Slowdown mechanisms of ultraintense laser propagation in critical density plasma

Laser Ion Acceleration in a Near Critical Density Trap

In order to accelerate ions by a laser, we go back to the original and the fundamental idea of how longitudinal field structure generation can be carried out in an ionized media and how particles may be trapped by the created wakefield. The latter condition is characterized by the phase velocity of the longitudinal structure vph be equal to the particle trapping width vtr. Since the trapping width is inversely proportional to the square-root of the mass of the accelerated particles, this width is much shorter for ions than for electrons. Thus, our dictum for laser ion acceleration is to impose a near critical density trap to decelerate laser group velocity, vg and subsequently to generate longitudinal wakefield to be able to trap ions under the condition of vtr = vph. We demonstrate this concept by PIC simulation and find that this method is effective, and the efficiency of laser ion acceleration is enhanced by a couple of orders of magnitude toward unity.

Stimulated Raman scattering in the relativistic regime in near-critical plasmas

Interaction of a high-intensity short laser pulse with near-critical plasmas allows us to achieve extremely high coupling efficiency and transfer laser energy to energetic ions. One-dimensional particle-in-cell simulations are considered to detail the processes involved in the energy transfer. A confrontation of the numerical results with the theory highlights a key role played by the process of stimulated Raman scattering in the relativistic regime. The interaction of a 1 ps laser pulse (I∼6×10^{18}Wcm^{-2} with an undercritical (0.5n_{c}) homogeneous plasma leads to a very high plasma absorption reaching 68% of the laser pulse energy. This permits a homogeneous electron heating all along the plasma and an efficient ion acceleration at the plasma edges and in cavities.