Laser impulse coupling measurements at 400 fs and 80 ps using the LULI facility at 1057 nm wavelength

At the École Polytechnique « LULI » facility, we have measured the impulse coupling coefficient C_m (target momentum per joule of incident laser light) with several target materials in vacuum, at 1057 nm and 400 fs and 80 ps pulse duration. A total of 64 laser shots were completed in a two-week experimental campaign, divided between the two pulse durations and among the materials. Our main purpose was to resolve wide discrepancies among reported values for C_m in the 100 ps region, where many applications exist. A secondary purpose was to compare C_m at 400 fs and 80 ps pulse duration. The 80 ps pulse was obtained by partial compression. Materials were Al, Ta, W, Au, and POM (polyoxymethylene, trade name Delrin). One application of these results is to pulsed laser ablation propulsion in space, including space debris re-entry, where narrow ranges in C_m and specific impulse I_sp spell the difference between dramatic and uneconomical performance. We had difficulty measuring mass loss from single shots. Imparted momentum in single laser shots was determined using pendulum deflection and photonic Doppler velocimetry. C_m was smaller at the 400 fs pulse duration than at 80 ps. To our surprise, C_m for Al at 80 ps was at most 30 N/MW with 30 kJ/m² incident fluence. On the other extreme, polyoxymethylene (POM, trade name Delrin) demonstrated 770 N/MW under these conditions. Together, these results offer the possibility of designing a C_m value suited to an application, by mixing the materials appropriately.