Effective generation of fast particles and X-ray from nano-structured targets irradiated by ultra-short intense laser pulses

Full text: Interaction of an ultrashort and ultraintense laser pulse with nano-structured targets is considered. Maximum energy of fast particles, conversion efficiency of laser energy to fast ion one and the divergence of particle beams are compared for various types of targets. The optimal is the target with the maximum absorption and the minimum mass. One variant of efficient target consist of a bunch of parallel or tapered carbon nanofibers. Such targets generate effectively a big current of relativistic electrons, propagating along the fibers and following their curvature. It follows that the optimal fiber thickness should be of several length of the skin layer of the electrical field and has to be in the order of magnitude of tens of nanometers. The distance between the fibers can vary between several Debye radiuses up to values of a few laser wavelengths. Collecting all fibers in only one allows to reduce the transverse size of fast electron cloud. The density of the energy flux of the hot electrons in such bunch, propagating along one nanofiber exceeds several times the intensity of the laser pulse. The absorption of laser energy in optimal nanostructure targets is about 100%. The factor of conversion of laser energy in energy of fast ions for such target can come approximately to tens percents. Efficient conversion of fundamental laser radiation into characteristic sub-femtosecond X-ray radiation and generation of tunable up-converted radiation are predicted. The results of the simulations were compared with the experimental data and have shown a good coexistence. (author)