Relativistically-Transparent Magnetic Filament Experiments at the BELLA iP2 Petawatt Laser

We present the results of experiments performed using the BELLA iP2 laser system, in which we interact a high-intensity (>10²¹ W/cm²) laser pulse with a relativistically transparent plasma (𝑛_𝑐<𝑛_𝑒<𝑛_𝑐𝑎₀, for critical density 𝑛_𝑐, electron density 𝑛_𝑒, and normalized laser amplitude 𝑎₀). Relativistic transparency allows the laser pulse to enter the plasma, where it drives an axial electron current and generates a magnetic filament with field strength of the order of the laser amplitude (>10⁵ Tesla). When the electron density is greater than 0.01 × 𝑛_𝑐𝑎₀, this magnetic filament traps the electron orbits, enabling direct laser acceleration of the electrons and efficient conversion of laser energy into MeV photons by synchrotron-like radiation. Analytical scaling laws in the ultrafast regime (τ ≪ 1 ps) predict that the re-radiated gamma-ray energy and the efficiency of radiation both scale inversely with density. We present results from irradiating microchannel targets of varying lengths (20—100 μm) filled with a range of foam densities (5—15 mg/cc). We compare the results with predictions from analytical scaling laws and 3-D particle-in-cell simulations to assess the dependence of electron acceleration on plasma density. This material is based upon work supported by the Department of Energy [National Nuclear Security Administration] University of Rochester “National Inertial Confinement Fusion Program” under Award Number(s) DE-NA0004144.