DIII-D physics research addresses critical challenges for the operation of ITER and the next generation of fusion energy devices. This is done through a focus on innovations to provide solutions for high performance long pulse operation, coupled with fundamental plasma physics understanding and model validation, to drive scenario development by integrating high performance core and boundary plasmas. Substantial increases in off-axis current drive efficiency from an innovative top launch system for EC power, and in pressure broadening for Alfven eigenmode control from a co-/counter-I (p) steerable off-axis neutral beam, all improve the prospects for optimization of future long pulse/steady state high performance tokamak operation. Fundamental studies into the modes that drive the evolution of the pedestal pressure profile and electron vs ion heat flux validate predictive models of pedestal recovery after ELMs. Understanding the physics mechanisms of ELM control and density pumpout by 3D magnetic perturbation fields leads to confident predictions for ITER and future devices. Validated modeling of high-Z shattered pellet injection for disruption mitigation, runaway electron dissipation, and techniques for disruption prediction and avoidance including machine learning, give confidence in handling disruptivity for future devices. For the non-nuclear phase of ITER, two actuators are identified to lower the L-H threshold power in hydrogen plasmas. With this physics understanding and suite of capabilities, a high poloidal beta optimized-core scenario with an internal transport barrier that projects nearly to Q = 10 in ITER at similar to 8 MA was coupled to a detached divertor, and a near super H-mode optimized-pedestal scenario with co-I (p) beam injection was coupled to a radiative divertor. The hybrid core scenario was achieved directly, without the need for anomalous current diffusion, using off-axis current drive actuators. Also, a controller to assess proximity to stability limits and regulate beta (N) in the ITER baseline scenario, based on plasma response to probing 3D fields, was demonstrated. Finally, innovative tokamak operation using a negative triangularity shape showed many attractive features for future pilot plant operation.

A theoretical investigation of the dissociative excitation by electron impact on the NO molecule is presented, aiming to make up for the lack of data for this process in the literature. A full set of vibrationally-resolved cross sections and corresponding rate coefficients are calculated using the local-complex-potential approach and five resonant states of NO-.

Observed turbulence in space and astrophysics is expected to involve cascade and subsequent dissipation and heating. Contrary to standard collisional fluid turbulence, the weakly collisional magnetized plasma cascade may involve several channels of energy conversion, interchange, and spatial transport, leading eventually to the production of internal energy. This paper describes these channels of transfer and conversion, collectively amounting to a complex generalization of the Kolmogorov cascade. Channels may be described using compressible magnetohydrodynamic (MHD) and multispecies Vlasov-Maxwell formulations. Key steps are conservative transport of energy in space, parallel incompressible and compressible cascades in scale, electromagnetic work on particles driving macroscopic and microscopic flows, and pressure-strain interactions, both compressive and shear-like, that produce internal energy. A significant contrast with the collisional case is that the steps leading to the disappearance of large-scale energy in favor of internal energy are formally reversible. This property motivates a discussion of entropy, reversibility, and the relationship between dissipation with collisions and in the Vlasov system without collisions. Where feasible, examples are given from MHD and Particle in Cell simulations and from MMS observations.

The phenomenology of reconnection events, associated to relaxations in high-current (~1.5 MA) plasmas of the reversed-field pinch device RFX-mod, is shown. Each relaxation event can be described as a series of stages starting in the core and propagating towards the plasma edge. In an initial stage (trigger), the evolution of the q profile brings resonant layers closer together, allowing for an interaction of the current sheets associated to tearing modes at the respective rational values of q. The phase locking of the resonant modes then initiates the reconnection process that, once started, changes the magnetic topology bringing the initially helical state to a more chaotic configuration.

High temperature and low density plasmas are ubiquitous in the Universe. These systems often exhibit a turbulent dynamics, characterized by the cross-scale coupling of fluid and kinetic scales and by the inhomogeneous development of coherent spatial and temporal structures. At smaller scales the energy of fluctuations is dissipated and plasma is eventually heated. Despite collisions are usually ruled out from the description of these systems, it has been recently shown that plasma collisionality may be locally enhanced, owing to the presence of fine structures in velocity space. In this perspective, further insights are here given by comparing the characteristic times of collisional dissipation with the time scales of other collisionless processes, such as nonlinear coupling and linear instability onset. When taking into account fine velocity structures, these characteristic times could be comparable. A novel scenario for the description of heating in weakly-collisional plasmas, even including inter-particle collisions, is finally proposed.

Kinetic simulations based on the Eulerian Hybrid Vlasov-Maxwell (HVM) formalism permit the examination of plasma turbulence with a useful resolution of the proton velocity distribution function. The HVM model is employed here to study the balance of energy, focusing on channels of conversion that lead to proton kinetic effects, including growth of internal energy and temperature anisotropies. We show that this Eulerian simulation approach, which is almost noise-free, is able to provide an accurate energy balance for protons. The results demonstrate explicitly that the recovered temperature growth is directly related to the role of the pressure-strain interaction. Furthermore, analysis of local spatial correlations indicates that the pressure-strain interaction is qualitatively associated with strong-current, high-vorticity structures although other local terms-such as the heat flux-weaken the correlation. These numerical capabilities based on the Eulerian approach will enable a deeper study of transfer and conversion channels in weakly collisional Vlasov plasmas.

In preparation for the upcoming JET D-T campaign, great effort has been devoted during the 2015-2016 JET campaigns with the ITER-like wall (ILW) to the extension of the high performance H-mode phase in baseline and hybrid scenarios. Hybrid discharges were the only ones that have been stopped by the real-time vessel protection system due hot-spot formation on the outboard poloidal limiter. Generation of hot-spots was linked to the application of high neutral beams injection and ion cyclotron resonance heating (ICRH) power. In tokamaks with high-Z plasma components, the use of ICRH heating is also accompanied by an increased metallic impurity content. Simultaneous control of hot-spot temperature and the core impurity content was crucial due to the fact that the same plasma-wall interaction mechanism is responsible for both phenomena. Impurity data collected by SXR, EUV and VUV diagnostics were able to provide for the first time comprehensive information concerning tungsten and mid-Z impurities such as nickel, iron, and cooper. To determine absolute mid-Z impurity concentrations a new relative calibration technique, compatible with JET-ILW, has been developed based on cross-calibration with a calibrated spectrometer via the quasicontinuum of W in the 200-400 Å wavelength range. In hybrid discharges, it was found that local D2 gas injection, plasma current, separatrix density, and fast ion losses appeared to impact hot-spot temperature and core impurity levels. Analysis showed a reduced maximum hot-spot temperature and impurity concentration at higher gas rate. Changes in the plasma current had a strong impact on the plasma-wall interaction, both via modifications in the edge density and in the fast ion losses. At constant gas injection rate, both the hot-spot temperature and the core impurity content decreased with the separatrix density. The main mechanism responsible for the formation of the hot-spots was found to be linked to the fast ion losses, but RF sheath effects may also be playing a role in the high limiter temperatures observed in these experiments. © Institute of Plasma Physics and Laser Microfusion.

The hybrid Vlasov-Maxwell system of equations is suitable to describe a magnetized plasma at scales on the order of or larger than proton kinetic scales. An exact stationary solution is presented by revisiting previous results with a uniform-density shear flow, directed either parallel or perpendicular to a uniform magnetic field, and by adapting the solution to the hybrid Vlasov-Maxwell model. A quantitative characterization of the equilibrium distribution function is provided by studying both analytically and numerically the temperature anisotropy and gyrotropy and the heat flux. In both cases, in the shear region, the velocity distribution significantly departs from local thermodynamical equilibrium. A comparison between the time behavior of the usual "fluidlike" equilibrium shifted Maxwellian and the exact stationary solutions is carried out by means of numerical simulations of the hybrid Vlasov-Maxwell equations. These hybrid equilibria can be employed as unperturbed states for numerous problems which involve sheared flows, such as the wave propagation in an inhomogeneous background and the onset of the Kelvin-Helmholtz instability.

The non-local expression proposed by Luciani-Mora-Virmont is implemented in a one dimensional fluid model for the scrape-off layer. Analytical solutions for heat equation are discussed as well as the impact of sheath boundary conditions on the continuity of the temperature profile. The non-local heat flux is compared to the Spitzer-Härm heat flux for different collisionality.

TCV (Tokamak à Configuration Variable) is a tokamak device capable of many different plasma shape s and positions, equipped with a flexible system of Electron Cyclotron (EC) antennas and a new Neutral Beam (NB) injector [1] . The auxiliary power from the beam can reach 1 MW and it is injected tangentially co - current , coupling mostly with ions . This heating system allows n e w insights on advanced tokamak scenarios in TCV which, up to now , have been performed only with EC heating (ECH) . These scenarios have high ? N , high non - inductive current fraction and a relevant energetic particle (EP) population fraction (?10 %) . An internal transport barrier can be generated by reversing the q - profile using EC current - drive (ECCD) [2] . In this work we show that the effect of the sum of the two heating sources (NBI and EC H ) in TCV high ? N plasmas is not linear, and interpretative modelling is carried out to understand the behaviour of the NB EP s when ECH is present . A statistical study on a set of experiments with both ECH and NBI is presented to show t he effect of NB injection (NBI) on plasma performance: ? N and the plasma stored energy do n o t increase linearly with NB power. Furthermore, the contribution to the total plasma current from ohmic transformer, bootstra p current and current drive are respectively estimated , showing that EC has a strong impact on Z eff , modifying therefore the plasma resistivity and the ohmic contribution to the current. This effect is taken into account when applying the Monte Carlo inter pretative NBI code NUBEAM. It results that with the combined application of ECRH and ECCD , the electron temperature and plasma equilibrium change significantly, impacting on the NB power deposition: CX and orbit losses tend to increase, reducing by 20% the power deposited on plasma species. Modelling suggests that the variation in T e changes the EP power redistribution among the species, transfe rring more power to the ions. The impact on EP s orbit given by ECCD equilibrium modification is performed with the Monte Carlo ASCOT code for NBI modelling , capable of solving the EP full gyro - motion.

The description of the Moffatt and Parker problem recently revisited by O. Pezzi et al. [Astrophys. J. 834, 166 (2017)] is here extended by analyzing the features of the turbulence produced by the interaction of two colliding Alfvenic wave packets in a kinetic plasma. Although the approach based on the presence of linear modes features is still helpful in characterizing some low-energy fluctuations, other signatures, which go beyond the pure linear modes analysis, are recovered, such as the significant weakening of clear dispersion relations and the production of zero frequency fluctuations.

Eulerian simulations of the Vlasov-Poisson equations have been employed to analyze the excitation of slow electrostatic fluctuations (with phase speed close to the electron thermal speed), due to a beam-plasma interaction, and their propagation in linear and nonlinear regimes. In 1968, O'Neil and Malmberg [Phys. Fluids 11, 1754 (1968)] dubbed these waves "beam modes." In the present paper, previous analytical results on the beam modes in both linear and nonlinear regimes have been revisited numerically, pointing out that, when an electron beam is launched in a plasma of Maxwellian electrons and motionless protons and this initial equilibrium is perturbed by a monochromatic density disturbance, the electric field amplitude grows exponentially in time and then undergoes nonlinear saturation, associated with the kinetic effects of particle trapping and phase space vortex generation. Moreover, new numerical results give evidence that, when the initial density perturbation is setup in the form of a low amplitude random phase noise, the whole Fourier spectrum of wavenumbers is excited. As a result, the electric field profile appears as a train of isolated pulses, each of them being associated with a phase space vortex in the electron distribution function. At later times, these vortical structures tend to merge and, correspondingly, the electric pulses collapse, showing the tendency towards a time asymptotic configuration characterized by the appearance of electric soliton-like pulses. This dynamical evolution is driven by purely kinetic processes, possibly at work in many space and laboratory plasma environments. Published by AIP Publishing.

Electron cyclotron resonance ion sources (ECRIS) are widely used plasma based machines for the production of intense ion beams in science and industry. The performance of modern devices is limited by the presence of the density cut-off, above which electromagnetic (EM) waves sustaining the plasma are reflected. We hereby discuss the systematic data analysis of electrostatic wave generation in an ECR prototype operating at 3.75 GHz-0.1 THz. In particular, electron Bernstein waves (EBW) have been excited. EBW have already been generated in ;large-scale plasma devices for thermonuclear fusion purposes. In ion sources where L-c similar to lambda(RF) (Lc being the plasma chamber size and lambda(RF) the pumping wave wavelength) the EM field assumes a modal behaviour; thus both plasma and EM field self-organize so that no optical-like wave launching is possible (i.e. the cavity effect dominates on the optical path). The collected data, however, supported by 3D full-wave simulations, actually demonstrate that a Budden-type X-B conversion scenario can be established above some critical RF power thresholds, operating in an off-ECR regime. The generation and absorption of the EBW has been demonstrated by the presence of three peculiar signatures: along with the establishment of an overdense plasma, generation of supra-thermal electrons and modification (non-linear broadening) of the EM spectrum measured within the plasma have been observed. At the threshold establishing such a heating regime, the collected data provide evidence for a fast rotation of the electron fluid.

In this work a low-temperature plasma source for the generation of plasma activated water (PAW) is developed and characterized. The plasma reactor was operated by means of an atmospheric-pressure air dielectric barrier discharge (DBD). The plasma generated is in contact with the water surface and is able to chemically activate the liquid medium. Electrodes were supplied by both sinusoidal and nanosecond-pulsed voltage waveforms. Treatment times were varied from 2 to 12 min to increase the energy dose released to the water by the DBD plasma. The physics of the discharge was studied by means of electrical, spectroscopic and imaging diagnostics. The interaction between the plasma and the liquid was investigated as well. Temperature and composition of the treated water were detected. Images of the discharges showed a filamentary behaviour in the sinusoidal case and a more homogeneous behaviour in the nanosecond-pulsed one. The images and the electrical measurements allowed to evaluate an average electron number density of about 4 × 1019 and 6 × 1017 m-3 for the sinusoidal and nanosecond-pulsed discharges respectively. Electron temperatures in the range of 2.1÷2.6 eV were measured by using spectroscopic diagnostics. Rotational temperatures in the range of 318-475 K were estimated by fitting synthetic spectra with the measured ones. Water temperature and pH level did not change significantly after the exposure to the DBD plasma. The production of ozone and hydrogen peroxide within the water was enhanced by increasing the plasma treatment time and the energy dose. Numerical simulations of the nanosecond-pulsed discharge were performed by using a self-consistent coupling of stateto- state kinetics of the air mixture with the Boltzmann equation of free electron kinetics. Temporal evolution of the electron energy distribution function shows departure from the Maxwellian distribution especially during the afterglow phase of the discharge. When limited deviations from Maxwellian distribution were observed, calculated electron temperature is in good agreement with the one measured by means of spectroscopic diagnostics. Computed temporal evolution of the energy delivered to the discharge is comparable with the one obtained from electrical measurements. The electrical discharges supplied by both voltage waveforms produce plasma activated water with negligible thermal effects and pH variations.

In vari esperimenti operanti nel settore della fusione nucleare, è necessario son dare il plasma di bordo, dove avvengono fenomeni interessanti dal punto di vista della fisica del plasma. Per fare questo si utilizzano delle teste di misura con molte diagnostiche operanti in un piccolo volume, movimentate da manipolatori veloci che fanno rimanere la teste di misura all'interno del plasma per periodi molto brevi. Questo per evitare che la test di misura sia danneggiata dal plasma durante l'inserzione. In questo ambito si inserisce la costruzione del manipolatore FaRM, oggetto di questa spe cifica tecnica.

The existence of several characteristic times during the collisional relaxation of fine velocity structures is investigated by means of Eulerian numerical simulations of a spatially homogeneous force-free weakly collisional plasma. The effect of smoothing out velocity gradients on the evolution of global quantities, such as temperature and entropy, is discussed, suggesting that plasma collisionality can locally increase due to velocity space deformations of the particle velocity distribution function. These results support the idea that high-resolution measurements of the particle velocity distribution function are crucial for an accurate description of weakly collisional systems, such as the solar wind, in order to answer relevant scientific questions, related, for example, to particle heating and energization.

The initial state recurrence in numerical simulations of the Vlasov-Poisson system is a well-known phenomenon. Here, we study the effect on recurrence of artificial collisions modeled through the Lenard-Bernstein operator [A. Lenard and I. B. Bernstein, Phys. Rev. 112, 1456-1459 (1958)]. By decomposing the linear Vlasov-Poisson system in the Fourier-Hermite space, the recurrence problem is investigated in the linear regime of the damping of a Langmuir wave and of the onset of the bump-on-tail instability. The analysis is then confirmed and extended to the nonlinear regime through an Eulerian collisional Vlasov-Poisson code. It is found that, despite being routinely used, an artificial collisionality is not a viable way of preventing recurrence in numerical simulations without compromising the kinetic nature of the solution. Moreover, it is shown how numerical effects associated to the generation of fine velocity scales can modify the physical features of the system evolution even in nonlinear regime. This means that filamentation-like phenomena, usually associated with low amplitude fluctuations contexts, can play a role even in nonlinear regime. (C) 2016 AIP Publishing LLC.

Plasma properties can be useful in a wide spectrum of applications. Experimental projects on controlled nuclear fusion are the most challenging of these applications and, at the same time, the best way to approach plasma science. Since nuclear fusion reactors can ensure a large-scale, safe, environmentally-friendly and virtually inexhaustible source of energy, several fusion-oriented megaprojects and innovative companies are appearing all over the world. PROTO-SPHERA (Spherical Plasma for HElicity Relaxation Assessment) is the first plasma project with a simply connected configuration, namely not requiring additional objects inside the plasma volume. This is obtained by a plasma arc, shaped as a screw pinch, acting as the centerpost of a spherical torus with minimal aspect ratio. Due to its intrinsic physical, engineering and economic advantages, this new approach is attractive also on an industrial scale and with several developments that still needs to be explored. This paper presents the PROTO-SPHERA basic principles, its first encouraging results and its expected and potential evolutions.

The effects are presented of including electron-electron collisions in self-consistent Eulerian simulations of electrostatic wave propagation in nonlinear regime. The electron-electron collisions are approximately modeled through the full three-dimensional Dougherty collisional operator [J. P. Dougherty, Phys. Fluids 7, 1788 (1964)]; this allows the elimination of unphysical byproducts due to reduced dimensionality in velocity space. The effects of non-zero collisionality are discussed in the nonlinear regime of the symmetric bump-on-tail instability and in the propagation of the so-called kinetic electrostatic electron nonlinear (KEEN) waves [T. W. Johnston et al., Phys. Plasmas 16, 042105 (2009)]. For both cases, it is shown how collisions work to destroy the phase-space structures created by particle trapping effects and to damp the wave amplitude, as the system returns to the thermal equilibrium. In particular, for the case of the KEEN waves, once collisions have smoothed out the trapped particle population which sustains the KEEN fluctuations, additional oscillations at the Langmuir frequency are observed on the fundamental electric field spectral component, whose amplitude decays in time at the usual collisionless linear Landau damping rate. (C) 2015 AIP Publishing LLC.

A detailed comparison between the Landau and the Dougherty collision operators has been performed by means of Eulerian simulations, in the case of relaxation toward equilibrium of a spatially homogeneous field-free plasma in three-dimensional velocity space. Even though the form of the two collisional operators is evidently different, we found that the collisional evolution of the relevant moments of the particle distribution function (temperature and entropy) are similar in the two cases, once an 'ad hoc' time rescaling procedure has been performed. The Dougherty operator is a nonlinear differential operator of the Fokker-Planck type and requires a significantly lighter computational effort with respect to the complete Landau integral; this makes self-consistent simulations of plasmas in presence of collisions affordable, even in the multi-dimensional phase space geometry.