Pietro Fiorentin

The plasma system of RFX

Abstract The “plasma system” of RFX includes the first wall, the vacuum vessel, the stabilizing shell, the vacuum and gas injection system, the remote handling system and the relevant auxiliary plants (cooling, baking, glow discharge cleaning, etc.). Since no limiters are provided, the full energy and particle flux from the plasma must be absorbed by the first wall. Taking into account the significant power losses from the plasma in nominal conditions, the high peaking factors due to “dynamo” effects, field errors and plasma shift, and possible sudden losses of configuration, the first wall was designed to be able to withstand high thermal loads. Also the need to use a low Z plasma-facing material led to the choice of pure, microcrystalline graphite. The wall is subdivided into 2016 tiles, individually clamped to the vacuum vessel by means of a special bayonet system suitable for remote handling. The remote handling system, which was designed to facilitate the replacement and inspection of all 2016 graphite tiles, comprises an arm with five degrees of freedom, two grippers for different tasks, two television cameras, and a control system for automatic or teleoperated motions. The vacuum vessel is a stiff sandwich structure with 72 solid poloidal rings bearing the first wall tile supports. It is cooled by CO 2 flowing through the interspace; baking up to 350°C is also provided by means of heating cables attached to the outer surface. The vessel is closely surrounded by a shell, giving passive stabilization against magnetohydrodynamic modes; the shell is made of aluminium alloy, 65 mm thick, and, to allow fast field penetration, it is electrically subdivided into four sections, by means of epoxy-glass layers. The RFX vacuum system is a combination of 12 cryopumps and 12 turbomolecular pumps, giving a total pumping speed at the vessel of 4500 1 s −1 for nitrogen. Because of the short plasma pulse duration, the gas injection system must produce high gas throughputs (up to 700 Pa m 3 s −1 ) with very short response times (some milliseconds). The glow discharge cleaning system utilizes four anodes toroidally equispaced and located at the centre of the vessel minor cross-section, is r.f. assisted and allows wall conditioning in hydrogen, helium or methane. In this paper we present the basic concepts underlying the main design choices and then, for the main components and plants, we describe the significant design features, as well as the methods and results of the structural, thermal and electromagnetic calculations. We also deal with preliminary tests on prototypes and with the most important manufacturing technologies, production cycles, and assembly procedures. Finally, the preliminary design of the control system for the plasma main electromagnetic parameters is presented.

Characterization and calibration of a CCD detector for light engineering

This paper describes the methodology developed for characterizing a commercial charge-coupled device (CCD) camera as a luminance meter for analyzing lighting systems and especially for measurements in road light plants. Today, several luminance meters based on commercial CCD cameras are on the market. They are very attractive for the lighting engineer: The availability of a high number of closely spaced small detectors (pixels) on a single chip permits analyses almost impossible with a traditional luminance meter. These commercial-industrial CCD cameras are sold at prices lower than scientific grade ones. They are factory equipped with a dedicated filter to reach the correct photopic sensitivity V(/spl lambda/), and they are factory calibrated in luminance SI units. The main counterparts in using these cameras are in the difficulties to define the measurement accuracy and the influence of the environment luminance on the measured values of the framed scene, in the low resolution of their A/D converter (usually 8 or 12 bit), and the higher noise level (usually the CCD chip is not cooled). To reach the measurement accuracy required by lighting norms, it is necessary to characterize metrologically a camera and quantify all the possible external influences which could degrade its performances, in real measurement situations, and which could affect the measurement results. A carefully controlled measurement set up and operating procedure could limit the causes of errors and improve the accuracy of measurements obtained in operating conditions. In this way, the measurement uncertainties might be evaluated completely, and considerations on the results could suggest particular operating practices to limit the causes of error due to measurement setup and environmental conditions.

RFX machine and power supply improvements for RFP advanced studies

Experimental results and theoretical studies call for Reversed Field Experiment (RFX) machine and power supply improvements to allow studies that go beyond those of a conventional Reversed Field Pinch (RFP) with passively stabilized turbulent MHD dynamo. The new paths opened by recent results in RFX and other RFP machines are introduced; then the goals and the design lines of the technical modifications of RFX, mainly addressed to improve the first wall, the plasma magnetic boundaries and to increase the operational flexibility of the toroidal field circuit power supply, are reported.

Analogue feedback control system of RFX

Abstract The paper deals with the design and the experimental results of the first feedback control system of the main plasma quantitites for a reversed field pinch (RFP) machine. It was designed for RFX (Reverse Field eXperiment), the largest presently operating RFP device. The peculiar aspects of RFP with respect to Tokamaks are highlighted by considering their influence on the specifications of the control system design and the choice of the controlled quantities. Three feedback loops were implemented: the control of the plasma current, the toroidal magnetic field at the wall and the poloidal magnetic field configuration. The main purpose of the last loop is to minimize the field errors at the poloidal cuts of the conducting shell which surrounds the RFX vacuum vessel. The evaluation of the transfer functions of the systems and the regulator design are presented together with the experimental results obtained till now. The validation of the models used for the regulator synthesis was also performed and its results are reported.

Detector-Based Calibration for Illuminance and Luminance Meters—Experimental Results

The connection between illuminance and luminance measurements is considered to create a traceability link among a highly accurate illuminance meter, which is considered as an internal standard of the Photometric Laboratory, University of Padova, Padova, Italy, and other illuminance meters or photometric heads and luminance meters. After an introduction on the main involved definitions and equations linking the illuminance and the luminance measurements, this paper presents the instruments to which the procedures are applied and, at the end, results regarding the absolute calibration of the instruments under test, the check of their linearity, and the analysis of their spectral responsivity within the visible range. Following this way, this paper shows the main results in the connection among instruments devoted specifically to illuminance and luminance measurements. Then, it also introduces a comparison to other photometric heads, e.g., sensors that are part of more complex systems, like an integrating sphere for luminous flux measurement. In this last case, it has no significance to evaluate the difference between the absolute outputs, but it is interesting to compare the response of the two devices considering their linearity and their relative spectral response. A special section devoted to the evaluation of the spectral response presents a new simple but effective method. The application of a detector-based method allows a significant reduction of the uncertainty in the internal calibration of the photometric instruments, granting the continuity of the Photometric Laboratory operation contemporarily.

Design of a new electromagnetic diagnostic for RFX

A new set of electromagnetic probes has been designed for the RFX modified toroidal assembly. Typology and layout of the probes have been revised based on the analysis carried out in previous experiments, and taking into account the foreseen scenarios of operation. A large number of probes are required to correctly identify the complex spatial structure of the plasma column. An accurate calibration and a careful probe mounting is required. Probes will be installed inside the stabilising shell, which will be placed very close to the vessel. Very little space is available. A two axes pick-up probe has been developed: probe materials and construction techniques have been optimised in order to minimise coupling between the two coils and the dependence of the measurement on the temperature. Extensive tests have been carried out on probe prototypes to characterise their electrical and thermal behaviour. In the paper, the design of the new set of probes is presented, and the results of the tests performed on prototypes are reported.

A system for in situ measurements of road reflection properties

The characterization of the photometric properties of a road surface is of prime importance in the design of lighting plant and when the real vision condition should be determined by computer simulation. The measurement could be done in laboratory but the in situ measurement are very interested because it permit to test several zone on the road and there is no mechanical starch on the surface of the sample. This work describes an innovative portable system based on a CCD luminance meter able to obtain uncertainty comparable in traditional laboratory systems.

Comparison of luminance measurement based on illuminance and luminance detectors

The connection between illuminance and luminance measurements is considered to create a traceability link among an highly accurate illuminance meter, considered as an internal standard of the Photometric Laboratory of the University of Padova, and other illuminance meters or photometric heads and luminance meters. Examples are presented in the paper. The application of a detector-based method allows a significant reduction of the uncertainty in the internal calibration of the photometric instruments, granting the continuity of the Laboratory operation, contemporarily.

Internal Traceability between Luminous Flux and Illuminance Standards

Two methods for the measurement of the luminous flux are presented: the first is based on an integrating sphere and a luminous flux standard lamp, the second utilizes an accurate goniometer and an accurate illuminance meter. A comparison between the two methods is carried on for two different light sources: an LED and a halogen source, presenting different luminous fluxes. The measuring procedure and an evaluation of the measurement uncertainties are described for both the methods. The comparison shows a good agreement between the two measuring processes: both of them allow the identification of the luminous fluxes with relative uncertainties of about 1.5%, covering the differences between the luminous flux values. The comparison is considered a valid possibility of verifying the calibration constancy of the laboratory luminous flux and illuminance standards.

On the use of broadband ultraviolet detector calibration

Measurement of ultraviolet radiation is a common issue in many fields of applications. The easy way to approach to ultraviolet measurement is by using broad band probes. Their use should be straightforward, but their calibration is not always simple to be applied in small industrial, test and research laboratories. The paper presents the two main approaches to the calibration of ultraviolet probes, with their related limitation on their use and difficulties in the extension of the calibration to the cases considering radiating source different from the one used in the calibration. Starting from the approach applied at metrological laboratories, an attempt to overcome this problem is presented which takes advantage of the information commonly provided by manufacturer on data sheets of probes and sources, i.e. data easily achievable by all laboratories.