THE TRAPPING CONDITION AND A NEW INSTABILITY OF THE ION MOTION IN THE ION-CYCLOTRON RESONANCE TRAP

In analogy to the critical mass, m(crit), a critical voltage, U(crit), (and a general trapping parameter, pi(trap)) is defined, above which the ion motion in an ion cyclotron resonance (ICR) trap is unstable and the ions are lost from the trap. The theoretical values for the critical voltage are confirmed by experimental results. Singly charged gold cluster ions, Au(n)-, of several sizes, n = 50, 60, 76, 100, 110, and 145 (the latter corresponding to an ion mass of 28 560 u), were injected into an ICR trap, stored, and detected by axial ejection and single ion counting using a microchannel plate detector. During the storage period the trapping voltage, U, was varied for extended durations (from a few to over one hundred milliseconds), to probe its effect on the trapping efficiency. In addition to the expected instability above the critical trapping condition (m/m(crit) = U/U(crit) = pi(trap) = 1), a new instability was observed below the critical value. This is explained in terms of a resonance effect occurring for a trapping parameter pi(trap) = U/U(crit) = m/m(crit) = 8/9, which corresponds to the smallest possible integer ratio of the eigenfrequencies of ion motion, i.e. omega+ = omega2 = 2omega- (where omega+, omega2 and omega- stand for the reduced cyclotron, the trapping and the magnetron frequency, respectively).