Ion energy distribution in an electron beam ion trap inferred from simulations of the trapped ion cloud

We have inferred the energy distribution of trapped ions in an electron beam ion trap (EBIT) from simulations of the spatial distribution of Fe13+ ions and a comparison with measured visible light images of the ion cloud. We simulated the cloud of Fe13+ ions by computing ion trajectories in the EBIT for different ion energy distributions used to initialize the trajectories. We then performed a least-squares fit to infer the ion energy distribution that best reproduced the measured ion cloud. These best-fit distributions were typically non-Maxwellian. For electron beam energies of 395-475 eV and electron beam currents of 1-9 mA, we find that the average ion energy is in the range of 10-300 eV. We also find that the average ion energy increases with increasing beam current approximately as < E > approximate to 25I(e) eV, where I-e is the electron beam current in mA. We have also compared our results to Maxwell-Boltzmann-distribution ion clouds. We find that our best-fit non-thermal distributions have an < E > that is less than half that of the T from the best-fit Maxwell-Boltzmann distributions (< E >/q)/T = 0.41 +/- 0.05.