Lasergate: a windowless gas target for enhanced laser preheat in Magnetized Liner Inertial Fusion*

At Sandia's Z Facility, the Magnetized Liner Inertial Fusion (MagLIF) program aims to study inertial confinement fusion in deuterium-filled gas cells by implementing a three-step process on the fuel: premagnetization, laser preheat, and Z-pinch compression. In the laser preheat stage, the Z-Beamlet laser focuses through a polyimide window to enter the gas cell and heat the fusion fuel. However, the presence of the few µm thick window reduces the laser energy coupled to the gas and causes window material to mix into the fuel. The Lasergate concept is designed to avoid these detrimental effects by "cutting" the window and allowing the interior gas pressure to push the window material out of the beam path just before the heating laser arrives. We present proof-of-principle experiments to evaluate a laser-cutting approach to Lasergate and explore the subsequent window and gas dynamics. Further, an experimental comparison of gas preheat with and without Lasergate gives clear indications of an energy deposition advantage using the Lasergate concept, as well as other observed and hypothesized benefits. While Lasergate was conceived with MagLIF in mind, the method is applicable to any laser or diagnostic application requiring direct line of sight to the interior of gas cell targets.