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Generation of picosecond high-density ion fluxes by skin-layer laser-plasma interaction

Published online by Cambridge University Press:  07 June 2005

J. BADZIAK ,
S. GŁOWACZ ,
S. JABŁOŃSKI ,
P. PARYS ,
J. WOŁOWSKI  and
H. HORA
J. BADZIAK
Affiliation:
Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland
S. GŁOWACZ
Affiliation:
Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland
S. JABŁOŃSKI
Affiliation:
Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland
P. PARYS
Affiliation:
Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland
J. WOŁOWSKI
Affiliation:
Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland
H. HORA
Affiliation:
Department of Theoretical Physics, University of New South Wales, Sydney, Australia
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Abstract

The possibilities of producing ultrahigh-current-density ps ion fluxes by the skin-layer interaction of a short (≤ 1ps) laser pulse with plasma were studied using two-fluid hydrodynamic simulations, and the time-of-flight measurements. Backward-emitted ion fluxes from a massive (Au) target as well as forward-emitted fluxes from various thin foil targets irradiated by a 1-ps laser pulse of intensity up to 2 × 1017 W/cm2 were recorded. Both the simulations and the measurements confirmed that using the short-pulse skin-layer interaction of a laser pulse with a thin pre-plasma layer in front of a solid target, a high-density collimated ion flux of extremely high ion current density (∼ 1010 A/cm2 close to the target), can be generated at laser intensity only ∼ 1017 W/cm2. The ion current densities produced by this way were found to be comparable to (or even higher than) those estimated from recent short-pulse experiments using a target normal sheath acceleration mechanism at relativistic laser intensities. The effect of the target structure on the current densities and energies of forward-emitted ions is demonstrated.

Keywords

Ion fluxesLaserPicosecond pulsesPlasma
Type
Research Article
Information
Laser and Particle Beams , Volume 23 , Issue 2 , June 2005 , pp. 143 - 147
Copyright
2005 Cambridge University Press

1. INTRODUCTION

High-peak-power laser generating short (≤ 1ps) pulses enable the production of fast ion beams (Snavely et al., 2000; Clark et al., 2000; Badziak et al., 2001a; Borghesi et al., 2002; Hegelich et al., 2002; Zepf et al., 2003; Cowan et al., 2004; Pegoraro et al., 2004; Roth et al., 2005) with a potential to be applied in various fields of research including, in particular, high energy-density physics (Patel et al., 2003), and fast ignition of inertial fusion targets (Roth et al., 2001). However, these applications require high-density ion beams of extremely high current densities (∼ 1010 A/cm2 or higher), and of very short ion pulse duration (in the ps time scale). The above requirements are far beyond the present possibilities of conventional accelerators; they are attainable with the use of laser-driven ion sources. A possible way of producing ion beams with such extreme parameters is ballistic focusing (within a distance ≤ 1 mm) of energetic ions, driven by target normal sheath acceleration (TNSA) mechanism at relativistic laser intensities (Wilks et al., 2001; Patel et al., 2003).

In this paper, we consider another method, where the production of ps 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, hereinafter referred to as skin-layer ponderomotive acceleration (S-LPA), uses non-linear ponderomotive forces induced at the skin-layer interaction of a short laser pulse with a proper pre-plasma layer in front of a solid target (Badziak et al., 2004; Osman et al., 2004).

2. SKIN-LAYER PONDEROMOTIVE ACCELERATION OF IONS: A SIMPLIFIED PHYSICAL MODEL

A simplified model of production of high-density fast ion beams by the S-LPA mechanism was presented by Badziak et al. (2004). Very briefly, on the target surface the laser pre-pulse produces a pre-plasma layer of the thickness Lpre, at least several times smaller than the laser focal spot diameter df. The main short laser pulse interacts most intensely with the plasma in the skin layer near the surface of the critical electron density nec and the geometry of the interaction is almost planar (Lpre << df). The high plasma density gradient in the interaction region induces two opposite non-linear forces which break the plasma and drive two thin (∼ λ) plasma blocks toward vacuum and toward the plasma interior, respectively (λ is the laser wavelength). As the density of the plasma blocks is high (the ion density ni ≈ nec/z, where z is the ion charge state) even at moderate ion velocities vi ∼ 107–108 cm/s, the ion current densities js = zenivi can be very high (∼ 109–1010 A/cm2 or higher). The time duration of the ion current flowing out of the interaction region (being the ion source) is approximately equal to the laser pulse duration. Due to almost planar acceleration geometry, the angular divergence of the ion beam is small.

For subrelativistic laser intensities: I << Irel ≈ 4.1 × 1018/λ2 (W/cm2, μm), the ion energies, Ei, and the ion current densities, js, of the plasma blocks can be estimated from the equations (Badziak et al., 2004):

where s = S for the forward-accelerated ions or s = S–1 for the backward-accelerated ions, S is the dielectric swelling factor and A is the atomic mass number. For instance, at sz/A = 1, a laser pulse of λ = 1 μm and of I = 1017W/cm2 produces the ion current density of js ≈ 2.3 × 1010 A/cm2.

3. RESULTS OF NUMERICAL SIMULATIONS

The production of high-density ion fluxes by the S-LPA mechanism at subrelativistic laser intensities was investigated in detail using the advanced two-fluid plasma hydrodynamic model (Hora & Aydin, 1992; Boreham et al., 1997). Both one- and two-dimensional codes were worked out for numerical calculations. An example of 2D calculations, performed for the case of 200-fs, 1.05-μm laser pulse of intensity 1016 W/cm2 interacting with a hydrogen plasma ramp of linear initial profile, is shown in Figure 1. It illustrates the spatial distribution of the ion current density near the critical plasma surface and its cross section at y = 0 (along the laser beam axis). We can see the generation of the under dense plasma block moving against the laser (negative ion current densities) and the over dense plasma block behind the critical surface moving in the forward direction (positive ion current densities). The transverse distribution of ion current density (along the y-axis) follows the intensity distribution of the laser beam and the effective widths of the plasma blocks are close to the beam size.

Two-dimensional spatial distribution of the ion current density near the critical surface and its cross section at Y = 0 produced by a 200-fs laser pulse of intensity 1016 W/cm2.

The main conclusions from our numerical simulations are as follows: (a) the current densities and the velocities of backward-accelerated ions follow approximately a square-root dependence on the laser intensity as predicted by Eqs. (1) and (2); however, for the forward-accelerated ions, they increase faster with I and they attain higher values; (b) both for backward-accelerated and forward-accelerated ions there exist optimum values of the plasma density gradient scale length, Ln, and high plasma density gradients (Ln< λ) are preferable for the forward-accelerated ions; (c) the laser pulses of shorter wavelength produce ion fluxes of lower ion velocities but of higher current densities (Fig. 2, Eqs. (1) and (2)); (d) the current densities and the velocities of forward-accelerated ions continuously increase with an elongation of the laser pulse, but in the case of backward-accelerated ions, distinct maxima of the ion current densities and the ion velocities occur (Fig. 2); (e) at laser intensities ∼ 1016–1017 W/cm2, the ion current densities attain values in the range of 109–1011 A/cm2 in accordance with the simplified model of S-LPA.

The current densities of ions driven by a short laser pulse as a function of the laser pulse duration calculated for the first and the second harmonics of Nd:glass laser. Ln/λ = 1.

4. RESULTS OF MEASUREMENTS

The experiment was performed with the use of Nd:glass CPA laser, generating a 1-ps, 1.05-μm pulse of a long-time scale (≥ 1 ns) intensity contrast ratio higher than 108 (Badziak et al., 1997, 2001b). The intensity of the short lasting (∼ 10−10 s) pre-pulse was ∼ 104 times lower than the intensity of the main pulse (Badziak et al., 2001b). For measurements of backward-accelerated ion fluxes, the linearly polarized laser beam was focused by an on-axis f/2.5 parabolic mirror, with a hole in the center, onto a massive Au target at an angle of 0° with respect to the target normal. The maximum intensity of the focused laser beam (df ≈ 20 μm) was about 1017 W/cm2. For the measurements of forward-accelerated ion fluxes, the ps laser beam was focused by f/1 aspheric lens on a thin foil target normal to the target. The maximum laser intensity was 2 × 1017 W/cm2. For both kinds of the targets used the short-lasting pre-pulse produced pre-plasma on the target surface of the thickness Lpre ≤ 5 μm (Badziak et al., 2003, 2004).

The measurements of the ion flux parameters were performed with the use of ion collectors (ICs) and an electrostatic ion-energy analyzer (IEA) (Badziak et al., 2001b). The IEA and the ring ion collector (IC1) measured the backward-accelerated ions passing through the hole in the parabolic mirror along the target normal and the laser beam axis (Fig. 3). For a rough estimation of the angular distribution of ion emission, two additional collectors viewing the target at angles of 26° and 34° with respect to the target normal were applied. The forward-accelerated ions were recorded with the ICs and the IEA situated behind the thin foil target in the similar geometry.

Simplified scheme of the experimental arrangement. IEA–electrostatic ion-energy analyzer; IC1, IC2, IC3–ion collectors.

For measurements of forward-accelerated ions, both single-layer and double-layer targets were used, and particularly: (1) polystyrene (PS) targets of the thickness, LT, of 0.5 μm and 1 μm (marked as PS0.5 and PS1, respectively); (2) Al target of LT = 0.75 μm (marked as Al0.75); (3) double-layer target with a 0.05-μm Au front layer and a 1-μm PS back layer (marked as Au0.05/PS1); (4) double-layer target with a 1-μm PS front layer and a 0.05-μm Au back layer (marked as PS1/Au0.05).

To estimate the fast ion current density at the source in the close vicinity of the target surface on the basis of our time-of-flight measurements we use the formula (Badziak et al., 2004):

where: Qi is the total charge of fast ions measured in the far expansion zone, τis is the duration of fast ion generation at the source, which is roughly equal to the laser pulse duration, τis ≈ τL; Ss is the area of the fast ion source. We assume τis = τL, Ss = Sf and Qi = QIC1, where QIC1 is the fast ion charge passing through the IC1 collector, seen within the angle of 3° from the source, and Sf is the focal spot area.

Our measurements of backward-accelerated ions, performed in the laser intensity range of 1016–1017 W/cm2, confirmed the linear dependence (Eq. (1)) of the mean ion energy on the intensity: Ei ∝ I1.02 ± 0.18 as well as the square-root dependence (Eq. (2)) of the ion current density at the source on the intensity: js ∝ I0.57 ± 0.08 with js ≈ 1010 A/cm2 at I = 1017 W/cm2 (Badziak et al., 2004).

The measurements of forward-accelerated ions from thin foil targets revealed the fact that for all the targets used, only a single, highly collimated fast proton group, well separated from other, slower ion groups, is generated. The energy distributions of protons emitted from various thin foil targets as well as the proton current densities at the source, emitted within the 3° angle cone, are presented in Figure 4. The highest proton energies (both mean and maximum ones) were recorded for the Au0.05/PS1 target. In turn, the highest proton current densities (> 2 × 109 A/cm2) were achieved for the thinnest PS target and the lowest ones (for the Al target).

The proton energy distributions (a) and the current densities at the source (b) of protons produced form various thin foil targets. τL = 1 ps, IL = (1.2 ± 0.2) × 1017 W/cm2.

The ion current densities at the source achieved in our experiment (js ∼ 1010 A/cm2) were found to be comparable to (or even higher than) those estimated from Eq. (3) for recent short-pulse experiments using the TNSA method at relativistic laser intensities. For instance, the data from the petawatt experiment (Snavely et al., 2000) result in a similar (∼1010 A/cm2) value of js, while js for the ballistic focused ion beam produced in the 100 TW/100 fs experiment (Patel et al., 2003) was estimated to be at least a few times smaller.

5. CONCLUSIONS

It was shown that the S-LPA mechanism makes it possible to produce highly collimated high-density ion beams (plasma blocks) of extremely high ion current densities at the source, comparable to those produced by TNSA at significantly higher energy and power of a laser pulse. Apart from the simpler physics of the laser-plasma interaction, the advantage of S-LPA 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 high energy-density physics, ICF, or X-ray laser studies.

ACKNOWLEDGMENTS

This work was supported in part by the International Atomic Energy Agency in Vienna under Contract Number 11535/RO and by the State Committee for Scientific Research (KBN), Poland under Grant Number 1 PO3B 043 26.

Footnotes

This paper was presented at the 28th ECLIM conference in Rome, Italy.

References

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Figure 0

Two-dimensional spatial distribution of the ion current density near the critical surface and its cross section at Y = 0 produced by a 200-fs laser pulse of intensity 1016 W/cm2.

Figure 1

The current densities of ions driven by a short laser pulse as a function of the laser pulse duration calculated for the first and the second harmonics of Nd:glass laser. Ln/λ = 1.

Figure 2

Simplified scheme of the experimental arrangement. IEA–electrostatic ion-energy analyzer; IC1, IC2, IC3–ion collectors.

Figure 3

The proton energy distributions (a) and the current densities at the source (b) of protons produced form various thin foil targets. τL = 1 ps, IL = (1.2 ± 0.2) × 1017 W/cm2.