Energy Increase in Multi-MeV Ion Acceleration in the Interaction of a Short Pulse Laser with a Cluster-Gas Target

An approach for accelerating ions, with the use of a cluster-gas target and an ultrashort pulse laser of 150-mJ energy and 40-fs duration, is presented. Ions with energy 10–20 MeV per nucleon having a small divergence (full angle) of 3.4° are generated in the forward direction, corresponding to approximately tenfold increase in the ion energies compared to previous experiments using solid targets. It is inferred from a particle-in-cell simulation that the high energy ions are generated at the rear side of the target due to the formation of a strong dipole vortex structure in subcritical density plasmas.

Figure 1

(a) Schematic of the experimental setup. (b) Measured (solid curve) and calculated (dotted curve) soft x-ray spectra for the subcritical density plasma of the order of ${10}^{20}{\mathrm{cm}}^{-3}$ which is about $0.1n_c$. (c) Typical images of the etched pits registered in the 11th layer of CR39.

Figure 2

(a) The shadowgraph image for a mixture of He gas and ${\mathrm{CO}}_2$ clusters. The red (or gray) line shows the initial atom density profile. (b) The shadowgraph image for a pure He gas target.

Figure 3

The ion energy spectrum obtained by the TOF method. The inset shows TOF spectrum obtained in one laser shot which registers $15\mathrm{MeV}/u$ ion signal. A saturated signal around the flight time $t=5$ is caused by hard x rays emitted from the laser-cluster interaction region.

Figure 4

The results of 2D PIC simulation. (a) The electron density $N_e$ normalized by $n_c$. The red (or gray) line shows the initial density profile. (b) The magnetic field $B_z$ normalized by the laser field, and (c) the longitudinal electric field $E_x$ normalized by the laser field at $t=900\mathrm{fs}$, respectively. (d) The calculated spectra of the ions in units of $\mathrm{MeV}/u$ and the electrons in MeV at $t=6\mathrm{ps}$.