3D CFD modeling of flowing-gas Rb DPALs: effects of buffer gas composition and of ionization of high lying Rb states

Abstract

A comprehensive three-dimensional computational fluid dynamics (3D CFD) modeling of flowing-gas Rb diode pumped alkali laser (DPAL) is carried out. The cases of H e / C H 4 and pure He buffer gases are investigated, and the output power and optical efficiency are calculated for various pump powers, mole fractions of methane, buffer gas pressures, and flow velocities. The model considers the processes of excitation of high levels of Rb, ionization, ion-electron recombination, and heating of electrons, which affect the diffusion coefficient of Rb ions. Two types of Rb DPAL were studied: a low-power laboratory-scale device with pump power of several tens of watts and a high-power multi-kilowatt laser. Efficient operation of the Rb laser using pure He as buffer gas can be achieved only in a large-scale laser with a pump beam cross-sectional area of several c m 2 . The calculated results for such a device were compared with those reported by Gavrielides et al. [J. Opt. Soc. Am. B 35, 2202 (2018)JOBPDE0740-322410.1364/JOSAB.35.002202], where a simplified three-level model based on the one-dimensional gas dynamics approach was applied.

Dissociative Recombination of K2+ and Rb2+ in Diode-Pumped Alkali Lasers: Potential Energy Curves and Atomic Products

“…Therefore, most of the Rb atoms produced by DR are in the 6 2 P excited state, while a small fraction of the atoms are in the 5 2 P state. Just as for K, this conclusion is somewhat contrary to the assumption made in the kinetic scheme of Rb DPAL …”

“…As shown in refs , , the DR pathway is dominant, whereas the rate of another three-body recombination pathway involving X + ions is negligibly small. Therefore, the rate of ion–electron recombination in DPAL is determined by the DR rate, which affects the laser power in different DPALs. − It has a particularly strong influence on the operation of K DPAL …”

“…Ion–electron recombination reduces the depletion of neutral alkali atoms and therefore has a strong effect on the performance of K and Rb DPALs using pure He buffer gas. In Cs DPAL where hydrocarbon–He mixtures are used as buffers, the influence of the recombination is much weaker. , It proceeds via the sequence including fast conversion of the atomic alkali ions X + to the molecular ions X 2 + ( X = Rb or K) ­( M is either X or He buffer gas atom) and dissociative recombination (DR) of X 2 + where X * is an excited state of atom X , and X ** is another excited or ground state of this atom. As shown in refs , , the DR pathway is dominant, whereas the rate of another three-body recombination pathway involving X + ions is negligibly small.…”

“…In Cs DPAL where hydrocarbon–He mixtures are used as buffers, the influence of the recombination is much weaker. , It proceeds via the sequence including fast conversion of the atomic alkali ions X + to the molecular ions X 2 + ( X = Rb or K) ­( M is either X or He buffer gas atom) and dissociative recombination (DR) of X 2 + where X * is an excited state of atom X , and X ** is another excited or ground state of this atom. As shown in refs , , the DR pathway is dominant, whereas the rate of another three-body recombination pathway involving X + ions is negligibly small. Therefore, the rate of ion–electron recombination in DPAL is determined by the DR rate, which affects the laser power in different DPALs. − It has a particularly strong influence on the operation of K DPAL …”

“…Thus, it is of interest to perform quantum mechanical computations of this rate constant and to determine what excited states of the alkali atoms X * are the products of the DR process (2). In refs , , kinetic schemes for K and Rb DPALs were suggested, and it was assumed that K and Rb excited atoms produced in reaction are K (4 2 D 3/2,5/2 , 5 2 P 3/2,1/2 , and 6 2 S 1/2 ) and Rb (5 2 D 3/2,5/2 , 6 2 P 3/2,1/2 , and 7 2 S 1/2 ), respectively. Since there are no experimental data on the recombination pathways in K and Rb in the literature, an additional goal of this study is to examine the validity of these assumptions.…”