Dependence of static K DPAL performance on addition of methane to He buffer gas: 3D CFD modeling and comparison with experimental results

Abstract

Recently it was observed that adding a small amount of methane to the He buffer gas of a static potassium diode pumped alkali laser (K DPAL) increases considerably the laser power. Further increase in the amount of methane leads to a moderate decrease in power. In the present work the effect of methane addition was investigated theoretically applying a 3D computational fluid dynamics (CFD) and potassium kinetics model, which was supplemented by the analysis of the electron temperature and K ion ambipolar diffusion. It was found that for a pure He buffer the K DPAL power is lower than for ${\rm He}/{{\rm CH}_4}$He/CH4 mixtures due to slow ion-electron recombination and high electron temperature exceeding 3000 K. The high electron temperature in pure He results in fast ambipolar diffusion of K ions to the wall and depletion of the neutral K atoms in the lasing region. These effects are mitigated when methane is added to the buffer gas. The calculated results for the normalized laser power are in satisfactory agreement with the experimental ones.