Ionization dynamics inside femtosecond enhancement cavities

Intra-cavity high harmonic generation utilizing femtosecond enhancement cavities (fsEC) has been shown as a route to generate frequency combs in the vacuum-ultraviolet. Such VUV frequency combs have the potential to enable precision spectroscopy in this otherwise difficult to access spectral region. Pulse energies exceeding 25 $\mu $J are achievable inside a fsEC with peak intensities at the intracavity focus above 1$\times $10$^{14}$W/cm$^{2}$. At these intensities, we identify fundamental limitations to the intracavity pulse evolution due to ionization induced phase shifts and spectral blue shifting. Numerical simulations and experimental measurements of the intra-cavity ionization dynamics will be presented. We show that the fsEC can itself be used for precise measurements of extreme optical nonlinearities.