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We have proposed a compact scheme for generating high-density and high-energy positrons by irradiating different shaped targets with an ultra-intense laser pulse, using 2D particle-in-cell simulations and numerical analysis. Our simulations show that the Breit–Wheeler process dominates positron production during laser-target interaction when a laser with an intensity of 4 × 10 23 W cm − 2 is used. We obtain a positron beam with an energy of 2.5 GeV and a total number exceeding 109, which is an improvement by two orders of magnitude compared to previous work with the same laser intensity. By using different shaped targets, we are able to generate a high-energy positron beam with low divergence. The yield of positron pairs depends on both the target and the laser parameters, and we have investigated how the shape of the target can enhance pair production and reduce divergence. Both of our analytical and simulation results demonstrate that this high-flux and low-divergence positron beam has direct applications in the field of medical and astro-physics that can be explored in the upcoming high intensity laser facilities. [ABSTRACT FROM AUTHOR]

We report measurements of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma‐ray Burst Monitor that was produced during a negative cloud‐to‐ground (CG) lightning leader. This is the first report of a downward directed TGF occurring during a CG flash but detected by a space‐based instrument. The gamma ray photons are produced 3 ms preceding a return stroke (−146 kA) and are essentially simultaneous with an isolated low frequency radio pulse. Based on timing, the pulse is estimated to initiate at approximately 6 km altitude, and its polarity indicates downward moving negative charge, the opposite of regular satellite‐detected upward TGFs. A likely scenario is that the runaway electrons accelerate into the upper, positively charged end of the leader in a high field region, with the reverse positron beam generating upward gamma rays detectable from space. A search for similar waveform features indicates that this type of downward CG‐TGF may occur prior to 1% of high peak current CG strokes. Extrapolating gives a global rate of 5–10% of previously known TGFs and potentially a significant fraction of global TGFs. Key Points: A TGF detected by Fermi GBM occurred 3 ms before a −CG stroke during the descending negative leader, coincident with a 120 μs radio pulseThe radio pulse polarity indicates that it is likely a downward TGF produced at ~6 km altitude with a reverse positron beam detected by Fermi GBMThis type of downward CG‐TGF may occur prior to 1% of high‐intensity CG strokes, constituting 5–10% of the previously known TGF population [ABSTRACT FROM AUTHOR]