S. Jablonski

Production of ultrahigh ion current densities at skin-layer subrelativistic laser–plasma interaction

Some applications of fast ions driven by a short (≤1 ps) laser pulse (e.g. fast ignition of ICF targets, x-ray laser pumping, laboratory astrophysics research or some nuclear physics experiments) require ion beams of picosecond (or shorter) time durations and of very high ion current densities (∼10 10 A cm -2 or higher). A possible way of producing ion beams with such extreme parameters is ballistic focusing of fast ions generated by a target normal sheath acceleration (TNSA) mechanism at relativistic laser intensities. In this paper we discuss another method, where the production of short-pulse ion beams of ultrahigh current densities is possible in a planar geometry at subrelativistic laser intensities and at a low energy (≤ 1 J) of the laser pulse. This method-referred to as skin-layer ponderomotive acceleration (S-LPA)-uses strong ponderomotive forces induced at the skin-layer interaction of a short laser pulse with a proper preplasma layer in front of a solid target. The basic features of the high-current ion generation by S-LPA were investigated using a simplified theory, numerical hydrodynamic simulations and measurements. The experiments were performed with subjoule 1 ps laser pulses interacting with massive or thin foil targets at intensities of up to 2 x 10 17 W cm -2 . It was found that both in the backward and forward directions highly collimated high-density ion beams (plasma blocks) with current densities at the ion source (close to the target) approaching 10 10 A cm -2 are produced, in accordance with the theory and numerical calculations. These ion current densities were found to be comparable to (or even higher than) those estimated from recent short-pulse TNSA experiments with relativistic laser intensities. Apart from the simpler physics of the laser-plasma interaction, the advantage of the considered method is the low energy of the driving laser pulses allowing the production of ultrahigh-current-density ion beams with a high repetition rate. It opens a prospect for unique tabletop experiments in various fields of physical and technological research.

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).

Studies on laser-driven generation of fast high-density plasma blocks for fast ignition

Proton fast ignition (FI) of fusion targets [1] requires ps proton beams of PW power and of extremely high proton current densities (ji >10 12 A/cm 2 ) and proton beam intensities (Ii > 10 19 W/cm 2 ) [2, 3], which are not attainable at present even with the biggest conventional accelerators [4]. Potentially, such extreme proton beam parameters can be achieved using ballistic focusing of fast proton beams generated by the target normal sheath acceleration (TNSA) mechanism [5] driven by a short (≤ 1ps) laser pulse of relativistic intensity. Achieving required proton beam intensities with sufficiently high efficiency with this method can encounter, however, severe difficulties. One of the reasons is relatively low density of accelerated protons at the source (in a close vicinity of the rear target surface), which typically is ~ 10 19 cm -3 [6 – 9] or less (see further). As the relation between the ion beam intensity, Ii, the ion density, ni, and the ion (mean) energy, Ei, can be expressed by the equation (see e.g. [9]):

Progress and prospect of fast ignition of ICF targets

Fast ignition (FI) is an innovative approach to inertial confinement fusion, which has the potential for higher energy gain at lower overall driver energy and cost. If realized at full scale, it could open a route to inertial fusion energy. This paper is a brief review of basic FI concepts and the progress in FI research which has taken place in recent years. The requirements for the DT fuel and the ignitor (electron or proton beam) as well as for the laser drivers are discussed. Key issues related to electron FI and proton FI are considered. Prospects for FI-related experiments using next generation laser facilities just being constructed or designed are outlined.

Numerical modelling of production of ultrahigh-current-density ion beams by short-pulse laser-plasma interaction

The basic features of generation of ion beams of ultrahigh (>109 A/cm2) current densities by the skin-layer subrelativistic interaction of a short (≤1 ps) laser pulse with an inhomogeneous plasma layer are studied with the use of a two-fluid hydrodynamic 1-D computer code. The calculations performed for pico—and subpicosecond 1.05—μm laser pulses of intensity ∼1016∨1017 W/cm2 confirm that non-linear ponderomotive forces induced by the laser pulse accelerate two thin (a few μm) plasma blocks of current densities in the range of 109∨1011 A/cm2, propagating in forward and backward directions. The effect of initial plasma density gradient as well as the laser intensity and the pulse duration on characteristics of ion beams is demonstrated and discussed.

Production of high-intensity proton fluxes by a 2ω Nd:glass laser beam

The results of numerical and experimental studies of high-intensity proton beam generation using a 2ω or 1ω Nd:glass laser beam irradiating a thin hydrogen-rich target are reported. The effect of the laser wavelength (λ), intensity (IL) and pulse duration as well as the target thickness, and the preplasma density gradient scale length on proton beam parameters, and the laser-protons energy conversion efficiency were examined by particle-in-cell simulations. Both the simulations and measurements, performed on the LULI 100 TW laser facility at IL up to 2× 10 19 W/cm 2 , prove that at the ILλ 2 product fixed, the 2ω laser driver can produce proton beams of intensity, current density and energy fluence significantly higher than the ones which could be achieved using the 1ω driver. In particular, at ILλ 2 ∼(0.5–1)× 10 20 Wcm −2 μm 2 the 2ω picosecond driver makes it possible to generate multi-MeV proton beams of intensity and current density in excess of 10 21 W/cm 2 and 10 14 A/cm 2 , respectively, with the conversion efficiency above 10%.

Effect of laser light polarization on generation of relativistic ion beams driven by an ultraintense laser

The effect of laser light polarization on properties of proton and carbon ion beams generated from a CH target irradiated by a 130 fs laser pulse of ultra-relativistic intensity (∼1022–1023 W/cm2) is investigated using particle-in-cell simulations. It is shown that only circular light polarization ensures the production of quasi-monoenergetic relativistic beams of both protons and carbon ions from such a target while using the linear one results in the generation of quasi-monoenergetic protons accompanied with carbon ions of complex and broad energy spectrum. The influence of the target thickness and laser intensity on the ion energy spectrum and the laser-ions energy conversion efficiency is examined.

Laser-driven generation of ultraintense proton beams

The results of experimental and numerical studies of high-intensity proton beam generation driven by a short laser pulse of relativistic intensity are reported. In the experiment, a 350 fs laser pulse of 1.06 or 0.53 μm wavelength and intensity up to 2×1019 Wcm−2 irradiated a thin (0.6–2 μm) plastic (PS) or Au/PS (plastic covered by 0.2 μm Au front layer) target along the target normal. The effect of laser intensity, the target structure and the laser wavelength on the proton beam parameters and laser-protons energy conversion efficiency were examined. Both the measurements and one-dimensional particle-in-cell simulations showed that MeV proton beams of intensity ∼1018 Wcm−2 and current density ∼1012 Acm−2 at the source can be produced when the laser intensity-wavelength squared product I Lλ2 is ∼1019 Wcm−2 μm2 and the laser-target interaction conditions approach the skin-layer ponderomotive acceleration (SLPA) requirements. The simulations also proved that at I Lλ2≥slant 5×1019 Wcm−2 μm2 and λ≤slant 0.53 μm, SLPA clearly prevails over other acceleration mechanisms and it can produce multi-MeV proton beams of extremely high intensities above 1020 Wcm−2.

Particle-in-cell simulation of acceleration of ions to GeV energies in the interactions of an ultra-intense laser pulse with two-species targets

This contribution presents the results of one-dimensional particle-in-cell simulations of ion acceleration in the interactions of an ultra-intense (IL ~ 1023 W cm−2) 130 fs laser pulse with thin hydrocarbon (CH) and hydride (CaH2, CsH, ErH3) targets. The influence of laser pulse polarization and target material on the mean ion energy and the ion energy spectrum is examined. It is shown that for circular polarization the ion energy spectra are narrower than those for the linear one and for this polarization protons from both CH and ErH3 targets of the areal mass density below 0.1 mg cm−2 have a very narrow energy spectrum of . However, for ErH3 targets, protons are clearly separated from Er ions and their mean energy equal to 9 GeV is several times higher than those for other studied targets.

Ultrashort-pulse generation in excimer lasers by fast mode locking using electrooptic deflector

A novel method for ultrashort-pulse (USP) generation in excimer lasers employing fast mode locking with the use of an electrooptic deflector (EOD) is investigated numerically. In this method, a USP is formed by a train of very short high-contrast-ratio transmission windows produced in a cavity by a modulator composed of an EOD and a diaphragm. The potential of this method and fundamental properties of USP generation with its use are demonstrated taking a KrF laser as an example. It is shown that the application of the EOD driven with both sine and square wave of voltage enables one to obtain high-contrast-ratio light pulses of duration in the range 1-10 ps at the gain duration of only 50-100 ns. The pulses produced with this method are two orders of magnitude shorter than those attainable with conventional mode locking. The method is capable of being an efficient way for USP generation also in other short-gain-duration lasers.