Studies of laser-driven ions and kilo-amp pulses in novel regimes

Abstract

The interaction of a high intensity (≥10¹⁸W/cm²) laser with a thin (≈μm) foil target generates a population of hot electrons, a fraction of which escape into the vacuum while setting up sheath electric fields at the target surfaces. These fields accelerate ions, preferentially protons, to multi-MeV energies via the target normal sheath acceleration (TNSA) mechanism, while the loss of electrons launches a neutralising, kilo-amp electromagnetic (EM) pulse away from the interaction region. This EM pulse has been used with suitable target geometries to tailor the spectral and spatial properties of the TNSA protons, with a view to their use in applications. Additionally, other acceleration mechanisms, such as radiation pressure acceleration (RPA) and transparency enhanced acceleration, have been explored as a way to more directly deliver ion beams with the desired features.

In the first experiment presented in this thesis, the high intensity, ultra-high contrast, 527nm wavelength laser of the Orion Laser Facility was utilised to attempt to improve the ion spectral features beyond those of the TNSA mechanism. This sub-picosecond pulse delivered an on-target intensity of ≈10²⁰W/cm², and ion acceleration was compared with that driven by the facility's lower contrast infrared pulse. Experimental and simulation results concerning the proton spectra suggested that the green pulse produced little to no preplasma, and would therefore not deform sub-micron thickness plastic targets. Ion acceleration from such targets showed bunch-like structures in the proton spectra, while C⁶⁺ spectra showed a maximum cut-off energy at an optimum target thickness. Supporting particle-in-cell simulations suggested that relativistically induced transparency inhibited RPA, while the direct exposure of target electrons to the laser led to an enhanced longitudinal accelerating electric field. This was responsible for bunching up the protons in the spectra, and the interplay between the transparency time and the number of electrons present was the reason for an optimum target thickness for the maximum C⁶⁺ cut-off energy. While the sub-picosecond green pulse was unsuitable for RPA, it was useful for studying transparency enhanced acceleration from sub-micron targets unperturbed by preplasma.

In the second experiment, the transport of the kilo-amp EM pulse across a vacuum gap between the target foil and a pick-off wire was investigated using proton radiography. This was done with the view to increasing the repetition rate of coil targets that use the pulse to tailor the features of TNSA protons. It was found that even up to a disconnect gap of ≈1mm, the EM pulse retained ≈25% of its amplitude compared to the connected case. Supporting numerical simulations found that the presence of ionised material on the pick-off wire was required to support the transfer of the EM pulse across the gap. The presence of electrons from this ionised material allowed the gap to ‘spark’ and for a current to flow across it. Without this ‘sparking’, the simulations showed that the gap behaved like a high-pass filter. This meant that the EM pulse could not then travel around bends in the wire, which behaved as a low-pass filter, without a flip in its polarity and the formation of a significant trailing negative peak being observed. Such a flip in polarity was not observed experimentally. The ‘sparking’ maintained power in the <0.2THz spectral components of the EM pulse, which allowed the pulse to travel around bends in the wire and for the experimental observations to be reproduced. It was also shown experimentally that the disconnect can be applied to coil targets, while the requirement for the presence of ionised material will be important for future numerical studies in this regime. Preliminary results collected during the experiment showed that the transfer of the EM pulse could be improved by varying the disconnect style; the addition of small foil flags to the pick-off wire made little difference to the magnitude of the EM pulse peak for gaps of <400μm, but did lead to a ≈25-50% increase in the peak at a gap of ≈500μm compared to the case without the foil flags.

Date of Award	Dec 2022
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Engineering & Physical Sciences Research Council
Supervisor	Satyabrata Kar (Supervisor) & Marco Borghesi (Supervisor)