Striated Jets and Anomalous Absorption Due to Nonlinear Ponderomotive Forces in Laser Produced Plasmas

Abstract

One of the peculiarities observed in laser produced deuterium plasmas is the correlation of neutron production with increased reflectivity of the laser light, the creation of a second elevated temperature of a few keV, and the generation of a fast group of plasma with 20 keV ions. Besides the very complex questions of the plasma dynamics characterized by nonlinear changes in the collision frequency, parametric instabilities, anomalous absorption and nonlinear forces, in this paper special mechanism is treated where the nonlinear ponderomotive forces are calculated for the field of an obliquely incident standing wave, generating a motion along the planes of the nodes of this standing wave, parallel or antiparallel to the direction of the plane of incidence. The electric vector of the light has to be parallel to the plane of incidence. These forces vanish at perpendicular incidence and are due to the longitudinal component of the electric vector of the wave. The forces cause a maximum laminar motion for angles of incidence of 25°. The motion needs a mean free path less than the thickness of the layers. The limitation toward turbulent motion is given. Laser intensitites around 1013 W/cm2 can build up striated jets with ion energies of a few keV. This model could explain a not purely thermonuclear generation of fusion neutrons and is consisted with a number of observations.