Long-term coherence of the cyclotron mode in a trapped ion cloud

An entire family of mass spectrometers relies on the excitation and subsequent detection of the cyclotron mode in a trapped ion cloud. The observed long-term coherence of this mode is critical to the success of these instruments. This paper analyzes the dynamics of the charge cloud after excitation and finds that they are mathematically equivalent to the dynamics of the cloud before excitation. A vortexlike rotation of the cloud prevents expansion under many conditions depending on the trap geometry and the properties of the cloud. The stability of an excited ion cloud with velocity dispersion and applied shear is analyzed and numerical predictions of relevance for ion cyclotron mass spectroscopy are made.