Abstract
In a conventional linear Paul trap (LPT), four electrode rods are placed symmetrically around the trap axis to generate a radio-frequency (rf) quadrupole field for transverse ion confinement. The periodic nature of the external focusing potential can give rise to serious ion losses under a specific condition. The loss mechanism is essentially the same as the coherent betatron resonance well-known in intense beam dynamics [1, 2]. In fact, the collective motion of an ion plasma in the LPT is shown equivalent to that of a charged-particle beam traveling through an alternating-gradient (AG) focusing lattice. In the present study, we perform the direct measurement of low-order coherent oscillation modes in the LPT by detecting image currents induced on the electrodes' surfaces. The four-rod structure of the LPT allows us to pick up weak signals from the dipole and quadrupole oscillations of a plasma bunch. These signals are Fourier analyzed to evaluate the coherent oscillation tune at different initial ion densities. The measured tune of the quadrupole mode is used to deduce the tune depression as a function of ion number stored in the LPT.
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