High Throughput and Contrast Enhancement from Ultrathin Liquid Crystal Films in a Double Plasma Mirror Configuration.*

High temporal pulse contrast is critical for experiments using ultraintense laser pulses. Prepulses or a slowly rising pulse pedestal can impair experiments using solid density targets. One method to improve the contrast is the implementation of a self-triggering plasma mirror (PM). A PM typically consists of a low reflectance substrate that is precisely placed such that only the main peak of the pulse will ionize it and is reflected, resulting in a higher contrast pulse. Here we report the use of ultrathin, free standing liquid crystal (LC) films formed \textit{in-situ} to implement a double plasma mirror (DPM) configuration at the BELLA PW facility. We characterize the DPM system including nanosecond and picosecond contrast enhancement, total reflection, far-field wavefront quality, focal spot mode, and beam pointing using 7 J p-polarized pulses. Notably, we observed \textasciitilde 80{\%} reflection and negligible deterioration of the focus. We also describe a new analytical model that predicts the total reflection as a function of intensity incident on each PM. This model was also validated against a previous PM experiment using LC.