Solar wind provides an example of a weakly collisional plasma expanding from a thermal source in the presence of spatially diverging magnetic field lines. Observations show that in the inner heliosphere, the electron temperature declines with the distance approximately as Te(r) ~ r^−0.3 . . . r^−0.7, which is significantly slower than the adiabatic expansion law r^−4/3. Motivated by such observations, we propose a kinetic theory that addresses the non-adiabatic evolution of a nearly collisionless plasma expanding from a central thermal source. We concentrate on the dynamics of energetic electrons propagating along a radially diverging magnetic flux tube. Due to the conservation of their magnetic moments, the electrons form a beam collimated along the magnetic field lines. Due to weak energy exchange with the background plasma, the beam population slowly loses its energy and heats the background plasma. We propose that no matter how weak the collisions are, at large enough distances from the source a universal regime of expansion is established where the electron temperature declines as Te(r) ~ r^−2/5. This is close to the observed scaling of the electron temperature in the inner heliosphere. Our first-principle kinetic derivation may thus provide an explanation for the slower-than-adiabatic temperature decline in the solar wind.

In this work I formulate collisionless 1-particle kinetic theory and ideal compressible fluid dynamics (for either dilute gases or electrostatic plasmas) as noncanonical Hamiltonian systems, where the dynamics is governed by a Poisson bracket and a Hamiltonian functional. I then consider a map from kinetic to hydrodynamic variables that preserve the Poisson brackets (i.e. a Poisson map). The novelty here is that in addition to the momentum and mass moments, the Poisson map includes entropy moments as well. The lack of moment closure under this map can then be attributed to the fact that the kinetic theory Hamiltonian is not expressible solely in terms of the fluid variables. More precisely, the kinetic definition of temperature coincides with that defined by a thermodynamic equation of state if and only if the distribution function is a local Maxwellian. If we "approximate" the Hamiltonian functional by discarding this difference, or equivalently constrain the distribution function to be a local Maxwellian, we obtain a manifestly Hamiltonian derivation of the compressible Euler equations.

Understanding turbulent transport physics in the tokamak edge and scrape-off lay- er (SOL) is critical to developing a successful fusion reactor. The dynamics in these regions plays a key role in determining the L-H transition, the pedestal height and the heat load to the vessel walls. Large-amplitude fluctuations, magnetic X-point geometry, and plasma interactions with material walls make modeling turbulence in the edge/SOL more challenging than in the core region, requiring specialized gyrokinetic codes. Electromagnetic effects can also be important in the edge/SOL region due to steep pressure gradients and line bending resulting from the coupling of perpendicular dynamics with kinetic shear Alfven waves. However, all gyrokinetic results in the SOL to date have assumed electrostatic dynamics, due in part to numerical challenges like the Ampere cancellation problem. We present the first nonlinear electromagnetic gyrokinetic results of turbulence on open field lines in the tokamak SOL, obtained using the Gkeyll full-f continuum gyrokinetic code. The results, which use a model helical SOL geometry and NSTX-like parameters, show magnetic fluctuations of up to dBperp/B ~ 1%. Line-tracing visualizations show that field lines are pushed and bent by radially prop- agating blobs. Comparisons to electrostatic simulations show that including electromagnetic effects can produce larger relative density fluctuations and more intermittent transport.

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Eugene Parker developed the first theory of how the solar wind interacts with the dynamo‐generated magnetic field of the Sun. He showed that the wind carries the magnetic field lines away from the star, while their footpoints are frozen into the corona and twisted into an Archimedean spiral by stellar rotation. The resulting magnetic topology is now known as the Parker spiral and is the largest magnetic structure in the heliosphere. The transition between magnetic field co‐rotating with a star and the field advected by the wind is thought to occur near the so‐called Alfvén surface ‐ where inertial forces in the wind can stretch and bend the magnetic field. According to the governing equations of magnetohydrodynamics, this transition in a magnetic field like the Sun's is singular in nature and therefore suspected to be highly dynamic. However, this region has yet to be observed in‐situ by spacecraft or in the laboratory, but is presently the primary focus of the Parker Solar Probe mission. Here we show, in a synergistic approach to studying solar wind dynamics, that the large‐scale magnetic topology of the Parker spiral can also be created and studied in the laboratory. By generating a rotating magnetosphere with Alfvénic flows, magnetic field lines are advected into an Archimedean spiral, giving rise to a dynamic current sheet that undergoes magnetic reconnection and plasmoid ejection. These plasmoids are born at the tip of the streamer cusp, driven by non‐equilibrium pressure gradients, and carry blobs of plasma outwards at super‐ Alfvénic speeds, mimicking the observed dynamics of coronal helmet streamers. Further more, a simple heuristic model based on a critical plasmoid length scale and sonic expansion time is presented. This model explains the frequencies observed in the experiment and simulations (10s of KHz) and is consistent with the 90 minute plasmoid ejection period of full‐scale coronal streamers as observed by the LASCO and SECCHI instrument suites.

Understanding the origin of magnetic fields in astrophysical unmagnetized plasmas is a problem of great interest, which has attracted considerable effort during the past years [1]. Various mechanisms leading to the field generation have been identified, but a clear comprehension of the process is still missing. Analytical and numerical works have suggested that the Biermann battery [2] and the Weibel/Current Filamentation instability [3, 4] are able to produce seed magnetic fields. Nowadays, the availability of multi-terawatt lasers with intensity higher than 10^19 W/cm2 and ultra-relativistic high-density particle beams allows probing these scenarios in the laboratory through properly scaled experiments. In this way, fine experimental diagnostics can grant access to a new series of unprecedented information on the magnetic field dynamics [5, 6]. During this talk, I will review how strong magnetic fields can be generated under realistic experimental conditions using available or soon to be available neutral electron-positron beams or intense laser pulses [7, 8]. Leveraging realistic kinetic simulations, novel experimental setups will be illustrated. It will be thus shown how fields similar to the one present in astrophysical contexts can be produced and explored in the laboratory.

The phenomenon of magnetic-field amplification due to the motion of turbulent plasma has been investigated in a series of experiments carried out at various high-energy laser facilities during the last five years. Plasma jets driven by intense laser irradiation pass through asymmetric grids, then collide head on, leading to developed turbulence. Thomson-scattering, soft-X-ray-imaging and proton-radiography diagnostics have allowed for a thorough characterisation of the plasma state, including measurements of temperature, flow velocities, turbulent spectra, and magnetic fields. Our key finding is that at sufficiently large magnetic Reynolds numbers, magnetic fields are amplified very efficiently, attaining dynamical strengths. The robustness of this conclusion has been confirmed subsequently via several extensions of the original experimental configuration. Our results lend support to theoretical expectations that plasma turbulence is responsible for the magnetic fields universally observed in various astrophysical environments, from stars to the intra-cluster medium.

Parker Solar Probe (PSP), launched in August 2018, is set to become the first spacecraft to fly through the solar corona, the tenuous outer atmosphere of the Sun. The spacecraft carries a comprehensive set of instruments to fully characterise the local plasma environment; it has currently reached 27 solar radii from the Sun and by 2024 will get to within 9 solar radii. One of the primary goals of the mission is to investigate the related open problems of coronal heating and solar wind acceleration. Plasma turbulence is thought to be one of the key processes underlying these phenomena. Here, I will present initial results from the first two orbits of PSP to investigate the properties of the turbulence near the Sun and the possible role it plays in the generation of the solar wind. Significant differences of the turbulence closer to the Sun include much higher energy levels, a greater level of imbalance, all Alfvenic fields taking a -3/2 spectrum, and a smaller slow mode energy fraction. Comparison to the solar wind model of Chandran et al. (2011) indicates fluxes consistent with a turbulence-driven solar wind and a generation of the inward Alfvenic fluctuations (necessary for the turbulence to occur) by reflection from the large-scale gradient in Alfven speed, meaning that turbulence driven by Alfven waves from the Sun remains a viable explanation for solar wind acceleration. If time permits, I will also discuss some more recent findings, and future plans for the mission.

The behaviour of a collisional plasma which is optically thin to cyclotron radiation is considered, and the distribution functions accessible to it on the various timescales in the system are calculated. Particular attention is paid to the limit in which the collision time exceeds the radiation emission time, making the electron distribution function strongly anisotropic. Unusually for plasma physics, the collision operator can nevertheless be calculated analytically although the plasma is far from Maxwellian. The rate of radiation emission is calculated and found to be governed by the collision frequency multiplied by a factor that only depends logarithmically on plasma parameters. Two broad classes of applications are also discussed.