Propagation of High-Intensity Ultrashort Laser Pulses in Plasmas.

The propagation of high-intensity, ultrashort duration laser pulses in plasmas is of critical importance to many experiments involving the interaction of intense laser light with matter. In this work, a variety of effects related to such propagation were studied including ionization blue-shifting, ionization refraction, and self-channeling using a state-of-the-art multi-terawatt laser system. In the first section of this dissertation I give a theoretical introduction to the propagation of high intensity laser pulses in gases. After describing the basics of gas ionization at high intensities, I then consider the refractive effects of ionization on propagation. Finally, I introduce the concept of short pulse self-channeling and provide a detailed theoretical model of this phenomenon. I describe the design and construction of a high -power, ultrashort pulse laser system in the second section of this dissertation. The system employs chirped-pulse -amplification in titanium-doped sapphire, and produces 3 TW, 100 fs pulses which can be focused to intensities of up 10^{19} W/cm ^2. A introduction to the design of laser amplifiers is presented along with a detailed description of the laser system. Many of the non-ideal effects found in a chirped-pulse-amplification system are also considered. Experimental work on the propagation of laser pulses in plasmas is studied by imaging the profile of the scattered radiation using a sensitive camera. The third section of this dissertation describes the experimental apparatus needed to perform these measurements, followed a description of the experimental results. My observations indicate that ionization refraction dominates the propagation of the pulses, causing the beam to defocus and limiting the maximum electron density which the laser can produce. An extensive attempt to observe theoretically predicted self -guided modes of propagation, or self-channeling, of the laser pulses yielded a null result under plasma conditions far above the threshold conditions. Spectroscopic measurement of the scattered and the transmitted laser light show the existence of strong blue-shifting due to rapid ionization of the gas by the laser. Intense stimulated Raman scattering was also observed, and is an indication of the presence of large amplitude electron plasma waves which may account for the lack of self-channeling. The final portion of this dissertation will deal with my observations of anomalously extended propagation, similar to what one might expect from self-channeling. These observations were made under conditions which were many times below the predicted threshold. In increase in the Rayleigh range of the laser beam by nearly a factor of three was obtained while propagating 0.5 TW pulses in a 100 torr pressure of helium.