Laser entrance window transmission and reflection measurements for preheating in magnetized liner inertial fusion

Laser-driven magnetized liner inertial fusion (MagLIF) is being developed on the OMEGA Laser System to study scaling. MagLIF targets require a preheat laser entrance window that can hold the gas in the target yet allow sufficient laser energy to enter the gas. For OMEGA MagLIF targets, 1.8-μm-thick polyimide foils were found to be sufficient to hold a fuel pressure of up to 14 atm. Transmission and reflection of an OMEGA beam incident on such foils were measured with a calorimeter and time-resolved spectrometers for 2.5-ns square-shaped pulses, with energies from 60 to 200 J, focused to intensities from 0.65 to 2.2 × 10¹⁴ W/cm². The laser energy transmitted in every case exceeded that required to achieve the goal of preheating the gas to 100 eV. The time-resolved measurements showed an initial period with very low, decreasing transmission, the duration of which decreased with increasing intensity, followed by a rapid transition to full transmission, accompanied by brief sidescattering of the transmitted light with a significant red shift. Reflection was always negligible. Two-dimensional radiation-hydrodynamic simulations, using 3-D ray tracing with inverse bremsstrahlung energy deposition, did not capture the rapid transition to full transmission, showing instead a slow increase in transmission, without significant sidescatter or red shift. We propose that full transmission is achieved by self-focusing followed by ponderomotive blowout of the plasma.