Spatial Compression of Bunched Crystalline Ion Beams

Recently, bunched crystalline ion beams have been realized in the table-top rf quadrupole storage ring PALLAS by means of laser cooling. Here, a novel method for the measurement of the spatial distribution of the ion bunch is presented which is based on the time-resolved analysis of the fluorescence emitted by the ions during cooling. Thus, the phase transition from a gaseous to a crystalline beam can be followed monitoring the whole phase space, and structural transitions can be studied as a function of the variable longitudinal confinement. Surprisingly, the length of crystalline bunches was found to be considerably shortened by a factor of three with respect to the length expected for space charge dominated bunches.