Alberto Maistrello

The neutral beam heating systems for ITER will be based on radio-frequency driven sources, accelerating a negative ion beam current of tens of amperes for up to one hour. In its second operation phase SO-2, the full-scale prototype source SPIDER was operated for the first time in multi-beamlet configuration at ∼70 kW driver ⁻¹ , extracting a H ^— current density of 215 A m ⁻² over ¼ of the extraction area at 0.3 Pa source filling pressure, providing a total accelerated beam power of ∼700 kW. Scaling of the results confirms the specifications of the on-going RF generators upgrade, indicating that target current density of 330 A m ⁻² in hydrogen is achievable by increasing the RF power to 100 kW driver ⁻¹ . Beam uniformity was assessed at the beamlet-group scale under moderate underperveance conditions: at high power row-averaged beamlet currents were within ±10% for fifteen out of sixteen rows. A lower homogeneity was observed at the single-beamlet level. The influence of the magnetic filter field on beam uniformity was also characterized, confirming the beneficial effect of multi-driver operation on homogeneity at the extraction region. A caesium conditioning procedure was developed and validated for the partial-source configuration. An anomalous amplification of the acceleration current peculiar of the vacuum-insulated source design specific to ITER sources, attributed to stray backstreaming charges and surface emission effects, was identified and mitigated by electrostatic shielding. Together with the ongoing installation of new solid-state RF generators and an improved pumping system, the results presented in this work open the way to full-source operation in the next experimental campaign and yield findings of direct relevance to the ITER injector design.

Source for production of ions of deuterium extracted from RF plasma (SPIDER), the full-scale prototype of the ITER ion source operated at the Neutral Beam Test Facility in Padua, Italy, experiences frequent high-voltage breakdowns between accelerator grids, which generate electrical fast transients on the power supply system. These events are driven by the discharge of the electrostatic energy of the high-voltage system through a strongly interconnected power plant and can overstress components, especially under very high breakdown repetition rates. This article investigates two critical issues and presents the design and in-plant validation of dedicated mitigation measures. First, an impulse phase-to-neutral overvoltage on the 6.6kV AC medium-voltage (MV) supply network feeding the ion source and extraction grid (EG) power supplies (ISEPSs) is explained through circuit modeling calibrated on dedicated tests; capacitor-based protection modules installed in the plant reduce the peak below the specified limit and are validated during plant commissioning. Second, accelerator breakdowns produce overvoltage surges at the output of the EG power supply that exceed component ratings when conventional varistor clamps are used. A fully passive semiconductor-based protection with external energy-absorbing resistors is developed, distributed over multiple installation points to share surge currents, and tuned through on-system breakdown testing. The proposed solutions provide practical design and qualification guidelines for robust operation under frequent breakdown conditions.

The magnetic energy storage and transfer (MEST) system is a newly developed scheme to supply the superconducting (SC) coils in large-size tokamak and reversed field pinch (RFP) reactors to cope with the issues related to electrical power handling in these applications. It is a very promising alternative to face both the issues related to high active power peaks, required during plasma formation and control, and the huge reactive power demand, linked to the use of thyristor converters. The MEST system operating principle and control strategy supplying a single SC coil have already been outlined and studied in previous works. From this starting point, the MEST control system has been expanded to supply a complete set of SC poloidal field coils, considering the magnetic coupling and the energies associated with the magnets-plasma system, essential to control the overall system energy and achieve a certain degree of decoupling between the grid and the coils. This article presents and discusses the principles of controlling the currents of a complete set of poloidal field coils using the MEST system with efficient electrical power management. The application study to PILOT fusion–fission hybrid reactor (FFHR) is reported using analytical calculations and numerical simulations to assess the control operation.

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.

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.

An overview of the recent operations and the main results of cesium injection in the Source for the Production of Ions of Deuterium Extracted from Rf plasma (SPIDER) negative ion source are described in this contribution. In experiments without cesium injection, all SPIDER plants were tested to verify the basic expectations on the operational parameters (e.g., electron cooling effectiveness of magnetic filter field) and to determine its operational region. For beam properties, it was shown that the current density varies across the beam in the vertical direction. In preliminary cesium experiments, the expected increase of negative ion current and simultaneous decrease of co-extracted electrons were found, along with the influence of the control parameters (polarization of the plasma electrodes, magnetic filter field) on the SPIDER beam uniformity in the horizontal and vertical directions. It was shown that non-Gaussian tails can be identified in the angular distribution on the plane perpendicular to the beam propagation direction. Stray particles, nonhomogeneous beam and large divergence might result in unexpected heat and particle loads over ITER neutral beam injector (NBI) accelerator grids; it is the goal of SPIDER to assess and possibly to identify suitable methods for controlling these beam features. A major shutdown, planned for late 2021, to solve the issues identified during the operation and to carry out scheduled modifications, is outlined. Such improvements are expected to allow SPIDER to pursue the ITER requirements in terms of negative ion current, electron-to-ion ratio, and beam duration.