Diagnostics of a diluted ultracold plasma using the autoionization effect of Rydberg states of $^{40}$Ca atoms

A sensitive method to study a dilute ultracold plasma based on the autoionization resonances of Rydberg atoms is demonstrated. Ultracold $^{40}$Ca Rydberg atoms are prepared in a continuously operating magneto-optical trap. A method that allows the detection of the plasma with ion and electron densities below $10^9$ m$^{-3}$ is shown. Experimental studies of resonances at the two-photon $4s3d\,{}^1D_2{-}90\,^1D_2$ Rydberg transition have been carried out using $672$ and $798$ nm resonant laser beams. The autoionization resonance is observed during fluorescence of single-charged $^{40}$Ca ions at a wavelength of $397$ nm. The dependence of the amplitude of autoionization resonance on the density of ions in the continuously generated ultracold plasma is obtained.