability of the GBS model in describing the RFX-mod SOL plasma dynamics. It is found that the simulations are able to quantitatively reproduce the RFX-mod experimental measurements of the electron plasma density, electron temperature, and ion saturation current density (jsat) equilibrium profiles. Moreover, there are indications that the turbulent transport is driven by the same instability in the simulations and in the experiment, with coherent structures having similar statistical properties. On the ot...

Imaging of helical coherent structures in the reversed field pinch

Images 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 lines.