On the role of focal spot size in ultra-intense laser-solid interaction physics

• Main Author: Wilson, Robbie
• Published: University of Strathclyde 2018
• Subjects: 530
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.742074

This thesis reports on experimental investigations examining the role of laser-pulse focal spot size on key aspects of laser-solid interactions, namely laser-driven proton acceleration and laser-energy absorption, at the current state-of-the-art peak laser intensities (1020-1021 W/cm2). This includes the development of new optical tools and diagnostics to achieve these intensities and explore the resultant physics. The interaction of intense laser pulses (>1018 W/cm2) with solid foils has received considerable attention over the last few decades, motivated by their ability to generate high energy particles (electrons and ions), photons (x-rays, γ-rays and THz emission) and gigagauss magnetic fields. The development of these novel sources of high energy particles and radiation requires understanding of the underpinning physics on parameters such as the laser focal spot size. The work reported here is structured into three main studies. The first study presents the development of an ellipsoidal F/1 focusing plasma mirror (FPM) capable of increasing the peak intensity achievable on a petawatt level laser system, through focal spot size reduction. A factor of 2.5 reduction in spot size (from 4.0 μm to 1.6 μm [FWHM]) is achieved when compared to F/3.1 focusing with a conventional (solid state) optic. This corresponded to a factor of 3.6 enhancement in peak intensity, taking into account changes in plasma mirror reflectivity and focal spot quality. The sensitivity of FPM operation to misalignment is also investigated, this is vital for its successful development. An example use of a FPM, in an investigation of laser-driven proton acceleration, is demonstrated. The intensity increase (3x1020W/cm2 to 1021 W/cm2) results in a factor of 2 increase in the maximum energy (from 27 MeV to 53 MeV) of sheath-accelerated protons from a foil target. This study helps to move the concept of focusing plasma mirrors beyond demonstration, towards routinely used tools in laser-plasma research, and enables a window into future research through the intensity enhancement achieved. The developed F/1 FPM is employed in the second study to investigate the inuence of using tightly focused (near-wavelength sized) focal spot geometry on the properties of beams of accelerated protons generated by the target normal sheath acceleration (TNSA) mechanism. When comparing beam measurements for tight focusing to relativity larger (x2.5 larger) focal spot irradiation, significant variations are found. These include, a slower maximum proton energy scaling with laser intensity (from I0:6 to I0:2), a x 2.5 enhancement in laser-to-proton energy conversion efficiency and a significant degree of target edge emitted protons with tight focusing compared to the larger focal spot measurements. The findings are explained in terms of changes to the evolution dynamics of the rear surface electron spatial distribution, with enhanced lateral electron spreading and subsequent recirculation, under tight focusing compared to the larger spot. Through 2D particle-in-cell simulations this effect is suggested to derive from the front surface interaction dynamics, with a wider angular distribution of fast electrons throughout the target observed with tight focusing. This study is timely considering the employment of low F/# optics is a pulse focusing scheme under consideration by several existing, and future laser facilities as a route to higher intensities. The final study reports on an investigation of laser-energy absorption into dense plasma. Using a suite of diagnostics, the total reected laser energy as a function of intensity is measured, distinguishing between the inuence of laser energy and focal spot size on energy absorption. Good agreement is found with previously published data on the scaling of absorption with intensity, by variation of pulse energy. However,when the intensity is controlled by variation of the focal spot size, higher absorption values are measured (45% with a relatively large [270 μm FWHM] focal spot, compared to 22% with a tight focus [7 μm FWHM], both at an intensity of ~5 x1017 W/cm2) and a slower absorption scaling is observed, relative to the pulse energy variation case. Through 2D particle-in-cell simulations this difference is shown to arise from additional energy gained by the population of electrons recirculating within the target due to multiple interactions with the laser pulse, a process dependent upon; the pulse duration, target thickness, focal spot size, and the energy spectrum and divergence of the fast electrons. A simple geometric electron recirculation model is presented to explore this absorption concept. This investigation has important consequences for fundamental understanding, application development and, most immediately, for experimental methodology.