US3641341A

US3641341A - Ion beam image converter

Info

Publication number: US3641341A
Application number: US887690A
Authority: US
Inventors: Douglas M Jamba; Robert M Ennis Jr
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1969-12-23
Filing date: 1969-12-23
Publication date: 1972-02-08
Anticipated expiration: 1989-02-08

Abstract

The ion beam image converter comprises a secondary emission screen in the ion beam path so that, when ions impinge upon the secondary emission screen, secondary electrons are emitted. An accelerator screen is positioned downstream from the secondary emission screen to accelerate the secondary electrons. Deflecting means is positioned downstream of the accelerator screen to deflect the electron beam onto a path away from the ion beam path. A phosphor image screen is placed on the deflected secondary electron path to convert the electron beam to visible image. Electron beam deflection can be either electrostatic or magnetic.

Description

United States Patent J amba et al.

[54] ION BEAM IMAGE CONVERTER [72] Inventors: Douglas M. Jamba, Woodland Hills; Robert M. Ennis, Jr., Malibu, both of 3,277,297 10/1966 Forrester et al. "250/495 Feb. 8, 1972 2,769,911 ll/l956 Warmoltz ..250/4l.9

Primary Examiner-William F. Lindquist Attorney-James K. Haskell and Allen A. Dicke, Jr.

[ ABSTRACT The ion beam image converter comprises a secondary emission screen in the ion beam path so that, when ions impinge upon the secondary emission screen, secondary electrons are emitted. An accelerator screen is positioned downstream from the secondary emission screen to accelerate the secondary electrons. Deflecting means is positioned downstream of the accelerator screen to deflect the electron beam onto a path away from the ion beam path. A phosphor image screen is placed on the deflected secondary electron path to convert the electron beam to visible image. Electron beam deflection can be either electrostatic or magnetic.

3 Claims, 2 Drawing Figures [ON BEAM IMAGE CONVERTER BACKGROUND This invention is directed to an ion beam image converter, which is capable of providing a visible image corresponding to the original ion beam so that gross beam characteristics, such as size and shape of the ion beam cross section, can be visualized, even for very low-current ion beams 0.l pA).

One group of prior devices used for observing ion beams depended upon ions striking material, such as quartz, or a phosphor, such that they emit visible light. Direct ion bombardment of phosphors gradually destroys the light-emitting properties, to thus limit the useful lifetime. Quartz detector plates are more resistant to bombardment, but the visible light is diffused and reflected, distorting the images, to make observation inaccurate and difficult. In another type of such device, a metal screen is placed directly in front of the phosphor. The ion beam striking the metal screen generates secondary electrons which are attracted through the screen and accelerated to the phosphor, producing an image of the ion beam. This device is usable to observe only relatively low-energy ion beams, such as are obtained in ion microscope studies of cesium ion emission from porous tungsten. The success of this device depends upon maintaining the positive potential on the phosphor screen higher than the beam energy, so that ions are decelerated and are not allowed to strike the phosphor. This type of device is limited to a maximum of about 20 k.e.v. beam energy. Above that energy level, the electrons striking the phosphor have a high enough energy to cause dangerous X- ray emission conditions. In all cases where higher energy ion beams are allowed to strike a surface, there is an inherent danger of generating X-rays of sufficient intensity to prohibit direct visual observations.

SUMMARY In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to an ion beam image converter. The image converter has a secondary electron emission screen, which is placed in the path of the ion beam to be observed. An accelerator screen is positioned to accelerate the secondarily emitted electrons. Deflecting means is positioned to deflect the electron beam away from the ion beam path and, finally, a phosphor screen is placed upon the electron beam path so that electrongenerated visible light corresponds in shape to the original ion beam so that the ion beam can be visualized. The preferred deflecting means is electrostatic deflection.

Accordingly, it is an object of the present invention to provide an ion beam image converter which makes use of secondary electrons to light a phosphor, and eliminates the possibility of ions striking the phosphor. It is another object to provide an ion beam image converter which employs a relatively low field to deflect secondary electrons away from the ion beam to a phosphor screen so that only the electron beam impinges upon the phosphor. It is a further object to provide an ion beam image converter which operates at a sufficiently low voltage that there are no dangerous X-ray emissions during observation. It is a further object of this invention to provide a visual image of the beam cross section, even at very low ion beam intensities. It is still another object to provide a phosphor screen which is bombarded only by electrons in such an ion beam image converter device so that the life of the phosphor screen is essentially unlimited.

Other objects and advantages of this invention will become apparent from a study-of the following portion of the specification, the claims andthe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of the ion beam image converter of this invention.

FIG. 2 is a section taken generally along the line 2-2 of FIG. I.

DESCRlPTlON Referring to the drawings, the ion beam image converter is generally indicated at 10. It is understood that, in use, the ion beam image converter is positioned in an ion beam device, on the path of an ion beam, such an ion beam being indicated at 12. Thus, the ion beam image converter is located in a housing which is evacuated. The housing is provided with a suitable side porthole for observing the phosphor screen of the image converter 10, and is equipped with means for providing electrical connection to the several parts of the ion beam image converter.

' Ion beam image converter comprises secondary emission screen 14 mounted upon screen support 16. The mesh size of this screen determines the smallest ion beam that can be observed. Screens having a mesh as fine as 2,000 mesh are available, and these provide a resolution approaching 10' meters. The open percentage or transmission of screen 14 should be as great as is consistent with the fineness, which is determined by the desired resolution. Secondary emission screen 14 is mounted upon its support 16 by means of ring 18. In view of the fineness of screen 14, securing ring 18 is secured thereon to distribute tension and provide adequate structural support for securement.

Accelerator screen 20 is a large mesh screen; for example, 10 mesh, which is directly supported upon screen support 22; for example, by spot welding. Screen supports 16 and 22 are secured to each other and are spaced from each other by means of ceramic

insulator spacer posts

24 and 26. These posts have threaded holes therein, so screws may be engaged thereon, with their heads engaged on the screen supports.

Deflection electrode 28 is the inner deflection electrode for deflecting the secondary emission electrons out of the ion beam. lnner deflection electrode 28 is mounted upon ceramic insulator support post 30, which passes through suitable openings in the screen support 22 and is secured upon screen support 16. Deflection electrode 28 is in the form of a fractional cylinder having its axis on the desired center of the radius of curvature of electron deflection.

Outer deflection electrode 32 also has a cylindrical surface. In this case, similarly to deflection electrode 28, the electrode is formed as part of a cylindrical thin metal tube. Opening 34 through electrode 32 is positioned on the axis of ion beam l2 so that the ion beam can pass therethrough. Screen 36 covers the opening 34. Screen 36 is a large mesh screen having a large transmission area, at least 90 percent, so that the ion beam is substantially unimpeded in its passage. Outer deflection electrode 32 is mounted upon ceramic insulator post 38, which passes through a hole in support 22, and is mounted on screen support 16. Outer deflection electrode 32 preferably has as the center of its radius the same center as the radius of deflection electrode 28.

Phosphor screen 40 is a glass plate with a layer of phosphor powder deposited thereon. The phosphor screen may be a commercial unit or may be prepared using conventional techniques of spraying or settling phosphor powder on a glass plate. The glass plate must first have a transparent conductive coating (i.e., tin oxide) in order to maintain the electric field and conduct charges away. The type of phosphor powder required can be any of the materials normally used for cathode ray tubes or display devices making use of electron beam exitation. Phosphor screen 40 is mounted by means of support wires 42 and 44 on screen support 22. Phosphor screen 40 is positioned in such a location as to receive the beam of deflected electrons, and to be seen through a porthole in the ion beam device. The purpose of the ion beam converter is to provide a nondestructive, indirect means of viewing an ion or electron beam cross section at any plane in a field-free region along the beam axis. It can be utilized to observe, with the naked eye, gross beam characteristics, such as the size and shape of beam cross section. Since the ion beam image converter is a continuous monitoring device, effects of changes of beam source and optical system parameters on the gross beam characteristics at some plane in the beam system can be observed. The ion beam image converter is independent of beam energy, and can be applied to beams of very low energy (below 1 k.e.v.) to very high energy (hundreds of m.e.v.).

lons striking secondary emission screen 14 generate secondary electrons in direct proportion to the ion current density. The electrons are accelerated to the right, as is seen in the drawing, by a uniform electric field established between screens 14 and 20. Preferably, the ion beam would be traveling in a grounded, metal chamber, and screen support 16 would be mounted in this chamber at ground potential, to maintain an electric, field-free region up to ion beam image converter 10. A positive voltage of between and 20 kv. is applied to the accelerator screen 20 to accelerate the electrons to provide the necessary electron energy to illuminate the phosphor in phosphor screen 40.

Equal positive and negative deflecting voltages, with respect to the potential on accelerator screen 20, are applied to the

deflection electrodes

28 and 32 to maintain a uniform bending of the electrons to the phosphor with no change in energy. Deflecting voltage should be approximately 30 percent of the accelerator voltage. For example, if accelerator screen 20 has a voltage of kv., the voltages on deflecting

electrodes

28 and 32 would be 13 kv. and 7 kv. respectively. The phosphor screen 40 is maintained at the same potential as screen 20, by means of support wires 42 and 44.

The ion beam image converter 10 can be used for essentially any ion beam energy. If the ion beam energy is lower than the potential of accelerator screen 22, the ions will be decelerated and reversed. Operation in this manner is useful in cases of low-current densities because additional secondary electrons are generated when the ions are reversed and return to strike screen 14.

For intermediate ion beam energies, comparable to the potential on screen 20, the ions can be deflected upward and strike deflection electrode 32. This will cause no difficulty, because the secondary electrons generated by this striking will not have enough energy to light the phosphor and will not normally follow trajectories allowing them to reach the phosphor. High-energy ion beams, of sufficient energy that they are unaffected by the deflection voltages, will pass through the ion beam image converter to the normal target. Similarly, in this case, any secondary electrons generated at deflection electrode screen 36, will have insufficient energy to affect the ion beam image on the phosphor.

There is a possibility of X-ray generation at high-beam energies, above 100 k.e.v., caused by ions striking the metal screens. To provide proper protection, the deflection plates in this device may be extended, allowing the phosphor to be mounted in a position where lead shielding of the main chamber will eliminate any hazard.

Since only the electrons are deflected, and the deflected electrons light the phosphor screen 40, the possibility of ions striking the phosphor is eliminated. This is accomplished because the voltages on the phosphor and other electrodes are such as to repel positively charged ions. The energy required for electrons to light the phosphor is in the range of 5 to k.e'.v., causing no X-ray problems during observation. The phosphor screen is bombarded only with electrons and its life is essentially unlimited. When the energy of the ion beam is sufficiently high to be undeflected by the deflection energy of the ion beam image converter, beam monitoring by the image converter 10 can be continuously carried out while the ion yerter. The net ion beam transmission through the ion beam image converter when there IS no electron beam deflection applied depends mainly on the size of the secondary electron emission screen 14, which is chosen to provide the desired image resolution. For example, an ion beam having a diameter of about 1 mm., can be resolved with a screen of mesh (100 lines per inch). The net ion beam transmission in this case would be about 70 percent. Accordingly, if the ion beam image converter 10 is going to be used fairly often, it can be left in the beam path. On the other hand, if it is only employed during setup to focus the beam and optimize the ion source, it can be removed after these functions have been accomplished.

While electrostatic deflection of the electron beam has been shown in the drawings, it is clear that electromagnetic deflection can alternatively be employed.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty.

What is claimed is:

1. An ion beam image converter, said ion beam image converter being positionable on the axis of an ion beam, said ion beam image converter comprising:

a screen positionable along the ion beam axis for impingement by the ion beam so that said screen emits secondary electrons in a pattern substantially corresponding to the beam impact pattern on said screen;

an accelerator electrode positioned downstream from said screen along the ion beam axis, said accelerator electrode being chargeable to a potential to accelerate the secondary electrons along the ion beam axis in the direction of ion flow;

deflection means positioned adjacent said ion beam axis downstream from said accelerator electrode, said deflection means comprising inner and outer electrostatic deflection electrodes respectively positioned interiorly and exteriorly of a curved electron beam path;

said outer deflection electrode having an opening therethrough positioned alongsaid ion beam axis, a screen in said opening to permit substantial passage of the ion beam along said axis and to maintain a uniform electric field across said opening, said inner and outer electrostatic deflection electrodes being respectively chargeable to potentials higher and lower than said accelerator electrode to cause deflection of the secondary electron beam away from the ion beam axis; and

a phosphor screen positioned on said electron beam path away from said ion beam axis so that electrons striking said phosphor screen represent the ion beam cross section.

2. The ion beam image converter of claim 1 wherein said phosphor screen is electrically connected to be at the same potential as said accelerator electrode.

3. The ion beam converter of claim 2 wherein said accelerator electrode comprises a screen.

Claims

1. An ion beam image converter, said ion beam image converter being positionable on the axis of an ion beam, said ion beam image converter comprising: a screen positionable along the ion beam axis for impingement by the ion beam so that said screen emits secondary electrons in a pattern substantially corresponding to the beam impact pattern on said screen; an accelerator electrode positioned downstream from said screen along the ion beam axis, said accelerator electrode being chargeable to a potential to accelerate the secondary electrons along the ion beam axis in the direction of ion flow; deflection means positioned adjacent said ion beam axis downstream from said accelerator electrode, said deflection means comprising inner and outer electrostatic deflection electrodes respectively positioned interiorly and exteriorly of a curved electron beam path; said outer deflection electrode having an opening therethrough positioned along said ion beam axis, a screen in said opening to permit substantial passage of the ion beam along said axis and to maintain a uniform electric field across said opening, said inner and outer electrostatic deflection electrodes being respectively chargeable to potentials higher and lower than said accelerator electrode to cause deflection of the secondary electron beam aWay from the ion beam axis; and a phosphor screen positioned on said electron beam path away from said ion beam axis so that electrons striking said phosphor screen represent the ion beam cross section.