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.

K-shell radiation of fast heavy ions penetrating solid matter was used to analyze the stopping dynamics of ions over more than 80% of the stopping path. The most important advantage of this method is that the data is obtained with a high spatial resolution directly from the interaction volume. In experiments 11.4  MeV∕u Ca projectile were slowed down in solid quartz and low-density SiO₂ aerogel targets. Characteristic projectile and target spectra in the photon energy range of 1.5–4 keV were registered by means of spherically bent crystal spectrometers with high spectral and spatial resolution in the direction of the ion beam propagation. K-shell spectra of heavy ions induced by close collisions with target atoms provided information about the projectile charge state and velocity dynamics. The line intensity distribution of the K-shell transitions arising from ions with different ion charges represents the charge state distribution along the ion beam track. The variation of the line Doppler shift due to the ion deceleration in the target material was used to determine the ion velocity dynamics. The spectroscopic analysis of the stopping process was complemented by measurements of the energy loss and ion charge state distribution after the ion beam emerged from the target using a standard time-of-flight method and magnet spectrometer.

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.

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.

We have studied states of ⁷He using the ⁹Be(¹⁵N,¹⁷F)⁷He reaction at two incident energies, E_lab=240 MeV and 318.5 MeV and observed clearly the first excited state of ⁷He. An extended study of the three-body background was required to determine the excitation energy and width. The resonance parameters obtained for l=1 are E_x=2.95(10) MeV, and Γ=1.9(3) MeV. There is also evidence of a broad excited state at E_x=5.8(3) MeV with Γ=4(1) MeV. From the analysis of the ⁷He ground state resonance, we extracted a value of 0.14(2) MeV for the width. In the calibration reaction ¹²C(¹⁵N,¹⁷F)¹⁰Be three high-lying states of ¹⁰Be are found at 13.6(1), 15.3(2), and 16.9(2) MeV.

We report on a method to measure charge-changing cross sections for ions passing through matter in nonequilibrium conditions. The charge states of an initial distribution are separated by applying a high voltage before they penetrate thin carbon foils; the charge states after those foils are identified using a high resolution magnetic spectrometer. Thus the full matrix of cross sections σ(q_i,q_f) and energy losses ΔE(q_i,q_f) is obtained. The data are compared to calculated charge-state distributions based on microscopic cross sections for charge-changing processes in individual interaction steps and with a Monte Carlo simulation.

A spectroscopic study of the proton-rich, particle unstable nucleus ¹¹N has been performed using the multinucleon transfer reaction ¹²C(¹⁴N,¹⁵C)¹¹N at 30A MeV incident energy at GANIL. Levels of ¹¹N are observed as well defined resonances in the spectrum of the ¹⁵C ejectiles. They are localized at 2.18(5), 3.63(5), 4.39(5), 5.12(8), and 5.87(15) MeV above the ¹⁰C+p threshold. The comparison of the measured widths with R-matrix calculations allows the estimation of spins and parities for these resonances.