Optical Pumping of Stripped Relativistic Ions

Selected states of ions in the sodium-like isoelectronic sequence (P through Ca) in a beam accelerated to relativistic energies can be excited by a beam of optical laser radiation that intersects their trajectory at an appropriate angle. Pumping rates can be estimated, assuming the line broadening mechanism to be Doppler and available laser intensities. We find that it is possible to generate population inversions between the 32D3/2 states and the 32p3/2 states of those ions by selectively exciting the 42P1/2 state. To a fixed observer, the radiation from transitions between those states will be Doppler-upshifted into the X-ray region and will be tunable. Moreover, given sufficiently high ion density and low beam emittance, and assuming that backward emission can be suppressed by means of a traveling-wave pumping system, moderate gain by stimulated emission along the ion column may be achieved.