Sliding-wave acceleration of ions in high-density gas jet targets

A hybrid mechanism of ion acceleration is investigated which demonstrates the higher spectral density of protons at high energies. The interaction of few-cycle terrawatt laser pulses with near-critical density gas target is studied with the help of two-dimensional particle-in-cell simulation. The generation of few MeV protons with high spectral concentration near cutoff is attributed to the propagation of solitary waves in the decaying density profile of the gas jet. Plasma dynamics at longer time scale is explained by semianalytical modeling and conditions for solitary wave breaking are presented.

Figure 1

Magnetic field generated at the back side of the target right after the exit of the laser pulse (a),(b). The white ellipse indicates the regions where the magnetic vortex is most effective. The spatiotemporal evolution of the longitudinal electric field along the laser axis (c),(d). The plasma density profile, shown by the red curves, is described by super-Gaussian function with exponents: $p=4$ (a),(c) and $p=8$ (b),(d).

Figure 2

Magnetic field distribution at $t=1$ ps in the case of smoother $[p=4$ (a)] and steeper $[p=8$ (b)] density profiles. The lateral propagation is more pronounced in the case of greater density gradient.

Figure 3

Phase-space distribution of protons at different time instances for $p=4$ (a),(b),(c) and for $p=8$ (d),(e),(f). The spatial profile of longitudinal electric field is also shown on the right axes. The protons are selected from a limited spatial domain: $65\mu \text{m}<y<75\mu \text{m}$.

Figure 4

Difference between the position of charge separation ($x_s$) and magnetic vortex ($x_m$) in the $(p,\sigma )$ parameter space. In the color code an upper cutoff at $x_s-x_m=19{\lambda }_{D0}$ is applied; thus in the dark red area the actual values are much larger.

Figure 5

Proton energy spectra at $t=4.5$ ps after the interaction for different density steepnesses of the gas jet. By dots the spatial distribution of the same protons are shown. Here protons accelerated within a cone angle of $\pm 2^{\circ }$ are included.