Partial Discharges detection in 1 MV power supplies in MITICA experiment, the ITER Heating Neutral Beam Injector prototype

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.Comment: Nine pages, twelve figures, accepted manuscript of a paper published in Fusion Engineering and Desig

Overview on electrical issues faced during the SPIDER experimental campaigns

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.Comment: 8 pages, 12 figures. Presented at SOFT 202

Design and Development of a Diagnostic System for a Non-Intercepting Direct Measure of the SPIDER Ion Source Beamlet Current

Stable and uniform beams with low divergence are required in particle accelerators; therefore, beyond the accelerated current, measuring the beam current spatial uniformity and stability over time is necessary to assess the beam performance, since these parameters affect the perveance and thus the beam optics. For high-power beams operating with long pulses, it is convenient to directly measure these current parameters with a non-intercepting system due to the heat management requirement. Such a system needs to be capable of operating in a vacuum in the presence of strong electromagnetic fields and overvoltages, due to electrical breakdowns in the accelerator. Finally, the measure of the beam current needs to be efficiently integrated into a pulse file with the other relevant plant parameters to allow the data analyses required for beam optimization. This paper describes the development, design and commissioning of such a non-intercepting system, the so-called beamlet current monitor (BCM), aimed to directly measure the electric current of a particle beam. In particular, the layout of the system was adapted to the SPIDER experiment, the ion source (IS) prototype of the heating neutral beam injectors (HNB) for the ITER fusion reactor. The diagnostic is suitable to provide the electric current of five beamlets from DC up to 10 MHz

Characterization of the SPIDER Cs oven prototype in the CAesium Test Stand for the ITER HNB negative ion sources

The ITER Heating Neutral Beam (HNB) injector is required to deliver 16.7 MW of power to the plasma from aneutralized beam of H 12/D 12negative ions, produced by an RF source and accelerated up to 1 MeV. To enhancethe H 12/D 12production, Cs will be routinely evaporated in the source by means of specific ovens embedded inthe source, to reduce its work function. Controlling and monitoring the evaporation rate of Cs inside the sourcewill be fundamental to achieve the desired performance for the ITER HNB.The prototype RF negative ion source SPIDER has been developed and built in the Neutral Beam Test Facilityat Consorzio RFX. In SPIDER, liquid Cs based ovens will be used to inject Cs vapors inside the source. TheCAesium Test Stand (CATS) has been specifically designed and set up for testing, commissioning, and char-acterizing Cs ovens in vacuum, but also to study the Cs evaporation and deposition onto surfaces. A SPIDER Csoven prototype has been manufactured and tested in CATS in order to characterize its thermal behavior, bymeans of thermocouples and thermal camera, and its Csflux, by means of Surface Ionization Detectors and LaserAbsorption Spectroscopy