Detector Development, Source Characterization and Novel Applications of Laser Ion Acceleration

Abstract: The main focus of the work presented in this thesis is on experimental studies oflaser acceleration of protons and other positive ions from solid targets. The topic is explored from three different angles: firstly, the development of diagnostics adapted to the ion pulses, secondly, the characterization of the source of the energetic particles and, finally, the application of laser-accelerated protons for time-resolved radiolysis of glass and water.Detectors that can efficiently record ion pulse parameters, such as the energyspectrum and spatial profile, were developed and implemented. One of these instruments was a modified version of a Thomson parabola spectrometer. Apartfrom the typical ion energy spectra provided by such an instrument, this modifieddiagnostic tool can also provide spatial information on pulse properties and on thespectrum of laser light transmitted through the laser-plasma interaction. The other diagnostic system developed and employed made use of plastic scintillators as a position-sensitive detector, to record either proton pulse profiles, or in combination with a dipole magnet, to record proton energy spectra, in a multiple-use, real-time feedback set-up. Laser acceleration of ions is a highly non-linear process and therefore pulse-to-pulse fluctuations are commonly large. When developing the detectors emphasis was thus placed on either being able to extract information about as many proton pulse parameters as possible simultaneously, or on being able to record large amounts of data efficiently (>100s of acquisitions over a few hours). Both these measures can be used to reduce the influence of pulse-to-pulse variations when analysing experimental results. However, they fulfil different needs, as the repetition rates of high-power lasers used for ion acceleration vary, from many laser pulses per second to one per hour or less. To be able to control and optimize the processes that occur when energyis transferred in a plasma, from a high-intensity laser pulse to a population ofenergetic protons, it is vital to understand as much as possible about the acceleration mechanisms. The so-called sheath field, a TV/m electric field, at the back of a solid target, in which the protons are accelerated to high energies is especially interesting. This sheath has been experimentally characterized in terms of its transverse expansion and the way in which this expansion influences the resulting proton pulse profile. It was shown that for an angle of incidence between the laser pulse and the target foil of 45 degrees, the transverse expansion of the sheath is asymmetric and its magnitude depends on the amount of energy contained in the laser pulse. Considerable correlation was found between the spatial properties of the laser pulse focus, the sheath and theresulting proton pulse. By splitting the laser pulse into two parts, and focusing them to two independent foci, separated by a few m, it was possible to manipulate the shape of the sheath and thereby also the transverse divergence of the proton pulse. Finally, experiments were performed on optically probed picosecond protonpulse radiolysis of various materials, such as glass and water. This was done bysplitting each laser pulse so that one part drove an acceleration process, whilethe other part could be used as an intrinsically synchronised optical probe. The measurements were resolved in time by using a chirped optical probe pulse. It was found that exposure to a pulse of energetic protons, induced changes in the optical absorbance of the materials. Through these measurements it was also possible to obtain information about the proton pulse itself; in particular, the duration. Under the specific conditions used in that experiment and for a narrow energy bandwidth, the duration was found to be only 3.5 +-0.7 ps. Compared to most other sources of high-energy protons, these laser-generated proton pulses can deliver extreme doses (kGy) in short (picosecond) pulses close to their source. In the pulsed proton radiolysis of water, indications were found that such a high dose rate affects the yield of solvated electrons, a radiolytic species.