Development and Applications of a Laser-Wakefield X-ray Source

Sammanfattning: In laser-wakefield acceleration (LWFA), a femtosecond laser pulse is tightly focused in a gas to intensities exceeding 1018 W/cm2 . The laser radiation ionizes the medium and excites a plasma wave that travels behind the laser pulse. Electrons can be trapped in the oscillations in the plasma density, where electric fields of the order of several hundreds of GV/m accelerate them to relativistic energies. Together with the longitudinal accelerating fields, transverse electromagnetic forces keep the electrons oscillating in the three-dimensional plasma structure around the direction of propagation of the laser. These betatron oscillations cause the emission of X-ray pulses with a broadband spectrum and femtosecond duration.The emission of such relativistic electron beams and ultrashort X-ray pulses, due to the high accelerating fields that can be sustained in the plasma, promises a new generation of compact electron accelerators. However, the complexity of the nonlinear laser–plasma interaction that produces LWFA poses challenges in the reproducibility and control of both the electron beams and the X-ray pulses.The main goals of the present work are the optimization and application of a LWFA-based X-ray source. The mechanism through which the electrons are trapped and accelerated in the plasma wave influences the final parameters governing both the electrons and X-rays. Several trapping mechanisms have been investigated: using gas mixtures, a plasma density down-ramp and a combination of the two. The advantages of these approaches were a general improvement in the shot-to-shot reproducibility and some control of the electron beam charge and spectrum. It is shown that an increase in the dopant concentration of the gas also affects the acceleration process, as it increases the contribution to the electron energy from direct laser acceleration (DLA), where the overlap of the laser fields with the charge trapped in the plasma wave led to higher transverse and longitudinal electron momenta. By the intersection of two laser pulses in the plasma, the merging of two laser wakefields is experimentally observed, producing the emission of one single electron beam and X-rays in the forward direction. Due to their effect on the electron transverse momentum, DLA and wakefield merging are expected to increase the total emission of X-rays produced by LWFA.Regarding potential uses of the X-ray source, several experiments presented here demonstrated the applicability of betatron X-rays for scientific studies in different disciplines. Phase-contrast imaging of an insect and an alloy demonstrated the resolution of a few micrometres that is possible with betatron radiation. The source was also used to test an X-ray absorption spectrometer designed for a femtosecond time-resolved study of hot plasmas with a LWFA-based X-ray source.