A. A. Andreev

Review of ultrafast ion acceleration experiments in laser plasma at Max Born Institute

New perspectives have been opened up in the field of laser–matter interactions due to recent advances in laser technology, leading to laser systems of high contrast and extreme intensity values, where the frontier of maximum intensity is pushed now to about 1022 W/cm2. Many striking phenomena such as laser-acceleration of electrons up to the GeV level, fast moving ions with kinetic energies of several 10s of MeV, as well as nuclear physics experiments have already actuated a broad variety of theoretical as well as experimental studies. Also highly relativistic effects like laser induced electron-positron pair production are under discussion. All these activities have considerably stimulated the progress in understanding the underlying physical processes and possible applications. This article reviews recent advances in the experimental techniques as well as the associated plasma dynamics studies at relativistic intensities performed at the Max-Born-Institute (MBI). Interactions of a laser pulse at intensities above 1019 W/cm2 with water- and heavy-water droplets, as well as, with thin foils are discussed. Rear and front side acceleration mechanisms, particle dynamics inside the dense target, proton source characteristics, strong modulations in proton and deuteron emission spectra, and finally generation of quasi-monoenergetic deuteron bursts are the topics covered in the article.

Enhanced laser ion acceleration from mass-limited targets

Laser interactions with mass-limited targets are studied here via numerical simulations using our relativistic electromagnetic two-dimensional particle-in cell code including all three-velocity components. Analytical estimates are derived to clarify the simulation results. Mass-limited targets preclude the undesirable spread of the absorbed laser energy out of the interaction zone. Mass-limited targets, such as droplets, are shown here to enhance the achievable fast ion energy significantly due to an increase in the hot electron concentration. For given target dimensions, the existence is demonstrated for an optimum laser beam diameter when ion acceleration is efficient and geometrical energy losses are still acceptable. Ion energy also depends on the target geometrical form and rounded targets are found to enhance the energy of accelerated ions. The acceleration process is accompanied by generation of the dipole radiation in addition to the ordinary scattering of the electromagnetic wave.

Ion acceleration by femtosecond laser pulses in small multispecies targets

Ion acceleration by ultrashort intense femtosecond laser pulses (∼4×1019W∕cm2, ∼30fs) in small targets of uniform chemical composition of two ion species (protons and carbon C4+ ions) is studied theoretically via a particle-in-cell code with two spatial and three velocity components. Energy spectra of accelerated ions, the number and divergence of fast protons, are compared for various target shapes (cylinder, flat foil, curved foil) and density profiles. Dips and peaks are observed in proton energy spectra due to mutual interaction between two ion species. The simulations demonstrate that maximum energy of fast protons depends on the efficiency of laser absorption and the cross section of the hot electron cloud behind the target. A rear-side plasma density ramp can substantially decrease the energy of fast ions and simultaneously enhance their number. These results are compared with analytical estimates and with previously published experiments.

Optimal ion acceleration from ultrathin foils irradiated by a profiled laser pulse of relativistic intensity

Recent investigations of relativistic laser plasmas have shown that the energy transfer from the laser field to the kinetic ion energy and therefore the attainable maximum energy of the ions increases when ultrathin targets are irradiated by laser pulse without prepulse. In this paper, the influence of the target thickness and laser pulse contrast on the energy of the accelerated ions has been studied theoretically as well as experimentally. An optimum target was searched if a real laser pulse with a certain prepulse irradiates the target.

Transient electric fields in laser plasmas observed by proton streak deflectometry

A novel proton imaging technique was applied which allows a continuous temporal record of electric fields within a time window of several nanoseconds. This “proton streak deflectometry” was used to investigate transient electric fields of intense (∼1017W∕cm2) laser irradiated foils. We found out that these fields with an absolute peak of up to 108V∕m extend over millimeter lateral extension and decay at nanosecond duration. Hence, they last much longer than the (approximately picosecond) laser excitation and extend much beyond the laser irradiation focus.

Nonlinear absorption of a short intense laser pulse in a nonuniform plasma

The analytical solutions of nonlinear relativistic hydrodynamic equations for inhomogeneous plasmas are considered and the model of laser pulse absorption when the laser and plasma parameters are varied over a wide range of values is developed. The absorption coefficient has a local maxima at an optimal plasma density gradient for the given laser intensity, and that absorption then increases with scale length, in agreement with our particle-in-cell simulation results. The simulations and model have shown that the absorption of the foil target increases with laser intensity because of plasma density deepness and the spreading of the resonant area of laser plasma wave interaction.

Calibration of one-dimensional boosted kinetic codes for modeling high-intensity laser–solid interactions

An efficient means of performing kinetic simulations of oblique-incidence laser–plasma interaction via a relativistic Lorentz transformation is described in detail. Comparisons are made between one-dimensional boost codes and previous two-dimensional particle-in-cell (PIC) simulations in an effort to define benchmarks for modeling high-intensity, short-pulse interactions. Apparent discrepancies between results obtained using PIC and Vlasov codes are resolved, and some pitfalls involved with both techniques are identified.

Influence of target system on the charge state, number, and spectral shape of ion beams accelerated by femtosecond high-intensity laser pulses

Specific ion spectra have been obtained by irradiating spherical and planar targets with 40fs Ti:Sa laser pulses at intensities of ∼1019W∕cm2. From the mass-limited spherical target, strong modulations in the proton/deuteron spectra and a high laser to ion energy conversion originate, whereas the planar target provides higher cutoff energies of protons. We compare qualitatively models in which the acceleration field is assigned to a multitemperature electron distribution and alternatively where multispecies ion acceleration is important, which both can account for the observed modulations in the spectra. The abundance of ion species and especially the observed strong suppression of the heavy ion species during the ion acceleration from planar targets are estimated on the basis of the interplay of ions with different mass during their ultrafast acceleration and the further ion-bunch propagation.

Parameters of a fast ion jet generated by an intense ultrashort laser pulse on an inhomogeneous plasma foil

Analysis and simulations of fast particles produced by a high-intensity short laser pulse interacting with a foil target are performed. Initially, the plasma density distribution of the foil target has a smooth gradient with the scale length of plasma density varying across it. The absorbed laser energy is transferred to fast electrons, which penetrate in the foil and are partially ejected from the foil rear. These electrons produce an electric field that causes an ion beam to be emitted from the foil. We analyze the mechanism of ion acceleration in the foil plasma and the influence of the density gradient and other laser and plasma parameters on ion acceleration. The angular distributions of the ejected electrons and ions are calculated.

Laser proton acceleration in a water spray target

Studies of interaction of a cloud of submicrometer water droplets with ultrashort (40fs) and intense (∼2×1019W∕cm2) laser pulses demonstrate an efficient acceleration of protons and oxygen ions. Due to a high ratio of the volume to the enveloping surface of a single droplet and a large number of droplets in a focal volume, efficient laser pulse absorption is enabled, which provides high electron temperatures and ion acceleration to high energies. The generation of ions with energies more than 1MeV per nucleon is demonstrated. The observed quasi-monoenergetic feature in the proton spectrum is discussed with the thermal expansion–Coulomb explosion model and numerical simulations.