RESUMEN
The basic theory of laser propagation in hollow waveguides is considered in the context of laser-plasma physics. The physical model of waves reflecting between the guide walls is used to show that there is a discrete series of modes, and to give the mode dispersion relation and losses in terms of a given reflectivity. The mathematical connection between this model and the solution of Maxwell's equations for lossless propagation in a cylinder is given. Thus the solutions for low loss propagation for any given reflectivity can be obtained, provided it is close to 1. Results are given using Fresnel reflectivity for perfect dielectric and finite conductivity waveguides. The relationship of the breakdown intensity in dielectric waveguides to known breakdown intensities is also derived. The practical implications for the guiding of intense laser pulses and the limitations of the model are discussed. The theory is shown to explain, at least qualitatively, a number of previous experimental results.
RESUMEN
Protons with energies up to 18 MeV have been measured from high density laser-plasma interactions at incident laser intensities of 5x10(19) W/cm(2). Up to 10(12) protons with energies greater than 2 MeV were observed to propagate through a 125 &mgr;m thick aluminum target and measurements of their angular deflection were made. It is likely that the protons originate from the front surface of the target and are bent by large magnetic fields which exist in the target interior. To agree with our measurements these fields would be in excess of 30 MG and would be generated by the beam of fast electrons which is also observed.
RESUMEN
We analyze recent experimental results on the increase of fast electron penetration in shock compressed plastic [Phys. Rev. Lett. 81, 1003 (1998)]. It is explained by a combination of stopping power and electric field effects, which appear to be important even at laser intensities as low as 10(16) W cm-2. An important conclusion is that fast electron induced heating must be taken into account, changing the properties of the material in which the fast electrons propagate. In insulators this leads to a rapid insulator to conductor phase transition.
RESUMEN
A new technique for the measurement in fluids of the acoustic non-linearity parameter B/A is presented, together with measured B/A values for several fluids. The non-linearity parameter is measured by phase locking radial modes within a PZT cylinder. The system, which implements the isentropic phase technique, uses continuous wave phase locking to measure the change in sound velocity that is typically associated with a change in ambient pressure under constant entropy. The method provides a means of measuring B/A in vitro both accurately and simply without the typical problems involved in time-of-flight systems. Fluid samples can remain small due to the nature of the cavity resonator, so the system is well suited to small volume, biological samples.
