Investigation of laser prepulse effects in plasma formation from laser-solid foil interactions and electron beam dynamics studies in the laser wakefield acceleration

Abstract

Particle accelerators have been one of the fundamental research facilities for various research. The concept of compact, laser-driven particle accelerators with plasma have been intensively researched since Tajima and Dawson conceived the idea in the late 1970's. In 2019, researchers at BELLA have accelerated electron beams to a 7.8 billion electron volts (GeV) energy gain over a 20-centimetre-long plasma waveguide. Laser-driven ion acceleration research was active since 2000, has reported the generation of almost 100 MeV protons by Higginson et al. in 2018. This shows that the laser-plasma accelerator (LPA) will be an alternative acceleration method to replace the conventional radio frequency (RF) technology accelerators. Despite the impressive progress to date, detailed studies to overcome some issues in laser wakefield acceleration (LWFA) and the ion acceleration are essential to enhance the realisation of this concept for stable user-facilities. With the state-of-the-art of the laser technology advancement, sub-picosecond or femtosecond (fs) high power laser system (~TW) are easily accessible in the university laboratories. A typical chirped-pulse-amplification-based laser pulse intensity of 10^19 W=cm2 with a contrast ratio 10^6-10^8 on average will give an intensity of amplified spontaneous emission (ASE) or prepulses in the range of > 10^11 - 10^13 W/cm2. Such intensity is sufficient to ionise the target before the main peak arrives and influence the outcome of the experiment. The plasma formation from the prepulse effects might enhance the coupling (absorption) of the laser to yield high energy ions during the arrival of the main laser pulse for the acceleration stage. Efforts to increase the maximum ion energy have largely involved thin foil targets. Therefore, we reported the prepulse effects on the plasma formation from thin foil targets commonly used in ion acceleration experiments. The time-space evolution of the plasma as a result of the inevitable ASE from chirped-pulse-amplification (CPA) lasers is performed using Mylar and aluminium. Time-resolved pump-probe technique was used and 1-D analytical model was developed for this work. The results from our studies are reported in this dissertation. Two separate beam lines, i.e., 1 TW and 20 TW laser systems were developed and characterised to support various laser-plasma interaction experiments in Laser Plasma Acceleration Laboratory (LPAL), Gwangju Institute of Science and Technology (GIST). The generation of terahertz (THz) radiation from air-plasma filamentation was performed using the newly developed 1TW/35 fs laser system. The amplified output laser pulse from the 4-pass laser amplifier was measured and the pulse characterisation results were reported in this dissertation. The uncompressed 4-pass amplifier output pulse also acts as a `seed pulse' to the 5-pass main laser amplifier in the 20 TW beam line. Final compressed laser pulse gave the pulse energy of maximum 1 Joule with centre wavelength (CWL) at 800 nm and pulse duration 35 fs at full-width-half-maximum (FWHM). The electron beam generation using a gasjet with premixed gas (99.5% He + 0.5% N2) was performed. The results from the electron beam diagnostics system were shown in this dissertation. This experiment is a good way to start the setup of the LWFA experiments. The multi-staged accelerators depend collectively on the parameters of the injected electron beam, the booster stage, and the nonlinear transverse dynamics of the electron beam in the laser pulse wake, and hence it is crucial to couple the consecutive accelerator stages effectively. The beam dynamics of the laser wakefield acceleration generated electron beam bunches in the plasma-vacuum (P-V) boundary was performed using two-dimensional particle-in-cell (2D-PIC) simulation code, ‘cplPIC’, developed by M. S. Hur at UNIST, Korea. The emittance growth of the accelerated electron beam bunches with various drift lengths at the exit of the P-V boundary was reported in this dissertation. The beam dynamics results allow us to design a better beam transport line and improve the quality of the beam bunches in terms of emittance, divergence, and brightness with the beam properties that fulfil the minimum conditions: transverse size of a few microns, beam duration of 10 fs or less and charge in tens of pico-Coulomb orders. From the simulation results, the longer drift length mitigates the electron beam emittance better. We reported that the emittance growth showed a gradient of approximately ~1:5 pi-mm-mrad per 1 mm propagation in the plasma region, while the gradient of emittance growth in vacuum is ~5:3 pi-mm -mrad/mm at drift length del_l= 60 um. With a longer drift length at del_l = 300 um, the gradient of the emittance growth is reduced to ~2:8 pi-mm-mrad/ mm. In conclusion, the work done in this dissertation aimed to provide an insight to improve the design of a novel laser-plasma-based particle accelerator.