The production of intense x-ray and particle sources is one of the most remarkable aspects of high energy laser interaction with a solid target. Wide application of these laser-driven secondary sources requires a high yield, which is partially limited by the amount of laser energy absorbed by the target. Here, we report on the enhancement of laser absorption and x-ray and particle flux by target surface modifications. In comparison to targets with flat front surfaces, our experiments show exceptional laser-to-target performance for our novel cone-shaped silicon microstructures. The structures are manufactured via laser-induced surface structuring. Spectral and spatial studies of reflectance and x-ray generation reveal significant increases of the silicon K a line and a boost of the overall x-ray intensity, while the amount of reflected light decreases. Also, the proton and electron yields are enhanced, but both temperatures remain comparable to those of flat foil targets. We support the experimental findings with 2D particle in cell simulations to identify the mechanisms responsible for the strong enhancement. Our results demonstrate how custom surface structures can be used to engineer high power laser-plasma sources for future applications.

The spatial-intensity profile of light reflected during the interaction of an intense laser pulse with a microstructured target is investigated experimentally and the potential to apply this as a diagnostic of the interaction physics is explored numerically. Diffraction and speckle patterns are measured in the specularly reflected light in the cases of targets with regular groove and needle-like structures, respectively, highlighting the potential to use this as a diagnostic of the evolving plasma surface. It is shown, via ray-tracing and numerical modelling, that for a laser focal spot diameter smaller than the periodicity of the target structure, the reflected light patterns can potentially be used to diagnose the degree of plasma expansion, and by extension the local plasma temperature, at the focus of the intense laser light. The reflected patterns could also be used to diagnose the size of the laser focal spot during a high-intensity interaction when using a regular structure with known spacing.

Laser-driven proton acceleration from ultrathin foils is investigated experimentally using f /3 and f /1 focusing. Higher energies achieved with f /3 are shown via simulations to result from self-focusing of the laser light in expanding foils that become relativistically transparent, enhancing the intensity. The increase in proton energy is maximized for an optimum initial target thickness, and thus expansion profile, with no enhancement occurring for targets that remain opaque, or with f /1 focusing to close to the laser wavelength. The effect is shown to depend on the drive laser pulse duration.

Multi-species ion acceleration from ultra-thin foils was studied experimentally, employing the Vulcan Petawatt laser at the Central Laser Facility, UK. Plastic (CH) foils with thicknesses in the range 10nm - 340nm were irradiated with intense, short (750 fs) laser pulses producing maximum energies of ∼65 MeV and 25 MeV/nucleon obtained for H+ and C6+ ions, respectively. Ion spectra obtained from high resolution spectrometers suggest differences in the acceleration dynamics for the two species. Comparisons are made with 2-dimensional Particle in Cell simulations which identify, for an optimal thickness, two main mechanisms contributing to the ion acceleration process, namely multi-species Target Normal Sheath Acceleration and Radiation Pressure Acceleration. Ion energies are further enhanced by the onset of relativistically induced transparency. A final stage in the acceleration is caused by the formation of electron jets (as the target undergoes transparency), which accelerate the ions off-axis. By analysing the spatial and temporal evolution of the accelerating field, we are able to infer the effect of the different mechanisms on each species and how this translates to the experimental observations. The two main mechanisms, TNSA and RPA, are seen to each produce a distinct population of high energy protons whereas a single population of carbon is accelerated by a summation of these effects. This species specific analysis sheds new light on the complex dynamics in a multi-species target expansion and on the contribution of different acceleration processes to the acceleration of the most energetic ions in the spectrum.

Increasing the intensity to which high power laser pulses are focused has opened up new research possibilities, including promising new approaches to particle acceleration and phenomena such as high field quantum electrodynamics. Whilst the intensity achievable with a laser pulse of a given power can be increased via tighter focusing, the focal spot profile also plays an important role in the interaction physics. Here we show that the spatial-intensity distribution, and specifically the ratio of the intensity in the peak of the laser focal spot to the halo surrounding it, is important in the interaction of ultraintense laser pulses with solid targets. By comparing proton acceleration measurements from foil targets irradiated with by a near-diffraction-limited wavelength scale focal spot and larger F-number focusing, we find that this spatial-intensity contrast parameter strongly influences laser energy coupling to fast electrons. We find that for multi-petawatt pulses, spatial-intensity contrast is potentially as important as temporal-intensity contrast.

J. H. J a r r e t t a n d D . W. B r i t e ABSTRACTThe r e s u l t s of an i n v e s t i g a t i o n of t h e r e a c t i o n of polonium-210 vapor w i t h r a r e -e a r t h f i l m s a r e r e p o r t e d . Hand l i n g t e c h n i q u e s and r e a c t i o n c o n d i t i o n s r e q u i r e d f o r p r e p a r at i o n of t h i n f i l m s of r a r e -e a r t h p o l o n i d e s w i t h good i n t e g r i t y and optimum thermal s t a b i l i t y above 1000 O C a r e d e s c r i b e d .

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The response of the BAS-TR image plate (IP) was absolutely calibrated using a CR-39 track detector for high linear energy transfer Au ions up to ∼1.6 GeV (8.2 MeV/nucleon), accelerated by high-power lasers. The calibration was carried out by employing a high-resolution Thomson parabola spectrometer, which allowed resolving Au ions with closely spaced ionization states up to 58+. A response function was obtained by fitting the photo-stimulated luminescence per Au ion for different ion energies, which is broadly in agreement with that expected from ion stopping in the active layer of the IP. This calibration would allow quantifying the ion energy spectra for high energy Au ions, which is important for further investigation of the laser-based acceleration of heavy ion beams.