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To reach fusion conditions and control the plasma configuration in ITER, the next step in tokamak fusion research, two neutral beam injectors (NBIs) will supply 16.5 MW each, by neutralizing accelerated negative hydrogen or deuterium ions. The requirements of ITER NBIs (40A/1 MeV D- ions for ≤1 h, 46A/870 keV H- ions for ≤1000 s) have never been simultaneously attained. So in the Neutral Beam Test Facility (NBTF, Consorzio RFX, Italy) the operation of the full-scale ITER NBI prototype (MITICA) will be tested and optimised up to full performances, focussing on accelerator (including voltage holding), beam optics, neutralisation, residual ion removal. The NBTF includes also the full-scale prototype of the ITER NBI source with 100 keV particle energy (SPIDER), for early investigation of: negative ion production and extraction, source uniformity, negative ion current density and beam optics. This paper will describe the main results of the first two years of SPIDER operation, devoted to characterizing plasma and beam parameters, including investigation of RF-plasma coupling efficiency and magnetic filter field effectiveness in reducing co-extracted electrons. SPIDER is progressing towards the first caesium injection, which aims at increasing the negative ion density. A major shutdown, planned for 2021, to solve the issues identified during the operation and to carry out programmed modifications, will be outlined. The installation of each MITICA power supply and auxiliary system is completed; in-vessel mechanical components are under procurement by Fusion for Energy (F4E). Integration, commissioning and test of the power supplies, procured by F4E and QST, as the Japanese Domestic Agency (JADA), will be presented. In particular, 1.0MV insulating tests were carried out step-by-step and successfully completed. In 2020 integrated tests of the power supplies on the accelerator dummy load started, including the assessment of their resilience to accelerator grid breakdowns using a short-circuit device located in vacuum. The aggressive programme, to validate the NBI design at NBTF and to meet ITER schedule (requiring NBIs in operation in 2032), will be outlined. Unfortunately, in 2020 the coronavirus disease infection affected the NBTF activities. A solution to proceed with integrated power tests despite the coronavirus is presented.

This paper presents the design criteria and the preliminary characteristics of the power supply and electrical systems of the Divertor Tokamak Test (DTT) facility. The power supply system has to feed: 6 superconducting modules of the central solenoid, 6 poloidal field superconducting coils, 18 toroidal field superconducting coils designed for a current up to 50 kA, some coils for plasma fast control and vertical stabilization, the electron (ECRH) and ion (ICRH) cyclotron additional heating systems designed to deliver about 25 MW to the plasma, further 20 MW to the plasma generated by a neutral beam injector (NBI) and all the auxiliary systems and services. The analysis was carried out on a reference scenario with a plasma current of 6 MA, mainly to estimate the electrical power needed to operate the facility, but also to identify some design choices and component ratings.

The Acceleration Grid Power Supply Conversion System (AGPS-CS) of the MITICA Neutral Beam Injector has been installed between late 2017 - early 2018 and commissioned in the first half of 2018. This paper is focused on the installation and commissioning activities, with emphasis on the integration issues. The results of the site and acceptance tests are presented, proving the capability of the system to deliver the specified performance.

SPIDER is the full scale prototype of the ITER Heating Neutral Beam Injector ion source and the most powerful RF driven negative ion source currently in operation. In SPIDER the negative ions are extracted from a plasma generated by a 4x2 array of drivers, relatively small highly engineered antennas, each designed to be supplied with up to 100 kW at the frequency of 1 MHz. The drivers are fed in pairs by 4 independent RF oscillators in the push-pull configuration through 4 RF rigid lines and 4 "normal" L-type matching networks on board of the ion source. The basic concepts, previously developed and tested on smaller scale ion sources, were further improved and combined in a single facility considering all the constraint and restrictions relevant for ITER. The RF generators interact through their loads and the effects observed during the experimentation are frequency and output power modulations, which affect the plasma parameters and the ion beam performances. Furthermore, during the experimental campaign a common mode stray RF current was observed in the power supplies and in the ion source diagnostics system, causing faults and making the measurements unreliable. These issues have different root causes located both on the power supplies side and on the ion source side: the RF stray currents are mainly due to the transducers for the oscillators output voltage measurement, which provides low impedance path for them; the interactions among the oscillators are influenced by the layout of the RF circuit on board on the ion source that introduces large loops, thus magnetic coupling. The paper describes the original RF circuit and system, presents the understanding of the issues identified during the experimental campaigns and discusses the effectiveness of the provisions introduced to improve the performance.

This paper compares different provisions to protect a special high power switching power supply from internal short-faults, in particular internal short-circuits. The power supply is based on a five three-phase NPC inverters, with common dc-link, rated for a total power of about 60MW and for an output voltage of 6.5kV, using IGCTs as switching components. A large, distributed, capacitor bank is connected at the dc-link side of the inverters, with huge amount of stored energy, as high as 760kJ during nominal operation. In case of internal short-circuit of one inverter leg, due for example to a failure of one IGCT, this energy is discharged into the faulty leg, potentially leading to explosion and fault propagation. Simulations have been performed in order to analyze, select and design the most suitable protection to avoid this risk.

The objectives of the nuclear fusion power plant DEMO, to be built after the ITER experimental reactor, are usually understood to lie somewhere between those of ITER and a 'first of a kind' commercial plant. Hence, in DEMO the issues related to efficiency and RAMI (reliability, availability, maintainability and inspectability) are among the most important drivers for the design, as the cost of the electricity produced by this power plant will strongly depend on these aspects. In the framework of the EUROfusion Work Package Heating and Current Drive within the Power Plant Physics and Development activities, a conceptual design of the neutral beam injector (NBI) for the DEMO fusion reactor has been developed by Consorzio RFX in collaboration with other European research institutes. In order to improve efficiency and RAMI aspects, several innovative solutions have been introduced in comparison to the ITER NBI, mainly regarding the beam source, neutralizer and vacuum pumping systems.

The Acceleration Grid Power Supply (AGPS) is a system devoted to supply the acceleration grids of the MITICA experiment, the full scale prototype of the ITER Neutral Beam Injector (NBI), being built in Padova (Italy) to test the NBI in advance of the operation in ITER. The procurement of the AGPS is split in two parts: the low voltage conversion system, namely the AGPS-Conversion System (AGPS-CS,) being procured by the European domestic agency, and the high voltage DC Generators (AGPS-DCG), being procured by the Japanese domestic agency. After the contract award, in late 2015, the executive design phase of the AGPS-CS has been finalized. The paper will present the final design of the AGPS-CS and will discuss the main choices in particular concerning the protection from internal faults of the inverters.

After 10 years of operation since its major modification, an upgrade of the RFX-mod experiment is presently under design. The scientific objective is the improvement of 3D physics studies through a more robust transition to higher confinement regimes in both Reversed Field Pinch (RFP) and Tokamak configuration obtained thanks to an advanced system for the active control of MHD instabilities. The main design driver requirements for this machine upgrade are the removal of the present resistive vacuum vessel and the enhancement of the ‘shell-plasma proximity’, to reduce the deformation of the last close magnetic surface and to improve the self-organized helical plasma regimes. The fulfillment of these requirements implies a major change of the internal components of the machine such as the replacement of the whole first wall, the change of the support system of the stabilizing shell and the modification of the present toroidal support structure to provide the function of vacuum barrier. In combination, other components of the machine will be upgraded, such as magnets and power supply, diagnostic systems and a NBI will be integrated. The paper presents an overview of the engineering design of the new components and highlights the critical aspects of the new torus assembly.

To reach fusion conditions and control plasma configuration in ITER, a suitable combination of additional heating and current drive systems is necessary. Among them, two Neutral Beam Injectors (NBI) will provide 33 MW hydrogen/deuterium particles electrostatically accelerated to 1 MeV; efficient gas-cell neutralisation at such beam energy requires negative ions, obtained by caesium-catalysed surface conversion of atoms inside the ion source. As ITER NBI requirements have never been simultaneously attained, a Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Italy), including two experiments. MITICA is the full-scale NBI prototype with 1 MeV particle energy. SPIDER, with 100 keV particle energy, aims at testing and optimising the full-scale ion source: extracted beam uniformity, negative ion current density (for one hour) and beam optics (beam divergence