Radiation pressure from a laser has been used to cool and compress small nonneutral plasmas of ${}_{}{}^9{}_{}{}^{}\mathrm{Be}_{}^{+}{}_{}{}^{}$ ions confined by static electric and magnetic fields. A second laser has been used as a probe to measure ion densities of 2×${10}^7$ ${\mathrm{cm}}^{-3\mathrm{}}$ and ion temperatures below 100 mK. A Coulomb coupling constant, $\Gamma$, as large as 10 has been measured indicating that the plasma is strongly coupled. In the future, values of $\Gamma$ large enough to observe a liquid-solid phase transition should be accessible.

• Received 7 May 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.348