Multiple pulse sheath acceleration : an optical approach to spectral control

Recent experimental results have shown that it is possible to produce laser-accelerated proton and ion beams with distinct quasi-monoenergetic features in the energy spectrum [1, 2]. As short-pulse ultraintense laser intensities exceed 1021Wcm−2, it may be possible to produce quasi-monoenergetic proton bunches with energies in the range of 100-200 MeV. This opens up the prospect of a new route to developing medical ion accelerators for oncology. In this paper we will briefly report on some of our recent work [3]. This showed that it is theoretically possible to produce laser-accelerated proton/ion beams with distinct spectral peaks by irradiating a solid target with two laser pulses that arrive in rapid succession. No special target composition or structure is required, unlike the other schemes that have been proposed [1, 2]. This may be advantageous for certain applications

Femtosecond, two-dimensional spatial Doppler mapping of ultraintense laser-solid target interaction

We present measurements of the spatio-temporal evolution of a hot-dense plasma generated by the interaction of an intense 25 femtosecond laser pulse with a solid target, using pump-probe two-dimensional Doppler spectrometry. Measuring the time-dependent Doppler shifts at different positions across the probe beam, we achieve velocity mapping at hundreds of femtoseconds time resolution simultaneously with a few micrometer spatial resolution across the transverse length of the plasma. Simulations of the interaction using a combination of 2D particle-in-cell (PIC) and 2D radiation hydrodynamics codes agree well with the experiment

Formation and evolution of post-solitons following a high intensity laser-plasma interaction with a low-density foam target

The formation and evolution of post-solitons has been discussed for quite some time both analytically and through the use of particle-in-cell (PIC) codes. It is however only recently that they have been directly observed in laser-plasma experiments. Relativistic electromagnetic (EM) solitons are localised structures that can occur in collisionless plasmas. They consist of a low-frequency EM wave trapped in a low electron number-density cavity surrounded by a shell with a higher electron number-density. Here we describe the results of an experiment in which a 100 TW Ti:sapphire laser (30 fs, 800 nm) irradiates a 0:03 gcm^-3 TMPTA foam target with a focused intensity I_l = 9:5x10^17 Wcm^-2. A third harmonic (lambda_probe ~ 266 nm) probe is employed to diagnose plasma motion for 25 ps after the main pulse interaction via Doppler-Spectroscopy. Both radiation-hydrodynamics and 2-D PIC simulations are performed to aid in the interpretation of the experimental results. We show that the rapid motion of the probe critical-surface observed in the experiment might be a signature of post-soliton wall motion

Investigation of jet formation from the blast wave of a locally heated laser-irradiated target

AbstractA possible mechanism responsible for the formation of jets observed near young stellar objects is thought to involve conically converging flows which are generated when the stellar wind encounters an inward facing shock at an oblique angle. While this mechanism of inertial collimation has been verified by simulations, it is not accessible to direct observations due to the small scales on which it operates. Until recently, laboratory experiments have only been able to reproduce the second part of the mechanism by directly creating a converging conical flow to produce a jet. In this contribution we present a conceptual numerical study proposing an new configuration to create jets that are able to reproduce both stages of the mechanism, including the inward facing reverse shock, from simple initial conditions. By selectively heating a small region inside a target, irradiated by a high-intensity laser pulse, a jet can be created inside the plasma behind the rear target surface. We present three dimensional simulations of the formation of the jet. We find jets with aspect ratios of over 15 and Mach numbers between 2.5 and 4.3. The influence of simulation parameters is investigated and the applicability of the jets to their astrophysical counterparts is discussed

Tunable mega-ampere electron current propagation in solids by dynamic control of lattice melt

The influence of lattice-melt-induced resistivity gradients on the transport of mega-ampere currents of fast electrons in solids is investigated numerically and experimentally using laser-accelerated protons to induce isochoric heating. Tailoring the heating profile enables the resistive magnetic fields which strongly influence the current propagation to be manipulated. This tunable laser-driven process enables important fast electron beam properties, including the beam divergence, profile and symmetry, to be actively tailored, and without recourse to complex target manufacture

Spectral control in proton acceleration with multiple laser pulses

We address the question of whether multiple high intensity (> 10(18) W cm(-2)) laser pulses ( each O( 100 fs)) can produce proton beams with a modified energy spectrum on irradiating a foil target. This has been studied in one dimension with both Vlasov and particle-in-cell codes. A reduction in the maximum proton energy, and the generation of spectral peaks, is observed. This is the first theoretical demonstration of spectral peak generation by purely optical means. The mechanism, which has been termed multiple pulse sheath acceleration, that produces these spectral peaks is described, and the same mechanism occurs in both sets of simulations

Scaling of the proton density reduction scheme for the laser acceleration of proton beams with a narrow energy spread

The laser acceleration of proton beams with quasi-monoenergetic features in the energy spectra from microdot targets is investigated by numerical simulation. The formation of these spectral peaks is strongly dependent on the interplay between different ion species in the target. The scaling of the spectral peak's energy, and number of protons in the spectral peak, with both microdot composition and laser intensity is considered. Particular attention is given to determining the proton concentration below which the number of protons in the spectral peak rapidly diminishes. It is shown that at proton concentrations of 1-5n(crit) a spectral peak is produced that reaches an energy up to 70% of the maximum proton energy, whilst still containing more protons than would be produced by a conventional target in this energy range

Vorticity deposition, structure generation and the approach to self-similarity in colliding blast wave experiments

AbstractWhen strong shocks interact with transverse density gradients, it is well known that vorticity deposition occurs. When two non-planar blast waves interact, a strong shock will propagate through the internal structure of each blast wave where the shock encounters such density gradients. There is therefore the potential for the resulting vorticity to produce pronounced density structures long after the passage of these shocks. If the two blast waves have evolved to the self-similar (Sedov) phase this is not a likely prospect, but for blast waves at a relatively early stage of their evolution this remains possible. We show, using 2D numerical simulations, that the interactions of two ‘marginally young’ blast waves can lead to strong vorticity deposition which leads to the generation of a strong protrusion and vortex ring as mass is driven into the internal structure of the weaker blast wave