Design of plasma shutters for improved heavy ion acceleration by ultra-intense laser pulses

In this work, we investigate the application of the plasma shutters for heavy ion acceleration driven by a high-intensity laser pulse. We use particle-in-cell (PIC) and hydrodynamic simulations. The laser pulse, transmitted through the opaque shutter, gains a steep-rising front and its peak intensity is locally increased at the cost of losing part of its energy. These effects have a direct influence on subsequent ion acceleration from the ultrathin target behind the shutter. In our 3D simulations of silicon nitride plasma shutter and a silver target, the maximal energy of high-Z ions increases significantly when the shutter is included for both linearly and circularly polarized laser pulses. Moreover, application of the plasma shutter for linearly polarized pulse results in focusing of ions towards the laser axis in the plane perpendicular to the laser polarization. The generated high energy ion beam has significantly lower divergence compared to the broad ion cloud, generated without the shutter. The effects of prepulses are also investigated assuming a double plasma shutter. The first shutter can withstand the assumed sub-ns prepulse (treatment of ns and ps prepulses by other techniques is assumed) and the pulse shaping occursvia interaction with the second shutter. On the basis of our theoretical findings, we formulated an approach towards designing a double plasma shutter for high-intensity and high-power laser pulses and built a prototype.Comment: 30 pages 13 figure

Evidence of resonant surface wave excitation in the relativistic regime through measurements of proton acceleration from grating targets

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, has been experimentally investigated. Ultrahigh contrast ($\sim 10^{12}$) pulses allowed to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultra-high intensity >10^{19} \mbox{W/cm}^{2}. A maximum increase by a factor of 2.5 of the cut-off energy of protons produced by Target Normal Sheath Acceleration has been observed with respect to plane targets, around the incidence angle expected for resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.Comment: 5 pages, 5 figures, 2nd version implements final correction

Electron Acceleration by Relativistic Surface Plasmons in Laser-Grating Interaction

The generation of energetic electron bunches by the interaction of a short, ultraintense (I>1019 W/cm2) laser pulse with "grating" targets has been investigated in a regime of ultrahigh pulse-to-prepulse contrast (1012). For incidence angles close to the resonant condition for surface plasmon excitation, a strong electron emission was observed within a narrow cone along the target surface, with energy spectra peaking at 5-8 MeV and total charge of ∼100 pC. Both the energy and the number of emitted electrons were strongly enhanced with respect to simple flat targets. The experimental data are closely reproduced by three-dimensional particle-in-cell simulations, which provide evidence for the generation of relativistic surface plasmons and for their role in driving the acceleration process. Besides the possible applications of the scheme as a compact, ultrashort source of MeV electrons, these results are a step forward in the development of high-field plasmonics

Efficient ion beam generation in laser interactions with micro-structured targets

The maximum ion energy and acceleration efficiency have to be increased for practical applications of intense ion beams produced by intense short laser pulses incident on a thin foil. For this aim, we propose to use foil with a microscopic structure on the front size. We have prepared such targets by depositing a monolayer of polystyrene nanospheres of a size comparable to laser wavelength on a thin foil. The damage threshold of the produced targets is found experimentally above 3.5 × 109  W/cm2 for a nanosecond pedestal and above 1011  W/cm2 for femtosecond prepulses

Enhanced electron acceleration via ultra-intense laser interaction with structured targets

International audienceThe generation of energetic electrons by the interaction of a short laser pulse with solid “grating” targets, having a periodic groove on the irradiated surface, has been investigated in a regime of ultrahigh contrast (1012) and relativistically strong intensity (> 1019W/cm2). A strong enhancement of both the energy and number of electrons emitted from the target, with respect to at targets, has been observed for incidence angles close to the resonant condition for surface wave excitation. In particular we identified bunches of electrons with energies exceeding 10 MeV which are emitted in a direction close to the target surface. The experimental results are well reproduced by a three-dimensional particle-in-cell simulation, which confirms the dominant role of the surface wave in accelerating the electrons. These results are a step forward the development of high field plasmonics for a number of applications

Mutagenesis of Nucleophilic Residues near the Orthosteric Binding Pocket of M1 and M2 Muscarinic receptors: Effect on the Binding of Nitrogen Mustard Analogs of Acetylcholine and McN-A-343

Investigating how a test drug alters the reaction of a site-directed electrophile with a receptor is a powerful method for determining whether the drug acts competitively or allosterically, provided that the binding site of the electrophile is known. In this study, therefore, we mutated nucleophilic residues near and within the orthosteric pockets of M1 and M2 muscarinic receptors to identify where acetylcholine mustard and 4-[(2-bromoethyl)methyl-amino]-2-butynyl-N-(3-chlorophenyl)carbamate (BR384) bind covalently. BR384 is the nitrogen mustard analog of [4-[[N-(3-chlorophenyl)carbamoyl]oxy]-2-butynyl]trimethylammonium chloride (McN-A-343). Mutation of the highly conserved aspartic acid in M1 (Asp105) and M2 (Asp103) receptors to asparagine largely prevented receptor alkylation by acetylcholine mustard, although modest alkylation still occurred at M2 D103N at high concentrations of the mustard. Receptor alkylation by BR384 was also greatly inhibited in the M1 D105N mutant, but some alkylation still occurred at high concentrations of the compound. In contrast, BR384 rapidly alkylated the M2 D103N mutant. Its affinity was reduced to one tenth, however. The alkylation of M2 D103N by BR384 was competitively inhibited by N-methylscopolamine and allosterically inhibited by gallamine. Mutation of a variety of other nucleophilic residues, some in combination with D103N, had little effect on M2 receptor alkylation by BR384. Our results suggest that BR384 alkylates at least one residue other than the conserved aspartic acid at the ligand-binding site of M1 and M2 receptors. This additional residue seems to be located within or near the orthosteric-binding pocket and is not part of the allosteric site for gallamine

Laser-driven high-energy proton beam with homogeneous spatial profile from a nanosphere target

A high-energy, high-yield proton beam with a good homogeneous profile has been generated from a nanosphere target irradiated by a short (30-fs), intense (7×10^{20}  W/cm^{2}) laser pulse. A maximum proton energy of 30 MeV has been observed with a high proton number of 7×10^{10} in the energy range 5–30 MeV. A homogeneous spatial profile with a uniformity (standard deviation from an average value within 85% beam area) of 15% is observed with the nanosphere dielectric target. Particle-in-cell simulations show the enhancement of proton cutoff energy and proton number with the nanosphere target and reveal that the homogeneous beam profile is related with a broadened angular distribution of hot electrons, which is initiated by the nanosphere structure. The homogeneous spatial properties obtained with the nanosphere target will be advantageous in developing laser-driven proton sources for practical applications in which high-quality beams are required