Development of HVDC Gas-Insulated Components for the Power Supply of Neutral Beam Injectors

The development of High Voltage Direct Current Gas-Insulated components that will be employed for the power supply of Neutral Beam Injectors, requires specific numerical tools to analyze the electric field distribution, with the purpose of supporting the engineering and operational phases.The tools must comply with the nonlinearity of the material properties of solid insulators and the transport mechanisms in the dielectric gas. Despite its high Global Warming Potential, SF6 will be considered as the baseline dielectric gas for the development of HVDC gas-insulated components to be used for the Acceleration Grid Power Supply of Neutral Beam Injectors. Attention especially to the mechanisms driving to an increase of leakage current flowing in the gas, thus giving rise to possible discharge phenomena is here placed, mainly focusing on radiation and injection regimes. The described methodologies are adopted for the numerical modeling of the 500 kV DC gas-insulated components that will be used for the power supply of the Neutral Beam Injector of the Divertor Tokamak Test facility. In particular, a conceptual design of the SF6 Transmission Line and the SF6-air bushing is conceived

Magnetic and thermo-structural design optimization of the Plasma Grid for the MITICA neutral beam injector

MITICA is a prototype of the heating neutral beam (HNB) Injectors for ITER, built with the purpose of validating the injector design and optimizing its operation. Its goal is to produce a focused beam of neutral particles (H or D) with energy up to 1 MeV and power of 16 MW for 1 h. MITICA includes a Radio Frequency (RF) Plasma Source for the production of negative ions, a multi-stage electrostatic accelerator (up to 1 MV and 40 A), a neutralizer, a residual ion dump and a calorimeter. A transverse magnetic field in the Ion source and accelerator, including both a long-range component and a local component is crucial for obtaining the required Ion current and accelerator efficiency. The long-range component is produced by the current flowing through the plasma grid (PG) and related bus-bars. The PG current distribution and the uniformity of the resulting magnetic field have been optimized by detailed finite element (FEM) models. Hollow volumes in the thick copper part of the PG among beamlet groups allow a more uniform PG current distribution and a consequently uniform magnetic field in front of the grid. The paper describes in detail the PG geometry optimization procedure and the related magnetic and thermo-structural FEM analyses

NBImag: A Useful Tool in the Design of Magnetic Systems for the ITER Neutral Beam Injectors

NBImag is a code suitable for the design and opti- mization of complex magnetic field configurations, such as that of a multiaperture, multistage negative ion source and accelerator. The NBImag code has been developed for the design of the ITER neutral beam injector, whose full-size prototype, MITICA, is presently under construction in Padua, Italy. The ITER injector shall produce a focused beam of neutral particles (H or D) having an energy of about 1 MeV and a total power of 16.5 MW for 3600-s continuous operation. The NBImag code is based on an integral formulation and allows an efficient calculation of any static magnetic field configuration on the basis of the geometry of the magnetic sources, with linear material and permanent magnets. NBImag also includes magnetic force and inductance calculations, based on the same formulation. Thanks to the capability of efficiently describing a large number of permanent magnets with limited computational effort, NBImag has also been integrated with different automatic optimization procedures for the solution of inverse magnetic problems. This paper describes the formulation of the code and optimization algorithms, the vali- dation against analytical models and experimental measurements, and the application to the design of MITICA

Realization and Tests of Prototype Fluxgate Magnetic Sensors for the ITER Neutral Beam Injectors

In the ITER neutral beam injectors (NBI), the presence of an external variable magnetic field generated by the ITER tokamak itself, could deflect the ion beam during acceleration and cause a loss of beam focusing. For this reason, the ion source, the accelerator and the neutralizer will be shielded from external magnetic field by means of a passive magnetic shield and a system of active correction and compensation coils (ACCC). The ACCC will operate in a feedback control loop and thus require the measurement of magnetic field inside the NBI vessel. Magnetic sensors for this application must be capable of measuring DC and slow variable magnetic fields, and be vacuum-compatible, radiation-hard and robust, since they will be subjected to neutron flux produced by fusion reactions in the tokamak and inaccessible for maintenance. This paper describes the realization and tests of fluxgate magnetic sensors prototypes specifically designed for this purpose before the installation in MITICA and ITER

Construction and testing of grid prototypes for the ITER neutral beam injectors

A comprehensive set of R&D activities has been carried out at Consorzio RFX regarding the construction of the grids for the ITER Neutral Beam Injectors, in order to validate the proposed manufacturing methodologies, to further develop the details of the engineering design, and to adapt existing production techniques to the specific case. In the framework of this R&D program, two Multi Channel Prototypes (MCPs) have been designed and manufactured. These prototypes feature all the possible manufacturing issues of the SPIDER grids. In fact, they reproduce the geometry of the Extraction Grid (EG) for SPIDER by having the same thickness, same cooling channels, same distributors, same aperture shape and same slots for the magnets of an EG segment. The differences from an actual EG segment regard only the dimensions, i.e. reduced width (one fourth) and height (one half). The construction and testing of these prototypes is here described in detail

Feasibility study of a flux-gate magnetic field sensor suitable for ITER neutral beam injectors

ITER Neutral Beam Injectors (NBIs) need to be shielded from the relatively strong stray magnetic field generated by the Poloidal Field Coils of the Tokamak. For this reason both the Heating Neutral Beams (HNB) and the Diagnostic Neutral Beam (DNB) will be provided with a Passive Magnetic Shield and with a system of Active Correction and Compensation Coils (ACCC). The ACCC will operate in feedback control and thus require the measurement of magnetic field inside the NBI vessel, i.e. in an environment subjected to the neutron flux coming from the Tokamak. To this purpose, magnetic sensors which are robust, radiation hard, drift-immune and remote-handling compatible are required. Flux-gate magnetic sensors are a good candidate for this task, but commercial sensors of this kind have typically a limited measured range (below 0.1 mT). The feasibility of a flux-gate sensor for the ITER NBI has been studied by developing a numerical model which includes magnetic core hysteresis, and which demonstrated that, by suitable choice of the core magnetic properties and geometry, it is possible to increase the measurement range by at least 2 orders of magnitude. On this basis, a flux-gate sensor prototype has been realized at Consorzio RFX. Experimental tests carried out so far have demonstrated that the measurement range can be increased to 3c10 mT with acceptable accuracy and frequency bandwidth

Improvements of the magnetic field design for SPIDER and MITICA negative ion beam sources

The design of the magnetic field configuration in the SPIDER and MITICA negative ion beam sources has evolved considerably during the past four years. This evolution was driven by three factors: 1) the experimental results of the large RF-driven ion sources at IPP, which have provided valuable indications on the optimal magnetic configurations for reliable RF plasma source operation and for large negative ion current extraction, 2) the comprehensive beam optics and heat load simulations, which showed that the magnetic field configuration in the accelerator is crucial for keeping the heat load due to electrons on the accelerator grids within tolerable limits, without compromising the optics of the negative ion beam in the foreseen operating scenarios, 3) the progress of the detailed mechanical design of the accelerator, which stimulated the evaluation of different solutions for the correction of beamlet deflections of various origin and for beamlet aiming. On this basis, new requirements and solution concepts for the magnetic field configuration in the SPIDER and MITICA beam sources have been progressively introduced and updated until the design converged. The paper presents how these concepts have been integrated into a final design solution based on a horizontal "long- range" field (few mT) in combination with a "local" vertical field of some tens of mT on the acceleration grids

Cancellation of the ion deflection due to electron-suppression magnetic field in a negative-ion accelerator

A new magnetic configuration is proposed for the suppression of co-extracted electrons in a negative- ion accelerator. This configuration is produced by an arrangement of permanent magnets embedded in one accelerator grid and creates an asymmetric local magnetic field on the upstream and downstream sides of this grid. Thanks to the \u201cconcentration\u201d of the magnetic field on the upstream side of the grid, the resulting deflection of the ions due to magnetic field can be \u201cintrinsically\u201d cancelled by calibrating the configuration of permanent magnets. At the same time, the suppression of co-extracted electrons can be improved