NICOP - Predictive CFD modeling and experimental studies of basic processes in static and flowing-gas K DPALs

Diode pumped alkali lasers (DPALs) are currently the most promising and extensively studiedgas lasers due to their great potential as High Energy Lasers (HEL) which are major componentsin Laser Weapons Systems. The leading lasers for these systems have been solid state lasers, fiberlasers and chemical lasers. The strengths of solid state and fiber lasers are electrical pumping,efficient operation, compact size and minimal logistics. The problems with solid state lasers areheat removal and poor beam quality and the problems with fiber lasers are small core, poor beamquality and nonlinear effects. The strengths of chemical lasers are high output powers, good beamquality and scalability. Their problems are hazardous chemicals, complicated logistics, largeweight, cumbersome size and high cost. Despite many achievements, efforts to eliminate theseproblems resulted only in limited success and they are not expected to have potential for scalingup to very high powers. DPALs combine the positive characteristics of gas/chemical lasers andsolid-state/fiber lasers and are scalable to > 1 MW level without substantial deterioration of theoutput beam quality and suffering the negative properties of the other types of lasers which makesthem unique candidates to the role of military lasers for the Navy. Of the three alkali atoms (Cs,Rb and K) used in DPALs, we will concentrate on Cs and K DPALs because they represent quitedifferent characteristics with respect to the energy gap between the 2P3/2 and the 2P1/2 levels (thepumped and the lasing levels, respectively) and therefore of the preferred buffer gas. TheseDPALs are of particular interest due to the noticeable results achieved for their performance: thehighest value of the output laser power demonstrated for Cs and K flowing-gas lasers were 1 kWand ~ 2 kW, respectively, whereas the optical-to-optical efficiency reached 60%. Combining 60%optical-to-optical efficiency with modern laser diodes' efficiency (> 60%), ~ 40% wall-plugefficiency should be achievable. Attractive features of Cs laser are high values of the fractionalpopulation inversion and of the small signal gain, resulting in low threshold pump power which islower than in Rb and K DPALs. K laser has great potential due to its unique kinetic andspectroscopic properties. It has very high quantum efficiency (99.6%) and can operate with lowpressure (~1 atm) He buffer gas. The use of pure He in K DPALs eliminates the problem ofchemical reaction of highly excited alkali atoms with hydrocarbons, limiting the output power ofCs DPALs operating with hydrocarbon buffer. The processes limiting the output power of DPALsare heating of the gas mixture, photoexcitation and ionization of the alkali atoms and chemicalreaction between the highly excited atoms and methane or ethane, usually present in the lasermedium. To avoid the temperature rise and replenish the lost alkali atoms, there is a need to flowthe gas mixture. Demonstration of subsonic flowing-gas DPALs and modeling of both supersonicand subsonic devices, taking into account fluid dynamics and kinetic processes in the lasingmedium, show the positive influence of the gas flow on the laser performance. The objectives ofthe proposed research are to theoretically and experimentally study basic processes and developnew concepts of Cs and K DPALs, in particular of flowing-gas ones. Cs DPAL studied by us inthe past will serve as a reference to determine the advantages of K DPAL. By studying theperformance of Cs and K DPALs (static as well as flowing-gas) under different conditions (lasercell dimensions, gas pressure, composition and velocity, pumping and laser beams cross section,etc.) we will determine under what conditions we can get high power and efficient K DPALs. Thework will be performed in close cooperation with the U.S. Air Force Academy where Cs and KDPALs has been studied experimentally during the last decade. The interaction with this lab willpermit identification of problems with t