Electron acceleration by a short relativistic laser pulse at the front of solid targets

Acceleration of electrons in a low-density plasma in front of a solid target by a propagating short ultraintense laser pulse is studied. When the laser is reflected at the target surface the accelerated electrons, with energy scaling as the laser intensity, continue to move forward inertially and thus escape from the pulse. Electrons accelerated backwards by the reflected light can attain even higher energies due to their longer acceleration length and their high initial momentum from a relativistic return current.     

Hot-electron energy coupling in ultraintense laser-matter interaction

We investigate the hydrodynamic response of plasma gradients during the interaction with ultraintense energetic laser pulses, using kinetic particle simulations. Energetic laser pulses are capable of compressing preformed plasma gradients over short times while accelerating low-density plasma backwards. As light is absorbed on a steepened interface, hot-electron temperature and coupling efficiency drop below the ponderomotive scaling, and we are left with a new absorption mechanism that strongly relies on the electrostatic potential caused by low-density preformed plasma. We describe this process, explain electron spectra, and identify the parameter regime where strong compression occurs. Finally, we discuss the implications for fast ignition and other applications.     

Laser-driven proton acceleration and applications: Recent results

The acceleration of high-energy ion beams following the interaction of short (t < 1 ps) and intense (Iλ² > 10¹⁸ W cm^-2 μm²) laser pulses with solid targets is a field of research currently attracting high interest in the scientific community, due to some of the unique properties of these ion sources, promising routes toward the optimization of their energy content, and a number of possible, innovative applications in the scientific, technological and medical areas. Work on the characterization and development of these sources has progressed enormously over the past few years, thanks to the contribution of many groups worldwide. This paper will report some recent results, obtained in experiments carried out at the RAL and LULI laboratories, in which we investigated the ion acceleration mechanism, developed a technique to control the ion beam divergence and energy spectrum, and applied a proton radiography technique to investigate electric and magnetic field production following laser-matter interaction.     

Electron acceleration in an ultraintense-laser-illuminated capillary

An ultraintense laser injected a 10 J of power at 1.053 microm in 0.5 ps into a glass capillary of 1 cm long and 60 microm in diameter and accelerated plasma electrons to 100 MeV. One- and two-dimensional particle codes describe wakefields with 10 GV/m gradient excited behind the laser pulse, which are guided by a plasma density channel far beyond the Rayleigh range. The blueshift of the laser spectrum supports that a plasma of 10(16) cm(-3) is inside the capillary. A bump at the high energy tail suggests the electron trapping in the wakefield.     

Long-scale jet formation with specularly reflected light in ultraintense laser-plasma interactions

Long-scale jetlike x-ray emission was observed in a 100-TW laser-plasma interaction. The jet was well collimated with a divergence of 30-40 mrad and continued from the target surface into underdense regions for a distance over 4 mm in the specular direction of the laser light. A two-dimensional particle-in-cell simulation shows an electron acceleration with the specularly reflected laser light and collimation of the electron stream by a self-generated magnetic field, resulting in the electron jet to the direction of the specularly reflected light.     

High-energy ion generation in interaction. of short laser pulse with high-density plasma

Multi-MeV ion production from the interaction of a short laser pulse with a high-density plasma, accompanied by an underdense preplasma, has been studied with a particle-in-cell simulation and good agreement is found with experiment. The mechanism primarily responsible for the acceleration of ions is identified. Comparison with experiments sheds light on the ion-energy dependence on laser intensity, preplasma scale length, and relative ion energies for a multi-species plasma. Two regimes of maximum ion-energy dependence on laser intensity, I, have been identified: subrelativistic, ∝I; and relativistic, ∝. Simulations show that the energy of the accelerated ions versus the preplasma scale length increases linearly and then saturates. In contrast, the ion energy decreases with the thickness of the solid-density plasma. Received: 13 December 2001 / Published online: 7 February 2002     

Hot and cold electron dynamics following high-intensity laser matter interaction

The characteristics of fast electrons laser accelerated from solids and expanding into a vacuum from the rear target surface have been measured via optical probe reflectometry. This allows access to the time- and space-resolved dynamics of the fast electron density and temperature and of the energy partition into bulk (cold) electrons. In particular, it is found that the density of the hot electrons on the target rear surface is bell shaped, and that their mean energy at the same location is radially homogeneous and decreases with the target thickness.     

Comparison of laser ion acceleration from the front and rear surfaces of thin foils

The comparative efficiency and beam characteristics of high-energy ions generated by high-intensity short-pulse lasers (approximately 1-6 x 10(19) 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.     

Focusing dynamics of high-energy density, laser-driven ion beams

The dynamics of the focusing of laser-driven ion beams produced from concave solid targets was studied. Most of the ion beam energy is observed to converge at the center of the cylindrical targets with a spot diameter of 30 μm, which can be very beneficial for applications requiring high beam energy densities. Also, unbalanced laser irradiation does not compromise the focusability of the beam. However, significant filamentation occurs during the focusing, potentially limiting the localization of the energy deposition region by these beams at focus. These effects could impact the applicability of such high-energy density beams for applications, e.g., in proton-driven fast ignition.