Apparatus For Producing An Ion Beam By Removing Electrons From A Plasma

Abstract

The present invention relates to a method of production of ions wherein a plasma is generated from a target and then subjected to a number of successive electric fields to sort the particles which form the plasma by providing the successive electric fields with increasing amplitudes to slow down the electrons, collecting said electrons on electrodes and then causing the ion-enriched beam to converge towards the exit aperture by means of a final electric field.

Description

The invention relates also an ion source for carrying out this method. This source comprises an assembly constituted by a casing, a metallic cylinder and a metallic target placed at the bottom of said cylinder and in coaxial relation with said assembly a perforated extraction plate having preferably a circular opening fitted with a grid, a preacceleration plate having an opening fitted with a grid, a concentration electrode formed of an annular sleeve and a definition plate provided with a slit, means being provided for exciting the target in order to produce a plasma.

This invention relates to a novel method of production of ions, an ion source for carrying out said method as well as a mass spectrograph for the utilization of said source, the ions of which are produced by the impact of a laser beam.

It is known to extract a beam of charged particles from a plasma burst or plasmoid derived from a suitably excited target, the initial distribution of ionization rates of the chemical elements which form the target being well defined. However, no attempts to extract the ions from the plasma while retaining the initial distribution of ionization rates have ever proved successful. Up to the present time, there has not existed any known method for removing the causes of differential ionization of the different elements such as some unwanted electric discharge phenomena.

The method of production of ions according to the present invention provides a remedy for the above-noted deficiencies of methods employed in the prior art.

According to said method which entails the use of a target, a plasma is generated from said target and then subjected to a number of successive electric fields. The method is distinguished by the fact that it consists in sorting the particles which form the plasma by providing the successive electric fields with increasing amplitudes in order to slow down the electrons and collecting said electrons by means of electrodes, then in causing the ion-enriched beam to converge towards the exit aperture by means of a final electric field.

The beam convergence electric field is produced by means of an annular electrode which is coaxial with the ion-enriched beam.

The plasma is generated by a beam which is produced by a laser and directed onto the target.

The invention is also concerned with the construction of a source for carrying out the method aforesaid. Said source essentially comprises an assembly constituted by a casing, a metallic cylinder and a metallic target placed at the bottom of said cylinder, and in coaxial relation with said assembly a perforated or so-called extraction plate having a preferably circular opening fitted with a grid, a so-called preacceleration plate having an opening fitted with a grid, a concentration electrode formed of an annular sleeve and a definition plate provided with a slit, means being provided for exciting the target in order to produce a plasma.

A number of known techniques are available for the purpose of producing a plasma from the target. Said target can be subjected to ion bombardment or "sputtering;" a high-frequency discharge can be produced between two electrodes; and finally, the target can be excited by means of a laser beam.

In the case last referred to, the plasma is generated by the impact of the laser beam on the target. The means for causing excitation of the target comprise a port sealed in the casing of the source and a lens which is also secured to said casing for concentrating said beam on said target by means of a plane mirror through a gridded window of the metallic cylinder at the bottom of which said target is disposed.

One of the main applications of a source of this type is the development of a mass spectrograph which permits accurate superficial isotopic analyses.

The apparatus under consideration comprises a laser, an ion source of the type hereinabove described, a pumping system connected to the casing of the ion source, a system for the magnetic deflection of the ion beam derived from the source and a device for recording on photographic plates.

Apart from the main arrangements mentioned above, the invention is also concerned with a number of different secondary arrangements which will be mentioned hereinafter, especially in connection with the source employed for carrying out the method according to the invention.

In order that the properties of the present invention may be more readily understood, there will now be described an ion source for the application of the method according to the invention as well as a spectrograph which makes use of said source, it being understood that neither the source nor the spectrograph are intended to set any restriction on the modes of execution or on the potential applications of the invention.

Reference is made to the accompanying drawings, in which:

FIG. 1 is a sectional view taken along an axial plane of symmetry and showing an ion source according to the invention, the target of which is excited by a laser beam;

FIG. 2 is a sectional view of the same source taken along a plane which is perpendicular to the first section plane;

FIG. 3 is a diagrammatic presentation of a spectrograph in which the ion source of FIGS. 1 and 2 is employed;

FIG. 4 is an isometric representation of the target, extraction plate and preacceleration plate of a modified embodiment of the present invention.

The source 2 is contained within an enclosure 4 which consists essentially of a cylinder 6. A plurality of tubes 8-10 (FIG. 1) and 9-11 (FIG. 2) having their axes at right angles to that of the first tube terminate in said cylinder and the top portion of the cylinder is closed by an insulating plate 16 having insulated passageways 18 through which are passed high-voltage conductors 20 for supplying the source electrodes.

The tube 8 serves to connect the source enclosure 4 to the pumping device 74 (which is not shown in the drawings). The tube 10 is closed by a detachable end plate 22 through which the target to be studied can be introduced.

It will be noted that in the example described, the target 15 is excited by a laser beam and that, as a result of the impact of said beam, neutral atoms are ejected in the form of vapor as well as positive and negative charges which form a plasma. The device for producing the excitation of the target will be described hereinafter.

The target 15 constitutes the base of a metal cylinder 24. All the electrodes of the source are associated coaxially with the assembly 15-24. A circular extraction plate 26 is provided with an opening which is also circular and fitted with a grid 28. An adjustable diaphragm 30 is associated with said opening. Diaphragm 30 has an opening for passage of the plasma and the size of this opening may be varied as required. The intended function of the grid is to make the electric field as uniform as possible in the vicinity of the axis of the source, especially when the positive and negative charges are of high density, and the grid consequently serves to separate the ions from the electrons.

The source further comprises a preacceleration plate 32 in which is pierced a wide slot fitted with a grid. This second electrode enhances the action of the extraction plate and has the effect of reducing the danger of breakdown and discharge by preventing the application of the entire acceleration voltage to electrodes located in the region reached by the vapor which is ejected from the target as a result of the laser impact.

The concentration electrode 34 consists of a coaxial sleeve and a ring which is fitted on the external surface of said sleeve.

A final electrode consists of an acceleration plate 36 in which is formed a slit for the definition of the beam, or object slit.

The electrodes of the source are brought to stabilized potentials whilst the definition plate 36 is connected to ground, the target is brought to the most positive potential by means of a series of cells associated with stabilizing capacitors. Connections on said series of cells serve to supply the extraction electrode 26, preacceleration electrode 32 and acceleration or definition electrode 36.

The concentration electrode 34 is maintained by means of a second series of cells at a positive potential with respect to the potential of the preacceleration electrode 32. Stabilization is also obtained in this case by means of a capacitor.

The operation of the source can be explained as follows.

Ejection of positive ions and electrons resulting from the impact of the laser beam on the target is accompanied by the appearance of an instantaneous target-electron current of very high intensity. The plasma which is generated and the expansion of which is facilitated by the equipotential cage constituted by the metal cylinder 24 is therefore of positive polarity but also contains a high proportion of electrons whose presence is even more troublesome than that of a vaporized mass. These phenomena have made it necessary to devote particularly close attention to the development of high-voltage supply arrangements.

The potential of the extraction plate is so determined that the majority of the electrons are subjected to a resultant force which tends to move them away from the axis and are therefore collected by said plate but do not pass through this latter. This potential must be determined with care in order to prevent any displacement of electrons and ions in opposite directions under the influence of the electric field and therefore any discharge into the vapor. It would consequently not be feasible to incorporate said source in a spectrograph since the initial distribution of ionization rates between the ions is disturbed.

The utilization of a preacceleration plate is not essential although an electrode of this type can perform a function which is complementary to the role of the extraction plate. In this case, said preacceleration plate collects the majority of electrons which have passed through the plate 26.

The present Applicant has given consideration to the shape of the concentration electrode 34 and has found that an annular shape was highly conducive to the results which it is sought to achieve. This shape of electrode is intended to reduce the solid angle of divergence of the incident ion beam and to permit of final acceleration with minimum divergence. It has already been explained that this electrode is brought to a potential which is more positive than the preacceleration plate 32 and therefore exerts a repulsive force on the ions. In short, the resultant of the slightly divergent force to which the ions are subjected at the level of the preacceleration plate and of the repulsive force produced by said electrode 34 causes the ions to converge towards the axis.

Finally, the definition plate 36 which is connected to ground and therefore has the highest potential difference with respect to the target defines the shape of the beam and selects that portion of said beam which passes through the slit formed in said plate. By reason of the fact that only the portion referred to is useful, the opening of the preacceleration plate has been given a rectangular shape which eliminates any nonuseful regions of the beam at the level of said opening and thus reduces space charge effects. On the other hand, openings of revolution have been maintained in the case of the extraction plate and the concentration electrode in order that focusing along one axis should not result in defocusing along the perpendicular axis.

Referring to FIG. 4, there is shown an embodiment of the invention in which the opening of the preacceleration plate 32' is rectangular and the target carries a tubular member 24' of rectangular cross section. The extraction plate 26' has diaphragm 30' removably secured thereto for selection of an appropriately sized opening in the diaphragm to provide an adjustable diaphragm.

Referring to FIG. 2, there will now be described the sectional view which is taken along a plane at right angles to the first section plane. It will be noted that the majority of the elements shown in FIG. 1 are also illustrated in this figure and are designated by the same reference numerals. There can be seen in this figure two new portions of the casing 6, namely the tubes 9 and 11 having axes at right angles to that of the tube 6 and to the axes of the tubes 8 and 10.

Said tubes contain respectively the optical system 38 for the introduction of a laser beam and the system 40 for the displacement of the target 15.

The first system comprises a port 42 which is secured to the end of the tube 9, a device 44 for fixing the lens 46 which serves to focus a laser beam 48 on the target 15. Said beam is directed towards the target by means of a silvered mirror 50 and a grated window 54 of the metal cylinder 24.

The target 15 is attached to a sample holder 56 provided with a toothed rack 58. The member 56 is held in a support 60. The target can be displaced either backwards or forwards with respect to the plane of FIG. 2 by virtue of the movement of rotation of a gearwheel 62 which drives the toothed rack 68 when the target position control arm 64 is caused to rotate.

One of the main uses of the ion source of FIGS. 1 and 2 consists in incorporating said source in a mass spectrograph of the type shown in FIG. 3.

This apparatus comprises a ruby laser 66, an ion source 68 having a target which is excited by a laser beam, said source being of the type illustrated in FIGS. 1 and 2, a device 70 for the magnetic deflection of ions, a device 72 for collection and photographic recording as well as a pumping device 74 connected to the casing of the ion source.

The ruby laser 66 which is cooled by evaporation of liquid nitrogen operates in the Q-switched state and is of the rotating mirror type.

The source 68 has already been described in the foregoing. The device 70 for magnetic deflection of the ion paths constitutes a magnetic prism having an angle equal to 60.degree.. Said device comprises an electromagnet 78 fitted with pole pieces 80 and between these latter a tube section 76 traversed by the ion beam which is derived from the source. A diaphragm is placed at the entrance of the prism and limits the angle of divergence of the ion beam. Another diaphragm which is narrower collects a lateral fraction of the ion beam which emerges from the magnetic prism.

The quantities of charges received by the definition plate and by the diaphragm last mentioned are measured at the same time by means of a dual-beam oscillograph.

The ions transmitted by the final diaphragm are received by a photographic plate 82 of the collecting and recording device 72. Said plate is tangent to the focal surface of the spectrograph. The impact of the ions on the sensitive film of the photographic plate results after development of said film in blackening which is proportional to the quantities of ions which strike the target. The shape of the blackened lines obtained reproduces the shape of the object slit. Each line corresponds to a ratio of m/e.

The present Applicant has constructed an actual spectrograph of the type described above. The laser employed delivers within a time interval of 4 microseconds a number of light pulses having a total energy which can attain 0.2 joule. The lens 46 which serves to concentrate the beam of the laser on the surface of the source target has a focal distance of 43 mm.

The dimensions of the principal elements of the source are as follows:

The extraction plate 26 has a circular opening fitted with a grid 28 having a transparency of 85 percent and with a diaphragm 30 having an internal diameter between 2 and 10 mm.

The preacceleration plate 32 has a wide slit measuring 8.times.15 mm. whilst the internal diameter and the length of the concentration electrode can vary between 5 and 15 mm. The definition slit of the acceleration plate has dimensions in the vicinity of 1.times.10 mm.

The voltages to which the different electrodes of the gun are brought have approximately the following values:

target: 8.5 kv.

extraction plate: 8.5 to 8.3 kv.

preacceleration plate: 8.3 to 7.5 kv.

concentration electrode: 8.5 to 7.5 kv.

definition plate: 0

This ion source has made it possible to obtain a focused accelerated ion beam having a mean intensity of 200 microamperes. Said source has been employed with a single-focus mass spectrograph with photographic plate detection.

The performances of the mass spectrograph as a whole were such that the recorded range of mass defined by the ratio of masses received at both ends of the plate was 1.6 and the resolving power was 200.

Claims

What we claim is:

1. Device for generating an ion beam, comprising, in an evacuated enclosure:

a target for emission of a plasma of ions and electrons and carried in a tubular member to provide an equipotential cage for said target,

an extraction plate formed with a circular opening aligned with said tubular member and in spaced relation thereto,

an electrically conductive grid disposed transversely of said opening and carried by said extraction plate,

a preacceleration plate in spaced relation with said extraction plate and formed with a wide slot aligned with said opening,

an electrically conductive grid disposed transversely of said slot and carried by said preacceleration plate,

an annular concentration electrode aligned with said preacceleration plate and in spaced relation thereto,

an acceleration and beam definition plate located in spaced relation to said electrode and formed with a slit narrower than said slot and aligned therewith,

electrical voltage means connected to said target, to said plates and to said electrode for bringing said preacceleration plate, said extraction plate and target to increasing positive potentials, respectively, with respect to said acceleration plate and bringing said concentration electrode to a potential slightly higher than that of the preacceleration plate, whereby electrons will be removed from said plasma by said extraction and preacceleration plates and the resulting ion beam will be focused by said concentration electrode at said slit in said acceleration and beam definition plate,

and means for exciting said target to produce the plasma.

2. A device as described in claim 1, the potential of said target, of said extraction plate, of said preacceleration plate, and of said concentration electrode with respect to said acceleration plate being respectively about 8.5 kv., from 8.5 to 8.3 kv., from 8.3 to 7.5 kv. and from 8.5 to 7.5 kv.

3. An ion source in accordance with claim 1, said tubular member having an internal rectangular cross section.

4. An ion source in accordance with claim 1, said tubular member having a circular cross section.

5. An ion source in accordance with claim 1, including a window in said tubular member and a grid in said window.

6. An ion source in accordance with claim 1, including a diaphragm in said extraction plate.

7. An ion source in accordance with claim 1, said means for exciting said target including a laser and means for directing the beam produced by said laser into said target.

8. An ion source in accordance with claim 7, said means for directing the laser beam onto said target includes a port in said enclosure, a lens secured in said port and a plane mirror reflecting the laser beam onto said target.

9. A mass spectrograph including the ion source of claim 1, wherein said exciting means comprises a laser, a vacuum pumping system connected to the enclosure of said source, means for the magnetic deflection of the ion beam derived from said source and means for recording the deflected ion beam on a photographic plate.