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

Numerical study of neutron beam divergence in a beam-fusion scenario employing laser driven ions

The most established route to create a laser-based neutron source is by employing laser accelerated, low atomic number ions in fusion reactions. In addition to benefiting from the high reaction cross-sections at moderate energies of the projectile ions, the anisotropy in the neutron emission is another important feature of beam-fusion reactions. Using a simple numerical model based on neutron generation in a pitcher-catcher scenario, anisotropy in the neutron emission was studied for the deuterium-deuterium fusion reaction. Simulation results are consistent with the narrow divergence ( 70 full width at half maximum) neutron beam recently obtained from an experiment employing multi-MeV deuteron beams of narrow divergence (upto 30 FWHM depending on the ion energy) accelerated by a subpetawatt laser pulse from thin deuterated plastic foils via the Target Normal Sheath Acceleration mechanism. By varying the input ion beam parameters, simulations show that a further improvement in the neutron beam directionality (i.e. reduction in the beam divergence) can be obtained by increasing the projectile ion beam temperature and cut-off energy, as expected from the interactions with higher power lasers at upcoming facilities

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

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

Borane (B m H n ), Hydrogen rich, Proton Boron fusion fuel materials for high yield laser-driven Alpha sources

We propose for the first time, a new fuel-material for laser-driven Proton Boron (P-B) fusion nuclear reactions. We propose, Hydrogen rich, Borane (B m H n ) materials as fusion fuel as compared to Boron Nitride (BN) presently used. We estimate more than 10-fold increase in the yield of nuclear fusion reactions, and Alpha-prticle flux, when, for example Ammonia Borane (BNH6) laser-target material will be used compared to the state of the art normalized flux ∼108 Alphas/sr/J from BN targets. BNH6 contains ∼1000× higher concentration of Hydrogen compared to BN. We report the manufacture of the first solid-pellets Ammonia Borane laser-targets. To obtain high Flux Alpha sources from repetitive lasers we propose new BNH6 target geometries: liquid (molten) droplets/jets; or translated tape- or disc-targets coated with BNH6 powder. Targets would be irradiated in low pressure, ambient buffer gas. To enhance the fusion/Alpha yield of ultra-high intensity PetaWatt laser-target interaction we propose nano- and micro-structured Borane targets. As applications, we propose to use the Alpha-driven nuclear reactions inside the laser-driven Borane targets for new schemes to produce short-lived medical radioisotopes. Such laser-driven radioisotope beamlines would be installed directly in hospitals. Borane materials, like Diborane (6), B2H6, are also proposed as nuclear-fuels for laser-driven Proton-Boron fusion energy generation. The high dilution of Boron in Hydrgen B/H = 33% would need to be further enahnced to B/H < 15% to cut radiation losses from the hot and dense fusion pellet

A cascaded laser acceleration scheme for the generation of spectrally controlled proton beams

We present a novel, cascaded acceleration scheme for the generation of spectrally controlled ion beams using a laser-based accelerator in a 'double-stage' setup. An MeV proton beam produced during a relativistic laser-plasma interaction on a thin foil target is spectrally shaped by a secondary laser-plasma interaction on a separate foil, reliably creating well-separated quasi-monoenergetic features in the energy spectrum. The observed modulations are fully explained by a one-dimensional (1D) model supported by numerical simulations. These findings demonstrate that laser acceleration can, in principle, be applied in an additive manner. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft