a, Simplified illustration of the electrostatic potential used for the 40Ar13+ transfer from the EBIT to the Paul trap. The entire ion inventory stored in the EBIT, with its charge-state distribution displayed as grey-shaded, is ejected by switching the axial trap to a repulsive potential. The charge states separate owing to their distinct initial kinetic energies. 40Ar13+ ions (red) are selected by an electrode used as a gate (not shown). The fast 40Ar13+ bunch is then slowed down upon entering the pulsed drift tubes. Having arrived there at the centre of a linear potential gradient, the electrode potentials are rapidly switched to ground, and a slower 40Ar13+ bunch leaves the pulsed drift tubes. At the Paul trap, the ions are further decelerated by an electrostatic potential and enter the trapping region with a reduced residual kinetic energy of 5q V to 10q V. They then pass a Coulomb crystal of 9Be+ ions and are reflected by an electrostatic endcap electrode biased to a potential of about 12 V above the biased common ground. Meanwhile, an electrostatic mirror tube in front of the Paul trap has been switched up to a confining potential at which 40Ar13+ is unable to escape the Paul trap. This causes an oscillatory motion along the trap axis. Through repeated interactions with the laser-cooled 9Be+ ions, 40Ar13+ dissipates its residual kinetic energy and joins the Coulomb crystal. b, Normalized ion yield as a function of the time of flight after ion ejection from the EBIT, measured by the first MCP detector in front of the Paul trap. The black curve shows the entire charge-state distribution, with Ar charge states from +7 through +15. Using the gate electrode, 40Ar13+ is chosen for passage, as shown by the red curve. a.u., arbitrary units. c, d, Normalized 40Ar13+ bunches as a function of time and position along the beamline axis (averaged over 16 shots). The FWHM of the fast bunch is about 250 ns (c) and that of the slow bunch is about 185 ns (d). e, f, Normalized kinetic-energy distributions of the 40Ar13+ bunches along the beamline axis: fast bunch (e) and slow bunch after deceleration and phase-space cooling using the pulsed drift tubes (f). The red circles show the integrated ion yield of an averaged 40Ar13+ bunch (16 shots) for a given retardation potential, measured by the retarding-field analyser. A Gaussian error function (red line) was fitted to the data and differentiated to obtain the Gaussian energy distribution (blue line) to show the mean kinetic energy and longitudinal energy spread.