Subpicosecond Thomson Scattering Measurements of Optically Ionized Helium Plasmas

We present the first subpicosecond time-resolved temperature measurements of plasmas produced by high-intensity optical ionization. Thomson scattering is used to measure electron and ion temperatures of helium plasmas created by 125 fs, 800 nm laser pulses focused to an intensity of 2 × 1017 W/cm2. We find that the electron temperature is accurately predicted by a tunneling ionization model. The measured ion temperature is consistent with direct heating by the laser pulse

Short-Pulse Driven Transport Measurements in Dense Plasmas

Accurate transport properties—such as opacity, and electrical & thermal conductivities—provide crucial input for the intricate physics models necessary to describe the dynamics of complex, high energy density (HED) systems. This includes stars, giant planets, and inertial confinement fusion plasmas. However, these theoretical transport models present challenges as the phase space often sits at the intersection of solid, liquid, gas, and plasma where many effects of comparable magnitude must be considered. Additionally, the transient nature of such high energy density materials complicates experimental measurement, and many theories remain sparsely benchmarked by data. In the laboratory, HED material must be created via some combination of material compression to very high densities or by adding large amounts of energy to the material in a very short time. This thesis focuses on experiments utilizing the second technique. X-ray free-electron lasers (tau < 100 fs) or short-pulse lasers (tau < 1 ps) are capable of heating materials from room temperature to tens or even many hundreds of eV while keeping densities at appreciable fractions of their ambient value. This allows for the probing of material properties before hydrodynamics phenomena become dominant. First, an experimental platform designed to constrain thermal conductivity models in warm dense matter is presented. Its basis relies on differentially heating multilayer targets (one high-Z layer and one low- to mid-Z layer) to generate a thermal gradient. This concept was first demonstrated using the Titan laser at the Jupiter Laser Facility, creating an intense proton beam to heat a gold/aluminum multilayer target. The temperature, reflectivity, and expansion of the rear surface were observed with time-resolved diagnostics as the thermal energy from the hot gold layer reached the coldest part of the aluminum layer. The data were compared with hydrodynamics models that xxiii self-consistently used the electrical and thermal conductivities to calculate observables. Measured temperatures were too low relative to predictions, possibly indicating the need to decrease tested conductivity models. This experiment was repeated using an X-ray free-electron laser at the Linac Coherent Light Source (LCLS) with gold/iron targets. Data are presented for this work along with calculations and a discussion of how the different drivers impact the experimental design and data quality. Finally, data from a platform designed to measure opacities using short-pulse lasers at the Orion Laser Facility are presented. Spectroscopic measurements of silicon’s K-shell that are both temporally and angularly resolved are benchmarked against the radiation transfer code Cretin. The validity of the commonly-used escape factor approximation is tested against the full solution of the radiation transfer equation and found to be in good agreement for presented experimental conditions. An analysis of the effects of radial gradients on spectroscopically inferred temperatures is found to lead to errors in the peak temperature as large as 50% as well as incorrect cooling rates. This emphasizes the importance of absolute emissivity calibrations and spatially resolved spot size measurements.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/153443/1/ajmckelv_1.pd

Innovative XUV and X.ray Plasma Spectroscopy to explore Warm Dense Matter

Zusammenfassend widmet sich die vorliegende Arbeit der Erforschung definierter Zustaende warmer dichter Materie. Dieses Wissen ist von herausragender Bedeutung für die Laser-Fusions-Forschung, die Erforschung von Materiezustaenden mit hoher Dichte, sowie eine Vielzahl astrophysikalisch-relevanter Fragestellungen der Plasmaforschung. Die vorliegende Arbeit zeigt innovative Methoden der Erzeugung und Charakterisierung warmer dichter Materie auf. Dieser Nicht-Gleichgewichts-Zustand zwischen kaltem Festkoeorper und idealem Plasma wird durch Experimente mit hoher Energiedichte erzeugt. Seine Eigenschaften wurden mit hoher Praezision studiert. Die Erzeugung eines definierten Zustandes warmer dichter Materie mit Hilfe optischer Kurzpluslaser hoher Intensitaeten ist herausfordernd. Energiereiche Elektronen aus einem kleinen, heißen Plasma auf der Targetvorderseite heizen die kalte Materie durch Stoßionisation auf. Hohe spektrale sowie raeumliche Aufloesungen sind noetig, um nicht ueber verschiedene Plasmazustaende zu mitteln. Der Autor wendet hoch entwickelde Roentgenspektroskopie charakteristischer Emissionslinien, gefolgt von numerischer Datenverabeitung, an. Das Ergebnis enthaelt viele Informationen, da die Linienform durch die Plasmaparameter beeinflusst wird und die Roentgenstrahlen zudem transparent für Festkoerperdichte sind. Alternative Wege zur Erzeugung warmer dichter Materie sind momentan im Fokus der Hohe-Energiedichte-Forschung. In der vorliegenden Arbeit wird erstmals die Erzeugung warmer dichter Materie durch direkte Photoionisation gebundener Elektronen mit den Femtosekundenpulsen des weltweit ersten weichen Roentgenlasers FLASH demonstriert. Innerhalb der durch die internationale Peak-Brightness-Collaboration geschaffenen Rahmenbedingungen war der Autor verantwortlich für Pionierarbeiten. Schließlich gelang die erste vollstaendige Charakterisierung warmer dicher Materie an FLASH sowie die erste Demonstration saettigbarer Absorption im weichen Roentgenbereich

TOPICAL REVIEW: Relativistic laser–plasma interactions

By focusing petawatt peak power laser light to intensities up to 1021 W cm −2, highly relativistic plasmas can now be studied. The force exerted by light pulses with this extreme intensity has been used to accelerate beams of electrons and protons to energies of a million volts in distances of only microns. This acceleration gradient is a thousand times greater than in radio-frequency-based accelerators. Such novel compact laser-based radiation sources have been demonstrated to have parameters that are useful for research in medicine, physics and engineering. They might also someday be used to ignite controlled thermonuclear fusion. Ultrashort pulse duration particles and x-rays that are produced can resolve chemical, biological or physical reactions on ultrafast (femtosecond) timescales and on atomic spatial scales. These energetic beams have produced an array of nuclear reactions, resulting in neutrons, positrons and radioactive isotopes. As laser intensities increase further and laser-accelerated protons become relativistic, exotic plasmas, such as dense electron–positron plasmas, which are of astrophysical interest, can be created in the laboratory. This paper reviews many of the recent advances in relativistic laser–plasma interactions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48918/2/d308r2.pd