In the linear theory, the stability of the mode is completely defined by the stability index connected to the change of the magnetic energy of the plasma due to the reconnetion. However, aner the magnetic reconnection, non-linear mechanisms start to play an important role so that the magnetic island grows if the destabilizing (linear and non-linear) drives overcome the stabilizing effects. The contribution of the phenomena which affects the island growth are described in the Generalized Rutherford Eqution (GRE). According with the GRE theory, the curvature of the magnetic field lines in a toroidal configuration has a stabilizing effect, while the reduction of the bootstrap current due to the flattening of the pressure profile inside the island produces the neoclassical destabilization of the magnetic tearing perturbation. In an inhomogeneous two-fluid plasma, difference in the drin motion of electrons and ions drives a parallel return current, called "polarization current", which depends on the island width w and can be either stabilizing or destabilizing depending on the ratio ?/?& where ? and ?& are the island frequency and the ion diamagnetic frequency. The ion polarization current is thought to play an important role at the onset of the mode because of its scaling a 1/w3 that makes it a dominant term in the GRE when the island width is small. The curvature and the bootstrap terms depend on the equilibrium profiles (safety factor, pressure) and the poloidal ?, so that they change following a slower equilibrium timescale. On the other hand, the magnitude of the ion polarization current contribution also depends on equilibrium quantities, but its sign directly depends on the island frequency shin from the diamagnetic frequency, which may respond to faster temperature changes at the rational surface. This suggests that in an essentially stable discharge, a rapid change of the local temperature due to different phenomena (e.g., impurities radiation, other MHD activities...), could lead to a sudden offset of the balance of the competing bootstrap and curvature effects, leading to the onset of the mode, and eventually to disruption. The effect of the ion polarization current is evaluated with a time resolution of the available diagnostic used to estimate the ion temperature (~10ms). Firstly the analysis is performed on four JET pulses, that develop a neoclassical tearing mode in the termination phase of the plasma shot, aner a strong reduction of the temperature at the edge due to the presence of impurities (Edge Cooling). The behavior of the ion polarization current contribution is then compared to stable pulses, which exhibit an edge cooling without developing a mode. The analysis on the unstable pulses shows that the temperature fluctuation due to the edge cooling increases the destabilizing contribution of the ion polarization current. When this happens, it is easier for a resonant helical perturbation of the rational surface to overcome the stabilizing contributions leading to an unstable mode. This is confirmed by the analysis on the stable pulses showing that the destabilizing contribution of the ion polarization current does not increase aner the edge cooling. The reason why the ion polarization current contribution seems not to be affected by the edge cooling is because the reduction due to the temperature fluctuation does not reach the rational surface. This finding has been tested on a more general database of selected pulses with edge cooling. The flattening width is evaluated and compared to the position of the rational surface of every pulse in the database, showing that the mode is triggered whenever the flattening is close enough to the rational surface, producing a result which is consistent with the physical interpretation.

Present contribution aims at comparing the different kinds of disruptions that occurred in the last JET with ITER-like wall (ILW) campaigns with Tritium and Deuterium-Tritium fuels. Last campaigns performed in JET-ILW with a D fuel showed that the majority (around 80%) of disruptions follow two main paths [1]. The first path (temperature hollowing, TH) is strictly related with the influx of high Z impurities, which can accumulate in the plasma core increasing the radiative losses and deteriorating the electron temperature (Te) profile [2]. The second path (edge cooling, EC) is instead related to the erosion of the edge Te profile; the contraction of Te profiles looks similar to that of a "density limit" disruption [3]. Both paths are found [4] to modify differently the current density profile but always in a way to destabilize a 2/1 mode, which locks before disruption. In T and in DT campaigns, around the 90% of disruptions can be explained by the occurrence of TH or EC. Furthermore, it is found that in DT the two main scenarios developed at JET [5] are characterized by disruptions following mainly one out of the two paths. In the DT experiments performed in baseline scenario (?N~1.8, q95~3), 12 disruptions out of 13 follow an EC; while in the DT hybrid scenario (?N~2-3, q95~4) 13 disruptions out of 15 occur after a TH in the ramp-down phase. The two different kinds of disruptions will be compared for the two high performance scenarios and for the different isotopes contents. The comparison will be performed considering the conditions at onset of the 2/1 modes before the disruption, in terms of the power balance and of the density levels.

The capability to terminate plasma pulses safely is an important goal towards the optimization of operational scenarios in tokamaks, so it is of great importance to study the physical phenomena involved in plasma disruptions and to develop precursors for avoidance and/or mitigation actions. The development of tearing modes (TMs) inside the plasma is a major cause of disruptions. It has been shown that there are two main TM destabilization paths in plasma termination on JET, connected to the problem of impurity control: the core accumulation of high-Z impurities, leading to a temperature hollowing (TH) and to a broadening of the current density profile, and the influx of low-Z impurities, which are mainly radiating at the edge, leading to an edge cooling (EC) and to a shrinking of the current density profile. The formation of an "outboard radiating blob" due to high-Z impurities accumulated in the low-field side can also be responsible for EC. Following the picture of TMs generated by changes in the current density profile, reflecting the changes in the electron temperature profile, two parameters have been defined from ECE radiometry to highlight the occurrence of TH and EC, and the possibility of identifying locked mode precursors based on the two parameters has been preliminary explored, by evaluating, for a large dataset of pulses, the characteristic time intervals between the increase of such parameters and the mode lock, which is widely adopted as disruption precursor to trigger mitigation actions. The obtained advances with respect to mode lock signal (1 s for TH, 100 ms for EC) are associated to the effective resistive diffusion time linking the changes in the electron temperature profile and the changes in the current density profile leading to large mode amplitudes. This is the reason why the real-time implementation of such parameters (as already done for a temperature hollowness indicator during the current ramp-up phase) would offer the capability of obtaining alerts before the mode onset, when stability analysis indicate a stable MHD scenario. In particular, the TH parameter could provide alerts useful to attempt to correct the termination, avoiding the disruption, whilst the EC parameter could provide alerts useful to anticipate mitigation actions. Additional information are provided by the dynamics of mode lock signals. Mode saturation is quite general for EC in peaked electron temperature profile and the thermal quench (TQ) is usually induced by disruption mitigation valve (DMV) intervention, so it is not crucial to anticipate DMV. However, an explosive growth of the mode amplitude is sometimes observed for EC in hollow electron temperature profiles, leading in some cases to unmitigated TQ, so even a small advance in DMV intervention would be essential. A detailed analysis of this behaviour is planned in view of ITER, where the unmitigated disruption rate should be reduced as much as possible. The possibility of combining information on electron temperature and radiation profiles is also discussed, and examples from recent DT and isotopic experiments on JET are shown.

The JET 2019-2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major neutral beam injection upgrade providing record power in 2019-2020, and tested the technical and procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle (?) physics in the coming D-T campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed shattered pellet injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILWplasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design and operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and D-T benefited from the highest D-D neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER.

The analysis of the current ramp-down phase of JET plasmas has revealed the occurrence of additional magnetic oscillations in pulses characterized by large magnetic islands. The frequencies of these oscillations range from 5 to 20 kHz, being well below the toroidal gap in the Alfvén continuum and of the same order as the low-frequency gap opened by plasma compressibility. The additional oscillations only appear when the magnetic island width exceeds a critical threshold, suggesting that the oscillations could tap their energy from the tearing mode (TM) by a non-linear coupling mechanism. A possible role of fast ions in the excitation process can be excluded, being the pulse phase considered in the observations characterized by very low additional heating. The calculation of the coupled Alfvén-acoustic continuum in toroidal geometry suggests the possibility of beta-induced Alfvén eigenmodes (BAEs) rather than beta-induced Alfvén-acoustic eigenmodes. As a main novelty compared to previous work, the analysis of the electron temperature profiles from electron cyclotron emission has shown the simultaneous presence of magnetic islands on different rational surfaces in pulses with multiple magnetic oscillations in the low-frequency gap of the Alfvén continuum. This observation supports the hypothesis of different BAE with toroidal mode number n = 1 associated with different magnetic islands. As another novelty, the observation of magnetic oscillations with n = 2 in the BAE range is reported for the first time in this work. Some pulses, characterized by slowly rotating magnetic islands, exhibit additional oscillations with n = 0, likely associated with geodesic acoustic modes (GAMs), and a cross-spectral bicoherence analysis has confirmed a non-linear interaction between TM, BAE and GAM, with the novelty of the observation of multiple triplets (twin BAEs plus GAM), due to the simultaneous presence of several magnetic islands in the plasma.

An overview of recent results obtained at the tokamak ASDEX Upgrade (AUG) is given. A work flow for predictive profile modelling of AUG discharges was established which is able to reproduce experimental H-mode plasma profiles based on engineering parameters only. In the plasma center, theoretical predictions on plasma current redistribution by a dynamo effect were confirmed experimentally. For core transport, the stabilizing effect of fast ion distributions on turbulent transport is shown to be important to explain the core isotope effect and improves the description of hollow low-Z impurity profiles. The L-H power threshold of hydrogen plasmas is not affected by small helium admixtures and it increases continuously from the deuterium to the hydrogen level when the hydrogen concentration is raised from 0 to 100%. One focus of recent campaigns was the search for a fusion relevant integrated plasma scenario without large edge localised modes (ELMs). Results from six different ELM-free confinement regimes are compared with respect to reactor relevance: ELM suppression by magnetic perturbation coils could be attributed to toroidally asymmetric turbulent fluctuations in the vicinity of the separatrix. Stable improved confinement mode plasma phases with a detached inner divertor were obtained using a feedback control of the plasma ?. The enhanced D ? H-mode regime was extended to higher heating power by feedback controlled radiative cooling with argon. The quasi-coherent exhaust regime was developed into an integrated scenario at high heating power and energy confinement, with a detached divertor and without large ELMs. Small ELMs close to the separatrix lead to peeling-ballooning stability and quasi continuous power exhaust. Helium beam density fluctuation measurements confirm that transport close to the separatrix is important to achieve the different ELM-free regimes. Based on separatrix plasma parameters and interchange-drift-Alfvén turbulence, an analytic model was derived that reproduces the experimentally found important operational boundaries of the density limit and between L- and H-mode confinement. Feedback control for the X-point radiator (XPR) position was established as an important element for divertor detachment control. Stable and detached ELM-free phases with H-mode confinement quality were obtained when the XPR was moved 10 cm above the X-point. Investigations of the plasma in the future flexible snow-flake divertor of AUG by means of first SOLPS-ITER simulations with drifts activated predict beneficial detachment properties and the activation of an additional strike point by the drifts.

The tokamak a configuration variable (TCV) continues to leverage its unique shaping capabilities, flexible heating systems and modern control system to address critical issues in preparation for ITER and a fusion power plant. For the 2019-20 campaign its configurational flexibility has been enhanced with the installation of removable divertor gas baffles, its diagnostic capabilities with an extensive set of upgrades and its heating systems with new dual frequency gyrotrons. The gas baffles reduce coupling between the divertor and the main chamber and allow for detailed investigations on the role of fuelling in general and, together with upgraded boundary diagnostics, test divertor and edge models in particular. The increased heating capabilities broaden the operational regime to include T (e)/T (i) similar to 1 and have stimulated refocussing studies from L-mode to H-mode across a range of research topics. ITER baseline parameters were reached in type-I ELMy H-modes and alternative regimes with 'small' (or no) ELMs explored. Most prominently, negative triangularity was investigated in detail and confirmed as an attractive scenario with H-mode level core confinement but an L-mode edge. Emphasis was also placed on control, where an increased number of observers, actuators and control solutions became available and are now integrated into a generic control framework as will be needed in future devices. The quantity and quality of results of the 2019-20 TCV campaign are a testament to its successful integration within the European research effort alongside a vibrant domestic programme and international collaborations.

Since the 2018 IAEA FEC Conference, FTU operations have been devoted to several experiments covering a large range of topics, from the investigation of the behaviour of a liquid tin limiter to the runaway electrons mitigation and control and to the stabilization of tearing modes by electron cyclotron heating and by pellet injection. Other experiments have involved the spectroscopy of heavy metal ions, the electron density peaking in helium doped plasmas, the electron cyclotron assisted start-up and the electron temperature measurements in high temperature plasmas. The effectiveness of the laser induced breakdown spectroscopy system has been demonstrated and the new capabilities of the runaway electron imaging spectrometry system for in-flight runaways studies have been explored. Finally, a high resolution saddle coil array for MHD analysis and UV and SXR diamond detectors have been successfully tested on different plasma scenarios.

The TCV tokamak is augmenting its unique historical capabilities (strong sh aping, strong electron heating) with ion heating, additional electron heating compatible with high densities, and variable divertor geometry, in a multifaceted upgrade program desig ned to broaden its operational range without sacrificing its fundamental flexibility. The TCV program is rooted in a three-pronged approach aimed at ITER support, explorations towards DEMO, and fundamental research. A 1-MW, tangential neutral beam injector (NBI) was recently installed and promptly extended the TCV parameter range, with record ion temperatures and toroidal rotation velocities and measurable neutral-beam current drive. ITER-relevant scenario development has received particular attention, with strategies aimed at maximizing performance through optimized discharge tr ajectories to avoid MHD instabilities, such as peeling-ballooning and neoclassical tearing modes. Experiments on exhaust physics have focused particularly on detachment, a necessary step to a DEMO reactor, in a comprehensive set of conventional and advanced divertor concepts. The specific theoretical prediction of an enhanced radiation region between the two X -points in the low-field-side snowflake-minus configuration was experimentally confirmed. Fundamental investigations of the power decay length in the scrape -off layer (SOL) are progressing rapidly, again in widely varying configurations and in both D and He plasmas; in par ticular, the double decay length in L-mode limited plasmas was found to be replaced by a single length at high SOL resistivity. Ex periments on disruption mitigation by massive gas injection and electron-cyclotron resonance heating (ECRH) have begun in earnest, in parallel with studies of runaway electron generation and control, in both stable and disruptive conditions; a quiescent runaway beam carrying the entire electrical current appears to develop in some cases. Developments in plasma control have benefited from progress in individual controller design and have evolved steadily towards controller integration, mostly within an environment supervised by a tokamak profile control simulator. TCV has demonstrated effective wall conditioning with ECRH in He in support of the preparations for JT-60SA operation.

Contributo alla ideazione, progettazione e realizzazione del nuovo sitoweb dell'ISTP.

The termination of high performance plasmas in tokamak devices with high Z metal plasma facing components presents challenges related to the influx of heavy impurities which, if not kept under control, cause an increase of the radiative losses, radiative cooling and high probability of disruption. A number of key players in these dynamics have been identified by intensive research performed after the first years of operation in machines as AUG and JET in preparation of ITER operation. Inward neoclassical convection related to the peaking of the density profile, poloidal asymmetries, plasma rotation and centrifugal effects, temperature screening, pedestal temperature, pedestal density and ELMs control are among them. The objective of D-T fuelled plasmas with high neutron yield in stationary conditions, foreseen in the near future at JET, focuses the operations towards high performance in terms of thermal energy content and plasma current and consequently with higher disruption risk. The reduction of such risks is being pursued for the specific features of the two plasma scenarios being developed, baseline (?N ~1.8, q95 ~ 3) and hybrid (?N ~2-3, q95~4). The analysis of the previous experimental campaign and the data so far collected in the present campaign indicate that the combination of edge and core W control is needed to obtain a safe plasma termination, with the optimized use of the available actuators: gas and pellet for ELMs control, ramp-down waveform of the NBI heating power while maintaining a relevant ICRH additional power, sweeping of the separatrix hitting point on the divertor to reduce the heat load and to decrease the W source.

The termination of high performance plasmas in tokamak devices with high Z metal plasma facing components presents challenges related to the influx of heavy impurities which, if not kept under control, cause an increase of the radiative losses, radiative cooling and high probability of disruption. A number of key players in these dynamics have been identified by intensive research performed after the first years of operation in achines as AUG and JET in preparation of ITER operation. Inward neoclassical convection related to the peaking of the density profile, poloidal asymmetries, plasma rotation and centrifugal effects, temperature screening, pedestal temperature, pedestal density and ELMs control are among them. The objective of D-T fuelled plasmas with high neutron yield in stationary conditions, foreseen in the near future at JET, focuses the perations towards high performance in terms of thermal energy content and plasma current and consequently with higher disruption risk. The reduction of such risks is being pursued for the specific features of the two plasma scenarios being developed, baseline (?N ~1.8, q95 ~ 3) and hybrid (?N ~2-3, q95~4). The analysis of the previous experimental campaign and the data so far collected in the present campaign indicate that the combination of edge and core W control is needed to obtain a safe plasma termination, with the optimized use of the available actuators: gas and pellet for ELMs control, ramp-down waveform of the NBI heating power while maintaining a relevant ICRH additional power, sweeping of the separatrix hitting point on the divertor to reduce the heat load and to decrease the W source.

Studies of the effects of hydrogen (H) isotopes are an important issue on achievement of high performance in the next Deuteriun-Tritium (DT) and Tritium-Tritium (TT) high beta operations at JET with ITER-like Wall. Experiments with mixed H-D plasmas have been recently performed at JET to understand the dependence onthe confinement on these isotopes varying the gas, beam and pellet fuelling. Since strong dependence on confinement has been found in mixed plasmas close to pure values of H or D and nodependence for other values of the isotope mixture, while opposite behavior is expected for any DT composition, the presence in these scenarios of magnetohydrodynamic instabilities could still play a key role in the confinement loss. Particularly, the Neoclassical Tearing Modes (NTMs) are responsible of a decrease of performances leading in some cases to disruptions. The goal of this work is to predict the effects of different isotope mixtures on NTMs onset and dynamics for the plasma stability, depending the modes appearance on the isotope ion mass mi . The NTM modelling of the island width evolutionis performed using in the NTM module,integrated in the European Transport Simulator (ETS),a generalized Rutherford equation where the bootstrap and the ionpolarization terms contain the mi dependence through the ion collision frequency and the ion Larmor radius. Dependence of sawtooth (ST) period on the isotopes is investigated as well, because NTMs can be triggered after a crash of a sawtooth period, occurring when the magnetic shear becomes larger than a critical one (s1,cr). TheST periods in H are smaller than in D, because s1,cr(H) < s1,cr(D) , as observed in isotope identity experiments. As the mode onset is usually observed after a crash of a long ST period, the NTMs destabilization should be more favourable in D plasmas than in H and more in T than in D leading to more confinement degradation. This isotope dependenceof the periodof sawteeth is investigated.

In this work the onset of tearing modes in the termination phase of plasma pulses on JET is investigated. It is shown that the broadening or the shrinking of the current density profile, as a consequence of a core hollowing or an edge cooling of the electron temperature profile, strongly increases the probability of destabilizing a 2/1 tearing mode also in absence of an external trigger (e.g. a sawtooth crash). Two parameters are defined to highlight changes in the shape of the temperature profile that can lead to MHD instabilities and an empirical stability diagram is introduced into the space of the two new parameters. A large data-set of pulses carried out in the high-current scenario at JET with ITER-like wall is analyzed and criteria for the development of disruption alerts based on the two risk indicators for MHD instabilities are discussed, taking into account the different dynamics of the observed phenomena leading to the onset of 2/1 tearing modes.

This paper presents the Mechanical Ventilator Milano (MVM), a novel intensive therapy mechanical ventilator designed for rapid, large-scale, low-cost production for the COVID-19 pandemic. Free of moving mechanical parts and requiring only a source of compressed oxygen and medical air to operate, the MVM is designed to support the long-term invasive ventilation often required for COVID-19 patients and operates in pressure-regulated ventilation modes, which minimize the risk of furthering lung trauma. The MVM was extensively tested against ISO standards in the laboratory using a breathing simulator, with good agreement between input and measured breathing parameters and performing correctly in response to fault conditions and stability tests. The MVM has obtained Emergency Use Authorization by U.S. Food and Drug Administration (FDA) for use in healthcare settings during the COVID-19 pandemic and Health Canada Medical Device Authorization for Importation or Sale, under Interim Order for Use in Relation to COVID-19. Following these certifications, mass production is ongoing and distribution is under way in several countries. The MVM was designed, tested, prepared for certification, and mass produced in the space of a few months by a unique collaboration of respiratory healthcare professionals and experimental physicists, working with industrial partners, and is an excellent ventilator candidate for this pandemic anywhere in the world.

Disruption avoidance strategies for DEMO.

Recently, during Collective Thomson Scattering (CTS) measurements at mm-waves aimed at studying the ion dynamics in fusion plasmas, strong signals of scattering of the injected beam with non-CTS origin have been detected. A possible explanation of these signals in terms of parametric decay instabilities (PDIs) of the injected wave with power threshold much lower than previously envisaged by theory was proposed [1, 2]. The experimental activity with CTS diagnostic at FTU is aimed at two purposes: the characterization of the thermal ion distribution function and the investigation of the possible low power PDIs processes foreseen by the recent models. In order to ease the data analysis, a set of data processing tools has been integrated on purpose, with an activity started in 2014. Here we present the last implementation of an integrated data analysis tool, aimed at the investigation of the signals detected with the CTS diagnostic. The last version of the software integrates information included in the raw spectra of scattered radiation with the modeled ECE emission, with the aim of providing calibrated spectra improved with respect to the ones provided up to now. The correct calibration of the signals on the real line of sight of the beams is helpful to better distinguish anomalous emissions from less powerful CTS radiation. In addition, the analysis tool compares the calibrated spectra with the ones predicted considering real scattering parameters evaluated by means of realistic beam-trajectories, changing during the pulse, allowing also to extract information on ion dynamics and plasma composition. The last version of the software, which takes into account also multi-reflection beam-tracing simulations in both polarization modes in support to the scattering experiments, is presented.

Disruptions represent the highest concern for next-step fusion devices based on the tokamak principle. Active disruption avoidance and off-normal event handling need to be developed in Plasma Control Systems (PCS) to predict and react when the plasma approaches dangerous operational boundaries. EUROfusion programme has put strong emphasis on disruption research, focusing on their mitigation and prevention, as well as on the study of relevant disruption paths, such as H-mode density limits. Future fusion power plants are foreseen to operate at high densities in the high confinement mode (H-mode). At densities close to the Greenwald limit, the plasma can exhibit a back transition from the H-mode to the low confinement mode (L-mode). In addition to confinement degradation, a radiation instability, the MARFE, can develop, showing a poloidally localized volume of "cold" and dense plasma. The onset of this edge radiative instability is a non-negligible issue during plasma current ramp down on devices like ITER and DEMO. After the installation of the baffles in the divertor, the development of the H-mode density limit on TCV has been found to be consistent with the one observed on ASDEX Upgrade (AUG). Similarly, to the framework implemented in this latter, a disruption avoidance tool to handle H-mode density limits has been ported to TCV and integrated in the real-time control system. Such an approach relies on a proximity measure with respect to the operational boundary defined in the H98y,2 vs edge normalized density state space, and allows identifying the energy confinement degradation with increasing density that is associated with approaching the density limit, MARFE formation and disruption. Main concepts and schemes for active avoidance of the density limits will be described with reference to the generic framework implemented in the two machines to handle off-normal events potentially leading to disruption.