Cold dense ion clouds in a cryogenic neutral plasma

We describe the production of dense clouds of a variety of cold atomic ions, cooled by collisions with cryogenic helium buffer gas. The ion clouds exist within a neutral plasma with a Coulomb coupling parameter $\Gamma_C \approx 0.5$. High optical depths can be achieved for the ion clouds, enabling convenient laser absorption measurements. We also discuss the first precise measurements of isotope shifts of Nd$^+$ ions, and a test of King plot linearity, performed using this system.