Abstract
The transverse expansion of the electrostatic sheath during target normal sheath acceleration of protons is investigated experimentally using a setup with two synchronized laser pulses. With the pulses spatially separated by less than three laser spot diameters, the resulting proton beam profiles become elliptical. By introducing a small intensity difference between the two pulses, the ellipses are rotated by a certain angle, except if the spatial separation of the two laser pulses is in the plane of incidence. The rotation angle is shown to depend on the relative intensity of the two pulses. The observed effects are found to require high temporal contrasts of the laser pulses. A simple model describing how the transverse shape of the electron sheath on the rear of the target depends on the relative intensity between the foci is presented. The model assumptions are verified, and the unknown dependence of the transverse extents of the sheaths are estimated self-consistently through a series of high resolution, two-dimensional particle-in-cell simulations. The results predicted by the model are also shown to be consistent with those obtained from the experiment.
Highlights
In a recent study,[12] we showed that, by varying the laser intensity distribution on the front of the target, the divergence of the resulting proton beam can be controlled
The transverse expansion of the electrostatic sheath during target normal sheath acceleration of protons is investigated experimentally using a setup with two synchronized laser pulses
the ellipses are rotated by a certain angle
Summary
In a recent study,[12] we showed that, by varying the laser intensity distribution on the front of the target, the divergence of the resulting proton beam can be controlled. Irradiating the target simultaneously at an oblique incidence with two identical, focused laser pulses, spatially separated by less than three spot diameters, resulted in an accelerated proton beam with an elliptical transverse profile, with its major axis perpendicular to the foci separation axis. Separating the foci by more than three spot diameters resulted in two independent proton sources on the rear of the target, and the proton beam profiles observed some centimeters away from the target were circular, just as if only one focus was used. We extend that study and present new experimental results, partly expounded in the theses by Svensson[13] and Senje,[14] obtained by altering the intensity ratio between the two separated laser foci.
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