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We present an experimental study of ion acceleration using the high repetition rate
petawatt BELLA laser [1] with an increased laser focal spot (compared to similar experiments) from micrometer thick metallic targets. Ion beams of unprecedented charge density with narrow and achromatic divergence were observed. A reduced curvature of the accelerating sheath field was found to account for these effects. The field dynamics inferred from 2D particle-in-cell simulations suggest an adiabatic treatment of the acceleration process. This is achieved by increasing the laser spot size w0 incident on the targets front side to values well above the target thickness ¹l << w0º. As the laser spot size is increased and hence, the virtual source size of the ion beam, the number of accelerated ions is escalated accordingly. By optimizing the target thickness, the contribution to the divergence by scattering of the hot plasma electrons propagating through the solid target could be adapted to yield proton beams with achromatic, narrow divergence. These findings are embedded in the first study of Target Normal Sheath Acceleration (TNSA) with statistical significance at petawatt laser power, consisting of several hundred target shots. We applied a tape drive target system [2, 3] that allows for conducting the experiments with Titanium tape of 5 _m thickness at repetition rates up to 0:5 Hz. Such ion beams are ideally suited for subsequent emittance preserving beam transport, with typically narrow acceptance, such as active plasma lenses [4] and many application in cell biology, high energy density - and material sciences.
This work was supported by the U.S. Department of Energy Office of Science Offices of
High Energy Physics and Fusion Energy Sciences, under Contract No. DE-AC02-05CH11231.
This research used computational resources (Edison, Hopper) of the National Energy Research
Scientific Computing canter (NERSC), which is supported by the Office of Science of the
U.S. Department of Energy under Contract No. DE-AC02-05CH11231. J.H.B. acknowledges
financial support from the Alexander von Humboldt Foundation.
References
[1] K. Nakamura, et al. Diagnostics, control and performance parameters for the bella high
repetition rate petawatt class laser. IEEE Journal of Quantum Electronics, 53(4):1–21,
2017.
[2] B. H. Shaw, et al. Reflectance characterization of tape-based plasma mirrors. Physics of
Plasmas, 23(6):063118, 2016.
[3] S. Steinke, et al. Multistage coupling of independent laser-plasma accelerators. Nature,
530(7589):190–193, 2016.
[4] J. van Tilborg, et al. Active plasma lensing for relativistic laser-plasma-accelerated electron
beams. Physical Review Letters, 115(18):184802, 2015.
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