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High voltage (HV) insulation across a single gap in vacuum and low-pressure gas is a critical issue in relation to the development and realization of the electrostatic accelerator for the ITER neutral beam injector (NBI) (Toigo et al 2017 New J. Phys. 19 085004). The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the prototype for the ITER NBI (De Lorenzi et al 2011 Fusion Eng. Des. 86 742–5). A voltage up to 800kVDC was achieved in the HVPTF during the experimental campaigns with a sphere-plane configurations having variable gap length (from 30 to 150 mm) and pressure ranging from high vacuum (10–7 mbar) to 10–3 mbar in argon. Such an experimental campaign represents one of the few examples where voltages higher than 500–550 kV DC are sustained by a single vacuum gap between electrodes (Rohrbach 1971 CERN Report 71-5) and therefore constitutes additional experience to improve the knowledge of voltage holding across large vacuum gaps. The results in high vacuum indicate that at the beginning of voltage conditioning the breakdown events occur at the same cathodic electric field irrespective of the electrode geometry. However, after sufficient conditioning time, the breakdown voltage distribution seems to depend also on the electric field at the anode and on the total voltage between electrodes. A benchmark between a numerical tool previously developed to predict the voltage holding in high vacuum (voltage holding prediction model, VHPM (Pilan et al 2011 Fusion Eng. Des. 86 742–5)) and the experimental results is also reported and discussed. [ABSTRACT FROM AUTHOR]

The rf source test facility SPIDER will test and optimize the source of the 1 MV neutral beam injection systems for ITER. Cavity ring-down spectroscopy (CRDS) will measure the absolute line-of-sight integrated density of negative (H- and D-) ions, produced in the extraction region of the source. CRDS takes advantage of the photodetachment process: negative ions are converted to neutral hydrogen atoms by electron stripping through absorption of a photon from a laser. The design of this diagnostic is presented with the corresponding simulation of the expected performance. A prototype operated without plasma has provided CRDS reference signals, design validation, and results concerning the signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

The charge exchange neutral particle outfluxes and ion temperature are important quantities for the determination of the power balances and the plasma’s energetic content; therefore they constitute a long term issue of reversed field pinch research. In order to improve the experimental knowledge of these plasma quantities, in the reversed field experiment a vertical time-of-flight (TOF) neutral particle analyzer (NPA) has been recently installed. It sees the plasma along a vertical chord, whose impact parameter is about half radius, and it is meant to complement the other two NPAs, which are installed on the equatorial plane. In this article, the new vertical TOF is reported, with particular attention to its more original components. An “in situ” method to calibrate the electron multiplier gain absolutely is described in detail. The first measurements of the ion temperature are reported, showing the reliable operation of the system and the acceptable signal-to-noise ratio obtained up to now. The results are also compared with the measurements of the horizontal NPAs mainly in the experimental regime of high plasma density. Possible improvements of the new diagnostic and its use in combination with a neutral particle beam are also briefly described and discussed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

• Spectroscopy is main diagnostic of ITER prototype negative ion beam source SPIDER. • Photodiodes measure visible radiation from source faster than spectrometers. • Custom electronics has low-noise variable gain amplifier, FPGA-based remote control. • Photodiode has14 bit dynamic range, 1.2–135 kHz bandwidth, 100 kΩ to 5 GΩ gain. • First measurements show dependencies with RF power and filter field current. The source prototype SPIDER in the Neutral Beam Test Facility at Consorzio RFX has been developed to demonstrate the target performances of the RF plasma source of ITER Heating Neutral Beam injectors. SPIDER diagnostics characterize the plasma in the source, optimize performances and provide signals for machine protection. Photodiodes measure the time evolution of lines of sight integrated visible radiation emitted by the plasma in the source. Some are equipped with interference filters to select specific spectral lines, e.g. those of cesium to control its evaporation, which assists the H¯ production, and of metal impurities, which might indicate wall damages. Other lines of sight are fed to grating spectrometers to measure the spectrum at a slower rate, either at low resolution for spectral survey of hydrogen Balmer lines and impurities or at high resolution for molecular Fulcher bands. The overall layout of the photodiode diagnostic and preliminary results during the first SPIDER operation are presented. The custom photodiode electronics is described in detail, including a low-noise variable gain amplifier, FPGA-based remote control and suitably shaped digital output for feedback control. Main characteristics are: linear response up to 10 V, 14 bit dynamic range and 1.2–135 kHz bandwidth depending on gain in range 100 kΩ to 5 GΩ. Measured light through the drivers has allowed to recognise plasma features and dependencies with RF power and filter field current, as well as dominant frequencies in signal fluctuations. [ABSTRACT FROM AUTHOR]

Abstract: In the MITICA research program for the construction of the ITER Neutral Beam Injector prototype, a Laboratory for the investigation on high voltage holding in vacuum has been set up. This Laboratory - HVPTF: High Voltage Padova Test Facility - is presently capable of experiments up to 300kV dc, and planned for the upgrade to 800kV. The specific mission for this ancillary lab is the support to the electrostatic design and construction of the MITICA accelerator and the development and testing of HV components to be installed inside the MITICA accelerator during its operation. The paper describes the structure of the lab, characterized by a high degree of automation and reports the results of the commissioning at 300kV and the first results of voltage holding between test electrodes. [Copyright &y& Elsevier]

The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the prototype for the ITER Neutral Beam Injector. Evidences of Electron Stimulated Desorption of gas released from anodic surfaces have been observed during the onset of high voltage Direct Current vacuum discharges between stainless steel electrodes, separated by vacuum gaps of a few millimeters and with voltages of about 200 kV. The count of x-ray photons, produced by the accelerated electrons impinging the anode surface and observed as breakdown precursor in the 40% of the cases, was analyzed together with the images taken by an ultra-high speed camera during the onset of breakdown events. Electron stimulated desorption of neutral gas from the anodic surface appears to be a candidate for the voltage breakdown. The dynamic of the observed phenomena has been compared with the results obtained by a numerical model. The latter assumes a Townsend like discharge which develops following the emission of gas from the anode surface. The comparison shows good agreement between the experimental data and numerical simulations. [ABSTRACT FROM AUTHOR]

• The insulation capability of a full silica kapton coated optical fiber in vacuum under DC voltage is about 160kV for 600mm length. • The commercial batteries providing the highest energy density for low power application are Li-SOCl2 battery • The PIC microcontroller proven to be a reliable device with respect to EMI irradiated from Surge pulses as well as it can be a very low power consumption ADC when the IDLE and Standby operational modes are exploited. • A solution to feed electronic devices operating at high voltage in harsh environments (vacuum, risk of discharges, x rays in the nearby,...) in a reliable way consists in enclosing the batteries in a vacuum tight commercial nipple closed by a dedicated feedthrough. • The precise synchronization of fast events within a system of distributed devices can be done with a distributed trigger network when a deterministic time delay is known. The trigger time can be also precisely time stamped by means of a fast transient FPGA recorder provided by a iW-RainboW-G36S board that is kept in sync with a GPS absolute time source with IEEE1588 protocol. ITER is an experimental fusion device and requires two Neutral Beam Injectors (NBIs) for plasma heating and current drive which have to supply a power of 16.5 MW each, delivered by a 1MeV deuterium beam. This D0 beam is generated starting from negative ions, produced inside a RF driven ion source, and accelerated by a Multi-Aperture Multi-Grid electrostatic accelerator insulated in vacuum. MITICA (Megavolt ITER Injector & Concept Advancement) is a full-scale prototype of the ITER Heating Neutral Beams (HNBs), devoted to test and optimize the design, under construction in Padova at the Neutral Beam Test Facility (NBTF) hosted by the Consorzio RFX. One of the most critical requirements in MITICA concerns the capability to withstand the nominal voltage of 1MV on a single long gap (≈1m), which separates the beam source and the vacuum vessel, insulated by vacuum or low pressure gas (10−6-10−3 mbar range) and with large size electrodes. In order to address this issue, a dedicated experimental test campaign exploiting a 1:1 mockup of the MITICA Beam Source will be carried out at NBTF before the installation of the actual Beam Source. During this test campaign, a set of electrical diagnostics will be installed to identify the location of the breakdowns as well as to measure the magnitude of the microdischarge currents (the precursors of vacuum breakdowns), and arc currents to provide useful information to optimize the electrostatic design of the ITER HNB and to validate the numerical models. This paper describes the development and the design of these diagnostics system based on electrical measurements. [ABSTRACT FROM AUTHOR]