Robert T. Brown

Aerosol−induced air breakdown with CO2 laser radiation

The breakdown threshold with 10.6−μ wavelength CO2 laser radiation induced by particles is reported. Single solid particles of different materials and sizes were suspended electrodynamically at the focus of a pulsed TEA laser and the thresholds for vaporization and also for particle−induced breakdown were measured. Th threshold intensity for breakdown is of the order 108 W/cm2, independent of particle size and material and represents the limiting intensity that can be propagated through an aerosol−laden atmosphere.

Laser‐induced gas breakdown in the presence of preionization

Experiments have been carried out to study the ionization buildup and gas breakdown processes in the focal volume of an intense 10. 6‐μ laser pulse in atmospheric‐pressure helium preionized to various levels of electron density. With no preionization the laser flux required for breakdown was in agreement with results obtained by previous authors, and showed the same dependence on the focal volume size. However, with a high initial electron density, the threshold was much lower and showed no volume dependence. The results indicate that with preionization the breakdown threshold is controlled by cascade ionization processes, while with no preionization it is dominated by other effects such as small impurity particles in the gas, and that these other effects may be responsible for the diameter dependence of the breakdown threshold.

Efficient XeCl(B) formation in an electron‐beam assisted Xe/HCl laser discharge

XeCl(B) formation processes are examined for conditions typical of a discharge‐excited laser using HCl as the chlorine donor. It is shown that vibrational excitation of HCl followed by dissociative attachment is a primary step in the reaction sequence resulting in Cl− . XeCl(B) formation is the result of a three‐body Xe+ ‐CL− recombination reaction. Experimental results are presented which demonstrate efficient (∼2%) XeCl laser operation in an e‐beam assisted discharge in which over 75% of the energy was deposited by the discharge.

Kinetic processes in the HgBr(B→X)/HgBr2 dissociation laser

Results are reported of an investigation of fundamental kinetic processes influencing discharge and laser properties typical of the 502‐nm HgBr(B→X)/HgBr2 dissociation laser. Specific attention is focused on conditions representative of electron‐beam‐controlled discharges. Experimental results and corresponding analysis and interpretation are presented for several laser mixtures, focusing particularly on the factors affecting discharge characteristics and HgBr(B) formation. A set of phenomenological electron‐HgBr2 cross sections inferred on the basis of analysis of experimental observations is presented, along with a discussion of the effect of electron‐electron collisions on medium properties at the level of fractional ionization typical of the HgBr(B)/HgBr2 laser.

Efficient HgBr (B→X) laser oscillation in electron‐beam‐controlled‐discharge‐excited Xe/HgBr2 mixtures

This letter reports the results of an investigation of HgBr(B2J+→X2J+) laser oscillation at 502 nm in Xe‐HgBr2 mixtures excited using an electron‐beam‐controlled discharge. Measured values of instantaneous electrical‐optical energy conversion efficiency were 2%, a level substantially higher than that typical of N2‐HgBr2 mixtures. Calculations show that efficiencies of 5–10% may be possible under optimized conditions.

Instability onset in electron‐beam‐sustained KrF* laser discharges

Measurements of instability onset in a spatially uniform electron‐beam‐sustained KrF* laser discharge have shown that the time at which instability occurs decreases from about 1 to 0.1 μsec as E/n is increased in the range required for efficient laser operation. This finding is in good agreement with computed ionization instability onset times determined on the basis of a comprehensive model of the discharge.

Stability enhancement in electron-beam-sustained excimer laser discharges

Techniques are described for prolonging the duration of stable e‐beam‐controlled excimer laser discharges, including: temporal tailoring of either the discharge voltage or the ionization source and kinetics modification by way of additives. Theoretical and experimental results are presented for KrF* laser discharges.

Aerosol−induced thermal blooming

Experiments have been carried out to study the nonlinear distortion of a laser beam due to absorption by aerosol particles in the air. A 15−W 10.6−μ beam was passed through a 1.5−m cell containing a carbon aerosol in air at STP, and various measurements of the distorted beam profile were made. The data show that aerosol absorption can lead to severe nonlinear distortion and that, for the regime in which the beam transit time is long compared to the interparticle thermal diffusion time, this distortion is qualitatively similar to that produced by molecular absorption, but has a magnitude somewhat below that predicted by conventional thermal blooming theory.

Laser propagation through an absorbing transonic flow

Experiments were carried out to investigate the propagation of a cw CO2 laser beam through an absorbing slightly supersonic flow. A 500‐W CO2 beam was propagated through a small blow‐down wind tunnel operating with an air/SF6 mixture in order to simulate a high‐power beam propagating through the atmosphere. A collinear schlieren system was used to observe the density gradients caused by the absorbed power. Large density gradients were observed at a Mach number of 1.17; however, they were not shock initiated and their magnitude was not inconsistent with a heat‐balance analysis. Under the conditions of the experiment no shock waves were observed, even though their occurrence was predicted by a simple one‐dimensional analysis. From this study it is concluded that shock waves will not occur under most conditions encountered by high‐power cw CO2 laser radiation in the atmosphere. The presence of subshock density gradients and their effect on the propagating beam require further study.

Optically pumped electric‐discharge uv laser

A new technique for pumping uv laser transitions is described. The technique consists of using an intense 10.6‐μ laser pulse to drive a diffuse discharge in a preionized uv laser medium. Because of its fast rise time, high electron density, and ability to operate at high pressure, this discharge is well suited to pumping uv laser transitions. Experimental results are shown for the application of this technique to the 3371‐A transition in nitrogen. A diffuse discharge at atmospheric pressure was obtained, and an intense pulse of 3371‐A radiation was observed.