METHOD AND DEVICE FOR THE PRODUCTION OF HIGHLY CHARGED IONS

Abstract

The invention relates to a novel ion source, which uses method for the production of highly charged ions in the local ion traps created by an axially symmetric electron beam in the thick magnetic lens. The highly charged ions are produced in the separate local ion traps, which are created as a sequence of the focuses (F1, F2, and F3) of the electron beam (EB) rippled in the magnetic field (B(z)). Since the most acute focus is called the main one, the ion source is classified as main magnetic focus ion source (MaMFIS/T), which can also operate in the trapping regime. The electron current density in the local ion traps can be much greater than that in the case of Brillouin flow. For the ion trap with length of about 1 mm, the average electron current density of up to the order of 100 kA/cm2 can be achieved. Thus it allows one to produce ions in any charge state for all elements of the Periodic Table. In order to extract the ions, geometry of the electron beam is changed to a relatively smooth electron beam by setting the potential of the focusing electrode (W) of the electron gun negative with respect to the potential of the cathode (C).

Description

BACKGROUND

It is known from E. D. Donets, USSR Inventor's Certificate No. 248860, Mar. 16, 1967; Bull. OIPOTZ 23, 65 (1969) that there is the electron-beam method for the production of highly charged ions in the trap formed by the electron beam of a constant radius propagating inside the drift tube, which consists of, at least, three sections with positive potentials applied at the edge sections with respect to the middle one. The control of axial movement of ions (injection, trapping and extraction of ions from the trap) is carried out by variation of the potential distribution on sections of the drift tube.

The axially symmetric magnetic field is used for focusing the electrons emitted from cathode of the electron gun into the electron beam of consistent diameter and of relatively large length. In this case, the current density of the electron beam, which determines the maximum achievable ion charge and the average ion beam current, cannot exceed of the so-called Brillouin electron current density. In real running electron beam ion sources, the current density turns out to be less than Brillouin limit by many times.

It is also known from M. Kleinod, R. Becker, O. Kester, A. Lakatos, H. Thomae, B. Zipfel, and H. Klein, Frankfurt EBIS development: fundamental research and new applications, Rev. Sci. Instrum. 65, 1069 (1994) that there is the method for the production of highly charged ions in crossover of the electron beam, which is focused by the system of electrostatic lenses. The drawbacks of this method are the aberrations of lenses and the need of high voltages, which result in relatively low current density.

It is well known that the electron beam, which is formed in electron gun with cathode placed in the zero magnetic field, is transformed into the sequence of optical focuses in the axially symmetric magnetic field of sufficient length (thick magnetic lens). Theoretically, the first (main) focus is the most acute. In the main magnetic focus, the electron beam reaches the current density, which is considerably greater than that in the case of the Brillouin flow.

In subsequent optical focuses, the current density of electrons drops with length of the beam due to influence of the thermal velocities of electrons and effects of aberrations of the electrostatic (anode of the electron gun) and magnetic (focusing magnetic field) lenses. This is described in K. Amboss, Studies of a Magnetically Compressed Electron Beam, IEEE Trans. Electr. Devices 16, 897 (1969).

BRIEF DESCRIPTION OF THE INVENTION

The electron beam of variable radius, which propagates in the drift tube of constant radius, creates a sequence of local ion traps, the positions of which coincide with the positions of maximum value of the electron current density. The highly charged ions are prepared in local ion traps with extremely high current density. Thus it becomes possible to create the ion trap with the electron current density, which is significantly greater than that of the Brillouin flow. For this purpose, any one of the first three magnetic optical focuses can be used in practice.

The electron beam may be created by the electron gun with a cathode, a focusing electrode, and an anode and the cathode is located in the magnetic field characterized by the strength in the range from 0 to 200 Gauss.

The potential distribution along the axis of the ion source is determined by the geometry of both the electron beam and the drift tube. In order to extract the ions out of the trap, one can change the geometry of the electron beam. Namely, one can transform the electron beam with variable radius into the beam with constant radius. Such transformation of the electron beam becomes possible for special distribution of magnetic field by applying of negative potential on the focusing electrode of the electron gun, relatively to the cathode of the electron gun (see FIGS. 1 and 2). For better extraction and for opportunities to work in the leaky mode, it is possible to design the anode with expansion into the side of ion output. In this case, the potential distribution along the axis of the ion source in the direction of ion extraction has a smooth gradient (see FIGS. 3 and 4). For adjustable control of ion extraction the extractor electrode with a negative voltage can be installed behind the anode (see FIG. 5).

DETAILED DESCRIPTION OF THE DRAWINGS

The method of the invention is explained in more details by examples below. The accompanying pictures are given as follows:

FIG. 1. Basic scheme of the invention with the anode and the drift tube of constant diameter;

FIG. 2. Potential distribution along the z axis for the case depicted in FIG. 1;

FIG. 3. Basic scheme of the invention with the anode of conical form;

FIG. 4. Potential distribution along the z axis for the case depicted in FIG. 3;

FIG. 5. Basic scheme of the invention with the anode of conical form and additional extractor placed directly behind the anode.

FIG. 6. Schematic representation of the basic design of MaMFIS/T;

FIG. 7. Schematic representation of the electron gun unit;

FIG. 8. Schematic representation of the ionization chamber;

FIG. 9. Schematic representation of the permanent magnet focusing system;

FIG. 10. Schematic representation for the assembly of anode, electron collector, and ion extractor;

FIG. 11. Schematic representation of the invention design.

The method of invention is schematically illustrated in FIG. 1. The electron gun G with the cathode C and the focusing (Wehnelt) electrode W, anode A, and drift tube D of constant diameter are installed on the z axis of ion source I. The distribution of the magnetic field B(z) along the z axis is chosen, so that the electron beam EB creates three optical focuses denoted by F1, F2, and F3, respectively. In this case, the potential Uc of cathode C is equal to the potential Uf of the focusing electrode W, (Uc=Uf). The potential distribution along the axis z of the ion source I is shown in FIG. 2. Obviously, the electron beam EB creates three local ion traps, in which the ions are confined. The confined ions are exposed to the additional ionization in the dense electron beam EB during the time, when this potential distribution exists (FIG. 2, broken line). The distribution of the magnetic field B(z) satisfies to the condition, that the electron beam EB becomes relatively smooth, if the potential of the focusing electrode W becomes negative with respect to the potential of the cathode C, (Uc>Uf). In this case, the potential distribution on the z axis does not create the local ion traps (FIG. 2, continuous line). Accordingly, the highly charged ions can leave the trap.

The invention with the conical anode A is shown in FIG. 3. The electron gun G with the cathode C and the focusing (Wehnelt) electrode W and the anode A with conical expansion in the direction of ion output O are installed on the z axis of ion source. The potential distribution for this geometry is shown in FIG. 4. This potential distribution has a general slope in the direction of ion output O. Therefore, the highly charged ions, which are being prepared in the process of ionization at a certain temperature, can always leave the local traps (see FIG. 4, broken and dotted lines). For comparison, the potential distribution corresponding to the cylindrical anode A is also depicted in FIG. 4 (see continuous line). The larger is the angular expansion of conical tube, the steeper is the slope of the corresponding potential distribution. In particular, the angular expansion of the anode A shown as conical anode 2 is larger than that of the anode A shown as conical anode 1 (compare dotted and broken lines in FIG. 4). Thus the leaky mode for the direct ion current can be implemented. In order to improve the quality of ion extraction, the special extractor electrode E can be installed behind the conical anode A (see FIG. 5).

In the following a device according to the invention will be described as shown in FIGS. 6 to 11.

The ion source I (FIG. 6) consists of three parts, namely, the electron gun unit (FIG. 7), the ionization chamber (FIG. 8) with magnetic focusing system (FIG. 9), and the unit of electron collector together with the ion trap and ion optics (FIG. 10). These three parts are combined together via the z-axis linear manipulators between them.

The base of design of the complete installation is the ionization chamber with magnetic focusing system.

All other parts of the ion source I are built up regarding the line of this unit. The first z-linear manipulator M1 changes the position of the cathode C in the magnetic field (see FIG. 6). The second manipulator M2 changes the cathode-anode distance. These routines allow one to manipulate and change the characteristics of the electron beam, such as the intercepted current, the maximum current density, the perveance, and the spatial position of the local ion traps. The ionization chamber (FIG. 8) has a number of ports for installation of the observation windows in the middle plane. One of the ports can be equipped with a high-voltage feedthrough for anode/trap.

The magnetic focusing system can be constructed either as an electromagnet (including superconducting magnet) or on the basis of radial permanent magnets. The focusing system (FIG. 9) is combined from the trapezoidal pieces of permanent magnets with the radial magnetization vector. Each individual trapezoidal part can be made of rectangular magnetic pieces (FIG. 9aa). These constituent elements are collected in two half rings C1 and C2, which are combined together by the half-cylinder made of a soft iron (FIG. 9a). The two half-cylinders C1 and C2 are installed on the body of ionization chamber by pressing on them in the radial direction (FIG. 9b). Finally, the complete magnet provides the axially symmetric focusing magnetic field in the space in between the magnetic coils (FIG. 9c).

The electron gun is installed in the cylindrical vacuum chamber V (see FIG. 7). The power is supplied with the use of the radial high-voltage feedthroughs. Design of this unit allows one to pump out the source along the axis through the electron gun chamber, which is transparent for the vacuum pumping. The cathode and the heather strips are insulated from the Wehnelt electrode W.

The anode A, electron collector, and extractor E are located in cylindrical vacuum chamber V (see FIG. 10). In the case of running mode with one anode, the radial high-voltage feedthroughs (at least one) are used. In the running mode with the conical anode and the extractor electrode behind it, two or more high-voltage feedthroughs are required. The electron collector is surrounded by a magnetically soft iron for cutting the magnetic field on the axis within the range of the electron collector. The iron magnetic shield has the axially symmetric canals for the electrical connectors and vacuum pumping. The whole assembly, which consists of anode A, electron collector with iron shield, and extractor E, can be shifted along the z axis. The simplified invention design is shown in FIG. 11. For given electron energy and current density the position of cathode is fixed in the magnetic field MF. In this case, all parts of the ion source I are also fixed together, without using the z-linear manipulators.

LIST OF REFERENCE SYMBOLS

A anode

B(z) magnetic field distribution

C cathode

D drift tube

E extractor electrode

EB electron beam

F1, F2, F3 optical focus, position of a local ion traps (electron beam crossovers)

G electron gun

I ion source

M1, M2 manipulator

MF magnetic field

O ion output

R1, R2 half ring

Uc potential of cathode

Uf potential of focusing electrode

V vacuum chamber

W focusing (Wehnelt) electrode

z axis along the electron beam

C1, C2 half cylinder

Claims

1. Method for the production of highly charged ions by generating an electron beam propagating along a drift tube and traversing a magnetic field; comprising forming the electron beam with variable radius varying along the drift tube in a trapping mode andchanging the geometry of the electron beam so that the electron beam with variable radius is changed into an electron beam with constant radius along the drift tube in an extraction mode.

2. Method according to claim 1, wherein the trapping mode is executed by creating a sequence of acute optical focuses forming local ion traps along the electron beam focused by the magnetic field.

3. Method according to any of the claim 1 or 2, wherein the electron beam is led to propagate in an axial direction within a drift tube of either cylindrical or conical form.

4. Method according to any of the claims 2 to 3, wherein the distribution of magnetic field for focusing the electron beam is determined, so that the electron beam is focused in a sequence of three sharpest optical focuses in the trapping mode and the distribution of magnetic field is determined, so that the electron beam is transformed into the beam without ripples, if potential of the focusing electrode becomes negative with respect to the potential of the cathode in the extraction mode.

5. Method according to any of the claim 3 or 4, wherein the extracting modes comprises extracting highly charged ions in the axial direction of the cylindrical drift tube, when a negative voltage is applied to the focusing electrode, so that the electron beam is smoothed.

6. Method according to any of the claim 3 or 4, wherein the extracting modes comprises extracting highly charged ions in the axial direction of the conical drift tube, where the angle of the cone expansion defines slope of the extraction potential.

7. Method according to any of the claim 3 or 4, wherein the extracting modes comprises extracting highly charged ions in the axial direction of either the conical or cylindrical drift tube with special extractor electrode, which is installed behind the anode.

8. Device for the production of highly charged ions, consisting of three parts, the electron gun unit, the ionization chamber with focusing magnet system, and an unit of electron collector with anode and ion optics, wherein these three parts are designed and combined to form the electron beam with variable radius varying along the drift tube in a trapping mode and to change the geometry of the electron beam so that the electron beam with variable radius is changed into an electron beam with constant radius along the drift tube in an extraction mode.

9. Device according to claim 8, wherein an ion source consists of the tree parts which are connected via z-axis linear manipulators between them, wherein a first manipulator is installed between the electron gun unit and ionization chamber and designed to change the position of the electron gun in the magnetic field and the cathode-anode distance of the electron gun and wherein a second manipulator is installed between the ionization chamber and an assembly of anode, electron collector, and extractor allows and designed to change the gap between anode and cathode of electron gun in the case of fixed position of the cathode in magnetic field.

10. Device according to claim 8 wherein an ion trap consist of the three parts which are fixed together and designed to define the electron energy, magnetic field distribution, and local ion trap position.

11. Device according to any of the claims 8 to 10 wherein the electron gun unit is designed to pump out the entire source in the axial direction and wherein the cathode of the electron gun is insulated from the focusing Wehnelt electrode in order to control the behavior of the electron beam.

12. Device according to any of the claims 8 to 11, wherein the ionization chamber is provided with a number of observation ports in a middle plane perpendicular to the z-axis and at least some of these ports are occupied by high-voltage feedthroughs with ceramic insulation.

13. Device according to any of the claims 8 to 12, wherein the magnet system is provided either with electromagnets, including superconducting magnets, or with permanent magnets both designed to generate the magnet field with a determined distribution focusing the electron beam.

14. Device according to claim 13, wherein the permanent magnet system consists of two half-cylinders made of iron, bonded together in the radial direction and mounted on the ionization vessel, each half-cylinder contains two half rings constituted from the permanent magnets with radial magnetization and the rings on both ends of the magnetic system have opposite directions of magnetization vector.

15. Device according to any of the claims 8 to 14, wherein the anode, electron collector, and extractor are mounted into the separate unit with at least one high-voltage feedthrough, wherein a cylindrical iron shield with longitudinal channels for high-voltage feedthroughs and additional pumping surrounds the water-cooled electron collector.