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 (F.sub.1, F.sub.2, and F.sub.3) 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/cm.sup.2 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).

Claims

1. A method for a production of highly charged ions in local ion traps by generating an electron beam with a variable radius propagating through a drift tube in a longitudinal magnetic field, wherein both the geometry of electron gun and the distribution of magnetic field of a single thick magnetic lens with the field strength of from 0 to 200 Gauss at cathode for focusing the electron beam are determined, so that the electron beam is focused in any one of a first three magnetic optical focuses or 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 in an extraction mode, if potential of a focusing Wehnelt electrode becomes negative with respect to the potential of a cathode.

2. The method according to claim 1, wherein the extraction mode 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.

3. The method according to claim 1, wherein the extraction mode comprises extracting highly charged ions in the axial direction of the conical drift tube, whose angle of the cone expansion defines slope of the extraction potential.

4. A device for a production of highly charged ions, consisting of three parts, an electron gun unit, an ionization chamber with a focusing magnet system, and an unit of electron collector with anode and ion optics, such that the parts are connected therebetween either via two z-axis linear manipulators or are fixed together, without using the z-line manipulators, wherein a first z-axis manipulator is installed between the electron gun unit and the 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 z-axis manipulator is installed between the ionization chamber and an assembly of an anode, an electron collector, and an extractor and designed to change the gap between the anode and the cathode of the electron gun in the case of fixed position of the cathode in magnetic field.

5. The device according to claim 4, wherein the electron gun unit is designed to pump out the entire source in the axial direction and wherein a cathode of the electron gun is insulated from a focusing Wehnelt electrode in order to control the behavior of the electron beam.

6. The device according to claim 5, 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.

7. The device according to claim 5, wherein the anode, the electron collector and the extractor are mounted into a 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 a water-cooled electron collector.

8. The device according to claim 4, 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.

9. The device according to claim 8, wherein the anode, the electron collector and the extractor are mounted into a 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 a water-cooled electron collector.

10. The device according to claim 4, wherein the anode, the electron collector and the extractor are mounted into a 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 a water-cooled electron collector.

Description

DETAILED DESCRIPTION OF THE DRAWINGS

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

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

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

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

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

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

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

(8) FIG. 7. Schematic representation of the electron gun unit;

(9) FIG. 8. Schematic representation of the ionization chamber;

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

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

(12) FIG. 11. Schematic representation of the invention design.

(13) 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.

(14) 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).

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

(16) 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.

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

(18) 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.

(19) 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 R1 and R2, 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).

(20) 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.

(21) 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

(22) A anode B(z) magnetic field distribution of thick magnetic lens C cathode D drift tube E extractor electrode EB electron beam F1, F2, F3 optical focus, position of a local ion trap (electron beam crossover) 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 L thick magnetic lens