Spatially confined propagation of intense ultraviolet radiation in plasmas

X-ray amplification requires a high energy deposition rate in a high aspect-ratio volume. High power lasers for x-ray laser pumping have become available with the development of the short pulse and high intensity laser technology capable of producing pulses with a peak power as high as 10{dollar}sp{lcub}12{rcub}{dollar} watts. Short pulses of high intensity x-ray have been observed in laser-plasma interactions, which encurages many scientists actively pursuing the goal of constructing practical x-ray lasers. Our approach has concentrated on producing high aspect ratio x-ray amplifying medium by spatially confined propagation of high power laser pulse in plasmas.; A high intensity laser beam induces nonlinear refractive index changes in plasma. In the case of subpicosecond ultrahigh power laser-plasma interaction, the dominant mechanisms responsible for the refractive index change in plasmas are: (1) the relativistic free electron mass increase due to the increase of electron oscillation velocity in the intense electromagnetic field of the laser pulses; and (2) displacement of free electrons out of the high intensity region of the laser beam by ponderomotive force. Both of the above effects lead to a refractive index change of the plasma, which in turn has a positive lensing effect on the beam. If the focusing effect is strong enough to overcome diffraction the beam will stay in a spatially confined mode of propagation. This confined propagation provides an effective method of concentrating energy. The field intensity associated with the confined propagation is so high that the highly excited medium with high aspect ratio suitable for x-ray amplification can be achieved.; In this research we have successfully demonstrated spatially confined propagation of 500 GW subpicosecond laser pulse in laser induced plasma. The measured diameter of the propagation is less than 2 {dollar}mu{dollar}m and the aspect ratio of the confined propagation is over 1000. The filed intensity associated with the propagation is estimated to the order of 10{dollar}sp{lcub}19{rcub}{dollar} W/cm{dollar}sp2{dollar}. Qualitatively good agreements are reached by comparing the experimental results with theoretical calculations involving the relativistic and ponderomotive nonlinearity of the medium.