Institute for the Science and Technology of Plasmas

Runaway electron (RE) interaction with plasma-facing components (PFCs) has been documented to lead to deep volumetric melting and thermal shock driven material explosions followed by extensive wall cratering. This work reports a post-mortem FTU investigation that covers the primary localized RE-induced damage directly caused by beams striking poloidal or toroidal molybdenum (Mo)-based limiters and the subsequent secondary non-localized RE-induced damage inflicted on nearby limiter tiles by the mechanical impact of fast up to ∼ 1 km/s solid debris violently ejected during the direct RE-PFC interaction. Early indications on the resilience of tin liquid limiters to RE incidence are also presented. • Post-mortem study of localized and non-localized aspects of RE-induced PFC-damage in FTU. • Morphological and mechanical analysis of primary damage on poloidal and toroidal limiters of FTU. • Morphological analysis of secondary damage to nearby PFCs due to the impact of fast solid dust. • Preliminary observation that the tin liquid limiter is not severely damaged by RE incidence. • Despite the severity of RE-induced PFC damage, there was no significant effect on FTU operation.

Abstract At JET, Deuterium-Helium-3 (D- 3 He) experiments using ion cyclotron resonance heating (ICRH) with the three-ion scheme were conducted, involving thermal Deuterium (D), thermal Helium-3 ( 3 He), and a fast D beam injected via Neutral Beam Injection (NBI) and further heated using ICRH. In these plasmas, intense neutron emission at the level of ≈ 10 16 n/s was observed, and a non-thermal neutron spectrum was measured with the JET time of flight neutron spectrometer. Furthermore, the spatial profile of ≈ 16.4 MeV gamma rays originating from the 3 He(D,γ) 5 Li reaction was observed. This is a weak branch (≈ 10 -5 relative probability) of the more common 3 He(D,p)α reaction and it can be used to assess the production of alpha particles in these experiments. In this work we analyse a set of data from nuclear diagnostics in D-D NBI - 3 He experiments at JET and compare them with synthetic diagnostics based on TRANSP simulations of the fast deuterium distribution function, using the so-called "kick operator" to model power transfer from the wave to the beam fast deuterons. The neutron rate predicted by the TRANSP simulations broadly agrees with data measured with fission chambers. The shape of the neutron emission spectrum is also well understood, once finite Larmor radius effects are introduced into calculations. Data from the gamma ray camera are used to infer the alpha birth profile through tomographic reconstructions based on the TREVISO code. These were compared with corresponding calculations starting from TRANSP results and showing an overall good agreement with the experimental tomographies. The validated TRANSP results are finally used to discuss the modifications of the fast deuterium distribution function and of the alpha birth profile in a set of discharges.

Abstract The Magnetospheric Multiscale (MMS) mission's extended phase introduced an “unbiased campaign” that provided the first high‐resolution data set of the terrestrial magnetosheath obtained without human selection, across complete crossings from the bow shock to the magnetopause. This is achieved by collecting 3 min of burst‐mode data every 9 min. Leveraging this novel data set, we report average plasma parameters and observe the emergence of a range in the magnetic field power spectrum. A distinct ordering of both bulk and turbulence quantities is demonstrated when the Alfvénic Mach number is used as a proxy for the distance from the bow shock. This ordering enables us to obtain clear insights into the occurrence of small‐scale current sheets throughout the magnetosheath by evaluating magnetic field kurtosis. Furthermore, the evolution of the correlation time is opposite to that of kurtosis and aligns with the principle of turbulence relaxation. This information is dispersed when quantities are sorted by geocentric distance. Crucially, striking differences emerge in turbulence‐related quantities when comparing unsupervised and supervised data sets. These findings provide valuable insights into magnetospheric turbulence, establishing the campaign as a critical resource for unbiased statistical analysis of this environment.

Abstract In the last few decades, lanthanum halide scintillators such as LaBr3(Ce), LaBr3(Ce+Sr) and LaCl3(Ce) have been extensively used for gamma ray spectroscopy measurements. They offer excellent energy resolution (about 3-4 % at 662keV) and fast scintillation time, allowing high counting rate measurements. These scintillators find applications in many research fields such as nuclear physics, astrophysics, nuclear fusion and nuclear medicine. More recently, thanks to the exploitation of the n-35Cl nuclear reaction channel, LaCl3(Ce) crystals have been used as neutron spectrometers. This opened up the opportunity to explore pulse shape discrimination algorithms for enabling the neutron/gamma discrimination. In this work, an innovative algorithm for particle discrimination applied to lanthanum halide scintillators is presented. The effectiveness of the algorithm has been demonstrated first on the intrinsic radioactivity measurement of the crystals, which is characterized by gamma ray emissions from 138La decay and alpha decays due to actinides contamination, and then with fast neutrons. The particle discrimination feature allows for a better description of the intrinsic activity of the crystal and improved background suppression when performing gamma ray spectroscopy measurements in low cross-section measurements or in a mixed gamma-neutron radiation field.

SPARC is a high-field, compact tokamak aiming to obtain a fusion gain Q > 1 in Deuterium-Tritium (DT) plasmas for the first time in a magnetic confinement fusion reactor. Fusion power measurement is a key diagnostic requirement. In this work we present a scoping study of a gamma-ray spectrometer for the determination of the fusion power at SPARC. Measurements are based on the detection of the 16.7 MeV and ∼ 14 MeV gamma-rays produced by the weak secondary channel T ( D , γ ) 5 He of the DT fusion reaction, whose branching ratio has been recently measured at the JET tokamak. Starting from the numerical calculation of the DT gamma source and a first evaluation of the sources of background radiation, we determine the fusion power range where measurements are possible at SPARC. We find that the fusion power can be determined in the broad range of 0.2 MW to 140 MW, when different neutron and gamma-ray attenuators are employed depending on the operational conditions. Potential additional applications of the gamma-ray detector scoped in this work are also briefly discussed. • Feasibility study of DT-gamma–based fusion power measurement capability at SPARC. • The method employs a LaBr 3 :Ce detector and suitable neutron and gamma attenuators. • Fusion power can be measured at SPARC in the range 0.2 MW to 140 MW.

Magnetic switchbacks are fluctuations in the solar wind in which the interplanetary magnetic field sharply deflects away from its background direction so as to create folds in magnetic field lines while remaining of roughly constant magnitude. The magnetic field and velocity fluctuations are extremely well correlated in a way corresponding to Alfvénic fluctuations propagating away from the Sun. For a background field which is nearly radial this causes an outwardly propagating jet to form. Switchbacks and their characteristic velocity jets have recently been observed to be nearly ubiquitous by Parker Solar Probe with in situ measurements in the inner heliosphere within 0.3 AU. Their prevalence, substantial energy content, and potentially fundamental role in the dynamics of the outer corona and solar wind motivate the significant research efforts into their understanding. Here we review the in situ measurements of these structures (primarily by Parker Solar Probe). We discuss how they are identified and measured, and present an overview of the primary observational properties of these structures, both in terms of individual switchbacks and their collective arrangement into “patches”. We identify both properties for which there is a strong consensus and those that have limited or qualified support and require further investigation. We identify and collate several open questions and recommendations for future studies.

Diagnostic Neutral Beam Injectors (DNBI), through the combined use of Charge Exchange Recombination Spectroscopy (CHERS) and Motional Stark effect diagnostics (MSE), are a well-known tool to access important information about magnetically confined plasmas, such as radial profiles of ion temperature, ion flow, impurity content and intensity and direction of the magnetic field. For this purpose, a DNBI was installed and operated in the RFX-mod experiment, which was designed to confine plasma mainly through the Reversed Field Pinch configuration. The DNBI, designed and built by the Budker Institute of Nuclear Physics (BINP), was based on a source of positive hydrogen ions, accelerated to 50 keV and for a maximum ion current of 5 A. The beam could be modulated and the maximum overall duration was 50 ms. With the upgrade of RFX-mod to the present RFX-mod2 machine, the DNBI is being renovated to solve several power units faults and improve the overall reliability of the system. The 50 kV power supply is being improved, as well as the power supplies in the high voltage deck and its insulation transformer. Magnetic field survival tests were performed on the toroidal-core-based DC–DC converters that should power the electronic boards in a reliable way. The control system, originally based on CAMAC technology, was redesigned to be fully replaced. This contribution reviews the technical criticalities emerged in the DNBI check-up and the new solutions adopted to make the DNBI operative and more reliable. • A Diagnostic Neutral Beam Injector (DNBI), in combination with optical diagnostics, can provide crucial measurements for Reversed Field Pinch Plasmas. • The DNBI at the RFX-mod2 experiment is in phase of extraordinary maintenance and upgrade. • New circuits and devices were tested. • The electrical systems and the control system are being redesigned in order to be more reliable and robust.

The EUropean DEMOnstration power plant (EU-DEMO) project, a EUROfusion initiative, seeks to advance fusion power technology by developing a reliable Electron Cyclotron (EC) heating and current drive system capable of delivering up to 130 MW of auxiliary heating power presently through six equatorial ports. This paper introduces an innovative alternative to the baseline ex-vessel waveguide (EW) system design, which could be major for effective EC operations. The proposed design reduces vacuum-sealed interfaces significantly — from 173 to seven — minimizing failure points and reducing the need for extensive monitoring in confined port spaces. Enhanced for remote maintainability, the design simplifies maintenance tasks and mitigates vacuum breach risks. The vacuum dimple-plated casing, acting as the primary confinement boundary, ensures nuclear safety and enables integration with current infrastructure, while addressing challenges like dynamic loads and vessel movements to maintain structural integrity. These advancements indicate that the modular and sealless waveguide approach offers a more reliable, maintainable solution. • Modular design approach ensures remote maintainability under high-radiation conditions. • Sealless waveguides cut vacuum-sealed interfaces by 96% for minimal failure points. • Dimpled-plate casing streamlines integration within the tokamak environment.

Abstract This study presents a detailed investigation of a nanosecond-pulsed volume dielectric barrier discharge (VDBD) plasma in saturated humid air at atmospheric pressure, combining experimental diagnostics with kinetic modeling. Electric field-induced second harmonic generation (EFISH), employing a spatial filter and a picosecond laser, enabled electric field measurements with high spatial (270 µm) and temporal (28 ps) resolution at three distinct positions across the inter-electrode VDBD gap. These measurements revealed previously unobserved features and consistent differences between the discharge center and regions near the ground and high-voltage (HV) electrodes. Near the ground electrode, a higher electric field was observed prior to the first positive plasma discharge, along with an overshoot up to 50 kV cm−1 during plasma breakdown, absent at the center and near the HV electrode. A sharper post-breakdown decrease and recovery in the electric field was also observed near the ground electrode, differently from the other positions. This pattern inverted during the subsequent negative discharge at the end of the HV pulse, with the higher electric field being measured near the HV electrode. To complement the electric field data, the concentrations of nitric oxide (NO) and hydroxyl radical (OH) were measured using laser-induced fluorescence (LIF), providing insight into their temporal dynamics during and after the discharge. The EFISH data served as input for a zero-dimensional kinetic model, which revealed the dynamics of both charged and neutral species, showing excellent agreement with the LIF data and validating the model accuracy. The simulation identified OH−, O 2 − , and O 4 − as the key negative ions responsible for the stable electric field after plasma breakdown. This research highlights the importance of understanding electric field dynamics and reactive species formation in low-temperature plasmas. Integrating EFISH and LIF data with kinetic modeling provides a robust framework to optimize plasma-based systems, improving their performance and safety in practical applications.