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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.

SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma) is the full-scale prototype for the ITER Heating Neutral Beam (HNB) source. Driven by four driver coil pairs powered by oscillators the SPIDER source plasma exhibits fluctuations in a range of frequencies that have been observed by Langmuir probes and Source Emission Spectroscopy. These fluctuations are believed to influence the extracted beam current, meniscus shape and beam features. Beam operation in SPIDER, with the eventual aim of achieving 100 keV, 46 A hydrogen negative ion beam for one hour, has mainly been performed in volume operation. Recently, with the introduction of caesium, SPIDER has moved to the surface operation, with a noted increase in beam current. Electrical and calorimetric measurements of the beam current have shown discrepancies between the two, possibly due to background gas interactions. Taking advantage of the reduced number of beamlets due to the Plasma Grid mask a series of Hall effect current transducers and AC current transformers have been installed in SPIDER. These Beamlet Current Monitors (BCMs), positioned downstream of the Grounded Grid, provide a direct measurement of the current of up to five individual beamlets, from DC up to 5MHz frequencies. This contribution describes the initial results from the BCM with and without caesium, focussing on the characterisation of the beamlet direct current component.

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.

ITER envisages the use of two heating neutral beam injectors plus an optional one as part of the auxiliary heating and current drive system, to reach the desired performances during its various phases of operation. The 16.5 MW expected neutral beam power per injector is several notches higher than worldwide existing facilities.In order to enable such development, a Neutral Beam Test Facility (NBTF) was established at Consorzio RFX, exploiting the synergy of two test beds, called SPIDER and MITICA. SPIDER is dedicated developing and char-acterizing large efficient negative ion sources at relevant parameters in ITER-like conditions: source and accel-erator located in the same vacuum where the beam propagates, immunity to electromagnetic interferences of multiple radio-frequency (RF) antennas, avoidance of RF-induced discharges on the outside of the source. Three years of experiments on SPIDER have addressed to the necessary design modifications to enable full perfor-mances. The source is presently under a long shut-down phase to incorporate learnings from the experimental campaign, in particular events/issues occurred during operation, which led to the identification of improvement opportunities/necessities (e.g. RF discharges, local burns, water leaks, other damages, configuration/design upgrades to maximize chances/margin to quest target parameters).Parallelly, developments on MITICA, the full-scale prototype of the ITER Neutral Beam Injector (NBI) featuring a 1 MV accelerator and ion neutralization, are underway including manufacturing of the beam source, accel-erator and the beam line components, while power supplies and auxiliary plants, already installed, are under final testing and commissioning.Integration, commissioning and tests of the 1 MV power supplies are essential for this first-of-kind system, unparalleled both in research and industry field. 1.2 MV dc insulating tests of high voltage components were successfully completed. The integrated test to confirm 1 MV output by combining invertor systems, DC gener-ators and transmission lines extracted errors/accidents in some components. To realize a concrete system for ITER, said events have been addressed and solutions for the repair and the improvement of the system were developed.Hence, NBTF is emerging as a necessary facility, due to the large gap with existing injectors, effectively dedicated to identify issues and find solutions to enable successful ITER NBI operations in a time bound fashion. The lessons learned during the implementation on NBTF and future perspectives are here discussed.

The electrical insulation of the Megavolt ITER Injector and Concept Advancement (MITICA) beam source (BS) at 1 MV in vacuum is a challenging issue, which could not be fully addressed so far on the basis of experimental results and of theoretical models available in literature. A specific high-voltage (HV) test campaign is being prepared to validate and optimize the voltage holding capability of the BS insulation under realistic conditions, using full-size mockup electrodes reproducing in detail the geometry of the BS and accelerator. The proposed test strategy will address both the single-gap and the multistage insulation, so as to obtain a verification of voltage holding at 1 MV before the installation of the real components. This approach is intended to reduce the risk related to the HV insulation at 1 MV and, if necessary, to allow the development of effective corrections. In this article, the test motivations and requirements are defined, and the electrode implementation and diagnostic setup are described. Finally, the test configurations and the experimental procedure are discussed.

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.

The voltage holding capability of the MITICA Beam Source at 1 MV is a very challenging issue, which could not be fully addressed so far on the basis of experimental results and of theoretical models available in literature. This paper describes a specific HV test campaign which is planned to be implemented in the MITICA Vacuum Vessel using mock-up electrodes reproducing in detail the geometry of the Beam Source and Accelerator. The test set-up will incorporate all essential features both of the single-gap and of the multi-stage insulation, so as to obtain reliable data on voltage holding at 1 MV before the installation of the real components. This approach is intended to reduce the risk related to the HV insulation at 1 MV and, if necessary, to allow to develop the most effective solutions. The tests will be performed in the MITICA Vessel (already available in the NBTF site) both in vacuum and in low-pressure gas. In the paper, the test strategy and requirements are first introduced. Then the sequence of test configurations is defined. Finally, some design solutions for the electrode realization are described and a preliminary plan together with a list of test equipment is given.

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 17MW each, by neutralizing accelerated negative hydrogen or deuterium ions. The requirements of ITER NBIs (40A/1MeV D- ions for

The negative-ion beam source SPIDER, which is the full-scale prototype source for the ITER neutral beam injector, recently started the operation with caesium. This experimental phase follows three years of volume operation, devoted to the commissioning of the plants and to the integrated test of the ion source and accelerator. The ion source, composed of eight RF drivers connected to a large plasma chamber from which the negative ions are extracted, was operated up to a RF power of 400kW, and beam energy up to 50kV, and plasma discharges limited to less than one minute This contribution will describe the main results of the first campaign with caesium in SPIDER. The repetition of short plasma and beam extraction blips with different injection rates was applied, to study the effect on plasma and beam parameters. The caesiation procedure adopted in SPIDER will be described (caesium injection rate, duty cycle of plasma-on, RF power, source gas pressure) together with the effects of the source parameters on the extracted beam and its uniformity. Even though the use of a much reduced number of beamlets was a strong limitation in terms of total accelerated current (a mask at the plasma grid covered 1252 apertures over 1280 to limit the gas load to the vacuum pumps), it provided advantages in the study of the caesium effect on the beam, such as the introduction of dedicated diagnostics for the single beamlets, and the identification of the beamlet current and optics.

Recent advances in acquisition equipment is providing experiments with growing amounts of precise yet affordable sensors. At the same time an improved computational power, coming from new hardware resources (GPU, FPGA, ACAP), has been made available at relatively low costs. This led us to explore the possibility of completely renewing the chain of acquisition for a fusion experiment, where many high-rate sources of data, coming from different diagnostics, can be combined in a wide framework of algorithms. If on one hand adding new data sources with different diagnostics enriches our knowledge about physical aspects, on the other hand the dimensions of the overall model grow, making relations among variables more and more opaque. A new approach for the integration of such heterogeneous diagnostics, based on composition of deep variational autoencoders, could ease this problem, acting as a structural sparse regularizer. This has been applied to RFX-mod experiment data, integrating the soft X-ray linear images of plasma temperature with the magnetic state. However to ensure a real-time signal analysis, those algorithmic techniques must be adapted to run in well suited hardware. In particular it is shown that, attempting a quantization of neurons transfer functions, such models can be modified to create an embedded firmware. This firmware, approximating the deep inference model to a set of simple operations, fits well with the simple logic units that are largely abundant in FPGAs. This is the key factor that permits the use of affordable hardware with complex deep neural topology and operates them in real-time., Comment: Proceeding of the 22nd IEEE Real Time Conference

In recent decades, muon imaging has found a plethora of applications in many fields. This technique succeeds to infer the density distribution of big inaccessible structures where conventional techniques cannot be used. The requirements of different applications demand specific implementations of image reconstruction algorithms for either multiple scattering or absorption–transmission data analysis, as well as noise-suppression filters and muon momentum estimators. This paper presents successful results of image reconstruction techniques applied to simulated data of some representative applications. In addition to well-known reconstruction methods, a novel approach, the so-called μCT, is proposed for the inspection of spent nuclear fuel canisters. Results obtained based on both μCT and the maximum-likelihood expectation maximization reconstruction algorithms are presented. This article is part of the Theo Murphy meeting issue ‘Cosmic-ray muography’.

The RFX-mod2 Nuclear Fusion experiment is an upgrade of RFX-mod. Among the other improvements in machine structure and diagnostics, a larger number of electromagnetic probes (EMs) is foreseen to provide more information about plasma instabilities and to allow an improved real-time plasma control. An Analog to Digital Converter (ADC) architecture able to provide both transient recording and real-time streaming is foreseen in RFX_mod2. Transient recording provides full speed data acquisition (up to 1 MSample/s) by recording data in local memory and reading memory content after the plasma discharge. Real-time streaming of subsampled data is required for active control. The chosen technology is based on the XILINX Zynq architecture that provides in the same chip a multicore ARM processor tightly couple to a FPGA. Time critical functions carried out by the FPGA in this context are: 1) The management of a circular data buffer and the DMA transfer in RAM of pre and post trigger samples after the trigger has been received; 2) Antialiasing filtering and subsampling of the samples to be streamed. The resulting samples are enqueued in a FIFO accessed by the processor. The functions carried out by the processor are: 1) The management of the configuration setting, received via TCP/IP or HTTP. The processor validates the configuration and write the appreciate registers in the FPGA; 2) Offline data readout of acquired samples in transient recording; 3) Network data streaming of subsampled data read from the FIFO and sent in UDP packets to the active plasma control system.

This report describes the main activities carried out at the High Voltage Padova Test Facility HVPTF in 2017. According with the Work Program (WP) 2017, the activities were focused mainly in: - Studying of the X-ray spectra to investigate the nature of the physical phenomena which occurs during the high voltage conditioning - Implementing an effective automatic procedure to define a reproducible high voltage test Method - testing alternative conditioning techniques to treat the electrode surfaces before applying the DC high voltage in high vacuum - comparing of breakdown voltage statistics from QST labs and HVPTF - calibrating the Voltages measured by the high voltage power supplies - Extending the duration of the High Voltage tests - Concluding the HV tests of a post insulator prototype foreseen in WP2016 As usual, activities concerning ordinary maintenance on the vacuum plant and controlling system have been done during the year.

A short description of the muon tomography demonstrator at the INFN Laboratori Nazionali di Legnaro near Padua, Italy, is given and the principal achievements owing to the data collected at that experimental facility are presented. In particular, the feasibility studies for several applications based on the muon-tomographic technology, within national and European projects, are discussed. The experimental problems and the procedures used to improve the performance are underlined. In addition, new activities and the related detector optimization are illustrated. This article is part of the Theo Murphy meeting issue ‘Cosmic-ray muography’.

The muon tomography technique, based on multiple Coulomb scattering of cosmic ray muons, has been proposed as a tool to detect the presence of high density objects inside closed volumes. In this paper a new and innovative method is presented to handle the density fluctuations (noise) of reconstructed images, a well known problem of this technique. The effectiveness of our method is evaluated using experimental data obtained with a muon tomography prototype located at the Legnaro National Laboratories (LNL) of the Istituto Nazionale di Fisica Nucleare (INFN). The results reported in this paper, obtained with real cosmic ray data, show that with appropriate image filtering and muon momentum classification, the muon tomography technique can detect high density materials, such as lead, albeit surrounded by light or medium density material, in short times. A comparison with algorithms published in literature is also presented.

The ITER Heating Neutral Beam (HNB) source prototype SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma), hosted at the Neutral Beam Test Facility (NBTF) in Padova, Italy, has recently started operating with evaporated caesium in the source. This moves the primary symbolscript production mechanism from volume to surface processes, increasing the extracted symbolscript current while decreasing the co-extracted electron current. As in volume operation, the beam exhibits inhomogeneities across the vertical profile due to magnetic drifts, a result of the transverse filter field, which is vital for reducing the electron temperature near the extraction region. To minimise the occurrence of electrical discharges, SPIDER has been operated with a diminished number of extraction apertures to minimise the vessel pressure/ion source pressure ratio by means of a mask, which reduces the gas flow conductance between the source and the vessel. Therefore, it has been possible to directly measure the current of individual beamlets, due to the increased room between the beamlets, using the non-invasive Beamlet Current Monitor (BCM) diagnostic. Using measurements of five individual beamlets the homogeneity of the SPIDER symbolscript beam has been assessed, in a range of operating conditions with caesium in the source. The dependence of the beam homogeneity on source parameters (bias, filter field, RF power) has been observed, while increasing the Cs evaporation rate and unbalancing the power of the RF generators have proven to be effective at mitigating the beam inhomogeneity.

SPIDER is the full scale prototype for the ITER Heating Neutral Beam source, hosted at the Neutral Beam Test Facility in Padova, Italy. The behavior of the beam must be thoroughly investigated to bring the machine's performance in line with ITER's requirements. In particular, ripples in the beam can affect its divergence. Measuring the AC component of the beam current can therefore help to understand the impact that the oscillations caused by the source power supplies or RF generators have on the beam optics.To minimize the occurrence of electrical discharges, SPIDER was recently operated with a mask designed to close a large number of extraction apertures. As a consequence, the space between each beamlet was substantially increased, allowing for the installation of the Beamlet Current Monitor (BCM) which enables a non-invasive direct measurement of the DC and AC components of the currents of 5 individual beamlets.A first assessment of the beamlet currents' AC components (up to 10 MHz) was performed during SPIDER's first campaign with cesium evaporation. Recurring oscillations were identified in various frequency ranges, with amplitudes reaching up to similar to 7% of the beamlet current DC value. When possible, the ripples were correlated to oscillations due to the RF oscillators and the beatings caused by their mutual coupling, or to SPIDER's power supply systems.

The negative-ion based neutral beam injector for heating and current drive of the ITER plasma (ITER HNB) is under development, at present focusing on the optimization of the full-scale plasma source in the SPIDER test stand. The production of H− or D− ions in the ion source is based on the low work function surfaces obtained by caesium evaporation. This paper describes the caesium conditioning procedure and the corresponding beam performances during the first operation of SPIDER with caesium. Technical solutions to overcome present limitations of the test stand are described. The influence of source parameters on the caesium effectiveness was investigated in short beam pulse operation; with total radio-frequency (RF) power of 400 kW and filling pressure below 0.4 Pa, and a limited number of extraction apertures, a negative ion current density of about 200 A m−2 was extracted in hydrogen, with beam energy lower than 60 keV. Beam optics and beam uniformity were assessed thanks to the acceleration of isolated ion beamlets. A possible procedure to accelerate a uniform beam was demonstrated at low RF power. The results obtained in this first investigation provided key indications on the operation of one of the largest existing sources of accelerated negative hydrogen-like ions.