  We have generated Coulomb crystals of ultracold $^4$He$^+$ ions in a linear
radiofrequency trap, by sympathetic cooling via laser--cooled $^9$Be$^+$.
Stable crystals containing up to 150 localized He$^+$ ions at $\sim$20 mK were
obtained. Ensembles or single ultracold He$^+$ ions open up interesting
perspectives for performing precision tests of QED and measurements of nuclear
radii. The present work also indicates the feasibility of cooling and
crystallizing highly charged atomic ions using $^9$Be$^+$ as coolant.

Atomic Physics;Plasma Physics