LSA waves have been generated by a pulsed chemical HF laser beam (2.8 μ, 45 J, 7 μsec) when focused on solid surfaces in air. The emission spectrum is found to contain mainly N+ and O+ lines broadened by the Stark effect and superimposed on a strong continuum. The LSA wavefront travels at an average speed of 0.72 cm/μsec as measured by the time-of-flight method. Target species such as atomic Al emit light for 600 μsec after the laser pulse. In contrast with the results from a CO2 laser, however, no forbidden transition of AlI is observed.
Performance characteristics are reported for a pulsed HF/DF laser initiated by photodissociation of F2. Laser energy of 292 J is obtained from a 12.8-l active volume for 2.5% HF content at 1-atm total pressure, giving 8.1% chemical efficiency at 29% electrical efficiency. Thirty-three percent electrical efficiency is obtained with and 109-J laser energy. DF lasing produces 144 J with 14% electrical efficiency. Far-field measurements show a uniform beam profile with angular half-angle divergence of 5.5 mrad, in good agreement with the value expected for multimode operation of the large-Fresnel-number stable resonator used.
Pulsed DF laser irradiation of 2024-T3 aluminum in normal air produced increases in air-plasma-enhanced energy absorption after repeated exposures of the same target. For a sequence of pulses, the absorbed fluence per pulse increased on each shot until it reached a value more than twice that seen on the first pulse. The effect is believed to be due to surface roughening caused by melting and vaporization.
Energy thresholds for the formation of HF and DF laser-supported absorption waves at target surfaces have been measured for several target materials. Chemical laser pulses with a FWHM of 4 μsec and energies up to 150 J were focused to an effective target spot diameter of 0.94 cm. Thresholds were determined by observing the cutoff of incident laser radiation transmitted through a pinhole in the target. The observed variation of the plasma ignition time with incident pulse energy Ei showed that neither the threshold fluence es nor the threshold irradiance qm is invariant under variation of Ei. The measured dependence of es and qm on Ei is compared to calculations based on two simple threshold models. The minimum values of fluence es (in J/cm2) for which surface plasmas are formed are as follows: in the format ’’material [es (HF), es (DF)]’’: aluminum (65, 71); titanium (51, 43); stainless steel (65, 51); nickel (94, 43); copper (110, 46); Grafoil (94, 43); Lucite (100, 100).
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