Laser cooling of an expanding ultracold plasma

We analyze the resonance absorption spectrum of ions in an expanding ultracold spherical plasma. It is shown that the spectrum width is controlled by the highest expansion velocity at the sphere boundary. As the expansion velocity grows, the initial optical thickness of the medium decreases and the medium becomes transparent. We suggest a broadband laser radiation in the red absorption spectrum of ions be used to decelerate ions in an expanding plasma in order to achieve crystallization.