Monoenergetic Ion Beam Generation by Driving Ion Solitary Waves with Circularly Polarized Laser Light

Experimental data from the Trident Laser facility is presented showing quasimonoenergetic carbon ions from nm-scaled foil targets with an energy spread of as low as $\pm 15\%$ at 35 MeV. These results and high-resolution kinetic simulations show laser acceleration of quasimonoenergetic ion beams by the generation of ion solitons with circularly polarized laser pulses (500 fs, $\lambda =1054\mathrm{nm}$). The conversion efficiency into monoenergetic ions is increased by an order of magnitude compared with previous experimental results, representing an important step towards applications such as ion fast ignition.

Figure 1

Experimental setup, with the laser (CP and LP) being focused by an $F/3$ off-axis parabolic mirror on a nm-scaled diamondlike target; ions are measured with a Thomson parabola positioned 1.2 m behind the target.

Figure 2

(a) $C^{6+}$ spectra measured with CP (red triangles, noise in gray) and LP (magenta squares) from a 5 nm DLC target and the ${\mathrm{C}}^{6+}$ spectrum obtained from $20\mu \mathrm{m}$ Pd, published in Hegelich et al. [15]; (b) ${\mathrm{C}}^{6+}$ spectra measured with CP from two different shots on 10 nm DLC showing deterioration of the monoenergetic structure at increased target thickness.

Figure 3

Results from a 2D simulation: (a) ${\mathrm{C}}^{6+}$ spectra at 305 fs (end of the acceleration phase) for CP (solid) and LP (dashed) laser obtained along a $1\mathrm{\text{−}}\mu \mathrm{m}$ average over $z$ centered at $z=0$ (black) and from the whole simulation domain (red); (b) time evolution of carbon ion (red curves) and electron density (black), both normalized to $n_{\mathrm{cr}}$, during the transparency phase (the blue trace is the proton density); (c) bipolar electric field structure in $E_x$ (green) comoving with the solitary carbon density spike (red); (d) monoenergetic ion energy (red squares) obtained from ion spectrum vs time during the acceleration phase as compared with the soliton energy $\frac{1}{2}{m}_i{v}_p^2$ (black triangles) estimated from the soliton speed $v_p$ (the laser intensity profile is indicated in arb. u. as gray dashed line).