Abstract
The collective response of electrons in an ultrathin foil target irradiated by an ultraintense ( ${\sim}6\times 10^{20}~\text{W}~\text{cm}^{-2}$ ) laser pulse is investigated experimentally and via 3D particle-in-cell simulations. It is shown that if the target is sufficiently thin that the laser induces significant radiation pressure, but not thin enough to become relativistically transparent to the laser light, the resulting relativistic electron beam is elliptical, with the major axis of the ellipse directed along the laser polarization axis. When the target thickness is decreased such that it becomes relativistically transparent early in the interaction with the laser pulse, diffraction of the transmitted laser light occurs through a so called ‘relativistic plasma aperture’, inducing structure in the spatial-intensity profile of the beam of energetic electrons. It is shown that the electron beam profile can be modified by variation of the target thickness and degree of ellipticity in the laser polarization.
Highlights
The interaction of ultraintense laser pulses (>1018 W cm−2) with thin foil targets results in the generation of high energy ion beams[1, 2], bright x-ray sources[3, 4] or in the production of high harmonics[5]
For targets which expand to densities close to the relativistically corrected critical density (l = 40 nm in the present study), for which radiation pressure is active for the duration of the interaction, the plasma electrons are swept from side to side in the plane of the linear polarization, resulting in an elliptical beam distribution, as first reported in Gray et al.[16]
As in the linear polarization cases, when elliptically polarized light is used the accelerated plasma electrons exhibit a density distribution which is predominantly elliptical, with the major axis parallel to the major axis of the polarization direction. These results indicate a strong interaction between the electron and laser electric field for the laser conditions and this particular target thickness investigated
Summary
The interaction of ultraintense laser pulses (>1018 W cm−2) with thin foil targets (nanometre–micrometre scale thickness) results in the generation of high energy ion beams[1, 2], bright x-ray sources[3, 4] or in the production of high harmonics[5]. For the l = 500 nm and l = 200 nm cases the laser produces radiation pressure induced hole boring[9, 11] into the target This results in the electron density at the front of the laser pulse being compressed. The degree of electron heating is large enough that the condition ne < nc is satisfied, the plasma becomes relativistically underdense, through a phenomenon known as relativistically induced transparency (RIT), enabling the remainder of the laser pulse to propagate through This principle holds for all target thicknesses, more detailed models have been developed to take into account additional phenomena affecting the onset of transparency in targets with thickness below the laser wavelength. Measurements of the spatial-intensity distribution of the beam of relativistic electrons produced with linear, elliptical and circular polarization, and for foil thicknesses on either side of the transparency threshold, are compared. It is shown that laser polarization provides a mechanism by which the collective plasma electron motion can potentially be controlled
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