Electron impact ion source

Details Electron impact ion source Electron impact ion source

2002-06-05

2004-04-06

Lee, John R. (Department: 2881)

Radiant energy

With charged particle beam deflection or focussing

Magnetic lens

C250S3960ML, C250S42300F, C250S464100

Reexamination Certificate

active

06717155

ABSTRACT:

BACKGROUND
The invention relates to an electron impact ion source that enables the generation of highly-charged ions, the extraction of those ions, serves as a source of UV-, VUV-, IR-rays and of characteristic X-radiation of highly-charged ions.
Arrangements are known of the EBIT (Electron Beam Ion Trap) type to M. A. Levine, R. E. Marrs, J. R. Henderson, D. A. Knapp, M. B. Schneider; Physica Scripta, T22 (1988) 157, in which multiply-charged ions are generated in an axial-symmetric high density electron beam that is accelerated by a system of successive drift tubes under ultra-high vacuum conditions and focussed by superconducting Helmholtz coils.
This arrangement comprises an electron gun, several cylindrical drift tubes, an electron collector, an extractor, a focussing magnet system and a system for the generation of ultra-high vacuum conditions in the arrangement.
The electron beam generates an ion trap in the central part of the arrangement, which holds the ions in radial direction by their space-charge forces. In axial direction the ions, which are created in the electron beam by electron impact ionisation are held by positive potentials at the ends of the drift tube structures according to E. D. Donets; USSR Inventors Certificate No. 248860, Mar. 16, 1967, Bull, OIPOTZ No. 23 (1969) 65.
The obtained highly-charged ions can be extracted from the ion trap by reducing the trap potential at the last drift tube. During the ion storage in the trap characteristic X-rays emitted by the stored ions and other long-wavelength electromagnetic rays are radiated in the meridian plane of the magnet system perpendicular to the source axis.
The maximum achievable ion charge is a function of the ionisation factor j&tgr;, i.e. of the product of the electron current density j and the ion residence time &tgr; in the electron beam of the trap. Processes that limit the achievable highest charge states are essentially processes of charge exchange of multiply-charged ions with residual gas atoms. Therefore, devices that create highly-charged ions based on the method described must enable the formation of a highly dense electron beam under ultra-high vacuum conditions.
In order to achieve the aims mentioned, in EBIT arrangements cryogenic methods in combination with superconducting methods are employed. Superconducting Helmholtz coils with inductions of the magnetic field of 3T to 5T are used here to focus the electron beam over the length of the ion trap, whilst this length does not extend the value of 25 mm in known arrangements. The current density of the electron beam is 2,000-5,000 A/cm
2
over the trap length with a total length of the electron-optical system (cathode—electron collector) of more than 30 cm. The cryogenic system, in addition to cryostatting the superconducting Helmholtz coils at a temperature of 4.2 K, serves as an efficient cryo pump in the region of the ion trap to create a vacuum of from ≧10
−11
to 10
−12
Torr.
The extremely demanding technical parameters of those arrangements result in complex, technically difficult and very expensive arrangements. Additional limitations are put by the demand of cryogenic and ultra-high vacuum equipment.
The reduction of the electron current density to 200 to 500 A/cm
2
leads to an increase of the time required to create a specific ion charge state in the trap and, hence, to a decrease of the mean beam intensity of extracted multiply-charged ions, which can be compensated, however, by increasing the total electron current.
For the formation of electron beams having the above-mentioned densities focussing magnet field strengths of from 0.2 T to 0.5 T are required, which can be generated by permanent magnet systems based on modern magnetic materials.
Modern vacuum technology makes it possible to achieve ultra-high vacua in the pressure range of 10
−12
Torr without cryogenic equipment.
This led to the construction of a so-called MICRO-EBIT, as described in H. Khodja, J. P. Briand; Physica Scripta, T71 (1997) 113. The basic idea of this arrangement is that a compact industrial klystron is used for the creation of an ion trap of EBIT type. The focussing magnetic field that limits the radial dimensions of the electron beam in the region of the ion trap is created by two C-shaped permanent magnets, which yield a magnetic induction of 0.25 T. For the generation of the electron beam the original cathode of the klystron with a maximum emissivity of 2.5 A/cm
2
is used. The ultra-high vacuum in the arrangement is achieved after heating at 300° C. using standard technology combining a turbomolecular and an ion getter pump.
In the MICRO-EBIT Ar
16+
ions were detected after an ionisation time of 1.2 s, i.e. an ionisation factor of approx. 1-10
20
cm
−2
was obtained, which corresponds to an electron current density of 14 A/cm
2
.
This arrangement has a low electron current density in the beam (100 times lower than of superconducting EBIT), with which a limitation to comparatively low ion charge states such as Ar
16+
is connected.
The selection of an unsuitable cathode with a comparatively low emissivity and, connected with it, the utilization of an electron gun with a relatively high electrostatic divergence of the electron beam is another decisive disadvantage.
As it is known from S. I. Molokovski, A. D. Suschkov; Intensive Elektronen- und Ionenstrahlen (Intensive electron and ion beams), Vieweg Verlag, Wiesbaden, 1999, the maximum current intensity in an electron beam focussed by an axial magnetic field can be maintained for Brillouin focussing, provided the magnetic field is zero at the cathode place. In such a system the so-called Brillouin density of the electron flow is limited by thermal velocity components of the electrons when they are exiting from the cathode (see also M. Szilagyi; Electron and Ion Optics, Plenum Press, New York and London , 1988) and by aberrations within the anode lens. A minimum value of the aberrations is possible in the case of paraxial and laminar flows, i.e. for an electron gun with minimum divergence (compression) of the electron beam and hence for a maximum efficient cathode, i.e. a cathode with maximum high emission density.
The objective of the invention is the creation of an effective electron impact ion source (WEBIT) without any cryogenic components and without superconducting equipment to obtain highly-charged ions, the X-ray and VUV-spectroscopy with these ions and the extraction of the highly-charged ions from the trap for different scientific, technological and technical applications.
SUMMARY OF THE INVENTION
According to the invention an arrangement for the axial-symmetric focussing of the electron beam comprises at least two rings radially magnetized in opposing directions and each ring encloses the electron beam, each two rings radially magnetized in opposing directions are connected by magnetic conductors to form a unified magnet system, whereby the closing magnetic field passes the ion residence zone in the ion trap, the cathode has a very high emissivity of ≧25 A/cm
2
with a small cathode diameter, and a vacuum of from 10
−7
to 10
−11
Torr in thereon residence zone can be set while operating the source.
Advantageously, magnetized permanent magnet blocks are connected to form rings and are enclosed by magnetic conductors of soft magnetic material so that a radial magnetization results.
Also advantageously, the magnetized permanent magnet blocks are cuboids of hard magnetic materials such as Sm
5
Co or NdFeB, so that the rings can be produced efficiently.
Advantageously, the ion trap, which may be opened and closed, consists of a three-part drift tube mounted on a high-voltage insulator. A controllable acceleration potential is applied to the central part and a settable trap potential to the outer parts.
In order to create a maximum vacuum in the ionization zone, the central part of the drift tube is provided with a number of longitudinal slots or other suitable openings along the axial electron beam, which make it possible to pump efficie

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