Radiant energy – Electrically neutral molecular or atomic beam devices and...
Patent
1990-04-11
1992-05-05
Anderson, Bruce C.
Radiant energy
Electrically neutral molecular or atomic beam devices and...
H05H 300
Patent
active
051110423
DESCRIPTION:
BRIEF SUMMARY
The invention relates to apparatus for generating atomic beams. With increasing demand for fast atom applications for surface analysis and other studies, a pulsed fast atom source is urgently needed. For example, in instruments employing time-of-flight techniques and using fast atoms as their incident projectile a pulsed fast atom source is essential. According to the present invention there is provided a source of atomic or molecular particles comprising a source of ionized particles, means to remove a beam of said particles from said source, focusing means to focus said beam of particles and filter means to select particles in said beam having a predetermined velocity.
An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:
FIG. 1 is a schematic section of a pulsed atom source
FIG. 2 is a block circuit diagram illustrating the method of pulsing the atom source of FIG. 1
FIG. 3 is a schematic diagram of an experimental arrangement used for the measurement of the current characteristics of the atom source of FIG. 1
FIG. 4 is a graphical representation of the proportion of neutrals in an atom beam at different line pressures
FIG. 5 is a plot showing how the secondary electron coefficient varies with beam energy
FIG. 6 shows the variation of neutral current with differential pumping line pressure
FIG. 7 is a schematic diagram showing the experimental arrangement for divergence measurement of the atom beam
FIG. 8 is a current amplifier used in the measurement of atom beam divergence
FIGS. 9 to 11 are oscilloscope traces
FIG. 12A is a schematic diagram showing the parameters used in the calculation of current density and FIG. 12B shows parameters used in current distribution
FIG. 13 s the result of a typical computation
FIG. 14 is a schematic diagram showing the geometrical relationship used in the calculation of beam divergence
FIG. 15 is a schematic diagram of the vacuum system of the time-of-flight facility
FIG. 16 is a schematic diagram of the electronic system of the facility
FIG. 17 is a modified control unit
FIG. 18 is a typical example of the time-of-flight spectrum of a total beam
FIG. 19 is a typical example of the time-of-flight spectrum of a neutral beam, and
FIG. 20 is a fast atom scattering spectrum for argon atoms incident on a gold surface.
Referring now to the drawings, the basic idea of pulsing is to generate ions only when a voltage pulse is applied. As shown in FIG. 1, ions are created by electron impact in an ionization cell 1. They are then extracted from the ionization cell by means of an extraction electrode 2 and focused immediately by an einzel lens 3. A Wien filter 4 then allows only one value of ion velocity to pass. Those ions emerging from the filter are subsequently deflected at an angle of about 5.degree. from the previous axis by deflecting electrodes 5. This is necessary because neutrals created in that section of the gun may have a wide energy spread. This feature thus serves as a neutral dump. A Bruch telefocus lens 6 is then employed to focus the ions through a charge exchange cell 7. Such a lens allows one to include a long length charge exchange cell between the lens and a target without losing the focused beam. The region occupied by the lens is kept under good vacuum conditions, so that probability of charge exchange is minimized at this stage. The charge exchange cell is so designed that either a resonance or an electron capture charge exchange process can take place inside: this corresponds to a high or low neutral current mode. The exit aperture of the cell incorporates a set of deflection plates 8 which remove residual ions from the neutral beam and also may be used to scan the ion beam when the source operates in an ion mode.
The ion source includes a heated filament 9 and a grid 10. Gas is ionized by electron impact. This configuration is particularly suitable for the pulsing method, simple, and easy to be operated.
Optionally, the atom source may include a stigmator S to correct for astigmatism
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Sullivan John L.
Xu Ning-Sheng
Anderson Bruce C.
National Research Development Corp.
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