Search Results (10 total)

This Letter demonstrates the transporting and focusing of laser-accelerated 14 MeV protons by permanent magnet miniature quadrupole lenses providing field gradients of up to 500  T/m. The approach is highly reproducible and predictable, leading to a focal spot of (286×173)  μm full width at half maximum 50 cm behind the source. It decouples the relativistic laser-proton acceleration from the beam transport, paving the way to optimize both separately. The collimation and the subsequent energy selection obtained are perfectly applicable for upcoming high-energy, high-repetition rate laser systems.

The comparative efficiency and beam characteristics of high-energy ions generated by high-intensity short-pulse lasers (∼1–6×10¹⁹  W/cm²) from both the front and rear surfaces of thin metal foils have been measured under identical conditions. Using direct beam measurements and nuclear activation techniques, we find that rear-surface acceleration produces higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. Our observations are well reproduced by realistic particle-in-cell simulations, and we predict optimal criteria for future applications.

The laminarity of high-current multi-MeV proton beams produced by irradiating thin metallic foils with ultraintense lasers has been measured. For proton energies >10  MeV, the transverse and longitudinal emittance are, respectively, <0.004  mm mrad and <10^-4  eV s, i.e., at least 100-fold and may be as much as 10⁴-fold better than conventional accelerator beams. The fast acceleration being electrostatic from an initially cold surface, only collisions with the accelerating fast electrons appear to limit the beam laminarity. The ion beam source size is measured to be <15   μm (FWHM) for proton energies >10  MeV.

The evolution of laser-generated MeV, MA electron beams propagating through conductors and insulators has been studied by comparing measurement and modeling of the distribution of MeV protons that are sheath accelerated by the propagated electrons. We find that electron flow through metals is uniform and can be laser imprinted, whereas propagation through insulators induces spatial disruption of the fast electrons. Agreement is found with material dependent modeling.

Order-of-magnitude anomalously high intensities for two-electron (dielectronic) satellite transitions, originating from the He-like 2s^{2 1}S₀ and Li-like 1s2s^{2 2}S_1/2 autoionizing states of silicon, have been observed in dense laser-produced plasmas at different laboratories. Spatially resolved, high-resolution spectra and plasma images show that these effects are correlated with an intense emission of the He-like 1s3p ¹P–1s^{2 1}S lines, as well as the K_α lines. A time-dependent, collisional-radiative model, allowing for non-Maxwellian electron-energy distributions, has been developed for the determination of the relevant nonequilibrium level populations of the silicon ions, and a detailed analysis of the experimental data has been carried out. Taking into account electron density and temperature variations, plasma optical-depth effects, and hot-electron distributions, the spectral simulations are found to be not in agreement with the observations. We propose that highly stripped target ions (e.g., bare nuclei or H-like 1s ground-state ions) are transported into the dense, cold plasma (predominantly consisting of L- and M-shell ions) near the target surface and undergo single- and double-electron charge-transfer processes. The spectral simulations indicate that, in dense and optically thick plasmas, these charge-transfer processes may lead to an enhancement of the intensities of the two-electron transitions by up to a factor of 10 relative to those of the other emission lines, in agreement with the spectral observations.

Collimated jets of carbon and fluorine ions up to 5  MeV/nucleon (∼100   MeV) are observed from the rear surface of thin foils irradiated with laser intensities of up to 5×10¹⁹   W/cm². The normally dominant proton acceleration could be surpressed by removing the hydrocarbon contaminants by resistive heating. This inhibits screening effects and permits effective energy transfer and acceleration of other ion species. The acceleration dynamics and the spatiotemporal distributions of the accelerating E fields at the rear surface of the target are inferred from the detailed spectra.

We present the results of a detailed study on the acceleration of intense ion beams by relativistic laser plasmas. The experiments were performed at the 100 TW laser at the Laboratoire pour L’Utilisation des Lasers Intenses. We investigated the dependence of the ion beams on the target conditions based on theoretical predictions by the target normal sheath acceleration mechanism. A strong dependence of the ion beam parameters on the conditions on the target rear surface was found. We succeeded in shaping the ion beam by the appropriate tailoring of the target geometry and we performed a characterization of the ion beam quality. The production of a heavy ion beam could be achieved by suppressing the amount of protons at the target surfaces. Finally, we demonstrated the use of short pulse laser driven ion beams for radiography of thick samples with high resolution.

The application of x-ray spectroscopy methods enabled a space-resolved observation of the high-energy Rydberg satellite series 1s2lnl^′→1s²2l+hν and the Ly_β satellites 2l3l^′→1s2l+hν in dense laser-produced plasmas. Ab initio atomic structure calculations including relativistic and QED effects show excellent agreement with the precise wavelengths measurements. Satellite transitions with n>3 split into three distinct groups. For n>5, the transitions are found to merge with the Ly_β satellite transitions 2l3l^′→1s2l+hν and the series limits approach the Ly_β line. This leads to emission on the red and even on the blue wing of the Ly_β line. Spatially and spectrally resolved intensity distribution discovered strong spatial restrictions of the Rydberg satellite intensities near the target surface. On the contrary, all resonance lines show strong emissions up to 10-mm distance from the target and large optical thickness. This enabled us to develop essentially opacity-free and spatially localized temperature diagnostics based on Rydberg satellite emission. Total x-ray emission spectra are established with the help of the MARIA suite of kinetic codes. The derived properties of the spectral distribution make the high-energy Rydberg satellites of general interest for the wide class of dense plasma experiments.

By means of high spectral and spatial resolution x-ray spectroscopic methods we have observed for the first time simultaneously two-electron and higher-order intercombination transitions from autoionizing levels in large scale optically thick laser produced plasmas. Intercombination transitions as well as the two-electron op-satellite transitions in Li-like ions have anomalous large intensities (up to a factor of 14). Sophisticated non-Maxwellian opacity simulations treating extended configuration interaction atomic data employing the MARIA-code reveal, however, good agreement among forbidden transitions.

A specifically tailored plasma lens could shape a high-energy, heavy-ion beam into the form of a hollow cylinder without loss of beam intensity. It has been experimentally confirmed that both a positive as well as a negative radial gradient of the current density in the active plasma lens can be the underlying principle. Calculations were performed that yield the ideal current density distribution for both cases. A numerical simulation of an experiment with an intense ion beam highlights that the shaping of the beam increases the achievable compression in a lead sample.