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MITICA (Megavolt ITER Injector & Concept Advancement), the full scale prototype of ITER Heating Neutral Beam, is under realization at the Neutral Beam Test Facility (Padova, Italy). It is designed to deliver 16.5 MW to ITER plasma, obtained by accelerating negative Deuterium ions up to 1 MeV for a total ion current of 40 A and then neutralized. MITICA Acceleration Power Supply is composed of several non-standard equipment, beyond industrial standard for insulation voltage level (-1 MVdc) and dimensions. Voltage withstand tests (up to 1.265 MVdc) have been performed in five subsequent steps (from 2018 to 2019), according to the installation progress, after connecting equipment belonging to different procurements. During integrated commissioning, started in 2021, two breakdowns occurred in a position of the HV plant not still identified, so they could be occurred either in air or in SF6. To identify the locations of possible weak insulation points, the existing diagnostics for partial discharge detection (the precursor of breakdowns) as a first step have been improved on air-insulated parts by consisting in a set of instrumentation, like capacitive probes and off-the-shelf instruments for AC application (acoustic and electromagnetic sensors). The paper deals with the instruments qualification to assess their suitability for DC usage and then with the investigation performed in MITICA, in particular: 1) sensitivity assessment campaign, with artificially produced corona effect to identify the minimum threshold of each diagnostics 2) voltage application to MITICA plant, moving the instrumentation around equipment and increasing progressively the voltage looking for corona phenomena to identify possible weak insulation points., Nine pages, twelve figures, accepted manuscript of a paper published in Fusion Engineering and Design

SPIDER is the full-scale prototype of the ion source of the ITER Heating Neutral Beam Injector, where negative ions of Hydrogen or Deuterium are produced by a RF generated plasma and accelerated with a set of grids up to ~100 keV. The Power Supply System is composed of high voltage dc power supplies capable of handling frequent grid breakdowns, high current dc generators for the magnetic filter field and RF generators for the plasma generation. During the first 3 years of SPIDER operation different electrical issues were discovered, understood and addressed thanks to deep analyses of the experimental results supported by modelling activities. The paper gives an overview on the observed phenomena and relevant analyses to understand them, on the effectiveness of the short-term modifications provided to SPIDER to face the encountered issues and on the design principle of long-term solutions to be introduced during the currently ongoing long shutdown., 8 pages, 12 figures. Presented at SOFT 2022

At the International Thermonuclear Experimental Reactor (ITER) Neutral Beam Test Facility in Padua (Italy), the ion source full-scale prototype--SPIDER--has been in operation since June 2018 and, as of March 2021, has begun to explore the range of beam energies exceeding 30 keV. This has revealed a number of novel issues with the power supplies of ion source and extractor, mainly associated with grid breakdowns. Those power supplies include a set of high current single quadrant resonant converters, known as ion source support power supplies (ISSSs) and a high voltage pulse step modulator feeding the extraction gap. Grid breakdowns at beam energies exceeding 30 keV have shown the capability to upset the output measurements of the ISSSs, limiting operation and reducing reliability of the whole system. This article describes work undertaken first to identify the issues, found to belong to two distinct categories: general failures of all ISSSs measurements and trips from the output voltage measurements of the extraction gap power supply. Further investigation has associated the failures of the ISSS measurements to transient voltage affecting the electronics during grid breakdowns. A number of modifications to both power circuit and electronics have been undertaken and successfully tested, achieving immunity down to the full SPIDER operating voltage (-96 kV). The extraction gap power supply overvoltage has been quantified and a temporary modification deployed, able to mitigate it significantly. A deeper redesign will be required in this case and is planned during the 2022 shutdown of the experiment.

In SPIDER, the full-scale prototype of the negative ion source of the ITER Heating Neutral Beam Injectors, negative ions of hydrogen or deuterium are extracted and accelerated by a set of grids. This article describes the process and the technical solutions that allowed achieving the asseveration to operate the Acceleration Grid Power Supply of SPIDER at full voltage (-96 kV), reports the first experimental results, and discusses some possible enhancements that have been identified.

The Acceleration Grid Power Supply of the MITICA test facility in Padova (Italy) is currently under commissioning. The power conversion system, the DC generator, and the High Voltage equipment have been individually commissioned, whereas the integration tests are ongoing. It is a challenging process due to the unconventional application, to the variety of different electrical technologies involved and to the complexity of the interfaces. During the integrated tests of the power supplies the achievement of 700kV stable operation has been demonstrated for the first time in a Neutral Beam Injector, but an unexpected event occurred, most likely a breakdown in the HV part, which resulted in a fault of the DC generator. A subsequent test using an auxiliary power supply was performed to check the voltage withstanding capability of the HV plant, but another breakdown occurred at around 1MV. This paper describes the activity performed to identify the location of the breakdowns affecting the integrated tests. A test campaign has been devised with increased diagnostic capabilities and specific strategy conceived to trigger intentional breakdowns in specific locations and collect measurement patterns for different cases. The results of the campaign will be presented and the current understanding of the issue will be described, with a view on future tests and further improvements of diagnostics.

This paper describes the development and validation of an innovative protection system based on medium-voltage fuses for a high-power switching conversion system. This special conversion system, rated to deliver about 56 MW to the load, is based on neutral-point clamped IGCT inverters, connected to the same dc link through a set of distributed busbars, with a dc-link voltage of 6.5 kV and a capacitive stored energy up to 837 kJ. The sudden release of this energy in case of a switch failure in one inverter and the subsequent short circuit of one leg can lead to destructive consequences. From the analysis of different protection strategies, performed by numerical simulations of the fault evolutions, the developed solution based on medium-voltage fuses was found the only provision able to cope with such high stored energy and uncommon circuit topology. Custom fuses were developed for this application, and a specially tailored test was designed for validating the fuse selection. The paper, after summarizing the work carried out to simulate the fault evolution and select the protection, presents the analyses carried out to set up the validation test, and describes and discusses the results of the test and the complementing numerical simulations, which demonstrated the effectiveness of the protection system.

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 Residual Ion Dump Power Supply (RIDPS) is devoted to feed the Electrostatic Residual Ion Dump, a device which shall collect the residual ions at the output of the gas Neutralizer in the ITER Heating Neutral Beam Injectors (HNBI) and their full-scale prototype (MITICA), presently under construction in the Neutral Beam Test Facility (NBTF) in Padua, Italy. The procurement of the three RIDPS, part of the Procurement Arrangement signed between Fusion for Energy (F4E) and ITER Organization (IO) in 2009, has been committed by F4E to OCEM Energy Technology s.r.l. (OCEM). This power supply has to provide an average voltage of up to 25 kV, plus an ac low-frequency voltage component with maximum amplitude of 5 kV. The nominal output current is 60 A, and the maximum pulse duration is 1 hr. The selected topology is the Pulse-Step-Modulator (PSM), including a multi-winding dry type transformer and 42 water-cooled switching power modules, connected in series at the output. Among the various advantages of this technology, the most important for this application are the modularity, allowing redundancy at module level, and the light output filter and short switch-off time in case of fault, both limiting the energy delivered to possible arches among RID plates. The manufacturing of the RIDPS for MITICA has been concluded in summer 2018. The Factory Tests on the single components and on the full system were finished in October 2018 and in the same month started the Site Installation activities at NBTF. At the end of November the installations were completed and the Site Tests started, planned to be completed by beginning of 2019. A close follow-up of the whole procurement process has been carried out by both F4E and Consorzio RFX. Particular effort has been devoted to the verification of the design, the definition of the test procedures and the integration of the system with the existing installations, including the management of layout interferences and the detailed scheduling of site works. In particular, during the factory tests, some problems appeared, requiring the definition of proper solutions. For example, in the original specification the frequency of the ac component of the output voltage was set to 50 Hz, while it has been found that this caused unwanted input current peaks due to synchronization with the grid voltage, with possible saturation of the transformer. Electromagnetic immunity issues have output filter caused a voltage ripple slightly exceeding the limit with reversed polarity, solved with the improvement of the filtering action. Finally, a suitable test procedure has been devised for the Electromagnetic Emission Tests at Site. In this paper, the most significant issues faced during the design, integration and tests and the solutions identified will be described. The layout adopted in the final installation will be commented and the most interesting results of the Factory and Site Tests will be presented, with particular emphasis to those proving the compliance of the RIDPS with the most critical requirements.

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.

The Reverse Field eXperiment (RFX) explores the physics of Reverse Field Pinch (RFP) plasmas as an alternative confinement option with respect to Tokamak and Stellarator concepts. The toroidal field power supply of RFX, modified and upgraded during a long shut-down in 1999, is based on IGCTs (Integrated Gate Commutated Thyristors) and combines an extremely complex circuit topology and a fast, reliable and flexible control system. It includes, among other devices, 12 dc/ac converters rated for 3kV, 6kA, pulse duty operation (0.5s/10′) and 14 static breakers rated for 4kV, 16kA, 128 MA2s. This paper will describe in detail the system, from the power to the control and protection sections. Experimental waveforms will also be reported and interesting indication about the system reliability and availability will be given, derived from more than ten years of operation.

The paper describes a proposal for the new power supply of the RFX toroidal field system based on IGCT semiconductors used in d.c./d.c. converters and in d.c. current interrupters. The analyses carried out asseses its capability to validate the performance of the previous scheme and to satisfy the new requirements. The design criteria of the main devices will be also reported.

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