Post-recombination effects in confined gases photoionized at megahertz repetition rates.

Recombination-driven acoustic pulses and heating in a photoionized gas transiently alter its refractive index. Slow thermal dissipation can cause substantial heat accumulation and impair the performance and stability of gas-based laser systems operating at strong-field intensities and megahertz repetition rates. Here we study this effect by probing the pulse-by-pulse buildup of refractive index changes in gases spatially confined inside a capillary. A high-power repetition-rate-tunable femtosecond laser photoionizes the gas at its free-space focus, while a transverse-propagating probe laser interferometrically monitors the resulting time-dependent changes in refractive index. The system allows convenient exploration of the nonlinear regimes used to temporally compress pulses with durations in the ∼30 to ∼300 fs range. We observe thermal gas-density depressions, milliseconds in duration, that saturate to a level that depends on the peak intensity and repetition rate of the pulses, in good agreement with numerical modelling. The dynamics are independently confirmed by measuring the mean speed-of-sound across the capillary core, allowing us to infer that the temperature in the gas can exceed 1000 K. Finally, we explore several strategies for mitigating these effects and improving the stability of gas-based high-power laser systems at high repetition rates.