Laser-Accelerated Proton Beams as a New Particle Source

The framework of this thesis is the investigation of the generation of proton beams using high-intensity laser pulses. Today's high power, ultrashort pulse laser systems are capable of achieving laser intensities up to 10^21 W/cm^2. When focused onto thin foil targets, extremely high field gradients of the order of TV/m are produced on the rear side of the target resulting in the acceleration of protons to multi-MeV energies with an exponential spectrum including up to 10^13 particles. This acceleration process, called Target Normal Sheath Acceleration (TNSA), generates high-current proton beams with characteristics that are superior to properties from conventional proton sources. In this work, an experimental method to fully reconstruct laser-accelerated proton beam parameters, called radiochromic film imaging spectroscopy (RIS), was developed. RIS allows for the characterization of proton beams concerning real and virtual source size, envelope- and microdivergence, normalized transverse emittance, phase space, and proton spectrum. Therefore, thin gold foils with a microgrooved rear side were manufactured and characterized. Calibrated GafChromic radiochromic films in stack configuration were used as spatial and energy resolved proton detector. The target rear side structure is transported by the beam, mapped into the detector and retains information about the beam. Since the proton beam expansion is a plasma expansion with accompanying electrons, a low-energy electron spectrometer was developed, built and tested to study the electron distribution matching to the proton beam energy distribution. Electrons in high-current proton beams have significant influence on beam neutralisation and space-charge effects. First experimental results show, that the observed electron spectrum is apparently not of the expected exponential shape, but more a peaked distribution around an avergage energy. Hence, it is not coupled to the proton spectrum, which argues for an adiabatic expansion of the electrons around the proton beam instead of co-moving electrons (v_e = v_p) with an exponential energy distribution. Two experiments were carried out at the VULCAN Petawatt laser with the aim of showing dynamic control and enhancement of proton acceleration using multiple or defocused laser pulses. Irradiating the target with a long pulse, low-intensity laser (10^12 W/cm^2) prior to the main pulse (ns), an optimum pre-plasma density scale length of 60 micrometer is generated leading to an enhancement of the maximum proton energy (25%), the proton flux (factor of 3) and the beam uniformity. The experimental results of the second campaign on defocusing of high-intensity laser pulses onto the target show significant improvements in proton flux. Proton beams were generated more efficiently than previously by driving thinner target foils at a lower intensity over a large area. The optimum condition was a 2 micrometer foil irradiated with an intensity of 10^19 W/cm^2 onto a 60 micrometer spot. Laser to proton beam efficiencies of 7.8% have been achieved (2.2% before) -- one of the highest conversion efficiencies ever achieved. RIS has contributed significantly to the understanding of the beam optimisation process and the proton parameters. In the frame of this work, two separate experiments at the TRIDENT laser system have shown that these laser-accelerated proton beams, with their high number of particles in a short pulse duration, are well-suited for creating isochorically heated matter in extreme conditions. Besides the manipulation of the proton beam parameters directly during the generation, the primary aim of this thesis was the capture, control and transport of laser-accelerated proton beams by a solenoidal magnetic field lense for further purpose. In a joint project proposal, the laser and plasma physics group of the Technische Universität Darmstadt initiated the development of a test stand to transport, focus and bunch rotate these beams by conventional ion optics and RF technology. In collaboration with the accelerator department of the GSI Helmholtzzentrum für Schwerionenforschung, a possible injection into a post-acceleration unit will be studied. Subordinated to this project, an experimental campaign to capture laser-accelerated proton beams was carried out at the PHELIX system. Compared to previous experiments, a significant increase in proton transmission through the solenoid could be achieved by maintaining the beam quality. The field strength of 7.5 T enabled collimation of protons with an energy of >10 MeV for the first time. In addition, the focusing capability of the solenoid provided a flux increase in the focal spot of about a factor of 174 at a distance of 40 cm from the source, compared to a beam without using the magnetic field. For a quantitative analysis of the experiment numerical simulations with the WarpRZ code were performed. The code, which was originally developed to study high current ion beams and aid in the pursuit of heavy-ion driven inertial confinement fusion, was modified to enable the use of laser-accelerated proton beams as particle source. The calculated energy-resolved beam parameters of RIS could be included, and the plasma simulation criteria were studied in detail. The geometrical boundaries of the experimental setup were used in the simulations. The results from the virtual simulated film stack show a remarkable agreement with the observed proton signals in the film stack of the experiment. The importance of the space-charge effects of the electrons (included as second particle species) could be pointed out showing a significant influence on the results. 2.99exp+9 collimated protons in the energy range of 13.5+-1 MeV (Delta E/E = 7%) could be transported over a distance of 40 cm. In addition, 8.42exp+9 protons in the energy range of 6.7+-0.2 MeV (Delta E/E = 3%) were focused into a spot of