Laser-Driven Particle Acceleration - Improving Performance Through Smart Target Design

University dissertation from Camilla Nilsson, Department of Physics, Division of Atomic Physics, Lund University

Abstract: Laser-driven particle acceleration makes use of sub-picosecond, pulsed, high-power laser systems, capable of producing intensities ~10^{19} W/cm^2 at the laser focus to form plasmas, and use ultra-relativistic and nonlinear dynamics to produce quasistatic acceleration fields. This allows electrons to be accelerated to ~100 MeV over sub-centimetre distances, while protons may be accelerated to the ~10 MeV regime. In addition, novel sources of x-ray radiation become available with these schemes. The topics covered in this thesis focus mainly on target normal sheath acceleration of protons in the overdense plasma regime and laser wakefield acceleration of electrons in the underdense regime. An experimental approach leads to novel acceleration concepts and investigations on properties of new target designs. In the overdense plasma regime, hollow microspheres were found to have the potential to enhance the conversion of laser energy into proton energy. The microscopic structure of the material used as target has impact on electron beam filamentation during electron transport through the target bulk. Long-range order was found to result in smoother beams of TNSA-produced protons as compared to amorphous structures. In addition it was demonstrated that short pulse (fs) laser-solid interactions produce magnetic fields, the strength of which can reach 10 kT, mimicking astrophysical conditions. In the underdense regime, it was found that when tailored appropriately, density ramps can provide means of dividing the laser wakefield acceleration process into four steps: nonlinear laser evolution, trapping, bunch transfer into the second bucket, and acceleration, resulting in beams with reduced relative energy spread and divergence compared to self-injection by a nonlinear plasma wave. It was further shown that capillaries can be used to improve efficiency by guiding and refocusing the laser light onto the central axis. Short bursts of soft x-rays were produced inside capillaries. Finally, the use of an asymmetric laser field at the focus facilitated off-axis electron injection into the accelerating phase of a plasma wake oscillation and enhanced x-ray emission.