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Abstract
The collimation of proton beams accelerated during ultra-intense laser irradiation of thin aluminum foils was measured experimentally whilst varying laser contrast. Increasing the laser contrast using a double plasma mirror system resulted in a marked decrease in proton beam divergence (20° to <10°), and the enhanced collimation persisted over a wide range of target thicknesses (50 nm–6 µm), with an increased flux towards thinner targets. Supported by numerical simulation, the larger beam divergence at low contrast is attributed to the presence of a significant plasma scale length on the target front surface. This alters the fast electron generation and injection into the target, affecting the resultant sheath distribution and dynamics at the rear target surface. This result demonstrates that careful control of the laser contrast will be important for future laser-driven ion applications in which control of beam divergence is crucial.
Highlights
In this paper we experimentally demonstrate an enhancement in proton beam collimation by control of the laser contrast
In conjunction with hydrodynamic and two-dimensional (2D) particle-in-cell (PIC) simulations we demonstrate that for the parameter range investigated, a change in the fast electron generation process due to the presence of the pre-formed plasma on the target front surface is the key factor in altering the proton beam emission profile
With the laser and target parameters being considered in this paper, in particular the large angle of incidence, the relatively thick targets and the laser polarization, it is expected that sheath acceleration of ions from the target rear surface will be the dominant method of ion generation normal to the target
Summary
The most thoroughly explored mechanism for laser-driven ion beams is sheath acceleration [6], in which the laser heated electrons drive plasma expansion from the target foil surface, accelerating surface ions and protons to high energies. One key parameter for laser-driven ion beams when considering their use for applications is the beam divergence from the source Due to their inherent large divergence, despite a high number of particles accelerated per laser shot the achievable flux decreases rapidly away from the source, presenting a severe limitation for many applications. In conjunction with hydrodynamic and two-dimensional (2D) particle-in-cell (PIC) simulations we demonstrate that for the parameter range investigated, a change in the fast electron generation process due to the presence of the pre-formed plasma on the target front surface is the key factor in altering the proton beam emission profile
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