A driven, trapped, laser cooled ion cloud: a forced damped oscillator

We have studied the efficiency of laser cooling on clouds of magnesium and beryllium ions held in a Penning trap. We applied a driving voltage to the trap electrodes to drive the ions near one of their motional resonances, and studied the difference between the phase of the applied driving voltage and that of the resultant ion motion. Just as expected for a forced damped oscillator, we find that the phase difference changes by a when the driving frequency is scanned through resonance. From the width of the measured phase change curves we determine the relative strength of the laser cooling on each of the three characteristic ion motions. (C) 1999 Elsevier Science B.V. AU rights reserved.