Topic/Type: 1.3 High intensity Laser Plasma Interaction, Poster

Simulations of laser ion acceleration in shaped mass-limited targets

A.A.Andreev1, 2, K.Yu.Platonov2

1 Max-Born Institute, Berlin, Germany
2 SIC S. I. Vavilov State Optical Institute, St. Petersburg, Russia

The laser driven ion acceleration arises from charge separation effect caused by an ultrahigh intensity (UHI) laser radiation. Most groups use in experiments thin foils as targets because of the possibility to characterize them well and they can be positioned easily. UHI laser pulses may strongly accelerate ions in foils and highly collimated ion beams may be formed. In order to avoid the slowly moved regions of a larger foil as charge separation effects it has been proposed to use a small target with radius about laser spot size, so named mass limited target (MLT). This is intermediate regime in target dimensions between continuous solid and small size cluster targets. Because these MLT are near solid density and with sizes that are comparable to the laser spot size, high laser pulse absorption and strong laser plasma interaction are expected, where the absorbed energy does not lose to its surroundings through rapid conduction processes. This may increase the strength of the accelerating electric field and affect the ion energy and number. Recent experiments and numerical simulations show a possibility to control the ion energy spectrum and the angular divergence by choosing the geometric shape of the target and its atomic content. Quasi-mono-energetic ion beams were generated recently from foils with specially treated rear surface and from the water droplets. Another important effect, which opens a possibility to control the ion spectrum, is the use of multi-species targets. It has been shown theoretically that the separation of ion species in the electric field created by energetic electrons produces large-amplitude electrostatic potential jumps, which enable formation of ion bunches with a narrow energy spread. In-flight interaction between the light and heavy ion bunches allows one to compensate partially the effect of Coulomb explosion and to maintain a narrow energy spectrum.
Ion acceleration in targets irradiated by short ultra-intense laser pulses has been studied here with analytical model and 3D3V PIC simulations, which describe the complete process from the electron acceleration in the laser field to the ion bunch formation. Simulations were performed for different scale sizes plane targets and curve foil sections. The simulations account for multiple ion species and the electron cooling in the expanding micro-plasma. It is shown that presence of two ion species with different charge-to-mass ratios facilitates the formation of persistent peak in energy distribution of lighter ions, while the heavier ions act like a piston. Energy spectra of fast ions, laser conversion efficiency to fast ions and the divergence of ion beams are compared for various types of targets. When MLT is irradiated by UHI laser pulse, the resulting pellet plasma is strongly accelerated forward. The kinetic energy of the ions in the bunch?s densest region can exceed hundreds MeV. If the laser spot size is bigger than MLT the radial force at vacuum MLT boundary can provide confinement. The regime of a most effective acceleration is realized in the case when laser field is about electrostatic field of ion core of MLT. The obtained ion flux has enough good directionality compare to Coulomb explosion. It is found that maximal ion energy can be significantly enhanced by choosing of MLT instead of foil of the same thickness and ion energy spectrum may be tailored by transporting the ion bunch through a secondary thin target.