Interaction of Lasers with Atomic Clusters and Structured Plasmas

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We examine the interaction of intense, short laser pulses with atomic clusters and structured plasmas, namely preformed plasma channels. In examining the laser pulse interaction with atomic clusters we focus on the optical response of an individual cluster when irradiated by a laser. Our analysis of the laser pulse interaction with plasma channels focuses on the mode structure of a laser pulse propagating within the channel. We then present a novel application of these channels: quasi-phased match acceleration of electrons.

The optical properties of a gas of laser pulse exploded clusters are determined by the time-evolving polarizabilities of individual clusters. In turn, the polarizability of an individual cluster is determined by the time evolution of individual electrons within the cluster's electrostatic potential. We calculate the linear cluster polarizability using the Vlasov equation. A quasi-static equilibrium is calculated from a bi-maxwellian distribution that models both the hot and cold electrons, using inputs from a particle-in-cell simulation [Taguchi, T. et al., Phys. Rev. Lett., 2004. 92(20)]. We then perturb the system to first order in field and integrate the response of individual electrons to the self consistent field following unperturbed orbits. The dipole spectrum depicts strong absorption at frequencies much smaller than omega_p/√2. This enhanced absorption results from a beating of the laser field with electron orbital motion.

The properties of pulse propagation within plasma are determined by the structure of the plasma. The preformed plasma channel provides a guiding structure for laser pulses unbound by the intensity thresholds of standard wave guides. In particular, the corrugated plasma channel [Layer et al. Phys. Rev. Lett. (2007)] allows for the guiding of subluminal spatial harmonics. These spatial harmonics can be phase matched to high energy electrons, making the corrugated plasma channel ideal for the acceleration of electrons. We present a simple analytic model of pulse propagation in a corrugated plasma channel and examine the laser-electron beam interaction. Simulations show accelerating gradients of several hundred MeV/cm for laser powers much lower than required by standard laser wakefield schemes.