The magnetic energy storage and transfer (MEST) system is a technology conceived to address the issues of high active power transients and huge reactive power demand of conventional coil power supplies (CPSs), thus providing an effective solution to be employed in future large fusion reactors. The MEST system exploits an additional superconducting coil sink coil (KC) as an energy storage to sustain the operation of the load coil (LC). Through a switching section and a capacitor bank, it is able to transfer energy bidirectionally between the two superconducting coils, to provide the required current to the LC. Thanks to this storage, the external AC grid must supply only the average power through a power supply (PS) that charges the KC. A small-scale prototype of the MEST system is nearing completion, serving as a proof of concept and enabling the investigation of scaling the design to higher power ratings. The prototype operates with a resistive sink and LCs and follows a modular approach for the switching section (combined with the capacitor bank), with two modules in total. The modules are designed for operation at 2 kV and 2 kA and can function independently or be connected to achieve higher ratings. This prototype will permit the evaluation of the advantages and limitations of the specific topology, examine various configurations, test different control strategies, and understand any unexpected experimental findings. The first tests focus on verifying the prototype’s functionality, particularly the integration of the components, and assessing the system’s ability to effectively control the currents in both coils. This work presents the results of the initial tests, the first characterization of the circuit, and the validation of the developed numerical models with experimental data.

Abstract This paper reports the main design keys and the challenging issues of the Heating and Current Drive System (HCD) of the Divert Tokamak Test (DTT) facility that is under construction at ENEA site in Frascati with the aim to perform studies on the power exhaust in a flexible and easily modifiable environment. The selected HCD systems for DTT are those with the most consolidated technology and expected to be relevant for the future reactor. The status of each system is reported, both in terms of design and procurement, which are well advanced for the system required in the first phase of the DTT exploitation: electron and ion cyclotron resonant heating. The third system is neutral beam injector, based on negative ion acceleration, which will be installed in DTT in a second phase, after the first five years of operation. The full heating power will be reached with the doubling of the radiofrequency power to reach the 45 MW at plasma needed to test the divertor with a power density at reactor level.

The EU-DEMO (EUropean DEMOnstration Power Plant) fusion reactor is one of the most ambitious energy projects aiming at demonstrating the feasibility of nuclear fusion to be a clean, safe, and sustainable source of energy. The distribution grid of the DEMO facility plays a crucial role in delivering the net generated power to the European transmission grid, so it is imperative for its operation to be reliable. The proper design of the electrical subsystems within the fusion power plant is a fundamental factor that must be considered to guarantee reliable and secure operations. This paper presents the models and the results of a sensitivity analysis on the design and sizing of the components of DEMO electrical power system, under different operating conditions, exploiting a Monte Carlo-based Probabilistic Power Flow methodology. This approach was selected to assess the impact of uncertainties on DEMO steady-state power distribution grid and to identify the critical components, which need special requests in their design. The results of the sensitivity analysis can provide valuable insights for the design and sizing of the distribution system of the facility.

Supercapacitors (SCs) are efficient energy storage devices ideal for handling abrupt power variations. However, challenges like high costs, low voltage ratings, and limited frequency response have currently restricted their widespread adoption. Some high-energy applications are starting to integrate SCs into designs, and thousands of SC modules consequently have to be directly connected or integrated in a power converter, such as a Modular Multilevel Converter (MMC), to meet energy and voltage demands. A critical issue arises from the degradation of SCs over time, as variations in capacitance and internal resistance can occur due to aging or production mismatches. Current studies emphasize redundancy and maintenance strategies to extend the lifespan of SC-based systems, but they lack insights into the coexistence of old and new SC modules. This paper addresses this gap by analyzing the impact on electrical quantities of unevenly degraded SCs integrated into an MMC-like converter and proposing an innovative control strategy to improve the system performance.

The Plant Electrical System (PES) of the European DEMOnstration fusion power plant (DEMO), presently under conceptual design, shall supply power to the loads and deliver net power to the Power Transmission Grid (PTG). Starting from the available requirements, the applicability to DEMO PES of the technologies and design approaches adopted in ITER and nuclear power plants (NPP) has been evaluated. This article presents the results of the survey and proposes alternative solutions to deal with the identified criticalities.

The Plant Electrical System (PES) of the European DEMOnstration power plant (DEMO) is under conceptual design. The fast discharge units (FDUs) are vital components within the coil power supply (CPS) system. This paper proposes a tentative backup protection scheme for FDUs in DEMO. The proposed FDU consists of a main circuit breaker (CB) and a backup CB, which can rapidly discharge the coil energy in case of a quench into discharge resistors. The design of the backup CB is determined by the rating of superconducting (SC) magnets, the configuration of main CBs, and the ambient conditions of FDUs. Different DC interruption technologies in fusion devices are evaluated. The design criteria for the backup CB are discussed. Based on this, the paper proposes a backup hybrid CB which can discharge the stored energy rapidly and safely in the event of main CB malfunctions. The proposed backup CB comprises bypass switches, integrated gate-commutated thyristors (IGCTs) -based solid-state branches, a counter-pulse circuit. It facilitates solid-state branches zero current switching (ZCS) operation during the current interrupting process. A tentative backup protection trigger scheme is presented, which considers both the main CB and the backup CB operations. Simulation tests are conducted to validate the effectiveness of the proposed backup protection CB, ensuring the reliable and safe operation of the FDU during coil quench.

The Fast Discharge Units (FDUs) of the Superconducting (SC) Toroidal Field (TF) coils in the European demonstration fusion power plant DEMO warrant the machine integrity over its full lifetime against severe failure events, such as SC coil quenches or any other plant events requiring the safe TF magnet system discharge. A low (75 kA) and a high current (105 kA) configuration are under study for the TF coils for DEMO. The FDUs must be extremely reliable for the purpose of commutating in short time (∼1 s) the currents and to discharge the TF magnets safely into resistors outside of the tokamak building. Malfunctions of the FDUs must be avoided. The FDUs are considered as Safety Important Class (SIC) components that need to discharge high amounts of energy of about 161 GJ (@75 kA) resp. 118 GJ (@105 kA) stored in the DEMO TF coils.The TF FDUs Circuit Breakers (CBs) shall be installed in the lower level of the tokamak to minimize the length of the connecting busbars. The FDUs integration is challenging because of the high neutron and gamma radiation and stray magnetic fields of the tokamak.Since in DEMO the neutron fluence over lifetime is much higher than in ITER, the problems of using FDUs with electronic subsystems was expected to be more severe, so that their integration has been considered from the beginning of the DEMO project. Sufficient shielding or possible re-positioning of the whole FDUs or sensitive FDU components compared to ITER are being investigated, to reduce the neutron fluxes and neutron and gamma ray fluences. Alternative concepts, e.g., fully mechanical CBs are studied in the EUROfusion Work Package Plant Electrical System (WPPES) in parallel.This paper presents the CAD integration work on the DEMO TF FDUs supported by neutronics assessments. It is assumed the same FDU technology as in ITER. The magnet feeder´s integration is commenced at the same time.

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.

The Divertor Tokamak Test facility (DTT) is the new tokamak under construction in Frascati (Italy), with the main aim of testing different divertor magnetic configurations and technologies in view of DEMO. The Heating and Current Drive function will be provided also by one Neutral Beam Injector (NBI), accelerating negative Deuterium ions with an energy of 510 keV, giving an output power of 10 MW maximum. The Acceleration Grid Power Supply (AGPS), with 3 stages rated 167 kV each, will feed the NBI accelerator. For optical reasons, the insulating gap among the acceleration grids is at the limit of the voltage holding in vacuum, therefore frequent arc breakdowns (BD) among the grids will occur, requiring the quick switch-off of the AGPS to limit the arc energy. For the AGPS, beside the ITER-like scheme including gas-insulated HV components, an innovative approach based on the Modular Multilevel Converter (MMC) technology is being considered. This would include air-insulated HV converters, requiring a huge building volume, which at present is not available in close proximity to the tokamak building. The alternative of installing the MMC converters in a new building to be erected in an available area located far from the tokamak, and connecting the AGPS to the load with HV coaxial cables, is under study. This paper presents the studies carried out to verify the impact of the cables length on the arc energy, in case of breakdown among the grids, and to estimate the magnitude of the voltage oscillations due to resonance effects. These oscillations can occur not only between each hot pole and the return conductor, endangering the cable insulation, but also between the return conductor and local ground, with potential consequences on the laying method, the human safety and the electromagnetic compatibility with sensitive apparatus nearby, which have all been evaluated.

The European DEMO will make use of a significant additional heating power, which could be partly provided by neutral beam injectors (NBIs), which accelerate negative ions by means of grids placed at increasing potentials. These grids will be fed by the acceleration grid power supply (AGPS), divided in a number of stages, which has to provide an overall dc voltage down to −1 MV and currents in the order of tens of Amperes. The AGPS will have to satisfy a set of static and dynamic requirements, mainly in terms of ripple, accuracy, and rise time. In addition, during the NBI operation, frequent losses of insulation between the grids, called breakdowns (BDs), are expected. The AGPS will be able to handle such events by nullifying the output currents as fast as possible in order to limit the energy discharged onto the grids. Adopting the modular multilevel converter (MMC) technology for the AGPS of DEMO and future tokamaks seems promising, due to its intrinsic properties of modularity, high efficiency, fast dynamic response and small energy transferred to the arc in case of BD. Since the converter will be air-insulated, one of the main drawbacks is the large volume occupied. This can be partly reduced by adopting alternative MMC schemes, to minimize the number of components and optimize the counter-voltage applied by the converter at BD. In this article, alternative topologies for the MMC submodules (SMs) or combinations of different schemes [full-bridge (FB), half-bridge (HB)] are investigated. After a preliminary design of the converter, the results of numerical simulations carried out with circuit models are shown, with control schemes customized for the NBI operation. The performance of the different solutions in steady-state, dynamic and anomalous conditions are discussed, with particular focus on BD events. Finally, thermal analyses on the converter are carried out, to verify whether natural convection of air can be a suitable cooling method for the power components.

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 present conceptual design of the Neutral Beam Injectors (NBIs) for DEMO foresees an acceleration voltage of 1 MV, subdivided in 5 steps of 200 kV each, as for the ITER NBI. This voltage is provided by the Acceleration Grid Power Supply (AGPS), which shall also guarantee the capability of handling repetitive grid breakdowns and provide the required high dynamic response, low ripple and high accuracy. These requirements addressed the ITER AGPS design towards a challenging solution based on multiple conversion stages, where the last rectification and filtering stage is gas-insulated. The research here presented aims at evaluating the possible utilization of Modular Multilevel Converters (MMCs) for the AGPS of future NBIs. A preliminary design of the DEMO AGPS has been carried out, where each 200-kV voltage step is generated by an air-insulated MMC stage operating as a rectifier. The main circuit parameters of the MMC have been determined, considering normal and anomalous working conditions. A numerical model has been developed to verify the converter performances in stationary and dynamic conditions and to simulate breakdown and beam-off events. The area occupied by the proposed MMC-based AGPS has been tentatively estimated and compared to that of the ITER solution. The results of the analysis are shown in this paper. Finally, strengths and criticalities of the MMC solution with respect to the ITER-like approach are discussed.

The main purpose of the Divertor Tokamak Test facility (DTT) [1], whose construction is starting in Frascati, Italy, is to study solutions to mitigate the issue of power exhaust in conditions relevant for ITER and DEMO. DTT will be equipped with a significant amount of auxiliary heating power (45 MW) to reach P-SEP/R = 15 MW m(-1) required to be DEMO-relevant [2]. DDT is characterized by high flexibility for the assembling and testing of divertor components and for the different magnetic configurations to address the integrated physics and technology problems. The current conceptual design of the beamline for the DTT Neutral Beam Heating system is here presented, with a particular focus on the effect on the DTT plasma and on the technical solutions adopted to maximize the RAMI indexes (Reliability, Availability, Maintainability and Inspectability) and minimize complexity and costs. Various design options were considered, and a comprehensive set of simulations was carried out using several physics and engineering codes to drive the choice of the most suitable design options and optimize them, aiming at finding a good compromise among different requirements. This paper describes the design of the main components of the DTT NBI beamline, explaining the motivations for the main design choices. (C) 2021 The Japan Society of Plasma Science and Nuclear Fusion Research

The Divertor Tokamak Test facility (DTT) is the new tokamak whose construction is starting in Frascati (Italy), with the main aim of testing different divertor magnetic configurations and technologies in view of DEMO. In the present design, the DTT heating mix includes one Negative Neutral Beam Injector (NBI), with a power injected in the plasma of 10 MW and a beam energy of 500 keV. The NBI conceptual design is under development and includes solutions already adopted for ITER (RF-driven ion source, gas neutralizer, electrostatic Residual Ion Dump), but with some simplifications to reduce costs and complexity deemed feasible for DTT. In particular, the beam source is air-insulated and fed by the gas-insulated Transmission Line through gas-air bushings. This paper presents the status of the conceptual design of the main power supplies of the DTT NBI. The main ratings are collected from physics requirements and NBI design. On this basis, the reference technical solutions are selected, considering the experience gained at the ITER Neutral Beam Test Facility in Padova (Italy), the specific design of the DTT NBI and the layout constraints of the DTT Site. For the Acceleration Grid Power Supply, besides the ITER-like design, an innovative approach based on the Modular Multilevel Converter technology, never adopted in this application, is considered; pros and cons of the two solutions are evaluated.

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 (HNBIs) and Megavolt ITER Injector and Concept Advancement (MITICA), the full-scale prototype located in Padua (Italy), as part of the Neutral Beam Test Facility. This power supply has to provide an average voltage of up to 25 kV, plus an ac low-frequency voltage component with a maximum amplitude of 5 kV, with sinusoidal or trapezoidal waveform. The nominal output current is 60 A, and the maximum pulse duration is 1 h. The RIDPS for both MITICA and ITER is being provided by OCEM Energy Technology s.r.l. The manufacturing of the RIDPS for MITICA has been concluded in summer 2018 and the Site Tests in Padua have been successfully completed in May 2019. In this article, the most significant issues faced during the design, integration and tests, and the solutions identified are described. The most interesting results of the Factory and Site Tests are presented, with particular emphasis on those proving the compliance of the RIDPS with the most critical requirements.

Appropriate disposal of the non-neutronic energy and particle exhaust in a reactor is universally recognized as one of the high priority challenges for the exploitation of fusion as an energy source. The Divertor Tokamak Test (DTT) facility will be built to study a solution suitable for the power exhaust in conditions relevant for DEMO. The tokamak will reach the needed condition of 15 MW/m power flow to the divertor by coupling up to 45 MW of additional power to the plasma. The selected Heating Systems to achieve this goal are Electron Cyclotron Heating (ECH), Ion Cyclotron Heating (ICH) and Negative Neutron Beam Injector (NNBI). The power systems will be installed in two stages: at a first stage 16 MW of ECRH power, 3 MW of ICRH and one 7.5 MW NNBI injector will be installed, making the DTT plasma suitable for relevant experiment at 6T, 4MA configuration. The EC system relies on a 170 GHz, 1 MW gyrotron, similar to those developed for ITER, while for the power transmission a Quasi Optical approach has been chosen, where a multi-beam mirrors will be installed under vacuum to reduce the overall transmission losses below 10%. The power will be injected exploiting independent front-steering antennas capable to steer in real-time all the beams. The module of the ICRH system will be based on transmitters, capable of a wide frequency range (60-90 MHz), connected to two movable antennas inserted in the equatorial ports of DTT. The choice of the antenna type will be based on reliability (i.e. power density) rather than on its performance in terms of peak coupled power. Fast variations of the antenna loading, as the one expected in presence of ELM, will be compensated exploiting an external matching system. The NNBI will be based on two RF plasma sources capable to produce a negative ion current that will be accelerated by a grids system up to 400 keV. The designed injector will reflect the experience gained in SPIDER and MITICA, with modifications aiming to a simplified and well performing system. The paper describes the main characteristics of the design of DTT additional heating system, that will be one of the most powerful between the tokamaks of the next generation.

In the present pre-conceptual design, the base converters to supply the main DEMO poloidal magnets are rated for 45 kA and about 10 kV. If the traditional design approach, based on thyristor bridges, was adopted, this would result in very large reactive power exchanged when low voltage values are required by the load. To satisfy the limitations imposed by National Grid Operators, the reactive power should be compensated inside the DEMO Plant Electrical System by means of a Reactive Power Compensation and Harmonic Filtering system. This could represent an additional cost and takes up a large area of the plant. This paper shows that the reactive power demand, estimated with an analytical model starting from the current/voltage waveforms foreseen in DEMO magnets, is huge. Therefore, an innovative approach, based on Active Front End converters, is proposed and compared with the traditional solution for coil power supplies.

Supervisory control system for superconducting magnet power supplies in JT-60SA is designed and developed. This system is in charge of many kinds of control; discharge sequence control, real-time magnet current control, data acquisition, machine protection and human safety. In the design, communication interface has to be optimized according to all requirements. The most important feature is real-time control with 4 kHz which is required for equilibrium control of plasma. In order to satisfy this requirement, reflective memory (RFM) network is applied for high speed data sharing with multiple controllers. This paper describes the architecture of the control system and the operation of the power supply system under the supervision.

In JT-60SA, the control of resistive-wall-mode (RWM) instabilities will also be realized with a dedicated active control system based on 18 in-vessel sector coils. Each coil will be fed independently by a dedicated fast inverter. Due to the outstanding dynamic performance required, new insulated-gate bipolar transistor modules based on silicon carbide (SiC) have been adopted. Being capable to switch at 30 kHz, these components, together with a very fast control system, allow reaching the required high dynamic performance with the simple and compact H-bridge topology. The RWM-PS will be the first power supply system for fast control of plasma instabilities in fusion experiments adopting SiC semiconductors. This paper gives an overview of the final design of the RWM-PS, with particular emphasis on its special features and the solutions implemented to satisfy the critical requirements. The issues related with the high switching frequency and the peculiar nature of the load will be treated in detail.

The Residual Ion Dump Power Supply (RIDPS) is part of the Ground Related Power Supplies (GRPS) of the MITICA experiment of the ITER Neutral Beam Test Facility (NBTF) and the two ITER Heating Neutral Beam Injectors (HNBI). The GRPS will be manufactured by OCEM Energy Technology s.r.l. (OCEM) via a procurement contract with F4E. The RIDPS is devoted to feed the electrostatic Residual Ion Dump (RID), which deflects and collects the beam residual ions after the neutralization process. The maximum average voltage of the RIDPS is 25 kV, to which can be superimposed a sinusoidal or trapezoidal alternate voltage at 50 Hz, 5 kV maximum. The nominal current is 60 A, with a maximum pulse length of 1 h. This paper describes the detailed design of the RIDPS, highlighting its peculiar aspects, and the expected performance resulting from simulations. (C) 2017 Consorzio RFX. Published by Elsevier B.V.