Investigation on terahertz and X-ray generation by laser-wakefield-accelerated electrons

Abstract

This dissertation presents the characteristics and principles of terahertz (THz) and X-ray pulses generated from the laser wakefield acceleration (LWFA) of electrons. The electrons accelerated by LWFA at 100-terawatt (TW) laser power was characterized by electron spectrometers and imaging plates (IP) for energy and charge. An adaptive optics system was implemented in the laser beam path to achieve high peak intensities at the focus, and various gas compounds and densities were tested to find the optimal conditions for generating high-quality electron beams. The accelerated electrons were observed to generate high-energy THz radiation, which was measured by a combination of pyroelectric detectors (PEDs) and THz filters. The energy of emitted THz radiation was more than 4 millijoules at frequencies below 10 THz. Such radiation was characterized by its spectrum, polarization, coherence, and dependence on the gas target and electron beam properties. The mechanism of THz radiation was also examined with analytic calculations and particle-in-cell (PIC) simulations. The work in this dissertation shows that the intense THz radiation observed in LWFA is attributed to coherent radiation by accelerating electrons. The electrons accelerated by the laser ponderomotive force and subsequent plasma wakefields radiate broadband emission continuously along the laser propagation direction, ultimately resulting in phase-matched conical THz radiation in the far field. This conclusion is drawn from an extensive analysis of the measured temporal and spectral THz waveforms, THz scalings with charge and polarization, and a comparison with theory. Such strong THz radiation can be potentially applied to molecular alignment, harmonic generation, and THz-driven electron acceleration. In addition, unusual X-rays, produced in the transverse direction in the process of LWFA driven at the petawatt (PW) level, are measured and characterized. Such X-rays originate from bremsstrahlung radiation produced by energetic electrons ejected in the transverse direction by the plasma wakefields and traversing a cell window. The measured X-ray photon spectra have peak energies of 150200 keV. A PIC simulation result confirms that multi-MeV electrons can be ejected perpendicular to the laser propagation direction in the LWFA process. In addition, a FLUKA simulation shows that such electrons can produce ~100 keV X-rays when they traverse a solid material such as a cell window. In practice, this radiation can be problematic as it imposes radiation concerns and also serves as background radiation noises, potentially degrading the signal to be detected. Research on terahertz and X-ray generation by laser-wakefield-accelerated electrons is expected to enable intense THz pulse generation experiments using PW-class high-power lasers that have not been previously studied. And using the generated strong THz radiation will open up a new horizon of high intensity-THz studies such as molecular alignment, harmonic generation, and electron acceleration.