Produced ions of the ion source Dresden EBIS/T
The periodic table of the elements represents the ions which so far have been produced at the Dresden EBIT, EBIS, EBIS-A and EBIS-SC.
Basic Idea
Ion production in an EBIS is effected by successive electron impact ionization in a high density electron beam, which is compressed by a strong magnetic field produced with a special magnet system (see Functional Principle of EBIS/EBIT).
The produced ions are trapped in radial direction by the space charge potential of the electron beam and in axial direction by a positive electrostatic potential wall. The time control of this potential offset defines the ionization time and thus the charge state distribution within the trapping region.
Ionisation Process
The ionisation process in the electron beam is determined by several elementary processes, which influence the possible ion charge state in the electron beam:
- electron-impact ionisation (main process),
- double electron-impact ionisation,
- radiative recombination,
- charge exchange between ions and neutral atoms
- double charge exchange between ions and neutral atoms,
- electron capture by a higher charged ion from a lower charged one,
- ion heating by the electron beam,
- ion-ion energy exchange,
- ion confinement in the trap and
- ion escape from the trap.
These processes are influenced and partially controlled by different ion source parameters as:
- Electron beam energy
- Electron beam current
- Compressing magnetic field
- Ionisation time (ion trap time)
- Background pressure in the ionisation zone
- Working gas pressure in the ionisation zone
An important limiting factor for the production of highly charged ions is the so called ionisation factor, which is the product of the electron beam current density je = neve (ne - electron density, ve - electron velocity) and the ionisation time τ, respectively.
Ionisation Factor
The ionisation factor determines the possible degree of ionisation in an ion source and can be written as:
with q - ionisation charge state and σK electron impact ionisation cross section. The production of a certain stage of ionisation is only possible, if for the ionisation factor it yields
Number of Produced Ions
The upper limit for the production of a certain number of ions is given by the electric ion trap capacity
more realistic we must calculate
(α - ratio of usable ions in the charge state distribution, f - compensation factor of the electron beam)