Thin Solid Targets Research Articles

Target normal sheath acceleration: theory, comparison with experiments and future perspectives

Ions can be effectively accelerated during the interaction of an ultra-intense ultra-short laser pulse irradiating a thin solid target via the so-called target normal sheath acceleration (TNSA) mechanism. One of the pivotal questions at this stage of the research is how to predict the properties of the accelerated ions, both from a fundamental point of view and in the light of foreseen applications. In this context, it is desirable to have a simple but reliable description to be used to extrapolate current results to future regimes, which will be made available in the near future, thanks to developments in laser technology. In this paper, the possible approaches for an analytical description of TNSA are discussed, and a theoretical TNSA model is developed. This model is then used to investigate the maximum ion energy as a function of laser parameters. Detailed comparisons with available experimental data and scaling laws are presented. In particular, the relative role played by both the laser pulse energy and irradiance in determining the ion features is investigated.

Electron acceleration by intense short laser pulse in the preplasma of a target

Electron acceleration in the low-density preplasma of a thin solid target by a short intense laser pulse is investigated by particle-in-cell simulation. Electrons in the preplasma are trapped and accelerated by the laser ponderomotive force as well as the wake bubble field. When the laser pulse is stopped and reflected by the target, the trapped electrons continue to move forward inertially, passing through the target with small energy spread and emittance. The use of the present scheme in practical applications is discussed.

Observations of plasma heating by fast electron transport at the back side of solid targets

Fast adiabatic plasma heating of a thin solid target irradiated by a high intensity laser has been observed by optical fast interferometry diagnostic. It is driven by the hot electron current induced by the laser plasma interaction at the front side of the target. Radial and longitudinal temperature profiles are calculated to reproduce the observed rear side plasma expansion. The main parameters of the suprathermal electrons have been calculated from these observations.

Cross-section for D(p,p)D elastic scattering from 1.8 to 3.2 MeV at the laboratory angles of 155° and 165°

The D(p,p)D cross-sections for elastic scattering of proton on deuterium over incident proton energy range from 1.8 to 3.2 MeV at both laboratory angles of 155° and 165° were measured. A thin solid state target Ni/TiD x /Ta/Al used for cross-section measurement was fabricated by firstly depositing layers of Ta, Ti and Ni film on the Al foil substrate of about 7 μm in turn using magnetron sputtering and then deuterating under the deuterium atmosphere. The areal density of metal element in each layer of film was measured with RBS analysis by using a 4.0 MeV 4He ion beam, while the areal density of the deuterium absorbed in the Ti film was measured with ERD analysis by using a 6.0 MeV 16O ion beam. The results show that the cross-sections of p-D scattering under this experimental circumstance were much enhanced over the Rutherford cross-section value. It was found that the enhancement increases linearly as the energy of the incident beam increases. The total uncertainty in the measurements was less than 7.5%.

Quasimonoenergetic proton beam from ultraintense-laser irradiation of a target with holed backside

A scheme for producing quasimonoenergetic proton beams is presented. In the scheme, a thin solid target with a tiny hole on its backside is employed. The optimal hole diameter is of the order of the laser spot size so that a localized uniform sheath field can be generated in the hole. Particle-in-cell simulations demonstrate that the highly localized uniform fields can produce monoenergetic target normal sheath acceleration protons in the hole. The transverse electric fields can well suppress the transverse divergence of the protons. The dependence of the proton beam quality on the focal radius and hole structure is also investigated. This special tailed target together with optimized laser parameters can serve as sources for collimated as well as quasimonoenergetic proton beams.

On the ion acceleration by high power electromagnetic waves in the radiation pressure dominated regime

Abstract When the laser pulse radiation pressure is dominant, the efficiency of the laser energy transformation into the energy of relativistic ions is very high. An analysis of the stability of a thin plasma slab accelerated by the radiation pressure shows that the onset of a Rayleigh–Taylor-like instability can lead to transverse bunching of the slab. At the nonlinear stage of the instability development the plasma slab breaks up into separated clumps, which are accelerated by the wave radiation pressure. An indication of the effect of radiation pressure on the bulk target ions is obtained in the experimental studies of plasma jets ejected from the rear side of thin solid targets irradiated by ultraintense laser pulses. To cite this article: S.V. Bulanov et al., C. R. Physique 10 (2009).

Laser Ion Acceleration and Its Medical Application

Approach to downsize an accelerator for particle-cancer therapy with the use of a very high acceleration-gradient by laser-plasma interaction has been performed combining laser-produced ions with a conventional RF acceleration technique. Protons produced by a high peak-power (∼a few tens TW) short-pulse (<50 fs) laser from a thin solid foil target several μm in thickness has been improved in its beam chracteristics by combination of a “Phase Rotation” and electron beam cooling in order to be accelerated with a short-rising-time (∼5 ms) synchrotron to the energy of ∼200 MeV needed for cancer-therapy. In the present paper, the scheme to improve the characteristics of the laser-produced protons in the longitudinal phase space by “Phase Rotation”, which has been demonstrated in its feasibility experimentally, is described together with the procedure of laser proton production.

Comparative study of laser produced Li plasma plumes from thin film and solid target

The evolution features of lithium ions and neutrals generated by two different schemes viz. the laser-blow-off (LBO) of multicomponent LiF–C thin film and conventional laser ablation (referred here as LPP) from solid lithium have been studied using optical emission spectroscopic technique. The optical signal emitted by Li I (at 670.8 nm) and Li II (548.4 nm) was monitored as a function of laser fluence, ambient gas pressure, and distance “z” from the target. Apart from their similarities, some interesting differences were noticed in temporal profiles of the plumes generated by LPP and LBO both in vacuum as well as in the presence of the ambient gas. A comparative analysis of experimental results indicates that the ablation mechanism and subsequent laser-plume interaction were responsible for the observed differences in LPP and LBO plumes. Expansion features of the plume are discussed in the light of two different models.

Ultraintense proton beams from laser-induced skin-layer ponderomotive acceleration

The results of studies of high-intensity proton beam generation from thin (1–3 μm) solid targets irradiated by 0.35 ps laser pulse of energy up to 15 J and intensity up to 2×1019 W/cm2 are reported. It is shown that the proton beams of terawatt power and intensity around 1018 W/cm2 at the source can be produced when the laser-target interaction conditions approach the skin-layer ponderomotive acceleration requirements. The proton beam parameters remarkably depend on the target structure and can be significantly increased with the use of a double-layer Au/PS target (plastic covered by 0.1–0.2 μm Au front layer).

Theory of Light-Ion Acceleration Driven by a Strong Charge Separation

A theoretical model of the quasistatic electric field, formed at the rear surface of a thin solid target irradiated by a ultraintense subpicosecond laser pulse, due to the appearance of a cloud of ultrarelativistic bound electrons, is developed. It allows one to correctly describe the spatial profile of the accelerating field and to predict the maximum energies and the energy spectra of the accelerated ions. The agreement of the theoretical expectations with the experimental data looks satisfactory in a wide range of conditions. Previsions of regimes achievable in the future are given.

Plasma Jets Driven by Ultraintense-Laser Interaction with Thin Foils

Experimental evidence of plasma jets ejected from the rear side of thin solid targets irradiated by ultraintense (>10(19) W cm(-2)) laser pulses is presented. The jets, detected by transverse interferometric measurements with high spatial and temporal resolutions, show collimated expansion lasting for several hundreds of picoseconds and have substantially steep density gradients at their periphery. The role played by radiation pressure of the laser in the jet formation process is highlighted analytically and by extensive two-dimensional particle-in-cell simulations.

Theoretical investigation on novel particle beams and radiation sources in relativistic laser-solid interactions

We report the recent theoretical and numerical studies of quasi-monoenergetic electron and proton beams in laser solid interactions, where the quasi-monoenergetic electrons are produce in laser interaction with wire-targets and the quasi-monoenegetic protons are produced by ultra-intense circularly polarized laser pulses in a new acceleration regime called phase-stability acceleration. For the generation of coherent radiation at high frequencies, attention is paid on the emission from electron plasma waves excited in a thin solid target.

Ultrahigh-current proton beams from short-pulse laser-solid interactions

The results of studies of high-current proton beam generation from thin (1–3μm) solid targets irradiated by 0.35-ps laser pulse of intensity up to 2×1019 W/cm2 are reported. It is shown that the proton beams of multi-MA currents and multi-TA/cm2 current densities at the source can be produced when the laser-target interaction conditions approach the skin-layer ponderomotive acceleration requirements. The current and energy spectrum of protons remarkably depend on the target structure. In particular, using a double-layer Au/PS target (plastic covered by 0.1 − 0.2μm Au front layer) results in two-fold higher proton currents and higher proton energies than in the case of a plastic target.

Fast electron transport and induced heating in solid targets from rear-side interferometry imaging

Fast adiabatic plasma heating of a thin solid target irradiated by a high intensity laser has been observed by an optical fast interferometry diagnostic. It is driven by the hot electron current induced by the laser plasma interaction at the front side of the target. Radial and longitudinal temperature profiles are calculated to reproduce the observed rear-side plasma expansion. The main parameters of the suprathermal electrons (number, temperature, and divergence) have been deduced from these observations.

High flux of relativistic electrons produced in femtosecond laser-thin foil target interactions: Characterization with nuclear techniques

We present a protocol to characterize the high energy electron beam emitted in the interaction of an ultraintense laser with matter at intensities higher than 10(19) W cm(-2). The electron energies and angular distributions are determined as well as the total number of electrons produced above a 10 MeV threshold. This protocol is based on measurements with an electron spectrometer and nuclear activation techniques, combined with Monte Carlo simulations based on the GEANT3 code. The method is detailed and exemplified with data obtained with polypropylene and copper thin solid targets at a laser intensity of 2x10(19) W cm(-2). Special care is taken of the different sources of uncertainties. In particular, the reproducibility of the laser shots is considered.

Cross-section for proton–tritium scattering from 1.4 to 3.4 MeV at the laboratory angle of 165°

The elastic scattering cross-section for proton scattering from tritium was measured at a laboratory angle of 165° and over an incident proton energy range from 1.4 to 3.4MeV. A thin solid target containing 1.62×1017Tatoms/cm2 was prepared by absorption of tritium into a film of titanium on aluminium foil backing. The cross-section increases almost linearly with decreasing energy in the higher energy region of 2–3.4MeV. The currently measured cross-section data are compared with data available in the literature values and they show a similarly linear trend in a similar higher energy range. The maximum difference in the cross-section at almost the same scattering angle between current data and the previous results is no worse than 2.3%.

Acceleration dynamics of ions in shocks and solitary waves driven by intense laser pulses

The acceleration of ions in collisionless electrostatic shocks and solitary waves, driven by ultrashort intense laser pulses in a thin solid target under different conditions, is investigated theoretically. When a shock is formed, ions with certain initial velocities inside the target can be accelerated by the electrostatic field at the shock front to twice the shock speed. When a solitary wave is formed, only ions located at the rear surface of the target can be accelerated by the solitary wave together with the sheath field formed there.

HIGH BRIGHTNESS LASER INDUCED MULTI-MEV ELECTRON/PROTON SOURCES

The chirped pulse amplification (CPA) technique has opened new perspectives in the radiation-matter interaction studies using ultra-short laser pulses at ultra-relativistic intensities. In particular the original idea, proposed by Tajima and Dawson, of accelerating electrons by the huge electric fields of plasma waves which develop in the wake of a laser pulse propagating in a plasma, become feasible. Some laboratories all over the world have produced by such a technique collimated electron busts of hundreds of MeV along acceleration lengths of a few hundreds of microns. In other experiments, using thin solid targets, intense bursts of energetic protons have been at the same time detected. The proton acceleration mechanism is essentially based on the Coulomb force appearing at the thin solid target surface as a consequence of the previous escape of the energetic electrons from the target. In the paper some experimental results will be presented as well as the opportunities the INFN PLASMONX project will offer in this research field at LNF.

Relativistic Buneman instability in the laser breakout afterburner

A new laser-driven ion acceleration mechanism has been identified in particle-in-cell simulations of high-contrast-ratio ultraintense lasers with very thin (10s of nm) solid targets [Yin et al., Laser and Particle Beams 24, 291 (2006); Yin et al., Phys. Plasmas 13, 072701 (2007)]. After a brief period of target normal sheath acceleration (TNSA), “enhanced” TNSA follows. In this stage, the laser rapidly heats all the electrons in the target as the target thickness becomes comparable to the skin depth and enhanced acceleration of the ions results. Then, concomitant with the laser penetrating the target, a large accelerating longitudinal electric field is generated that co-moves with the ions. This last phase has been termed the laser “breakout afterburner” (BOA). Earlier work suggested that the BOA was associated with the Buneman instability that efficiently converts energy from the drift of the electrons into the ions. In this Brief Communication, this conjecture is found to be consistent with particle-in-cell simulation data and the analytic dispersion relation for the relativistic Buneman instability.

Density effect on proton acceleration from carbon-containing high-density thin foils irradiated by high-intensity laser pulses

The acceleration of protons in dense plastic foils irradiated by ultrahigh intensity laser pulses is simulated using a two-dimensional hybrid particle-in-cell scheme. For the chosen parameters of the overdense foils of densities ρ=0.2, 1, and 3 g∕cm3 and of an ultrahigh intensity (2×1020 W∕cm2) laser pulse, our simulations illustrate that a high-density target is favorable to high collimation of the target-normal-sheath acceleration protons but less energy for a short acceleration time (&amp;lt;100 fs). In particular, the difference of strong local heating of the carbon ion for different plasma densities is clearly observed at both the front and rear surfaces of thin solid targets, suggesting that the effect of the density and composition of the targets are also important for correctly simulating energetic ion generation in ultraintense laser-solid interactions.