Sub-micron fast electron filamentation in high intensity laser-solid interactions

Summary form only given. The transport of relativistic electrons generated in the interaction of high intensity (I ~ 8times10¹⁹ W cm^-2) lasers with solid targets has been studied through measurements of the optical radiation emitted from their rear surface. The high degree of polarization of the radiation indicates that it is predominantly optical transition radiation. Measurements of the signal strength of the coherent transition radiation (CTR) with respect to the direction of observation have also been performed. These measurements allow to estimate the fast electron filament size with an unprecedented resolution that is of the order of the observation wavelength (lambda_obs= 0.527 mum). This technique has been used with different Cu wedge targets with a thickness of ~50 mum. We find evidence for the presence of sub-mum filaments (FWHM beam diameter ~0.7 mum) at the target rear side, although filaments of larger size also contribute to the signal. Results from particle-in-cell codes have demonstrated that mum-scale filaments originate from the region of laser-solid interaction and are spread over a wide range of angles. However, the resulting small scale structure after propagating through ~50 mum of solid copper must account also for collisions and resistive effects. Indeed, after considering the effects of angular scattering on the filament divergence, it will be shown that the fine structure of the electron filamentation at the target rear side must be the result of self-generated magnetic or electric fields inside the solid target.