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We report the first experimental results and simulations that demonstrate a substantial effect of large-scale front-surface target structures on high-intensity laser-produced positrons.Specifically, as compared to a flat target under nominally the same laser conditions, an optimized Si microwire array target yielded a near 100% increase in the laser-to-positron conversion efficiency and produced a 10 MeV increase in positron energy. Full-scale particle-in-cell simulations that modeled the entire positron production and transport process starting from laser-plasma interactions provided additional insight into the beneficial role of target structuring. The agreement between experimental and simulated spectra suggests future target structure optimization for desired positron sources.Electron-positron pair plasmas are found in various extreme astrophysical objects, such as pulsars, bipolar outflows, active galactic nuclei, and gamma ray bursts 1 . Producing a pair plasma

Focusing effect of laser-driven positron jets by self-generated target sheath fields has been observed for the first time experimentally and the results are supported by the computational studies. In the experiment, OMEGA EP short-pulse (0.7 ps, 500 J) irradiates mm-size gold targets with a concave back surface and reference flat-surface targets. Both targets exhibited positrons with quasi-monoenergetic energy peaks while targets with concave curvature also showed increased number of positrons at the detector. The data is consistent with hybrid-PIC simulations confirming that the time-varying electric fields driven by electrons escaping from the target significantly change the trajectories of positrons. These simulations show a small radius of curvature on the rear side increases the relative focusing effect and the positrons to electrons ratio in the escaping plasma. For the smallest radius of curvature, positron jets that are up to 10 times denser can be achieved.

We present a platform on the OMEGA EP Laser Facility that creates and diagnoses the conditions present during the preheat stage of the MAGnetized Liner Inertial Fusion (MagLIF) concept. Experiments were conducted using 9 kJ of 3ω (355 nm) light to heat an underdense deuterium gas (electron density: 2.5×1020 cm−3=0.025 of critical density) magnetized with a 10 T axial field. Results show that the deuterium plasma reached a peak electron temperature of 670 ± 140 eV, diagnosed using streaked spectroscopy of an argon dopant. The results demonstrate that plasmas relevant to the preheat stage of MagLIF can be produced at multiple laser facilities, thereby enabling more rapid progress in understanding magnetized preheat. Results are compared with magneto-radiation-hydrodynamics simulations, and plans for future experiments are described.