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Register a new userSpectral modification of shock accelerated ions using hydrodynamically shaped gas target
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We report on reproducible shock acceleration from irradiation of a $lambda = 10$ $mu$m CO$_2$ laser on optically shaped H$_2$ and He gas targets. A low energy laser prepulse ($Ilesssim10^{14}, {rm Wcm^{-2}}$) was used to drive a blast wave inside the gas target, creating a steepened, variable density gradient. This was followed, after 25 ns, by a high intensity laser pulse ($I>10^{16}, {rm Wcm^{-2}}$) that produces an electrostatic collisionless shock. Upstream ions were accelerated for a narrow range of prepulse energies ($> 110$ mJ & $< 220$mJ). For long density gradients ($gtrsim 40 mu$m), broadband beams of He$^+$ and H$^+$ were routinely produced, whilst for shorter gradients ($lesssim 20 mu$m), quasimonoenergetic acceleration of proton was observed. These measurements indicate that the properties of the accelerating shock and the resultant ion energy distribution, in particular the production of narrow energy spread beams, is highly dependent on the plasma density profile. These findings are corroborated by 2D PIC simulations.
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We report on the generation of impurity-free proton beams from an overdense gas jet driven by a near-infrared laser ($lambda_L=1.053$ $mathrm{mu} m$). The gas profile was shaped prior to the interaction using a controlled prepulse. Without this optical shaping, a 30$pm$4 nCsr$^{-1}$ thermal spectrum was detected transversely to the laser propagation direction with a high energy 8.27$pm$7 MeV, narrow energy spread (6$pm$2 %) bunch containing 45$pm$7 pCsr$^{-1}$. In contrast, with optical shaping the radial component was not detected and instead forward going protons were detected with energy 1.32$pm$2 MeV, 12.9$pm$3 % energy spread, and charge 400$pm$30 pCsr$^{-1}$. Both the forward going and radial narrow energy spread features are indicative of collisionless shock acceleration of the protons.
We report on the experimental observation of beam-like neutron emission with peak flux of the order of 10^9 n/sr, from light nuclei reactions in a pitcher-catcher scenario, by employing MeV ions driven by high power laser. The spatial profile of the neutron beam, fully captured for the first time by employing a CR39 nuclear track detector, shows a FWHM divergence angle of 70 degrees, with a peak flux nearly an order of magnitude higher than the isotropic component elsewhere. The observed beamed flux of neutrons is highly favourable for a wide range of applications, and indeed for further transport and moderation to thermal energies. A systematic study employing various combinations of pitcher-catcher materials indicates the dominant reactions being d(p, n+p)^1H and d(d,n)^3He. Albeit insufficient cross-section data are available for modelling, the observed anisotropy in the neutrons spatial and spectral profiles are most likely related to the directionality and high energy of the projectile ions.
High energy ion beams (> MeV) generated by intense laser pulses promise to be viable alternatives to conventional ion beam sources due to their unique properties such as high charge, low emittance, compactness and ease of beam delivery. Typically the acceleration is due to the rapid expansion of a laser heated solid foil, but this usually leads to ion beams with large energy spread. Until now, control of the energy spread has only been achieved at the expense of reduced charge and increased complexity. Radiation pressure acceleration (RPA) provides an alternative route to producing laser-driven monoenergetic ion beams. In this paper, we show the interaction of an intense infrared laser with a gaseous hydrogen target can produce proton spectra of small energy spread (~ 4%), and low background. The scaling of proton energy with the ratio of intensity over density (I/n) indicates that the acceleration is due to the shock generated by radiation-pressure driven hole-boring of the critical surface. These are the first high contrast mononenergetic beams that have been theorised from RPA, and makes them highly desirable for numerous ion beam applications.
The formation of unmagnetized electrostatic shock-like structures with a high Mach number is examined with one- and two-dimensional particle-in-cell (PIC) simulations. The structures are generated through the collision of two identical plasma clouds, which consist of equally hot electrons and ions with a mass ratio of 250. The Mach number of the collision speed with respect to the initial ion acoustic speed of the plasma is set to 4.6. This high Mach number delays the formation of such structures by tens of inverse ion plasma frequencies. A pair of stable shock-like structures is observed after this time in the 1D simulation, which gradually evolve into electrostatic shocks. The ion acoustic instability, which can develop in the 2D simulation but not in the 1D one, competes with the nonlinear process that gives rise to these structures. The oblique ion acoustic waves fragment their electric field. The transition layer, across which the bulk of the ions change their speed, widens and their speed change is reduced. Double layer-shock hybrid structures develop.
A new diagnosis method for high energy ions utilizing a single CR-39 detector mounted on plastic plates is demonstrated to identify the presence of the high energy component beyond the CR-39s detection threshold limit. On irradiation of the CR-39 detector unit with a 25 MeV per nucleon He ion beam from conventional rf-accelerators, a large number of etch pits having elliptical opening shapes are observed on the rear surface of the CR-39. Detailed investigations reveal that these etch pits are created by heavy ions inelastically backscattered from the plastic plates. This ion detection method is applied to laser-driven ion acceleration experiments using cluster-gas targets, and ion signals with energies up to 50 MeV per nucleon are identified.
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