Magnetic Fields and Non-Local Transport in Laser Plasmas

The first Vlasov-Fokker-Planck simulations of nanosecond laser-plasma interactions – including the effects of self-consistent magnetic fields and hydrodynamic plasma expansion – will be presented. The coupling between non-locality and magnetic field advection is elucidated. For the largest (initially uniform) magnetic fields externally imposed in recent long-pulse laser gas-jet plasma experiments (12T) a significant degree of cavitation of the B-field will be shown to occur (> 40%) in under 500ps. This is due to the Nernst effect and leads to the re-emergence of non-locality even if the initial value of the magnetic field strength is sufficient to localize transport. Classical transport theory may also break down in such interactions as a result of inverse bremsstrahlung heating. Although non-locality may be suppressed by a large B-field, inverse bremsstrahlung still leads to a highly distorted distribution. Indeed the best fit for a 12T applied field (after 440ps of laser heating) is found to be a super- Gaussian distribution – f0 α e−vm – with m = 3.4. The effects of such a distribution on the transport properties under the influence of magnetic fields are elucidated in the context of laser-plasmas for the first time. In long pulse laser-plasma interactions magnetic fields generated by the thermoelectric (‘∇ne × ∇Te’) mechanism are generally considered dominant. The strength of B-fields generated by this mechanism are affected, and new generation mechanisms are expected, when non-locality is important. Non-local B-field generation is found to be dominant in the interaction of an elliptical laser spot with a nitrogen gas-jet

Instabilities, anomalous transport, and nonlinear structures in partially and fully magnetized plasmas.

Plasmas behavior, to a large extent, is determined by collective phenomena such as waves. Wave excitation, turbulence, and formation of quasi-coherent nonlinear structures are defining features of nonlinear multi-scale plasma dynamics. In this thesis, instabilities, anomalous transport, and structures in partially and fully magnetized plasmas were studied with a combination of analytical and numerical tools. The phenomena studied in this thesis are of interest for many applications, e.g., plasma reactors for material processing, electric propulsion, magnetic plasma confinement, and space plasma physics. Large equilibrium flows of ions and electrons exist in many devices with partially magnetized plasmas in crossed electric and magnetic fields. Such flows result in various instabilities and turbulence that produce anomalous electron transport across the magnetic field. We present first principle, self-consistent, nonlinear fluid simulations that predict the level of anomalous current generally consistent with experimental data. We also show that drift waves in partially magnetized plasmas (which we called Hall drift waves), destabilized by the electron drift along with density gradients, tend to form (via inverse energy cascade) shear flows similar to zonal flows in fully magnetized plasmas. These flows become unstable due to a secondary instability (similar to Kelvin–Helmholtz instability) and produce large-scale quasi-stationary vortices. Then, it was shown that in nonlinear regimes, the axial mode instability due to electron and ion flows (along the electric field) forms large-amplitude cnoidal type waves. At the same time, the strong electric field produced by axial modes affects Hall drift waves stability and provides a feedback mechanism on density gradient driven turbulence, creating a complex picture of interacting anomalous transport, zonal flows, vortices, and streamers. In the case where axial modes are destabilized by boundary effects, the nonlinear dynamics result in a new nonlinear equilibrium or standing oscillating waves. The formation of shear flows (zonal flows) was also studied in the framework of the Hasegawa-Mima equation and it was established that zonal flows can saturate due to nonlinear self-interactions. Lastly, a novel approach for high-fidelity numerical simulations of multi-scale nonlinear plasma dynamics is developed which is illustrated with the example of an unmagnetized plasma

Revisiting the anomalous rf field penetration into a warm plasma

Radio frequency waves do not penetrate into a plasma and are damped within it. The electric field of the wave and plasma current are concentrated near the plasma boundary in a skin layer. Electrons can transport the plasma current away from the skin layer due to their thermal motion. As a result, the width of the skin layer increases when electron temperature effects are taken into account. This phenomenon is called anomalous skin effect. The anomalous penetration of the rf electric field occurs not only for transversely propagating to the plasma boundary wave (inductively coupled plasmas) but also for the wave propagating along the plasma boundary (capacitively coupled plasmas). Such anomalous penetration of the rf field modifies the structure of the capacitive sheath. Recent advances in the nonlinear, nonlocal theory of the capacitive sheath are reported. It is shown that separating the electric field profile into exponential and non-exponential parts yields an efficient qualitative and quantitative description of the anomalous skin effect in both inductively and capacitively coupled plasma.Comment: 44 pages, invited paper at "Nonlocal, Collisionless Phenomena in Plasma" worksho

Multiscale numerical simulations of the magnetized plasma sheath with massively parallel electrostatic particle-in-cell code

Understanding the physics of the plasma boundary and plasma-surface interactions is one of the key scientific challenges in fusion science and engineering. Large-scale integrated simulations and high-performance computing can provide valuable insights on the dynamic phenomena involved at the interface between the plasma and the material surface. Current state-of-the-art simulations of magnetically-confined fusion devices are typically performed using gyrokinetic approximations, aimed at resolving the physics of the core plasma, scrape-of-layer, and a portion of the divertor. However, the region of plasma near to the surface, called the plasma sheath, where the plasma ions accelerate from subsonic to supersonic conditions, is typically either not handled or treated with ad-hoc approximations. The characteristic scale of the near-surface plasma (sheath and presheath) is comparable to the Debye length, which is of the order of, or smaller, than the ion gyroradius. A detailed description of the kinetic processes occurring during the supersonic acceleration across the collisional and magnetic presheaths and requires a fully-kinetic model that is not present in any current fusion code, thus limiting a detailed evaluation of the energy-angle spectrum of the ions impacting on the surface of a tokamak. We have developed and verified a new massively-parallel Particle-in-Cell code, named hPIC, solving the multi-species Boltzmann-Poisson integro-differential set of equations. We give an overview of the model equations, of the architecture of the code, and summarize the verification tests, also presenting the scalability tests performed on the Blue Waters supercomputer at the University of Illinois. The model has been used for the numerical characterization of the plasma sheath and presheath in strong magnetic fields. Thanks to the new Particle-in-Cell, we have performed a systematic analysis of the structure of the magnetized plasma sheath, in order to determine the trends of the Ion Energy-Angle Distributions (IEAD) of the particles impacting on the wall after crossing the presheath and sheath regions. The model provides the dependance of the IEAD on the level of magnetization and magnetic inclination with respect to the surface. We have found that in regimes of intermediate-to-strong magnetization, the ion flow has a characteristic three-dimensional structure, which appears in all evidence within the magnetic presheath after the ions transition from sonic to supersonic. The model also suggests the disappearance of the electrostatic (Debye) sheath at high magnetic angles, with an interesting reduction of the ion flow down to subsonic conditions. Furthermore, detailed Particle-in-Cell simulations have been compared to simplified representations of the magnetized plasma sheath based on a set of fluid equations coupled to a Monte-Carlo particle-tracer for the reconstruction of the Ion Energy-Angle Distributions (IEAD) of the particles impacting on the wall, finding qualitative agreement and suggesting strategies of model reduction which could be used in Whole-Device Modeling. Finally, the model of the magnetic and collisional presheath has been validated against three-dimensional tomographic Laser-Induced Fluorescence measurements taken at the HELIX helicon facility at WVU. Our analysis highlights the role of neutral gas pressure, background neutral flow, and ambient electric field on the structure of the collisional and magnetic presheath, finding absolute quantitative agreement between our calculated data and experimental measurements. In particular, the work gives clear evidence of the three-dimensional structure of the magnetized plasma sheath, a unique feature not present in the classical thermal sheath in unmagnetized conditions

On the unconstrained expansion of a spherical plasma cloud turning collisionless : case of a cloud generated by a nanometer dust grain impact on an uncharged target in space

Nano and micro meter sized dust particles travelling through the heliosphere at several hundreds of km/s have been repeatedly detected by interplanetary spacecraft. When such fast moving dust particles hit a solid target in space, an expanding plasma cloud is formed through the vaporisation and ionisation of the dust particles itself and part of the target material at and near the impact point. Immediately after the impact the small and dense cloud is dominated by collisions and the expansion can be described by fluid equations. However, once the cloud has reached micro-m dimensions, the plasma may turn collisionless and a kinetic description is required to describe the subsequent expansion. In this paper we explore the late and possibly collisionless spherically symmetric unconstrained expansion of a single ionized ion-electron plasma using N-body simulations. Given the strong uncertainties concerning the early hydrodynamic expansion, we assume that at the time of the transition to the collisionless regime the cloud density and temperature are spatially uniform. We do also neglect the role of the ambient plasma. This is a reasonable assumption as long as the cloud density is substantially higher than the ambient plasma density. In the case of clouds generated by fast interplanetary dust grains hitting a solid target some 10^7 electrons and ions are liberated and the in vacuum approximation is acceptable up to meter order cloud dimensions. ..