Natsumi Iwata

On the nonlocal properties of relativistic ponderomotive force

A theory of relativistic ponderomotive force of transversely localized laser fields is presented by taking into account the nonlocal effects that correspond to higher order terms of the expansion parameter ϵ≡l/L, i.e., the ratio between particle excursion length and scale length of the field amplitude gradient, while the existing local theory is the first order of ϵ. As a method for preserving the Hamiltonian structure up to higher orders, we employ the variational principle in noncanonical phase space coordinates incorporated with the Lie transformation. By finding noncanonical coordinates and gauges, we obtain a new formula for the ponderomotive force that involves new terms in the third order represented by the second and third spatial derivatives. The force then depends not only on the local field gradient but also on the curvature and its variation which represent the effects of higher-order nonlocal particle motion. The higher-order terms are found to be subject to the symmetry of the field structur...

Broadening of cyclotron resonance conditions in the relativistic interaction of an intense laser with overdense plasmas

The interaction of dense plasmas with an intense laser under a strong external magnetic field has been investigated. When the cyclotron frequency for the ambient magnetic field is higher than the laser frequency, the lasers electromagnetic field is converted to the whistler mode that propagates along the field line. Because of the nature of the whistler wave, the laser light penetrates into dense plasmas with no cutoff density, and produces superthermal electrons through cyclotron resonance. It is found that the cyclotron resonance absorption occurs effectively under the broadened conditions, or a wider range of the external field, which is caused by the presence of relativistic electrons accelerated by the laser field. The upper limit of the ambient field for the resonance increases in proportion to the square root of the relativistic laser intensity. The propagation of a large-amplitude whistler wave could raise the possibility for plasma heating and particle acceleration deep inside dense plasmas.

Laser-matter interaction in cluster medium in the radiation dominated regime

We study the interaction between laser and cluster medium in high intensity regime 1022-24 W/cm2 using a particle based integral code (EPIC3D). By introducing four targets consisting of the same mass, i.e. same packing fraction, but having different internal structure, we investigate the effect of cluster on the acceleration dynamics comparing with that of thin film. In the radiation pressure dominated regime, the cluster medium exhibits a higher maximum energy than that achieved by the simple piston mechanism due to the additional accerelation by the Coulomb explosion. The optimum cluster radius for ion acceleration is found to exist depending on the laser power irradiated.

Retraction: Higher-Order Nonlocal Effects of a Relativistic Ponderomotive Force in High-Intensity Laser Fields [Phys. Rev. Lett. 112, 035002 (2014)].

Retraction of DOI: 10.1103/PhysRevLett.112.035002.

Theoretical Study of Particle Motion Under High Intensity Laser–Plasma Interaction Aiming for High Energy Density Science

In recent years, high power short pulse lasers in the range of 1018–22 W/cm2 have been developed and explored new science and applications. One of them is the fast ignition-based laser fusion, which is expected as one of clean and abundant energy sources. In determining the interaction between such high intensity lasers and plasmas, the ponderomotive force (light pressure) plays an essential role due to the strong non-uniformity of the laser field strength originated from tight focusing of the laser light. The force has been expressed as that proportional to the gradient of the laser field amplitude at the oscillation center of the particle. However, under the tight focusing, not only the gradient, which corresponds to the first-order perturbation to the uniform field, but also the higher order structures, e.g. field curvature to the second order, becomes important in determining the particle orbit. In order to precisely describe the relativistic ponderomotive force including such effects, here, we introduce the noncanonical Lie perturbation theory. We successfully derived the oscillation-center equation of motion up to the second order keeping the Hamiltonian structure rigorously. The resulting equation is found to be same as that of the first order indicating that no additional force appears up to the second order due to the symmetric nature of the field curvature.

Nonlocal properties of the ponderomotive force in high intensity laser fields-An approach based on the noncanonical Lie perturbation theory-

In order to study the nonlocal particle dynamics in tightly focused laser fields in the order of wave length, we present a theoretical framework of relativistic ponderomotive force by using the noncanonical Lie perturbation theory based on the phase space Lagrangian formalism that keeps the Hamiltonian structure rigorously. Introducing a smallness parameter e, the ratio of the particle excursion length to the scale length of the gradient of the laser field amplitude, we perform the perturbation analysis up to the second order e2, which contain the effect of the field curvature, and obtain the oscillation center equation of motion averaged over the fast laser period. A betatron-like oscillation which corresponds to a confinement state of the particle in the laser field without suffering ejection due to the field curvature is analytically found.