XEBIST: simplified EBIS/EBIT without magnetic field

Abstract

EBIS and EBIT usually need a longitudinal magnetic field to focus a high density electron beam along the ionisation region. An alternative to EBIT, which has a short trap length, is a device in which the electron beam is focused by convergent launching from the gun and inertia only. A carefully designed Pierce type electron gun produces a high current density region at the beam crossover, forming the ion trap. As obtained in welding guns, values of more than 10 3 A/cm 2 are possible at electron energies of 150 keV. As in any EBIS, the energy spread and the emittance can be given extremely small values by lengthening the extraction time, which is usually as short as 30 to 100 μs. This source therefore will have a special advantage for experiments with highly charged ions at low velocities. The device can be useful as an ion source (EBIS) as well as an ion trap (EBIT), therefore the new generic name EBIST (Electron Beam Ions Source and Trap) is being proposed. A prototype of an XEBIST (X for crossover) has been designed, constructed and tested. With a 20 mA electron beam at 3.5 keV and confinement times of about 50 ms ions like Ne 6+, Ar 10+, Kr 12+, and Xe 14+ have been obtained already (H. Thomae, dissertation, Fachbereich Physik der Universität Frankfurt am Main (1994) [1]). An upgrade program is underway to improve the residual gas pressure as well as the breakdown limitations in order to produce much higher charge states (at the time of writing 100 mA have been obtained at 10 keV).