Abstract
The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of μm-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given.
Highlights
IntroductionThe radiation is found to be uniformly polarized over the emission region, with a degree of polarization dependent on the orientation of the target rear surface, demonstrating that it is predominantly transition radiation (TR)
Where the first integral refers to the incoherent component of the transition radiation (ITR) and the second integral refers to the coherent one (CTR)
The polarization allows us to identify the emission as principally coherent transition radiation (CTR) and not SR, CWE or thermal radiation
Summary
The radiation is found to be uniformly polarized over the emission region, with a degree of polarization dependent on the orientation of the target rear surface, demonstrating that it is predominantly TR. The strong variation of the signal with the orientation of the target rear surface (or, in other words, of the angle of observation) reveals that, at high laser intensities, fast electrons propagate in micron-size filaments. G(p) is the momentum distribution function, E and E⊥ are the Fourier transforms of the electric fields in the plane parallel and perpendicular to the radiation plane (defined by the directions of the target normal and the direction of observation) and F is a coherence function that takes into account the exact time and position at which electrons reach the interface. For our purposes an interface that separates a perfect conductor from the vacuum can be assumed, in which case the Fourier fields simplify, becoming [17, 18]
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