X-Ray and Electron Generation in the relativistic Lambda-cubed Regime.

Abstract

Over the last two decades, laser-plasma interactions at relativistic intensities have been carried out using large laser facilities producing at least several hundred millijoule pulses at a repetition rate of 10 Hz or lower. A less explored regime is when intensities in excess of 10sup18 W/cm2 are attained by focusing millijoule-level femtosecond pulses to a spot with a diameter comparable to the laser wavelength. This so-called relativistic λ3 regime allows the study of certain laser-plasma experiments at kilohertz repetition rate. The present dissertation contributes to the understanding of the x-ray source and hot electrons produced in this regime.

The micron-sized λ3 focus engenders a comparably sized x-ray source that could be attractive for high resolution x-ray imaging applications. With this in mind, the source size is measured for various target materials using the knife-edge technique. Furthermore, the source spatial coherence properties are investigated by analyzing the diffraction pattern off a straight edge.

Also investigated are the spatial and energy distributions of hot electrons escaping the plasma. In the case of an Al plasma, the electrons have a Maxwellian-like energy distribution with a temperature that scales with (I λ2)~0.6 in the 10sup17 − 2×10sup18 W/cm2 intensity range. On the other hand, in the case of an SiO2 plasma with λ/2 scale-length, the electrons are emitted in a collimated relativistic jet having a non-Maxwellian distribution with Ei = 675 keV. This is the first demonstration of laser-generated relativistic electron beams at kilohertz repetition rate.

Additionally, this dissertation reports on two pioneering demonstrations in a related but fundamentally different regime, that of high-average power fiber lasers. In the first experiment, Ni Kα x-rays are produced using a fiber CPA system at the intensity of 2×10sup18 W/cm2, the highest reported to date from a fiber system. The conversion efficiency into the Kα photons is comparable to those obtained from Ti:sapphire lasers with similar pulse-energy.

In another experiment, extreme UV radiation is generated from a bulk Sn target using a nanosecond fiber laser. This result has confirmed that fiber lasers are a potential contender to drive the source of the next generation EUV lithography instrument.