The Influence Of Free Electrons On The Propagation Of Intense Subpicosecond 248 nm Radiation In Hydrogen Plasmas

The influence of free electrons on the propagation of intense subpicosecond 248 nm radiation in hydrogen plasmasT . S . Luk, X . J Pan, J . C . Solemf , K . Boyer, and C . K . RhodesLaboratory for Atomic, Molecular, and Radiation PhysicsDepartment of Physics, University of Illinois at ChicagoP.O. Box 4348, Chicago, Illinois 60680ABSTRACT contribution to index (An) of a medium inducedby a laser field is given by Studies of the spectral and spatial propertiesof propagation of intense (~ 1016 W/cm2 ) An = TT N Y (F2 /3) . (1) subpicosecond 248 nm radiation in hydrogenplasmas for electron densities in the 10 - In Eq . (1) N is the particle density in cm"3 , 1019 cm"3 range are reported. The influence Y is the nonresonant hyperpolarizability in of the ponderomotive potential on the character esu units, and F is the field strength in of the propagation is considered . statvolt/cm . The solution of the nonlinearwave equation containing such a nonlinearity1 . INTRODUCTION gives rise to a range of phenomena, includingdistortions of the near field pattern^ andThe basic task in the production of coherent supercontinuum generation . 8 /!0 in the present short wavelength radiation is the controlled work, the effect of ionization and the influence deposition of energy at high specific powers . of electrons on propagation and self-focusing This requires both (1) a local deposition in molecular hydrogen gas is discussed, rate of sufficient magnitude1' 2 and (2) theability to govern the spatial organization of 3 . EXPERIMENTAL STUDIES AND DISCUSSION the excitation . The issue of energy deposition3occurring from intense irradiation has been A set of experiments has been performed experimentally examined in the collisionless in order to examine the role of plasma electrons regime through the study of ion charge state on the propagation of intense subpicosecond spectra4 and electron energy distributions . b ultraviolet radiation . Using a subpicosecond It has been estimated2 that a state selective ultraviolet laser with a wavelength of X = energy deposition rate of ~ 101H W/cm3 is 248 nm focused into a gaseous hydrogen medium, required to generate significant gain (g0 ~ full ionization can be made to occur during 10 cm"1 ) at a quantum energy of ~ 1 keV . the early phase of the laser pulse, an intervalfor which the intensity is much lower than theAn analysis of nonlinear coupling3 indicates maximum value. Under these circumstances, it that, for heavy materials in the high intensity can be shown that no significant nonlinear regime (~ 1019 - 1020 W/cm2 ), energy deposition increase of the index can be induced arising rates may approach 0 .1 - 1 .0 W/atom for short from molecular hydrogen . Instead, the principal pulse ( 2 . (2)Self-focusing is a common feature of At the high intensities considered, thenonlinear propagation' occurring in all ponderomotive force has a strong influence onphases of matter . It is also known that the electronic motions . Due to the highstable noncatastrophic beam self-focusing mobility of the electrons, the ponderomotivecan develop and, although the paraxial force produced by the transverse gradient theapproximation to the wave equation is invalid, laser intensity at the focus tends to expela new numerical procedure^ has been developed the electrons from the high field region .which can describe propagation in this regime . Since this electronic response can occur rapidly, a new mode of radiative channellingIn customary treatments, the nonlinear appears as a possibility for short pulses of