Dielectric Nonlinear Forces and Dynamic Absorption

Abstract

If an electromagnetic wave propagates into a plasma with a density very close to the cut-off density, the absolute value of the refractive index |ñ| is much less than unity (Eq. 6.14)). The energy momentum flux density E² + H² is then greater by the factor ϑ = |ñ|⁻¹, the effective wavelength of the laser light is increased, and the effective laser intensity I = I_ν/|ñ| = ϑI_ν increases over its vacuum value I_ν by this factor. Spatial gradients of the time-averaged energy momentum flux density will cause forces corresponding to one hundred times the radiation pressure of the light or more. These forces can cause self-focusing and may be related to the Linlor effect and to the homogeneous heating of a laserproduced plasma. The same forces can, however, in determining the dynamics of the plasma, cause ablation of the plasma surface and result in recoil from compression of the plasma, and can cause disadvantageous energy transfer by dynamic anomalous nonlinear absorption. The nonlinear forces are mainly related to the fact that the swelling factor ϑ is larger than unity, but also arise if the factor is only a small quantity, as seen in dynamic absorption or in Chen’s general derivation¹⁷³ of instabilities causing anomalous absorption. The essential property is the dielectric change of the refractive index from unity. This is the reason for the more specific expression “dielectric” nonlinear force.