P. Bettini

Modeling of epoxy resin spacers for the 1 MV DC gas insulated line of ITER neutral beam injector system

The spacers for the gas insulated transmission line for the ITER neutral beam injector will be designed to withstand the operating voltage of 1 MV dc. Electric charging processes of the spacer surface are then expected to play an important role in the final electric field distribution. Aim of the paper is to investigate the effects of the different conductivity properties of the SF6-Spacer insulating structures on the surface charge set-up, and to identify a procedure to minimize this charge. Based on the most updated literature, the paper presents and discusses the results of two newly developed simulation tools: the first consisting of a quasi-static non linear model for epoxy spacer based on finite element method code ANSYStrade, the other consisting of a spacer profile optimization package, whose kernel is based on a genetic algorithm. The numerical tools have been applied to post and disk spacer models of the gas insulated line; in particular, the optimization package has been used on the disk spacer, and the comparison between un-optimized and optimized spacer in terms of electric charge accumulated are presented and discussed.

Electromagnetic analysis of superconducting cables and joints in transient regime

The paper deals with the electromagnetic analysis of the superconducting multistrand cables used to manufacture magnets for fusion research. An analysis method is described, in which the cable is represented as an equivalent lumped network, whose parameters are automatically computed starting from the cable geometrical data. The network is then solved in transient regime.

Analysis of the optimal location of magnetic field probes for the determination of the current distribution inside S/C cables

During the tests on superconducting cables, the magnetic field is often measured by means of pick-up or Hall probes and these data are used to reconstruct the current distribution inside the cable. The efficiency and the accuracy of the reconstruction procedure depend on the algorithm adopted and on the location of the field probes. For a given geometry of the lines of current, the determination of the current distribution can be mathematically formulated as a linear inverse problem, in which the residual between the measured and the computed field values at the probe locations is minimised. A linear least-squares problem results, which is solved by means of the truncated singular value decomposition technique, applied to the matrix G which relates the unknown currents to the computed magnetic field values. The condition number of G is proposed here as an index of the quality of the location of the probes: a good location corresponds to a well conditioned G, while a bad location gives a worse conditioned matrix. In this paper a reconstruction procedure is presented and a criterion for the optimal probe positioning is shown.

Two-dimensional non-linear model of reversed field pinch plasma evolution

A two-dimensional non-linear equilibrium code, able to simulate reversed field pinch (RFP) discharges, has been developed. It is based on the two-dimensional MHD equilibrium code MAXFEA, which has been properly modified by introducing RFP profiles of the poloidal current density flux function in accordance with the μ&p model. In particular, equation parameters consistent with available data from the RFX machine have been used. A complete mesh of the RFX poloidal field system has been prepared and the whole poloidal electrical circuit has been implemented into the code. The RFP equilibrium code has been successfully validated against some RFX experiments, with and without plasma.

Dynamic identification of plasma magnetic contour in fusion machines

The paper presents a method to identify the plasma magnetic contour in fusion machines, when eddy currents are present in the conducting structures surrounding the plasma. The approach presented is based on the integration of an electromagnetic model of the plasma with a lumped parameters model of the conducting structures around the plasma. This approach has been validated against experimental data from RFX, a reversed field pinch machine.

Optimization analyses of the magnetic measurements on multistrand SC cables

The paper presents a theoretical and numerical analysis of the performance of a magnetic field (MF) probe system for the tests on ITER-type cables. The analysis considers the effects of a set of typical cable geometrical parameters, like the cable twisting pitch, the length and the number of its bundles. For each given cable geometry, the analysis evaluates the effects of the position and accuracy of the MF probes in terms of error between the imposed and the identified bundle currents. An optimization technique for the optimal probe positioning is also presented, and its results are discussed.

Harmonic impedance measurements and calculations in the EHV transmission network

The results of a measurement campaign regarding the driving point harmonic impedance of one node of the 400 kV Italian transmission network are presented and discussed, using a method suitable for high order harmonics (up to 50/sup th/). A model of the network has been then set up and the relevant results compared with the measurement, showing the crucial effect of the equivalent impedance simulating the external network.

Parametric analysis of current extinction in counterpulse systems

Abstract The results of the parametric analysis of current extinction in counterpulse systems without saturable reactor are presented in this paper. The current dcrivative di/dt has been assumed as a free parameter to iiwestigate ils effect on the design of the counterpulse capacitor bank and tuning induetor. General design criteria of the counterpulse systems are presented and compared with the solutions employing saturable reactors.

Misfiring protection and monitoring of the RFX toroidal circuit ignitron crowbar

The paper summarizes the actions taken to make safer the operation of the ignitron main crowbar system used to protect the winding and the power supplies forming the circuit which produces the toroidal magnetic field for plasma confinement in the RFX machine. Such actions, which comprise a self-triggering system, a monitoring system and a discharge resistors system, are intended to minimise the probability of explosion of the crowbar ignitrons in case of misfiring and to maintain a high level of reliability for the crowbar system operation.

Thyristor making switch system for plasma startup control in RFX

The necessity of better plasma current control during the startup phase asks for a toroidal loop voltage modulation obtained with the multi step reduction of the transfer energy resistor in the OH circuit. The paper begins by discussing the characteristics of such a modulation and then describes the modifications realized and tested on the power supply and magnet systems to prove the feasibility of such an operation; finally, on the basis of the results obtained, we describe the design of the making switch system, based on a 100 mm thyristor valve.