This paper describes the status of the implementation of the upgrades for the RFX-mod2 device, the second major modification of the Reversed Field eXperiment (RFX). The machine’s core update consists of a thin copper stabilizing shell placed inside the vacuum vessel to ensure long-term stability of resistive wall modes (RWM) and enable regimes with fast-rotating tearing modes. A comprehensive modernization of the facility’s technological plants and diagnostics is being carried out within the NEFERTARI project (New Equipment For the Experimental Research and Technological Advancement for the RFX Infrastructure). Funded by the Italian National Recovery and Resilience Plan (NRRP) under Next Generation EU, NEFERTARI aims to strengthen the RFX infrastructure and its network of partners. Significant plant upgrades include: the refurbishment of the main vacuum system and the development of a new glow discharge cleaning (GDC) system based on eight fixed electrodes for more uniform wall conditioning; a major overhaul of the control and data acquisition system, now featuring a high-resolution multichannel acquisition system with over 1500 channels that eliminates the need for analogue integrators; the enhancement edge plasma characterization and of core diagnostics, such as the high-repetition-rate (up to 3 kHz) Thomson Scattering system and the expanded soft x-ray (SXR) tomography; the upgrade of the existing and installation of new reflectometric system; the restoration of the Diagnostic Neutral Beam Injector (DNBI) for spatially resolved ion temperature measurements. Furthermore, the paper highlights the contributions of the Italian laboratory network, including the BiGyM infrastructure for testing innovative plasma-facing components; the ROMAN remote handling facility for in-vessel maintenance development; dedicated laboratories for the development of neutron detectors and for optical diagnostics for the characterization of surfaces, and for high-voltage electrical insulation testing.

This work presents a data reconstruction framework based on a modified variational autoencoder (VAE) architecture (Kingma and Welling 2022 (arXiv:1312.6114)) and compares its performance with a traditional B-spline (Chaudhuri 2021 (arXiv:2108.06617)) interpolation approach. The proposed VAE model, inspired by the architecture developed in Garola et al 2021 (IEEE Trans. Nucl. Sci. 68 2165-72), incorporates two main enhancements: a stochastic masking layer that randomly removes 10% of the input vector components, and a beta-annealing strategy to progressively reduce the influence of the Kullback-Leibler divergence term during training. The model has been trained on two datasets: (i) synthetic curves generated via Gaussian processes, and (ii) experimental profiles obtained from soft x-ray measurements in the RFX-mod reversed-field pinch experiment (Sonato et al 2023 22nd Symp. on Fusion Technology vol 66-68 pp 161-8). Both datasets consist of partially incomplete profiles with missing data points. The reconstruction performance of the VAE is evaluated and compared against B-spline interpolation. Results show that the VAE consistently outperforms the spline-based method, particularly under moderate to high levels of data loss, demonstrating the potential of deep generative models for robust data imputation in fusion plasma diagnostics.

The MUTOMCA (MUon TOMography for shielding CAsks) international project explores the suitability of cosmic muon tomography, a non-invasive and non-destructive imaging method, for the re-verification of loaded spent fuel casks. Such casks are stored in dedicated interim storage facilities under continuous containment and surveillance by international safeguards authorities using unattended monitoring equipment. In the hypothetical case of a temporary failure of these instruments, resulting in a loss of continuity of knowledge, a re-verification of the spent fuel to fulfill international safeguard obligations would be required. The project aims to demonstrate the ability of muon tomography, a non-invasive and non-destructive imaging method, to distinguish between dummy elements and spent fuel assemblies based on their different densities. For this purpose, an experimental detector system based on drift tube technology was designed, developed, constructed, and installed for a field test at a dry storage facility in Germany (Grafenrheinfeld, operated by BGZ). The test examined two CASTOR (R) V/19 casks: one loaded with a mixed configuration of dummy elements and spent fuel assemblies and the other exclusively with spent fuel assemblies. This was the first real data reconstruction with muon tomography performed in an interim storage facility. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/).

A statistical analysis of plasma temperature profiles in RFX-mod plasmas reveals that improved thermal structures can be observed in chaotic magnetic topologies, specifically in the so-called multiple helicity (MH) states. The improved thermal structures feature high electron temperatures, up to 1.2 keV, enhanced temperature gradient and radial extension which have been conventionally only attainable in organized quasi-single helicity states. The analysis reported here suggests a possible role of the m = 1, n=-7 and the m = 1, n=-8 modes (where m and n are the poloidal and toroidal mode numbers, respectively) in MH states, where the plasma retains good temperature properties. These findings, also supported by numerical ORBIT simulations, indicate that the magnetic islands of the innermost resonant modes may act as regions of improved energy confinement, opening new avenues for understanding the connection between the plasma's magnetic topology and thermal properties. This paves the way for new sets of experiments in RFX-mod2, with the aim of exploring these mechanics in detail and improving the thermal properties of RFP plasmas.

Event-driven data acquisition is used to capture information from fast transient phenomena typically requiring a high sampling speed. This is an important requirement in the ITER Neutral Beam Test Facility for the development of one of the heating systems of the ITER nuclear fusion experiment. The Red Pitaya board has been chosen for this project because of its versatility and low cost. Versatility is provided by the hosted Zynq System on Chip (SoC), which allows full configuration of the module architecture and the OpenSource architecture of Red Pitaya. Price is an important factor, because the boards are installed in a hostile environment where devices can be damaged by EMI and radiation. A flexible solution for event-driven data acquisition has been developed in the Zynq SoC and interfaced to the Linux-based embedded ARM processor. It has been successfully adopted in a variety of data acquisition applications in the test facility.

The RFX-mod2 device, the upgraded version of the previous RFX-mod with a modified magnetic boundary, is presently under realization and will start to be operated in 2025. Significant upgrades of the diagnostic capabilities have been proposed and are under development. These include a largely increased number of in-vessel magnetic and electrostatic sensors, a new fast reciprocating manipulator for the exploration of the edge plasma in a wide range of experimental conditions, the improved Thomson scattering and soft x-ray diagnostics system for a detailed determination of the behavior of the electron temperature profile, new dedicated systems for the space and time resolved analysis of x-ray spectra and neutron rate, a reflectometric diagnostic for real-time determination of plasma position, two diagnostics devoted to the imaging of light impurities and influxes behavior along with arrays of halo current sensors. These diagnostic upgrades will be accompanied by a significant effort to improve the control of the electron density and of the impurity influxes by means of proper treatment of plasma facing components with in-vessel fixed electrodes distributed over the first wall. The described advancements will allow a deeper understanding of physics phenomena in the wide variety of magnetic configurations, including the tokamak, the reversed-field pinch and the Ultra-low q, which can be produced in RFX-mod2 thanks to its flexibility and unique MHD control capabilities.

Proper timing synchronization is important when data from sensors are acquired by different devices. This paper proposes a simple but effective solution for System on Chip (SoC) architectures that integrates a general-purpose Field Programmable Gate Array (FPGA) with a CPU. The proposed approach relies on a network synchronization protocol implemented in software, such as Network Time Protocol (NTP) or Precision Time Protocol (PTP), and uses the FPGA to generate a clock reference that is maintained in step with the synchronized system clock. The clock generated by the FPGA is obtained from the FPGA oscillator via appropriate fractional clock division. Clock drift is avoided via a software program that periodically compares the FPGA and the system counters, respectively, and adjusts the fractional clock divider in order to slightly adjust the FPGA clock frequency using a Proportional Integral controller. A specific implementation is presented on the RedPitaya platform, generating a 1 MHz clock in step with the NTP synchronized system clock. The presented system has been used in a distributed data acquisition system for fast transient recording in the neutral beam test facility for the ITER nuclear fusion experiment.

The experimental fusion reactor ITER will feature two (or three) heating neutral beam injectors (NBI) capable of delivering 33(or 50) MW of power into the plasma. A NBI consists of a plasma source for production of negative ions (extracted negative ion current up to 329 A/m 2 in H and 285 A/m 2 in D) then accelerated up to 1 MeV for one hour. The negative ion beam is neutralized, and the residual ions are electrostatically removed before injection. The beamline was designed for a beam divergence between 3 and 7 mrad. The ion source in ITER NBIs relies on RF-driven, Inductively-Coupled Plasmas (ICP), based on the prototypes developed at IPP Garching; RF-driven negative-ion beam sources have never been employed in fusion devices up to now. The recent results of SPIDER, the full size ITER NBI ion source operating at NBTF in Consorzio RFX, Padova, measure a beamlet divergence minimum of 12mrad and highlighted beam spatial non-uniformity. SPIDER results confirmed the experimental divergence found in smaller prototype sources, which is larger compared to filament-arc ion sources. Although prototype experiments have shown that the extracted current requirement can be achieved with minor design improvements, the beamlet divergence is expected to marginally achieve the design value of 7 mrad, which in multi-grid long accelerators results in unexpected heat loads over the accelerator grids. A contributor to the beam divergence is the energy/temperature of the extracted negative ions, so it is believed that plasma differences between the two source types play a role. Research is focused on the plasma parameters in the ion source. One RF driver, identical to the ones used in SPIDER, installed in a relatively small-scale experimental set-up, inherently more flexible than large devices, is starting operations devoted to the investigation of the properties of RF-generated plasmas, so as to contribute to the assessment of negative ion precursors, and of their relationship with the plasma parameters, particularly when enhancing plasma confinement. The scientific questions, that have arisen from the preliminary results of SPIDER, guided the design of the test stand, which are described in this contribution, together with the diagnostic systems and related simulation tools. The test stand, which shares with the larger experiment all the geometrical features and constraints, will allow technological developments and optimized engineering solutions related to the ICP design for the ITER NBIs.

ITER envisages the use of two heating neutral beam injectors plus an optional one as part of the auxiliary heating and current drive system. The 16.5 MW expected neutral beam power per injector is several notches higher than worldwide existing facilities. A Neutral Beam Test Facility (NBTF) was established at Consorzio RFX, exploiting the synergy of two test beds, SPIDER and MITICA. SPIDER is dedicated to developing and characterizing large efficient negative ion sources at relevant parameters in ITER-like conditions: source and accelerator 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 performances. The source is presently under a long shut-down phase to incorporate learnings from the experimental campaign. Parallelly, developments on MITICA, the full-scale prototype of the ITER NBI featuring a 1 MV accelerator and ion neutralization, are underway including manufacturing of in-vessel components, while power supplies and auxiliary plants are already under final testing and commissioning. Integration, commissioning and tests of the 1MV power supplies are essential for this first-of-kind system, unparalleled both in research and industry field. The integrated test to confirm 1MV output by combining invertor systems, DC generators and transmission lines extracted errors/accidents in some components. To realize a concrete system for ITER, 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.

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 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 H− production mechanism from volume to surface processes, increasing the extracted H− 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 H− 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.

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.

MITICA is one of the two ongoing experiments at the ITER neutral beam test facility (NBTF) located in Padua (Italy). MITICA aims to develop the full-size neutral beam injector of ITER and, as such, its Control and Data Acquisition System will adhere to ITER CODAC directives. In particular, its timing system will be based on the IEEE1588 PTPv2 protocol and it will use the ITER time communication network (TCN) on a dedicated Ethernet local-area network. Following the ITER device catalog, the National Instruments PXI-6683H precision time protocol (PTP) timing modules will be used to generate triggers and clocks synchronized with a PTP grandmaster clock (GMC). The "Lazy Trigger" technique, then, will be adopted to implement event-driven data acquisition without the need for any hardware link in addition to the Ethernet connections used to transfer data and timing synchronization. To evaluate the accuracy over time that can be achieved with different network topologies and configurations, a test system has been set up consisting of a GMC, two PXI-6683H devices, and two PTP-aware network switches. In addition, a detailed simulation of the network and the involved devices has been performed using the OMNET++ discrete event simulator. The simulation parameters include not only the network and switches configuration, but also the PID parameters used in the clock servo controllers. A comparison between simulated and measured statistics is reported, together with a discussion of the possible optimal configuration strategies.

The Neutral Beam Test Facility (NBTF), Padua, Italy, is a facility devoted to the test and optimization of the Neutral Beam (NB) Injector for ITER. A full-scale prototype, called MITICA, is under construction at NBTF and has to demonstrate all the design parameters for ITER, among which a stable beam operation at the energy of 1 MeV. To test and optimize the stable high voltage holding of 1 MV, a dedicated campaign is foreseen for 2022–2023, utilizing realistic mockups of MITICA ion source and accelerator. During the campaign, the mockup of the ion source will be biased with an increasing potential up to −1 MV and the discharge toward the grounded electrodes will be observed. In this way, possible weak points of the geometry will be detected and possibly mitigated. As diagnostics for arc discharge position and intensity, a system of cameras was designed, including visible cameras, high-speed camera, and IR camera, placed on the available viewports of beam source vessel, and pointed toward the areas with higher probability of electric discharge. The results of this optical diagnostics will be then compared with the electric measurements of the discharges taken on a dedicated ground electrode, for a cross check. This article describes the motivations of the choice of the type and number of diagnostic cameras adopted during the experiments, the optimization of their layout, which includes also the use of mirrors, and the design of mechanical supports.

The SPIDER experiment is the first of two experiments being held at the ITER Neutral Beam Test Facility in Padova (Italy). SPIDER has been operating since 2018, initially with pulse duration of a few seconds and currently with pulses lasting up to 3000 s. The paper reports the CODAS experience gained after three years of operation. In particular, Data Storage and Data Access adopted strategies will be discussed, that proved to be of high impact in overall system performance and maintainability. Regarding Data Storage strategy, a tradeoff must be defined between the continuous and event driven data acquisition. Continuous data acquisition, i.e. sampling data at a constant frequency, represents the normal operation in short experiments, but can easily lead to an unmanageable amount of data for long lasting experiments. On the other side, Data acquisition at a varying rate, that is increased upon the occurrence of given events leading to an improved signal dynamics, is required for a subset of signals that describe physical phenomena with fast dynamics. Several strategies have been adopted in SPIDER to handle varying rate data acquisition and are discussed here. Considering data access strategy, an important Use Case, especially when the pulse duration is long, is the concurrent access to the pulse file for online analysis and visualization. Concurrent data read and write is supported by MDSplus, the adopted data system, but performance can be affected by the required locks in file access. For this reason it is important to limit as far as possible useless data access. This has been achieved in different ways, such as setting a Region of Interest (ROI) in data access and by the extended usage of on-the-fly resampling in conjunction with the availability in the pulse file of different versions of the same data item, acquired at different sampling speeds.

At the ITER Neutral Beam Test Facility (NBTF), the source for the production of ions of deuterium extracted from radio frequency plasma (SPIDER) has been in operation since 2018 aiming at prototyping the heating and diagnostic neutral beam appliances in view of the ITER demanding requirements for plasma burning conditions and instabilities control. For the sake of safety, machine protection, and efficiency, it is necessary to follow an accurate planning strategy and approval action of the experiment parameter settings. Although the initial tools available for the SPIDER integrated commissioning and early campaigns offered the basic functionality to perform the necessary tasks, there were a set of relevant issues that were identified as needing improvements for an efficient and safe configuration environment. Namely, the fact that there were no tools indicating the parameters change since the previous pulse, or the lack of a comparison tool between a new set of parameters and a previous setup, demanded a tedious and error prone verification of all parameters in the sequence. Moreover, several verifications should be automated according to the machine safety limits, increasing the safety check by human and automated machine (algorithmic) validation. This contribution depicts: 1) the approval sequence designed for SPIDER safe operation; 2) the configuration environment requirements according to the SPIDER pulse preparation procedure; 3) the decision of the development tools used for implementation and design; and 4) the implementation details and preliminary tests of the global environment.

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.

Three years of experiments on SPIDER allowed characterization of the main features of the source plasma and of the negative ion beam, in the original design configuration. For the large dimensions of the source chamber, and of the extraction area, the investigation of the single-beamlet currents and of the source plasma uniformity had to be carried out to extend the knowledge gained in smaller prototype sources. The configuration of the multiple RF drivers and filter field topologies were found to cause a peculiar behavior in the plasma confinement in the drivers, creating left-right asymmetries which were also visible in the extracted negative ion currents, even after the early implementation of a new scheme of plasma-grid current send and return busbars that greatly improved performance at high filter fields. The plasma properties in the driver and expansion region as well as the positive ion energy at the extraction region were studied in different experimental conditions, and interpreted also with the support of numerical models, suggesting that an improved plasma confinement could contribute to the increase of the plasma density, and to a certain extent to a lowering of the plasma potential profile; both effects shall contribute to increase the presence of cold negative ions for the formation of low-divergence beamlets. Early results related to unwanted RF discharges on the back of the plasma source and the gas conductance of the beam source suggested the reduction of the vessel pressure as mitigation, leading to the definition of a new pumping system. The difficulties related to the simultaneous operation, stable control and high-power operation of multiple RF self-oscillating vacuum tube based RF generators were an unambiguous obstruction to the experimentation, calling for the implementation of RF solid-state amplifiers. The initial tests related to caesium management, the non-uniform plasma properties at different locations across the plasma grid, and the challenges in the measurement of the current and divergence of the accelerated beamlet, unambiguously resulted in the need of new diagnostic systems to investigate with better resolution the spatial uniformities. This contribution summarises how the main experimental findings in the previous experimental campaigns are driving modifications to the SPIDER experiment, during the present shut down, in view of future operations.

SPIDER, ITER's full-size beam source at the Neutral Beam Test Facility (NBTF), has been operational since 2018. SPIDER's mission is to optimize the operation of the beam source in order to reuse SPIDER's experience on MITICA, the full-size prototype of the ITER Neutral Beam Injector, and in the ITER injectors. The exploitation of SPIDER started with short, low-performance pulses lasting up to a few seconds and progressed to obtain long pulses up to 3000 seconds. Furthermore, the integration of plant and diagnostic systems has grown over time. The amount of data collected per pulse provide a measure of evolution: it has gone from a few tens of MB in the first campaign pulses to the current average value of over 130 GB, most of which produced by infrared and visible cameras. From the first operation onwards, the control systems have also evolved, including components and functions initially not foreseen or developed in a preliminary form. This includes the progressive integration of plant and diagnostic systems and of protection and safety functions. The paper initially focuses on the architecture of the SPIDER control systems, i.e. CODAS, the system delivering conventional control and data management, the central interlock system, delivering plant protection, and the central safety system delivering people and environment safety. Since we developed all systems according to the ITER guidelines for implementing control systems, the integrated SPIDER control, interlock and safety systems may provide an interesting example for developers of ITER plant systems. The paper then describes how the top-down definition and implementation of operating states and operational scenarios provides the framework for the integration of control, interlock and safety systems and the basic elements for successful operation. Finally, the paper reports the lessons learned during these years of operation with particular attention to the progressive, continuous evolution and recommissioning of systems.

For negative ion beam sources there are several methods of measuring the accelerated beam current, most commonly electrical measurements at the power supply and calorimetric measurements. On SPIDER, the ITER Heating Neutral Beam full-scale beam source prototype, electrical measurements at the acceleration grid power supply (AGPS) are complemented by polarizing the diagnostic calorimeter STRIKE to provide an additional electrical measurement of the accelerated current. This is in addition to the calorimetric measurements provided by STRIKE. These diagnostics give differing measurements of the beam current. Exploiting the reduced number of open apertures on SPIDER a new beam diagnostic has been installed to measure the individual beamlet currents directly. The so called Beamlet Current Monitor (BCM) has been used to measure the current of five beamlets during the most recent SPIDER campaign. This work compares the BCM current to the electrical measurements at the AGPS and STRIKE. The average BCM current agrees well with the STRIKE electrical measurements, indicating that the AGPS overestimates the beam current. The individual beamlets are compared to the STRIKE calorimetric measurements, showing similar current trends with the source parameters.