AXIAL AND RADIAL ION CLOUD COMPRESSION - COUPLING OF MAGNETRON AND CYCLOTRON MOTION TO AXIAL MOTION IN A SEGMENTED CUBIC FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE ION TRAP

A segmented cubic ion cyclotron resonance (ICR) ion trap configuration that generates an r.f. electric excitation potential of approximately xz (two-dimensional quadrupolar) symmetry near the center of the trap is proposed. A new ion excitation/detection scheme is based on the analysis of image current induced by the combination of axial and azimuthal (cyclotron and magnetron) ion motions on the same electrode configuration. Ions may thereby be resonantly excited or detected at each of the combination frequencies omega(z) + omega-, omega(z) omega-, omega+ omega(z), and omega+ - omega(z), in which omega(z) is the axial (i.e. z-motion) frequency, omega- is the magnetron frequency, and omega+ is the reduced cyclotron frequency. The trajectory of an ion subjected to resonant xz excitation is derived analytically with provision for collisional damping. For example, when ions are excited at the frequency omega(z) + omega-, magnetron motion is periodically interconverted into axial motion. The coupling of magnetron and axial motions in presence of ion/neutral collisions (treated analytically as a frictional damping force) provides a method for axialization and cooling of heavy ions. Conditions for such coupling and cooling are analyzed in detail: under representative experimental conditions, magnetron-to-axial conversion may be achieved by xz quadrupolar excitation amplitude of just a few millivolts. Combination of the present method and previously demonstrated azimuthal (xy) quadrupolar excitation should result in both axial and radial compression of an ion cloud for improved Fourier transform ICR sensitivity and mass resolution.