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Optical Response of Plasmas From Moderate Intensity To The Relativistic Regime

Dec 31, 2021. Dissertations


This thesis explores the interaction of pulsed lasers with plasmas in the relativistic regime, i.e. when the kinetic energy of electrons in the laser field exceeds mc². Plasmas in such conditions can be created in the laboratory using petawatt class lasers and high-power lasers are rapidly being built around the world to explore this regime. The experimental capabilities needed to explore laser plasma interactions at solid density currently lag behind the capabilities of the lasers themselves, leading to possible gaps in exploring fundamental processes. In addition, exciting applications for intense laser-matter interactions require multiple technical breakthroughs to achieve long term goals. Here, improved target and plasma mirror technology that increases experimental capabilities in both the low and high plasma density regimes required for applications are presented, along with a novel study that exposes new characteristics of an often cited and important effect called relativistic transparency (RT). At low density, petawatt lasers can be used to drive Laser Plasma Accelerators (LPA). LPA's are currently capable of producing electron beams with multi-GeV energies from gas targets over acceleration lengths on the order of 10 centimeters, more than 100x shorter than conventional methods. The continued development of this scheme requires redirection of laser pulses near the laser focus, where the high intensity is certain to damage traditional optics. At high (solid) density, intensities often exceed 10^21 W/cm², and excellent pre-pulse contrast is essential for understanding many experiments because target damage thresholds are near 10^12 W/cm² (corresponding to a 10-9 contrast ratio). The use of a plasma mirror is a common method for increasing pulse contrast but traditionally operates at repetition-rates on the order of a few shots per hour. Presented in this work is the development and characterization of a new device called the Spinning Disk Inserter (SDI) based on liquid crystal films capable of producing ultra-thin plasma mirrors (<30 nm) at a >1 Hz repetition-rate. For reflected beam quality, the flatness and angular stability of SDI plasma mirrors are analyzed. The measured flatness is better than 200 nm peak-to-valley over a 3 mm diameter area and the angular variation in the reflected beam is about 4.1 mrad. Because the thickness of these plasma mirrors is an important feature for contrast enhancement and applications to LPAs, a new diagnostic for repetition-rate remote thickness measurement with a resolution <3 nm is presented. The SDI is used in a proof of principle experiment to redirect a petawatt-scale pulse, rejecting up to 94% of the energy, and enable a measurement of the normalized emittance a 0.8 GeV electron beam from an LPA. The normalized emittance is found to be ~4.0 micrometers. A new model combining a numerical approach and analytical theory is also developed to predict the performance of plasma mirrors with thicknesses on the order of the skin depth (~10 nm), in the intensity range of 10^14 -10^16 W/cm², a range typical for plasma mirror operation for both contrast enhancement and beam redirection applications. The model agrees with both particle-in-cell simulations and experiment better than ~5% over this intensity range. Pushing to higher repetition-rate will enable experiments not otherwise possible. RT is a far-reaching effect with implications for laser ion acceleration, high energy photon generation, relativistic laboratory astrophysics, and spatial and temporal pulse shaping. However, because of a lack of diagnostic tools, direct measurements are rare. More often, time integrated diagnostics of secondary processes are used in conjunction with simulations to infer aspects of the ultrafast RT process. Methods and results of an all optically diagnosed experiment utilizing a pump-probe geometry are presented here. This experiment measures the transparency over time of a relativistically hot target seen by a low intensity probe. Due to pre-expansion of the target, the interaction studied was with a relatively thick target, ~2 micrometers, with density a few times larger than the critical density. Ultimately, this regime gives rise to unexpected physical behavior including a return to opacity after the ultra-intense pump pulse subsides. The timescale of transparency of the target is as short as the resolution of the setup, with rise and fall times of order 50 fs. Additionally, an average rotation of the probe polarization ellipticity angle of ~7.8° at zero delay is measured for the highest pump intensities, serving as the first ever experimental evidence of a newly predicted effect, termed relativistic birefringence, due to the combination of relativistic mass increase and the anisotropic electron momentum distribution generated by an intense laser-target interaction. These results serve as an exciting development with possible applications in ultrafast optical shuttering and high intensity plasma optics. Additionally, relativistic birefringence, along with new pump-probe techniques particularly suited for high power applications developed herein, can also serves as a new diagnostic tool for relativistic laser-plasmas.


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