During the annual NIST calibration testing done at the LHMEL facility in FY06 on its high energy Carbon-Dioxide lasers, the LHMEL II device suffered severe damage to the internal surface of its ZnSe output
coupler optics. The damage occurred during a high power, short duration run and it was believed to have
been the result of a significant amount of surface contaminants interacting with the LHMEL cavity beam.
Initial theories as to the source of the contamination led to the inspection of the vacuum grease that seals
the piping that supplies the source gases to the laser cavity. Other contamination sources were considered,
and analysis was conducted in an effort to identify the material found at the damage sites on the optic, but
the tests were mainly inconclusive. Some procedure changes were initiated to identify possible
contamination before high energy laser operation in an attempt to mitigate and possibly prevent the
continued occurrence of damage to the output coupler window. This paper is to illustrate the type and
extent of the damage encountered, highlight some of the theories as to the contamination source, and serve
as a notice as to the severity and consequences of damage that is possible even due to small amounts of
foreign material in a high energy laser environment.
Laser-boosted lightsail experiments were carried out on 4-8 December 2000 with the 150 kW LHMEL II carbon dioxide CW laser at Wright Patterson Air Force Base - in their 2.74 m long, 2.13-, diameter vacuum chamber. All 5-cm diameter sail specimens were fabricated by ESLI. The prior Dec. '99 pendulum tests used ultralight carbon microtruss discs, sputter-coated with molybdenum on the front face to improve reflectivity at 10.6 um - and are the first known measurements of high power laser photonic thrust with real candidate lightsail materials. The Dec. '00 vertical wire- guided tests employed an improved moly-coated carbon-foil material with the same basic microtruss substructure. The performance of this new carbon foil sail was superior to the earlier specimens.
Laser-boosted light sail experiments were carried out on 13 - 20 Dec. 1999 with the 150 kW LHMEL II carbon dioxide CW laser at Wright Patterson Air Force Base, using a 2.74-m long, 2.13-m diameter vacuum chamber evacuated to 36 - 44 microTorr. The 5-cm diameter laser sail discs (i.e., the test articles) were fabricated from an ultralight carbon microtruss fabric that was sputter-coated with molybdenum on one side only, to improve its reflectivity to 10.6 micrometer laser radiation. Four laser sails discs with three different areal densities (one at 6.6 g/sq.m., two at 27 g/sq.m., and one at 28 g/sq.m.) were tested as magnetically-supported pendulums with an overall length of 18 cm. Pendulum deflections for the three heavier sails, ranged from 2.4 to 11.4 degrees, measured as a function of incident laser powers from 7.9 to 13.9-kW. These pendulum sails had masses of 83.7, 87.3, and 88 milligrams each; their center-of-mass was located at 11.5, 11.7, and 11.9 cm (respectively) below the magnetic bearing. Laser photon thrust ranged from 3.0 to 13.8 dynes, as calculated from pendulum deflections. Seven of the 10 data points fell in the feasible range of 3.3 to 6.67 N/GW for photon propulsion physics; the other three (higher laser power) data points exceeded the 6.67 N/GW limit by as much as 50%. From this data set, the onset for significant ablation was clearly identified to be 12.9-kW. Laser sail temperature was monitored with an optical pyrometer, and fell in the range of 2270- K to above 2823-K for laser powers from 8-kW to 20.8-kW, respectively. The experiments are the first known measurements of laser photonic thrust performance with real candidate light sail materials.
When developing a high-heat-flux system, it is important to be able to test the system under relevant thermal conditions and environmental surroundings. Thermal characterization testing is best performed in parallel with analysis and design. This permits test results to impact materials selection and systems design decisions. This paper describes the thermal testing and characterization capabilities of the Laser Hardened Materials Evaluation Laboratory located at Wright-Patterson Air Force Base, Ohio. The facility features high-power carbon dioxide (CO<SUB>2$</SUB> and neodymium:glass laser systems that can be teamed with vacuum chambers, wind tunnels, mechanical loading machines and/or ambient test sites to create application-specific thermal and environmental conditions local to the material sample or system. Representative results from recently conducted test series are summarized. The test series described demonstrate the successful use of a high power CO<SUB>2</SUB> laser paired with environment simulation capability to : 1) simulate the expected in-service heat load on a newly developed heat transfer device to ensure its efficient operation prior to design completion, 2) simulate the heat load expected for a laser diode array cooler, 3) produce thermal conditions needed to test a radiator concept designed for space-based operation, and 4) produce thermal conditions experienced by materials use din solid rocket motor nozzles. Test diagnostics systems used to collect thermal and mechanical response data from the test samples are also described.