The dynamics of bunched laser-cooled ion beams at relativistic energies

We discuss the axial dynamics of laser-cooled relativistic C³⁺ ion beams at moderate bunching voltages. Schottky noise spectra measured at a beam energy of 122 MeV/u are compared to simulations of the axial beam dynamics. Ions confined in the bucket are addressed by the narrow-band force of a laser beam counter-propagating to the ion beam, while the laser frequency is detuned relatively to the cooling transition frequency in the rest frame of the bucket.

At large detuning comparable to the momentum acceptance of the bucket, the axial dynamics can be well explained by the secular motion of individual non-interacting ions. At small detuning, corresponding to a small axial momentum spread Δp_axial/p_axial < 10⁻⁶ of the ions, the measured Schottky noise spectra can no longer be explained using an approach which neglects the ion-ion interaction. Instead, the model fails when the ion bunch enters the space-charge dominatedregime, at which the mutual Coulomb-energy of the ions becomes comparable to the kinetic energy of the ions.