The physics of streamer discharge phenomena

In this review we describe a transient type of gas discharge which is commonly called a streamer discharge, as well as a few related phenomena in pulsed discharges. Streamers are propagating ionization fronts with self-organized field enhancement at their tips that can appear in gases at (or close to) atmospheric pressure. They are the precursors of other discharges like sparks and lightning, but they also occur in for example corona reactors or plasma jets which are used for a variety of plasma chemical purposes. When enough space is available, streamers can also form at much lower pressures, like in the case of sprite discharges high up in the atmosphere. We explain the structure and basic underlying physics of streamer discharges, and how they scale with gas density. We discuss the chemistry and applications of streamers, and describe their two main stages in detail: inception and propagation. We also look at some other topics, like interaction with flow and heat, related pulsed discharges, and electron runaway and high energy radiation. Finally, we discuss streamer simulations and diagnostics in quite some detail. This review is written with two purposes in mind: First, we describe recent results on the physics of streamer discharges, with a focus on the work performed in our groups. We also describe recent developments in diagnostics and simulations of streamers. Second, we provide background information on the above-mentioned aspects of streamers. This review can therefore be used as a tutorial by researchers starting to work in the field of streamer physics.Comment: 89 pages, 29 figure

Collisional effects on electrostatic shock waves and heating in laser-generated plasmas

Electrostatic shock waves are associated with an electrostatic field structure propagating at supersonic speed through laboratory or astrophysical plasmas. Shock ion acceleration schemes, based on the strong electrostatic field in the shock structure, show promising potential due to the narrow energy spread of accelerated ions – which can be applied in plasma diagnostics, the generation of warm dense matter or medical purposes. The use of high-intensity laser pulses to generate shocks in the laboratory commonly result in plasmas which are weakly collisional; thus collisions are usually neglected in the corresponding theoretical, kinetic studies. By contrast, this thesis considers the effects of collisions on the structure and dynamics of electrostatic shocks as well as laser absorption and subsequent plasma heating.First, the structure of electrostatic shocks is considered in weakly collisional plasmas, via a semi-analytical model. Collisions are found to cumulatively affect the shock structure on longer time scales, despite the low collisionality. Then, the impact of collisions on laser-driven plasmas is analyzed via numerical, particle-in-cell, simulations. The importance of collisions is heightened in plasmas comprising highly charged ions at solid density. Collisional inverse Bremsstrahlung heating is found to be able to generate well-thermalized electrons at energy densities relevant for warm- and hot-dense-matter applications. The strong electron heating also creates favorable conditions for electrostatic shocks. Collisions between shock-accelerated and upstream ions are found to increase the fraction of accelerated ions, thus bootstrapping the shock ion acceleration. Lastly, collisional ion heating is studied in connection to the shock. Different modeling approaches available to treat the highly collisional, solid density plasmas may predict qualitatively different shock dynamics, providing an opportunity for experimental model validation

Collisional effects and attosecond diagnostics in laser-generated plasmas

When matter is radiated by laser light of extreme intensity, it is rapidly ionized, thereby forming a plasma. Such laser-generated plasmas can be used as sources of energetic particles and radiation, or to study astrophysically relevant phenomena in the laboratory and the behavior of matter under extreme conditions. This thesis considers the dynamics and diagnosis of laser-induced plasmas, with focus on the effect of Coulomb collisions on electrostatic shocks and laser-energy absorption, as well as ultra-rapid plasma diagnostics using attosecond pulses.Electrostatic shocks in plasmas have the potential to accelerate ions with a very narrow energy spread. First, collisional effects on electrostatic shocks are studied in two regimes of low and high collisionality. In the former, we show that even rare collisions can significantly affect the structure of the electrostatic shock over long time scales due to an accumulation of trapped ions. The high-collisionality case was studied using particle-in-cell simulations of laser foil targets. Effective ion acceleration by electrostatic shocks relies on a high electron temperature. Heating of the upstream ions, through collisions with the shock-accelerated ions, creates a self-amplifying process that increases the fraction of accelerated ions. However, this unstable condition rapidly depletes the energy of the shock, which transitions into a blast wave, unable to accelerate ions.An additional study of the same laser--solid interaction shows that, unlike the commonly held knowledge, collisions may dominate the energy absorption of ultraintense laser pulses through inverse bremsstrahlung, and also causing rapid thermalization of the target electrons.Finally, two diagnostic methods for the electron density utilizing attosecond extreme-ultraviolet pulses, are presented. The first method is based on the dispersion of a probe pulse, which can be used to infer information about the peak density and line-integrated density of the probed plasma. The second method is based on stimulated Raman scattering, which uses two pulses, and can give a localized reading of the electron density in the interaction regions where the two pulses meet

Laser-Cooled Ion Beams and Strongly Coupled Plasmas for Precision Experiments

The first part of this thesis summarizes the results of laser-cooling of relativistic C3+ ion beams at the ESR/GSI. It is shown that laser cooling at high beam energies is feasible and that momentum spreads much smaller than those observed for electron cooling can be achieved. Resulty indicate that space-charge dominated beams have been observed, reaching the regime of strong coupling which is an essential prerequisite for beam crystallization. Moderate electron cooling was employed to create three-dimensionally cold beams. With the laser cooled beams it was possible to perform precision VUV spectroscopy of the cooling transition. In the second part results on large-scale realistic simulations on the stopping of highly charged ions in a laser-cooled one-component plasma of 24Mg+ ions confined in a harmonic potential are presented. It is shown that cooling times short enough for cooling unstable nuclei can be achieved and fast recooling of the plasma is possible. With this cooling scheme highly charged ions for precision experiments such as mass spectrometry in Penning traps at millikelvin temperatures can be delivered

Quantized vortices in superfluid helium and atomic Bose-Einstein condensates

This article reviews recent developments in the physics of quantized vortices in superfluid helium and atomic Bose-Einstein condensates. Quantized vortices appear in low-temperature quantum condensed systems as the direct product of Bose-Einstein condensation. Quantized vortices were first discovered in superfluid 4He in the 1950s, and have since been studied with a primary focus on the quantum hydrodynamics of this system. Since the discovery of superfluid 3He in 1972, quantized vortices characteristic of the anisotropic superfluid have been studied theoretically and observed experimentally using rotating cryostats. The realization of atomic Bose-Einstein condensation in 1995 has opened new possibilities, because it became possible to control and directly visualize condensates and quantized vortices. Historically, many ideas developed in superfluid 4He and 3He have been imported to the field of cold atoms and utilized effectively. Here, we review and summarize our current understanding of quantized vortices, bridging superfluid helium and atomic Bose-Einstein condensates. This review article begins with a basic introduction, which is followed by discussion of modern topics such as quantum turbulence and vortices in unusual cold atom condensates.Comment: 99 pages, 20 figures, Review articl