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Reconnection events in high current reversed field pinch plasmas are often associated to the partial or total transitions from a helical topology with conserved flux surfaces to a configuration characterized by a chaotic magnetic field. The electron temperature dynamic together with the magnetic energy reconstructions are used to evaluate the energy balance during these events and to quantify the associated dissipated power and released energy. A fraction of the energy released during reconnection events is involved in ion heating, as estimated by the energy distribution function of neutral atoms, a rather interesting feature in a reactorial perspective.

The RFX-mod Reversed Field Pinch device passive boundary is being improved: - Drastic reduction of resistivity of first shell surrounding the plasma; - Reduction of plasma-stabilizing conductor distance from b/a=1.11 to b/a=1.04. The RFX-mod core upgrades consist of: - Removal of Inconel vacuum vessel; - Modification of the stainless steel Support Structure to ensure Vacuum Tightness (VTSS); - Modification of the copper Passive Stabilizing Shell (PSS); - Installation of upgraded sensors inside the vacuum vessel. Initial main points of investigation in the new device are discussed.

The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal plasma facing components. As regards impurity transport research, a number of working lines exploring several recently discovered effects have been developed: the effect of tangential drifts on stellarator neoclassical transport, the impurity flux driven by electric fields tangent to magnetic surfaces and attempts of experimental validation with Doppler reflectometry of the variation of the radial electric field on the flux surface. Concerning gyrokinetic simulations, two validation activities have been performed, the comparison with measurements of zonal flow relaxation in pellet-induced fast transients and the comparison with experimental poloidal variation of fluctuations amplitude. The impact of radial electric fields on turbulence spreading in the edge and scrape-off layer has been also experimentally characterized using a 2D Langmuir probe array. Another remarkable piece of work has been the investigation of the radial propagation of small temperature perturbations using transfer entropy. Research on the physics and modelling of plasma core fuelling with pellet and tracer-encapsulated solid-pellet injection has produced also relevant results. Neutral beam injection driven Alfvénic activity and its possible control by electron cyclotron current drive has been examined as well in TJ-II. Finally, recent results on alternative plasma facing components based on liquid metals are also presented., Nuclear Fusion, 59 (11), ISSN:0029-5515, ISSN:1741-4326

Sheared radial electric field is an important parameter in regulating cross-field transport in fusiondevices. The edge and scrape-off layer (SOL) turbulence coupling, as well as the parallel dissipation,are responsible for the SOL width maintenance, which, in turn, is a critical parameter for particle andenergy exhaustion in tokamaks \cite{Grenfell:2019}. This coupling is usually attributed to the trans-port of streamers or blobs (also called filaments). They are mostly created in the plasma edge, in highgradient free energy regions, and transported radially by ExB drift. The turbulent filaments prop-erties, e.g. velocity and size, and ultimately their penetration into the SOL, are defined by the typeof instability that generates them as well as how they interact with the background \cite{Myra:2013}.High beta filaments, on the other hand, tend to be more resilient to the background. This is thecase of the Edge-Localized Mode filaments in H-mode regimes, one of the current threats for thefeasibility of future fusion reactors. In this work, we study the filament or blob cross-field transportin the RFX-mod operating as tokamak in different plasma regimes by using a set of electrostatic andmagnetic probes \cite{Grenfell:2018}. It has been shown recently that H-mode ELM-free and ELMyare induced in the RFX-mod tokamak by electrode biasing technique \cite{Spolaore:2017}. We showthat the properties of filaments in these regimes, as well as in L-mode, are distinct. While in L-modeblobs propagate almost freely into the SOL scaling as sheath-connected filaments \cite{Manz:2013},in the ELM-free H-mode, filaments are trapped (their cross-field velocity approaches to zero) and dis-sipated around the plasma boundary, where the ExB shearing rate is marginally comparable to theireffective convective time \cite{Grenfell:2018}. ELMs filaments propagate further into the SOL carry-ing a larger parallel current density associated with the edge transport barrier relaxation.

Reconnection is a basic phenomenon in magnetized plasmas by which the plasma relaxes to a state oflower magnetic energy. It plays a fundamental role in a variety of instabilities common to most fusiondevices, as sawtooth or tearing modes. Often it occurs in impulsive events (crash phase), following alonger period in which the magnetic fields evolve slowly. Energy conversion and topological change are two highly linked aspects of reconnection.Hot fusion plasmas may contain multiple reconnection sites and this is particularly true for the Reversed Field Pinch (RFP), due to the peculiar current density distribution, associated to a variety ofresonant tearing modes. In addition, large RFPs involve large plasma currents and therefore a largeamount of available magnetic energy. This makes the RFP a good device to observe and analyze reconnection events.In this contribution the phenomenology of the impulsive and periodic reconnections in the RFX-mod device is analyzed in detail, through an ensemble average of the data from many reconnection eventsin high plasma current discharges (1.5 MA). Spatial and temporal scales are invoked as fundamental observables. Roughly speaking, an almost stationary helical state is considered typical of phases before the crash. The crash destroys this symmetry, and the plasma topology undergoes a transition to a more chaotic state with broad MHD spectra. The crash evolution is described as a sequence of events. A trigger of the crash has been associated for the first time to a slight change in the helical equilibrium, which pushes closer the inner most important resonant flux surfaces in the core. Reconnection starts when the resonant surfaces - and therefore the intense related current sheets- are close enough and a precursor of the crash is found in the locking in phase of these resonant modes. After these slowly evolving phases, fast reconnection takes place, which lasts about 1 ms.Plasma parameters and magnetic measurements suggest that crashes are related with the interaction between tearing modes at different radii, and the reconnection process evolves according to the safety factor profile.

The onset of Internal Transport Barriers (ITBs) is routinely observed during Quasi Single Helicity States in the Reversed Field Pinch experiment RFX-mod. ITBs appear as steep electron temperature gradients that enclose thermal structures. The characteristics of such thermal structures vary considerably, so that there are both narrow thermal structures enclosed by steep gradients and large thermal structures enclosed by milder gradients. The complexity increases considering that, while large thermal structures develop on helical states with a single helical axis (SHAx), narrow thermal structures can have both a single or double magnetic axis (DAx). In this paper we characterize the behavior of thermal structure width Wth and of ITB gradients, estimated from 84-point Thomson Scattering profiles, and of their helical equilibrium. Furthermore we present data analyses that show as Wth and the ITB gradients have a strong relationship with the magnetic turbulence of secondary modes [1,2]. In particular, as long as the secondary modes decrease, the DAx and SHAx narrow thermal structures widens (Wth increases) and their gradients become increasingly steeper. However, despite the very low value of secondary modes, the SHAx wide thermal structures exhibit gradients clearly milder than the gradients of DAx and SHAx narrow cases. The Wth behavior is quantitatively analyzed introducing the Quasi Separatrix Layer concept, used in astrophysics to explain the occurrence of solar flares in the absence of separatrix. This model provides also a qualitative explanation for the peculiar behavior of gradients.

In toroidal magnetic confinement fusion research, 1D transport models rely on one radial coordinate that labels nested toroidal flux surfaces. The presence of magnetic islands in the magnetic geometry does not impede making 1D transport calculations if the island regions are excluded and then, if necessary, treated separately. In this work we show a simple way to modify the flux-surface coordinate and corresponding metric coefficients when an island region is excluded. Comparison with the metrics obtained from Poincaré plots are shown, as well as applications to two types of plasma: Heliac (TJ-II, CIEMAT, Spain), where the geometrical effects alone cannot explain the experimental results when islands move throughout minor radius; and Heliotron (LHD, NIFS, Japan), where we estimate the effect of possible heat losses in flux-gradient relations.

One dimensional (1D) energy and particle di ffusion in a magnetized plasma rely on the presence of nested flux surfaces, which, fro m the geometrical point of view, provide a monotonic radial coordinate from the magnetic axis to the plasma edge. Resonant layers in the plasma may lead to the growth of magnetic islands, with the consequent breaking of the magnetic flux surfaces in that region of the plasma. This brings to multiple magnetic axes in the plasma domain, and to an intrinsically thre e dimensional (3D) nature of the transport problem. Since 1.5 transport codes need a monotonic radial coordinate, they cannot manage topologies with more than one ma gnetic axis; on the other side a proper fully 3D treatment of the problem is difficult and CPU-time demanding . In order to preserve a 1D geometrical description, a piecewise treatment where different radial coordi nates are defined in different domains around their own magnetic ax es has been developed [1]. In this contribution we present an altern ative method [2] based on the idea that the presence of magnetic islands does not impede making 1D transport calculations if the island region is excluded and then, eventually, treated separately. We present a simple way to modify the geometry (radial coordinate and relate d metric coefficients) in order to exclude the island region from the 'principal' plasma, wher e 1D transport is considered. The method would equally apply to a fully chaotic region, excluded from the conserved plasma. Comparison with the metrics obtained from Poincaré plots (in presence of magnetic islands) are shown, as well as applications to two types of plasma using the ASTRA transport shell [3] with appropriately modified metrics: Heliac (TJ-II, CIEMAT, Spain), and Heliotron (LHD, NIFS, Japan).

In magnetically confined experiments the presence of rational surfaces modifies the magnetic configuration with one or more islands embedded in the main plasma. The islands strongly influence particle and heat fluxes, hence their role must be included in the study of the transport properties. For this reason a new approach in modelling transport in presence of islands is presented: this method, dubbed Multiple Domains Scheme (MDS), with some simplifying assumptions, is directly applicable to 1.5D transport codes. In MDS three regions (core, island and outer part) are identified inside the plasma, all interfaced by the separatrix that acts as common boundary through which thermal and particle fluxes can be exchanged. In each domain a monotonic radial coordinate can be chosen and the metric elements can be computed accordingly. The MDS has been implemented in a new numerical tool, called Multiple Axis Solver (MAxS) [1]. To build a straight-field-line coordinate system and to compute the metric elements needed by MAxS, a new method that starts from discretized (Poincaré) magnetic field maps has been also developed [2]. Thanks to its flexibility, MAxS can be exploited to study transport in various plasma configurations: its application to LHD, a heliotron device, and to RFX-mod, a Reversed Field Pinch machine, is here presented. In LHD, a large m/n=1/1 magnetic island can be induced in the edge region by means of external coils. In stationary condition, the island alters the kinetic quantities appearing, for example, as a flat region in the electron temperature profile. To properly test the cross-field diffusivity in background plasma and inside of the island region, perturbative methods, such as heating modulation or pellet injection, are preferable. In previous works [1,3] a clear reduction of the transport in the island region has been found. Those studies have been performed, for simplicity, starting with a magnetic field given by the vacuum component. Now, the comparison of the transport properties of plasma with islands in LHD using the vacuum field and the more realistic finite beta configuration will be presented.

The study of the physical mechanisms that influence the tearing mode (TM) rotation is of interest because, while in present day devices, a significant TM rotation can be induced by Neutral Beam Injection, future reactors, ITER included, are not expected to provide enough induced momentum. We present a study of tearing mode dynamics in the RFX-mod device, a Reserved Field Pinch in Padua (Italy) that can be run as low-current, circular tokamak. Magnetic, flow and kinetic measurements are integrated to characterize the (2,1) and (3,2) TMs fast rotation. We are especially interested to study the role played by the diamagnetic electron drift on the TM rotation, including the slowing down and the wall-locking phases. When the latter occurs, the radial magnetic field penetrates the shell and the TM amplitude increases at a rate given by the wall resistive time constant. This phenomenon can lead to a rapid discharge termination via a disruption. A comparison of experimental data with a two-fluid MHD cylindrical model [Nucl. Fusion 54 (2014) 122001] has been used to interpret the observed TM fast rotation frequencies.

Characterization of fast ion dynamics and neutron production in RFX-mod

The occurrence of improved confinement regime, H - mode, in magnetic plasma confinement devices is characterized by the creation of an edge transport barrier and a reduction in the turbulence level s. Such reduction is usually associated with a massive shear in plasma flow at the edge, impacting on dynamic and features of blobs or filament. These structures emerging from turbulence background in the SOL are responsible for a huge loss of particles an d energy . Edge Localized Modes (ELMs) large filaments often appear at the plasma edge during the H - mode and are related to a quasi - periodic relaxation of the transport barrier. Although much information about ELMs has been gathered in the last few years, e nabling even the successful mitigation of them, their underlying physical characteristics ( i.g. their electromagnetic behaviour in different scales) are, in a certain sense, still obscure. Filament behaviour has been reported in the ELMs, bringing an inter esting parallel with the turbulent structures in the L - mode . In this contribution w e have been studying transition s to H - mode induced by electrode biasing at the edge of the RFX - mod experiment operating in tokamak configuration. The electromagnetic fluctua tion s and the equilibrium of the plasma parameters at edge were measured by insertable probe head, U - probe, before and during the biasing phase. They have been addressed to determine in which conditions the H mode is reached in the RFX (i.g. if there are i ntermediate phases ) . The different st ructures occurring during the H - Mode phase have been studied, including ELM and the inter ELM phase . Advanced statistical methods have been applied to simultaneous analysis of different plasma parameters such as the electron temperature , the electron density and the vorticity, as well as magnetic signals measured by a set of pick - up coils installed in the probe head . A detailed comparison of the electromagnetic features of the filaments occurre nce before and during the H - mode has been done, with a special attention to the electromagnetic ELM fine structure , through the analysis in different scales.

We describe a method to build a straight-field-line coordinate system from discretized magnetic field maps. This method, valid for generic plasma domains with conserved, nested flux surfaces, is particularly useful to calculate the metric tensor coefficients, the Jacobian, and the flux surface average of any plasma quantity in magnetic islands. In this work, we will show examples for tokamak, heliotron, and reversed field pinch plasmas with m=1 islands in low beta plasmas.

The presence of magnetic islands in fusion plasmas modifies the topology of flux surfaces and impacts on the transport properties. Hence, to study transport phenomena in plasma with islands, 3D transport codes have to be applied. Such tools are powerful but also complex and time consuming. Conversely 1.5D transport codes (like ASTRA, CRONOS) are fast, user-friendly and well spread in the fusion community, but they are applicable only for nested flux surfaces. In this work we present a new approach, compatible with 1.5D geometry, for modelling transport in presence of islands and its implementation in a numerical code dubbed Multiple Axis Solver (MAxS). The main idea is to divide the plasma in three regions, having the separatrix as common boundary. In each domain the metric elements are computed, considering the discontinuities on the separatrix. The transport equations for the kinetic quantities are, then, solved in each domain and the fluxes are matched at the common boundary. This scheme, being independent from the magnetic configuration, can be applied to Tokamaks, Stellarators and Reversed Field Pinches (RFPs). Two examples are then reported. The first presents the study of cold pulse propagation in LHD plasmas featuring a large externally induced 1/1 island, where the thermal diffusivity results lower than in the background plasma. The second shows the MAxS application to RFP plasmas where a spontaneous m=1 island has a beneficial impact on the energy confinement, featuring temperature gradients of several keV/m.

In magnetically confined experiments the presence of rational surfaces allows perturbations (either self generated by the plasma or externally induced) to grow and break the magnetic topology of flux surfaces nested around a single axis. The bifurcated magnetic configuration takes the form of one or more islands embedded in the main plasma, each with their own magnetic axis. Stochastic layers develop around the flux surface containing the island X-point (the separatrix) and divide the nested flux surfaces of the islands from the nested flux surfaces of the bulk plasma. The presence of islands strongly modifies particle and heat fluxes. Transport studies in fusion community benefit of the availability of several well developed 1.5D transport codes (ASTRA, CRONOS, JETTO, RITM ...) that solve the flux surface averaged transport equations across shaped flux surfaces, on the grounds that the variation of physical quantities along the flux surfaces is small. They constitute a powerful and relatively user-friendly tool since they are able to capture the main features of transport phenomena without the complexity of 2D transport codes. However 1.5D codes are applicable nowadays only for flux surfaces nested around a single axis. Our goal is to develop a user friendly modelling tool applicable to 1.5D transport codes to be exploited in plasmas in the presence of single harmonic magnetic islands. To this aim a multiple domains scheme is adopted and three regions inside the plasma are identified: the first around the main magnetic axis (core), the second around the island axis (island) and the third that includes the separatrix stochastic area and the outer plasma (edge). In each domain the magnetic flux can be chosen as an effective radial coordinate. This scheme is independent from the equilibrium magnetic configuration hence it can be applied to tokamak, stellarator and Reversed Field Pinch. In this work we present the first study in this direction. We mimic the ASTRA transport module solving, in each domain, the 1.5D equation for the heat diffusion by means of a dedicated numerical code. A predictive simulation is performed providing the code with the diffusivity profile for the core, island and edge region, studying the mathematical issues arising in the context of the equation solution, in particular the matching at the common boundaries, in steady-state as well as time-dependent cases. Our study is carried out in synthetic but realistic scenarios from the RFX-mod Experiment. We show that qualitative agreement with experiments is quite easily recovered.

A complete description of the effects of magnetic perturbation on the edge region of RFX-mod is here reported. The flexibility of the RFX-mod device [1] allows for the operation of the machine both as a reversed field pinch (RFP, with maximum current 2 MA) and as a low-current, circular ohmic tokamak (I-p,I-max = 0.15 MA). The present paper summarizes the most recent results obtained in both configurations with either spontaneous or induced edge radial magnetic perturbation. Emphasis will be devoted to the experimental characterization of the edge flow, focusing on the phase relation between flow and perturbed magnetic field. These informations are provided for natural and stimulated helical discharges in RFPs, and for tokamak safely operated, thanks to the unique RFX-mod MHD control system, in a wide range of edge safety factor 1.9 less than or similar to q(a)

An experimental analysis of particle dynamics is presented in both tokamak and reversed-field pinch (RFP) Deuterium plasmas produced in the RFX-mod device. Particular attention is dedicated to effect of magnetic reconnection processes occurring spontaneously in the plasma. The diagnostics is made of a Neutral Particle Analyzer (NPA) and of a couple of neutron and gamma-ray detectors. The NPA can resolve the energy distribution of the neutral atoms produced by charge-exchange processes and exiting the plasma on the low field side on equatorial plane of the machine on 11 energy channels, being the energy and mass dispersion produced by a E//B combination. Neutron and gamma-ray fluxes are measured by means of 2 scintillators (EJ-301 liquid and NaI(Tl)) coupled to flat-panel photomultipliers, suitable for operation under magnetic fields. The production of neutrons and gamma rays in RFP plasmas is found to be strongly influenced by the Ohmic input power, with a threshold value of about 1.2MA in terms of plasma current level. Below such current value a low rate of gamma rays and almost no neutrons are detected. Neutral Deuterium atoms fluxes, neutron and gamma productions are strongly determined by the MHD activity measured in the plasma. In particular, a bursty behavior of neutron yields, correlated to the spontaneous magnetic reconnection events, occurring almost cyclically in the RFP plasmas are observed. Magnetic reconnection also modifies the energy distribution of neutral atoms having impact on both the Maxwellian component and on the observed higher energy tail. This is true also in tokamak plasmas during the sawtoothing activity mostly seen in low q(a) condition. The role of particle acceleration and ion heating during such events is discussed.