Hybrid acceleration of compact ion bunches by few-cycle laser pulses in gas jets of two atomic species

Extreme states of matter that exist for a short time need probing with compact proton sources. Here, the generation of a compact MeV-energy proton source from few-cycle laser pulse interaction with narrow gas-jet target is demonstrated numerically. We realize such proton source by incorporating a gas mixture which is optimal for generation of quasimonoenergetic proton bunches. In the presented laser-plasma interaction we identify the ion acceleration from magnetic vortex, charge separation, and collisionless shock wave. The proposed particle source is excellent for applications where moderate-energy proton bunches are used at high repetition rate. We prove the applicability of this scheme for generating a pulsed spherical neutron burst with $\sim 100\mathrm{ps}$ duration.

Figure 1

A proposed experimental scheme for the generation of a narrow energy-spread ultrashort ion (deuteron) bunches and their applicability to the production of ultrafast neutron bursts. A few-cycle laser pulse is focused onto a gas jet, which is tilted towards the incident laser pulse.

Figure 2

The on-axis density profile in the down ramp (a) seen by the laser pulse at the exit side is much steeper when the gas jet is tilted (see text for details). The energy density of the ion bunch is shown in (b), which shows a very narrow spatial extension, thus the accelerated bunch is very compact. The energy spectrum of the ion bunch is shown in Fig. 5. This short ion bunch is capable of producing a 14.1-MeV (c) neutron burst (d) upon the interaction with a tritium-rich bulk target. The fast neutron bunch resembles a spherical shell (20 ns after the interaction) with a bunch length of the order of 200 ps.

Figure 3

(Top) Distribution of the transverse magnetic field at two time instances in 3D. The laser pulse (visible at $t=500$ fs) propagates in the positive $x$ direction. Azimuthal magnetic field (a), (b) and electric fields (c), (d) are shown at two time instances. In (c) and (d) below the axis of symmetry ($y=0$) the longitudinal field is shown, while above the transversal (or radial) electric field. In (b) the red dashed line represents the derivative of the density profile (in arbitrary units), which indicates that the magnetic vortex is generated at the position of the maximum density gradient.

Figure 4

Momentum and angle distribution of protons (a), (b) and ${\text{O}}^{6+}$ (e), (f). In all pictures particles with propagation angle less then ${60}^{\circ }$ are included. The upper half plane shows the actual angle of propagation in the $xy$ plane, while the lower half plane presents the distribution of longitudinal momentum of ions, which is multiplied by a numeric factor to be presented by the same color scale. In (c) and (g) the spatial density distributions are shown at $t=2.4\mathrm{ps}$, on logarithmic scale. In (d) and (h) the proton and oxygen ion phase-space distributions are shown at different time instances.

Figure 5

Proton angular-energy distribution (from $x>90\mu \mathrm{m}$) at 6 ps with (a) and without oxygen (b) present in the gas. The white lines show the integrated spectra in the angular interval below the red dashed lines. In (c) the deuteron spectrum is for the case presented in (a), but instead of hydrogen, deuterium was used. After the interaction with a tritium-rich target placed at 1 cm distance the resulting neutron distribution in the transversal plane is shown in (d) at the time instance when the slowest ion reaches the target surface. In the neutron production only energetic ions are included with energy larger than 200 keV.

Figure 6

Phase-space distribution of deuteron (upper) and oxygen (lower) ions at different time instances.

Figure 7

Red lines represent the longitudinal electric field profiles, while the blue lines depict the ion phase-space distribution for short and long density ramps.

Figure 8

Longitudinal (left column) and transverse (right column) current density components in the $xy$ plane at different time instances.

Figure 9

Azimuthal magnetic field (a)–(c) and forward accelerating electric field (d)–(f) in the $xz$ plane. The pictures are produced from the simulations presented in Figs. 2 and 3. Below the electron density distributions are shown at the same time instances.

Figure 10

Propagation angle of protons in the $xz$ cross section of the simulation domain.

Figure 11

Azimuthal magnetic field (upper row) and electric fields (lower row) in the $xy$ plane. These pictures present the continuation of Fig. 9. The time instance is 700 fs (left) and 800 fs (right). The red ellipse illustrates the position where the high-energy protons originate from. Focusing and accelerating fields are both present in this region.

Figure 12

Evolution of the electron density in the $xy$ plane.