Skin-layer Ponderomotive Acceleration Research Articles

Generation of ultra-intense ion beams by a short-pulse laser

This paper summarizes briefly the main experimental and numerical results of the IPPLM team studies on the generation of ultra-intense ion beams by a short (≤1 ps) laser pulse. Basic laser-driven ion acceleration schemes capable of generating such ion beams are described including the target normal sheath acceleration (TNSA) scheme, the skin-layer ponderomotive acceleration (SLPA) scheme and the laser-induced cavity pressure acceleration (LICPA) scheme. It is shown that an efficient way for achieving high ion beam intensities and fluencies lies in using a short-wavelength laser driver of circular light polarization. In such a case, SLPA clearly dominates over TNSA, and dense and compact ion bunch is generated with high energetic efficiency. The LICPA scheme operating in the photon (radiation) pressure regime can be even more efficient than SLPA. As it is demonstrated by particle-in-cell simulations, the LICPA accelerator with a picosecond, circularly polarized laser driver of intensity ∼ 1021 W/cm2 can produce sub-picosecond light ion beams of intensity ∼ 1022 W/cm2 and fluence > 1 GJ/cm2 with the energetic efficiency of tens of percent. Laser-driven ion beams of such extreme parameters could open up new research areas in high-energy-density science, inertial fusion or nuclear physics.

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.

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).

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.

Three — fluid hydrodynamic simulation of proton beam generation by a subpicosecond laser pulse

The skin-layer ponderomotive acceleration (S-LPA) induced by a short laser pulse is considered to be a promising method of producing dense high-current ion beams having the potential to be applied in high energy density physics or for fast ignition of inertial fusion. In this contribution properties of proton beam generation by S-LPA are studied using a three-fluid relativistic hydrodynamic model of laser-plasma interaction. The numerical calculations were performed for the subpicosecond neodymium-glass laser pulses of intensities up to 1018 W/cm2 interacting with inhomogeneous plasma consisting of three fluids: electron, proton and Au ion fluid. The impact of the initial plasma density gradient and of the different ratio of fluids constituting the plasma slab on proton velocities and proton current densities is presented. In particular, it is shown that the proton velocity increases and the proton current density decreases when the heavy ion fluid content increases. It results in the proton beam intensity having the maximum for the specific ratio of proton and Au ion fluids. The numerical computations were compared to the results of experiment, in which a 1 ps laser pulse of subrelativistic intensity (1017 W/cm2) interacted with a single-layer (plastic) or a double-layer (plastic + Au) thin foil target.

2D hydrodynamic simulations of generation of high-current proton beams by relativistic skin-layer laser-plasma interaction

The properties of fast proton beam generation by skin-layer ponderomotive acceleration (SLPA) induced by a short (< 1 ps) laser pulse are studied using 2D two-fluid relativistic code. It is found that both for subrelativistic and relativistic laser intensities, the crucial parameter determining the spatial structure and angular divergence (Θ b) of the proton beam is the ratio d L/L n, where d L is the laser beam diameter and L n is the density gradient scale length of preplasma in front of the target. When this ratio is relatively small (< 10), the proton beam generated at relativistic laser intensities has a bubble-like structure and Θ is large. When d L is at least tens times greater than L n, the transverse distribution of the proton density and the proton current density in the beam follow the spatial shape of the laser beam and Θ b is small. In the last case, highly collimated high-current (MA) proton beams can be produced by SLPA at relativistic laser intensities.

Generation of highly collimated high-current ion beams by skin-layer laser-plasma interaction at relativistic laser intensities

Generation of fast ion beams by laser-induced skin-layer ponderomotive acceleration has been studied using a two-dimensional (2D) two-fluid relativistic computer code. It is shown that the key parameter determining the spatial structure and angular divergence of the ion beam is the ratio dL∕Ln, where dL is the laser beam diameter and Ln is the plasma density gradient scale length. When dL⪢Ln, a dense highly collimated megaampere ion (proton) beam of the ion current density approaching TA∕cm2 can be generated by skin-layer ponderomotive acceleration, even with a tabletop subpicosecond laser.

Fast ignition of precompressed fusion target by laser-driven plasma block: A concept and preliminary studies

The possibility of fast ignition of a fusion target by a high-density plasma (proton) block driven by the laser-induced skin-layer ponderomotive acceleration (S-LPA) mechanism is considered. The estimates show that using such a plasma block, the ignition threshold for precompressed DT fuel can be reached at the short-pulse laser energy ≤ 100 kJ.

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

The properties of plasma (proton) block driven by the laser-induced skin-layer ponderomotive acceleration (S-LPA) mechanism are discussed. It is shown that the proton density of the plasma block is about a thousand times higher than that of the proton beam produced by the target normal sheath acceleration (TNSA) mechanism. Such a high-density plasma (proton) block can be considered as a fast ignitor of fusion targets. The estimates show that using the S-LPA driven plasma block, the ignition threshold for precompressed DT fuel can be reached at the ps laser energy ≤ 100 kJ.

Analytical description of rippling effect and ion acceleration in plasma produced by a short laser pulse

In this paper we present the analytical description of two processes dealing with the skin-layer ponderomotive acceleration method of fast ion generation by a short laser pulse: ion density rippling in the underdense plasma region and generation of ion beams by trapped electromagnetic field in plasma. Some numerical examples of hydrodynamic simulation illustrating these processes are shown. The effect of using the laser pulse consisting of different frequency components on the ion density rippling and on phenomena connected with trapped electromagnetic field is analyzed.

Laser-driven generation of high-current ion beams using skin-layer ponderomotive acceleration

Basic properties of generation of high-current ion beams using the skin-layer ponderomotive acceleration (S-LPA) mechanism, induced by a short laser pulse interacting with a solid target are studied. Simplified scaling laws for the ion energies, the ion current densities, the ion beam intensities, and the efficiency of ions' production are derived for the cases of subrelativistic and relativistic laser-plasma interactions. The results of the time-of-flight measurements performed for both backward-accelerated ion beams from a massive target and forward-accelerated beams from a thin foil target irradiated by 1-ps laser pulse of intensity up to ∼ 1017 W/cm2 are presented. The ion current densities and the ion beam intensities at the source obtained from these measurements are compared to the ones achieved in recent short-pulse experiments using the target normal sheath acceleration (TNSA) mechanism at relativistic (&gt;1019 W/cm2) laser intensities. The possibility of application of high-current ion beams produced by S-LPA at relativistic intensities for fast ignition of fusion target is considered. Using the derived scaling laws for the ion beam parameters, the achievement conditions for ignition of compressed DT fuel with ion beams driven by ps laser pulses of total energy ≤ 100 kJ is shown.

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 (∼1010 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 × 1017 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 1010 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.