Spectra of hollow ions in an ultradense laser plasma

Experiments with an ultrahigh-contrast femtosecond laser have yielded a new type of X-ray emission spectra with a complex structure consisting of resonance lines against the quasicontinuous background. Some recent work has shown that such spectra can only be accounted for by considering the radiation emitted by multiply charged hollow (empty K shell) ions in an ultradense plasma. This review discusses the observation of the spectra of hollow ions in an ultradense laser plasma and considers the types of such ions and how they are excited. Considerable opportunities of hollow ion spectra-based diagnostic techniques are emphasized.