JRM Logo Introduction to CRYEBIS Physics

A cross-sectional view of CRYEBIS
The KSU-CRYEBIS, a CRYogenic Electron Beam Ion Source, produces highly charged ions with an electron beam which strips many or even all electrons from atoms which happen to cross its path. Observing the ion production yields allows us to investigate many interesting features of electron impact ionization processes as well as features of electronic recombination processes.

The electron beam emerges from the hot spherical cathode of the electron gun. The gun focuses the electrons down to a 1 mm spot size at the entrance of the superconducting solenoid. The electrons tend to follow the magnetic field lines and therefore the 3-5 Tesla solenoidal field compresses the electron beam to a diameter of less than 0.1 mm, producing electron beam densities of many hundreds of amps per square centimeter. The electron beam expands as it leaves the solenoid, where it is absorbed by the collector.

Picture The high electron current density inside the solenoid results in an intense electron bombardment of any atom which passes through the beam. Once ionized, the ion's positive charge is trapped radially by the negative space charge of the electron beam and axially by the dam and gate potentials which are more positive than the trap area. A tiny flow of a gas containing the desired element is leaked into the seed trap, where the ions accumulate during the confinement and expulsion time periods. During the injection time the gate is lowered to let the ions which have accumulated in the seed trap drift into the main trap. Raising the gate at the beginning of the confinement time then traps the ions in the main trap where they undergo an intense electron bombardment stripping the ions in rapid succession of their remaining electrons. At the beginning of the expulsion period, the potential of the main trap is raised above that of the dam, allowing the ions to escape the main trap and to pass through the (electron) collector and (electron) repeller. A magnet analyses the escaped ions according to their charge state, energy, and mass, so that only one specific charge state is observed at the 90 degree exit port. There they are measured with a Faraday cup, or a Channeltron multiplier. Scanning the magnetic field allows one to observe the different charge states in succession. The yields of the different charge states vary strongly with the confinement time as well as with the energy of the electrons in the main trap, due to the nature of the electron impact ionization as well as recombination processes.

Picture The electron impact ionization cross section varies over many orders of magnitude. Ionizing neutral Argon, for example, has a cross section of 10-16 sq cm, while the cross section for removing the last electron from Ar17+ is 10-21 sq cm. The smaller cross sections require more close hits by electrons, which are correspondingly less frequent. This means that the ionization rate decreases rapidly with increasing charge state, requiring only 1 ms to produce Ar 8+, but 100 ms to remove eight more electrons for Ar16+, or even 1000 ms to remove the last two electrons for bare Ar18+. As the number of hits encountered by each individual ion is statistically distributed, several different charge states coexist in the trap at any given time. The average charge state, however, increases with increasing confinement. This is roughly proportional to the logarithm of the confinement time as one can see from the required ionization factor, the product of the electron beam current density and the confinement time.

Picture The Charge State Distributions experiment allows you to explore this increase of the average charge state as well as the width of the charge state distribution, which represents the number of simultaneously populated charge states. This width changes with the ratio of sequential ionization cross sections sigma(n,n+1)/sigma(n-1,n). Hence you will find broad distributions when the average charge state represents a roughly half full shell and narrow distributions when the average charge state is close to a full inner shell.

As electrons are removed, the remaining electrons are more strongly bound to the ion and therefore require more energetic electrons, or closer hits, to be removed. The binding, or ionization, energy of the remaining electron is the minimum energy required for its removal. It represents a threshold which becomes visible when, for example, one tries to remove the inner-most electrons from Argon, which have a binding energy of 4.12 keV for Ar16+ and 4.43 keV for Ar17+. Probing Ionization Thresholds allows one to observe the production of Ar17+ once the electron energy is raised above the 4.12 keV threshold, which cannot be observed below the threshold, even with excessively long confinement times. The electron energy in the main trap can be determined by adding the electron gun cathode voltage to the selected main trap voltage.

The most comprehensive picture of the sequential electron impact ionization of ions can be obtained by studying the Evolution of Charge States, where the computer automatically measures a series of charge state distributions for a range of confinement times and plots the populations of the different charge states versus confinement time. This allows one to watch the lower charge states fade away as the ongoing ionization process converts them to higher charge states.

The electron impact causes the ions to lose their electrons, at least in most cases. Less likely, but also possible, is the reverse process where the ions capture an electron from the electron beam. The momentum, or velocity, mismatch makes direct (or radiative) capture very unlikely. More likely is dielectronic capture where an energetic beam electron interacts with an electron of the ion. The electron from the ion is promoted to an excited state, while the beam electron loses enough energy to be captured into a bound level. As all levels have very specific energies, this process is resonant and can only be observed at very specific electron beam energies. The process is less likely than ionization but becomes obvious when observing terminal charge state distributions, which are charge state distributions of ions that have reached their equilibrium after a long bombardment. The Dielectronic Recombination experiment allows one to ionize Ar ions with an electron beam energy between 2 and 4 keV. This is below the ionization threshold of Ar16+ which will become the dominant charge state after a long confinement time. A small fraction of Ar15+ can always be observed due to capture of electrons from the residual gas. However, whenever the electron beam energy matches a dielectronic resonance, the dielectronic capture dramatically boosts the capture rate. This enhances the Ar15+ population while depleting the Ar16+ population. One can watch for the dramatic changes in the Ar15+ and Ar16+ populations while the computer scans the electron beam energy.

Last updated on Monday, 06-Oct-2008.

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