Ion gauges

Abstract

An ion gauge in which the collector is arranged outside the ionisation region and the ions representing the gas pressure are extracted from the ionisation region via an aperture in an X-ray screen and directed by a reflector electrode onto the collector electrode. The ion reflector electrode surrounding the collector electrode is constructed and shaped so that soft X-rays passing through the ion-exit aperture undergo multiple reflections at the reflector electrode before they are able to be reflected onto the collector. A cylindrical reflector electrode is disclosed provided with an internal truncated conical X-ray reflecting member which intercepts soft X-rays and reflects them back and forth several times between the cylindrical and conical reflecting surfaces. The end of the cylindrical reflecting electrode nearest the ion-exit aperture is tapered inwardly to reflect onto the conical reflector X-rays falling on that part of the surface.

Description

This invention relates to ionisation vacuum gauges of the kind employing an external collector electrode.

Low gas pressures in a region of space are frequently measured by ionisation methods in which electrons emitted by a cathode are accelerated so that they pass through a mesh wall into an ionisation space bounded by said mesh wall and located in the region, with sufficient energy to ionise gas present therein. Ions so formed are drawn by an electrostatic field towards a suitably biassed collector electrode and the collector current forms a measure of the amount of gas present in the ionisation space and hence of the gas pressure in the said region of space. Unfortunately other concurrent phenomena also contribute to the collector current and limit the accuracy and range of gas pressures over which the gauge will operate satisfactorily.

One such phenomena is the generation of soft X-rays from the collision of electrons with the mesh and other metal structural parts surrounding the ionisation space, and interception of this X-radiation by the collector electrode causes the emission of photo-electrons thereby. As attempts are made to measure lower gas pressures, this photo-electron current eventually swamps the ion measurement current.

Early attempts to reduce the magnitude of the X-ray photocurrent include the use of a thin wire collector as described by R. T. Bayard and D. Alpert in Review of Scientific Instruments Volume 21 page 571, and the use of an additional modulator electrode by P. A. Redhead Review of Scientific Instruments Volume 31 page 343. Thus by reducing the surface area of the collector electrode the photocurrent was reduced by comparison with the collected ion current, and by modulating the ionisation current component it was made possible to measure a smaller ion current in the presence of a given residual current due to photo-emission and other factors.

A further reduction in X-ray photo-emission from the collector electrode was achieved by mounting the collector electrode outside the ionisation space and withdrawing ions from the ionisation space via an aperture in an X-ray screen. Examples of such gauges are due to P. A. Redhead in U.S. Pat. No. 3,465,189 and to J. Groszkowski described and claimed in British Pat. No. 1,173,354.

In a development of the external-collector ion gauge, described in U.S. Pat. No. 3,742,343, the ionisation space is formed within a substantially equipotential boundary wall by employing a fine aperture mesh which effectively prevents an external electrostatic field from penetrating the ionisation region and withdrawing ions which would otherwise pass to the collector. The apertured X-ray shield is maintained at the same potential as the mesh, and ions are drawn out towards the collector by means of a suitably biassed extractor electrode. The ions are then concentrated onto a collector electrode by means of a deflector electrode. However there is a tendency for X-rays to pass through the aperture in the X-ray shield and to be deflected at the surface of the reflector electrode and thus concentrated onto the collector electrode.

It is an object of the invention to provide an improved ion gauge employing an external collector, in which the residual X-ray photocurrent can be significantly reduced.

According to the invention there is provided an ion gauge comprising a source of electrons, an enclosure formed of electrically conducting walls surrounding an ionisation space in which in operation a gas whose pressure is to be measured is present, the walls of said enclosure including a peripheral apertured region forming an electron accelerating grid such that in operation electrons are directed thereat by the application to said source mounted outside said enclosure of a negative bias relative to said enclosure sufficient to cause at least some of said electrons to pass into the ionisation space and to ionise gas present therein, said enclosure having an end wall substantially opaque to the passage of soft X-rays and provided with an ion-exit aperture through which ions formed in said ionisation space by said electrons can pass out of said ionisation space, a thin rod-like collector electrode mounted externally with respect to said ionisation space and directed along the axis of said ion-exit aperture and biassed in operation to collect ions from said ionisation space via said ion-exit aperture to provide a current dependent on the pressure of gas in said ionisation space, and a deflector electrode arranged to surround said collector electrode and so biassed that said ions are caused to converge and strike said collector electrode, said deflector electrode being so formed that at least the major portion of X-radiation formed by collision of electrons with the walls of said ionisation space and capable of striking the collector electrode after passing through said ion-exit aperture, is caused to suffer multiple reflections at said deflector electrode and thereby to be substantially reduced in intensity before being incident on said collector electrode.

The deflector electrode can be formed so that the major portion of the X-radiation passing through the ion exit aperture is caused to be reflected more than three times before reaching the collector electrode and the X-ray reflecting portions of said deflector electrode preferably has an X-ray reflection coefficient of less than 0.5. The deflector electrode can include a cylindrical surface and a further surface of revolution arranged within said cylindrical surface coaxial both with said cylindrical surface and with said collector electrode, so that at least the major portion of said X-radiation passing through said aperture and directed onto the inward facing said cylindrical surface, is reflected onto the outward facing said further surface of revolution and thereafter suffers multiple reflection between said surfaces. The said further surface of revolution can comprise a truncated conical member the apex of which is directed towards the ion exit aperture. The inward facing cylindrical surface can be tapered gradually inwards towards the end adjacent the ion exit aperture so that X-rays incident thereon which would in the absence of said taper be reflected onto the collector electrode, are reflected onto the outward facing conical surface instead.

The end wall of the ionisation space having therein the ion-exit aperture, can be electrically connected to the remaining walls of the ionisation space so as to be maintained at the same potential as said walls and an annular extractor electrode can be arranged adjacent the ion exit aperture outside the space enclosed by said walls and biassed negatively with respect to the walls of the ionisation space so as to extract ions from the ionisation space via said ion-exit aperture.

In order that the present invention may be clearly understood and readily carried into effect an embodiment thereof will now be particularly described by way of example, with reference to the accompanying drawings of which:

FIG. 1 is a longitudinal section of an ion gauge embodying the invention,

FIG. 2 is a longitudinal section of the extractor and deflector electrode assembly of a prior form of external collector gauge illustrating the paths taken by X-rays entering via the ion-exit aperture, and

FIG. 3 is a longitudinal sectional outline diagram of part of the ion gauge of FIG. 1 illustrating the paths taken by X-rays entering the collector region via the ion-exit aperture.

An ion gauge embodying the invention is illustrated in longitudinal section in FIG. 1 to which reference will now be made. The ion gauge shown in FIG. 1 is normally called a nude ion gauge since it is not provided with an airtight surrounding envelope and is intended to be mounted in apparatus, within which the pressure is to be measured, by means of a flange 23.

Cathodes 1, 2 in the form of electron emissive filaments are connected to support and connection wires 21 mounted in glass seals 25, and which project therefrom so that heating and biassing potentials can be fed thereto. It is desirable to employ filaments which can be operated at a relatively low temperature, for example a rhenium filament coated with lanthanum hexaboride, to prevent dissociation of hydrogen present, however any convenient filament and coating material can be employed.

The glass seals 25, which form insulating supports for various electrodes via respective support wires, are attached to and each form a seal with a metal plate 26 which latter is sealed to a conducting cylinder 24. The metal flange 23, welded to the cylinder 24, enables the ion gauge to be attached to a vacuum system to form a seal therewith. The glass employed for the seals 25 is chosen to have a coefficient of thermal expansion which is compatible with that of the plate 26.

An ionisation space 13 is bounded by a conductive electron acceleration grid assembly 3 comprising a peripheral wall 4 and one end wall 5, both formed of a closely woven wire mesh, conveniently of Tungsten, and a further end wall 6, substantially opaque to soft X-rays, and formed of thin sheet metal, conveniently stainless steel. The mesh size of the closely woven wire mesh forming the walls 4 and 5 is such that the largest dimension of each aperture in the mesh is small enough to prevent the field present, in operation, outside the electron accelerating grid 3 due to a potential difference applied between the grid 3 and the cathodes 1, 2 and between the grid 3 and the surroundings which comprise principally the grounded electrically conducting cylindrical screen 24, conveniently of stainless steel, from penetrating the ionisation space 13 to a significant extent. In this way conditions can be provided in normal operation so that, when an electron acceleration field is employed which is sufficient to provide optimum ionisation of the gas whose pressure is to be measured, the collector output current does not fall significantly when the electron accelerating field is increased while other parameters, such as gas pressure and other electrode potentials, are maintained constant. Other forms of apertured conducting material can alternatively be employed for the electron transparent wall region 4, 5, for example a suitable metal such as copper, electrodeposited on a mesh base and then stripped from the base, provided that the largest dimension of each of the apertures is small enough to prevent the electrostatic field set up outside the grid 3 from significantly penetrating the ionisation space 13.

The end wall 6 of the electron accelerator grid 3 is provided with an outwardly directed truncated conical portion 20 surrounding an ion-exit aperture 7. A generally cylindrical deflector electrode 31, formed conveniently of stainless steel or alternatively of molybdenum, tungsten, tantalium or other conducting material having a relatively low X-ray coefficient of reflection, and having at one end a wall 32, is conductively attached, for example by welding, to a connection and support rod 34 by which means the deflector electrode 31 can either be connected to the electron accelerator grid 3 or to another suitable source of ion repelling potential.

An annular extractor electrode 39, formed conveniently of stainless steel and having a V-shape cross-section, is mounted between the conical surround 20 of the ion-exit aperture 7 and the open end portion 33 of the deflector electrode 31 but is insulated therefrom and is provided with a conducting support 30 which forms the connection to a source of ion extracting bias potential, not shown. The inner radial extremity of the annular extractor electrode 39 projects inwardly, in a radial sense, slightly beyond the edge of the aperture 7 and the edge of the open end portion 33 towards the axis of the path taken by ions extracted via the aperture 7. The outer diameter of the electrode 39 is greater than that of the surrounding edge of the aperture 7 and is therefore partly screened by the end wall 6 from ions within the ionisation space.

A collector electrode 10 in the form of a wire, conveniently a tungsten wire extends along the axis of the extracted ion path within the deflector 31 and is mounted on a conductive support member 44, which extends through an aperture in the wall 32 and through a seal 25 to form a terminal connection for the collector electrode.

The deflector electrode 31 includes a hollow truncated conical member 35 electrically connected thereto and arranged so that the collector electrode projects through the aperture formed by the truncated apex for a short distance. The member 35 can be made of the same substance as the remainder of the deflector electrode 31 and should exhibit a relatively low reflection coefficient to X-rays directed at the outer surface thereof.

The open end 33 of the cylindrical portion of the deflector electrode 31 is preferably formed so that the inward facing cylindrical surface is tapered gradually inwards towards the end adjacent the extractor electrode 39.

A bead 36 of an insulating substance, such as glass or ceramic, is provided on the collector electrode 10 and arranged to screen the end of the collector support 44 from X-rays directed thereat from the walls of the ionisation region 13 via the ion-exit aperture 7. That part of the collector support 44 situated between the end wall 32 of the deflector 31 and the seal 25, is shielded from the incidence of charged particles present outside the extraction reflection system by a surrounding conducting cylindrical shield 37 attached and connected to the end wall 32.

In operation the cathodes 1, 2 are biassed negatively with respect to the grid 3 so that electrons are accelerated towards the mesh wall 4 and penetrate the apertures therein with sufficient energy to ionise gas present in the space 13. The screen 24 is connected to ground and the cathodes 1, 2 are biassed positively with respect thereto. The extractor electrode 39 and the collector 10 are biassed negatively with respect to the cathodes 1 or 2. The deflector 31 is biassed positively with respect to the collector 10, to a potential equal to or greater than that of the grid 3. Electrons present within the grid 3 form a space charge which, together with the screening effect of the closely woven mesh walls 4 and 5, produce an electric field within the ionising region 13 which tends to prevent positive ions formed therein from escaping via apertures in the walls 4 and 5 and to cause the positive ions to tend to move towards the central part of the ionisation region.

The potential on the extractor 39 sets up an ion extraction field which draws positive ions out of the ionisation region 13 via the aperture 7 towards the collector 10. The converging effect of the electron lens formed between the extractor 39 and the conical portion 20 of the wall 6 causes the ions to follow paths which, in general, avoid collision with the extractor 39. The further converging lens formed between the extractor 39 and the deflector 31 further assists in directing ions away from the extractor 39 and towards the collector electrode 10.

The collector electrode 30 is so situated that soft X-rays generated by the collision of electrons with the peripheral grid portion 4 of the acceleration grid 3 are substantially shielded therefrom by the end wall 6. However, soft X-rays generated at the end wall 5 and in that part of the peripheral grid 4 adjacent thereto, can pass through the ion-exit aperture 7, strike the inside surface of the cylindrical portion of the deflector electrode 31, and be reflected therefrom.

In the prior form of external-collector ion gauge described in copending British patent application No. 53007/69 (PHB 32008), the soft X-rays which pass through the ion-exit aperture and strike the inside cylindrical surface of the deflector electrode, tend to be reflected and concentrated onto the collector electrode because of the rotational symmetry about the axis of the extracted ion beam, and the end wall of the deflector electrode.

FIG. 2 illustrates in longitudinal section typical paths taken by soft X-rays passing through the ion-exit aperture 7 from the walls of the ionisation space 13 into an ion extractor assembly similar to that illustrated in FIG. 5 of the aforesaid British patent application No. 53007/69 (PHB 32008). Two ray paths are shown. X-rays following the path 46 will be reflected by the cylindrical inner surface of the deflector electrode 51 and, because of the rotational symmetry, will tend to be concentrated onto the surface of the collector electrode 50 where, of course, the incident X-rays will cause the photoemission of electrons and hence add an undesired component to the collector current which is not pressure dependent and will tend to swamp the pressure-dependent ion current to be measured at lower pressures. Another typical X-ray path is indicated by the reference 47, and this undergoes two reflections, at the inner cylindrical surface of the deflector 51 and at the end wall 52, before reaching the collector electrode.

Most metals have a relatively low reflection coefficient, in the region of from 0.2 to 0.3 for polished surfaces of for example molybdenum tungsten or tantalum, but since a large proportion of the soft X-rays passing via the aperture can readily reach the collector electrode in the arrangement shown in FIG. 2 after only one or two reflections, the residual intensity of the X-rays incident on the collector electrode can cause a significant undesired residual collector current component.

Referring now to FIG. 3, which similarly illustrates typical paths taken by soft X-rays passing through the ion-exit aperture 7 from the walls of the ionisation space 13 into the ion extractor assembly shown in FIG. 1 and embodying the invention, it will be seen that the inner surface of the cylindrical deflector 31 and the outer surface of the truncated conical member 35 cooperate to cause the possible X-ray paths to undergo at least three and preferably more than three reflections before reaching the collector electrode. Thus the rays 55 and 56 suffer a plurality of reflections between the cylindrical inner surface of the deflector 31 and the outer conical surface of the member 35 before reaching the surface of the collector electrode 10. Since the reflection coefficient for each reflection is small, the intensity of the reflected beam will decline rapidly as the number of reflections suffered by the X-radiation increases.

The deflector 31 can take the cylindrical form shown in FIG. 2, and the open end is indicated by the dotted profile 33' in FIG. 3. However, X-rays entering at an oblique angle via the aperture 7, such as the ray 57, would tend to be reflected onto the collector electrode 10 after only one reflection, as the ray 57'. In the preferred embodiment, therefore, the inward facing cylindrical surface of the deflector electrode 31 is tapered gradually inwards over the region 33 towards the end adjacent the extractor electrode 39 and the ion-exit aperture 7. It can be seen that, as a result, the reflected beam path 57" is directed onto the outward facing surface of the conical member 35 and is repeatedly reflected between the two surfaces a plurality of times, after which the resultant intensity is negligible.

The extractor electrode 39 is preferably shaped so that the tip 41 of the V-shaped cross-section has as small a diameter as possible. When the extractor electrode is shaped with a near-cylindrical portion at the tip, as shown at 40 in FIG. 2, there will be presented a relatively large area from which X-radiation can be reflected directly onto the collector electrode in a manner similar to that of the ray 57' in FIG. 3.

It should be noted that the inner surface of the deflector electrode 31 including the portion 33, and the outer surface of the truncated conical member 35 should be polished to ensure reflection of X-radiation in the desired direction. An unpolished and therefore diffusing surface would give uncontrolled scattering and would increase the chance that a significant amount of X-radiation would reach the collector after only one or two reflections.

As is well known, the best sensitivity for low pressure measurement can be obtained by employing a modulator electrode, and this can be introduced into the ionisation space 13 via an aperture in the end wall 5 of the grid 3. It has been found by experiment that a gauge embodying the invention can enable the lower limit of pressure measurement to be extended by a factor of 5.

Claims

I claim:

1. An ion gauge having a low x-ray photo-electron current, comprising:

a source of electrons;

a conductive electron accelerating grid cage adjacent said source which cage may be suitably biased so as to accelerate electrons from said source into said cage to collide with and ionize gas molecules therein, said cage having a conductive wall substantially opaque to any x-rays generated by collision of said accelerated electrons with said cage, said wall having an aperture through which gas ions may be extracted from the interior of said cage, the mesh of said cage being sufficiently small so as to shield the interior of said cage from electron accelerating fields exterior thereto;

an annular extractor electrode exterior to said cage and adjacent and surrounding said aperture but insulated therefrom, which extractor electrode may be suitably biased to draw gas ions from said cage;

a thin rod-like collector electrode mounted external to said cage with the axis thereof directed toward said aperture, which collector electrode may be suitably biased to collect gas ions drawn from said cage by said extractor electrode; and

a hollow deflector electrode positioned adjacent to but insulated from said extractor electrode for receiving gas ions drawn from said cage by said extractor electrode, said deflector electrode surrounding said collector electrode so as to deflect gas ions toward said collector electrode upon suitable biasing of said deflector electrode, said deflector electrode having at least two interior surfaces which converge away from said aperture to form a relection trap for soft x-rays, the interior surfaces of said deflector electrode being so angled that most soft x-rays coming from said cage through said aperture and striking said deflector electrode suffer multiple reflections in said reflection trap before striking said collector electrode, thereby substantially reducing the intensity of soft x-rays striking said collector electrode and the photo-electron current generated thereby.

2. An ion gauge as defined in claim 1 wherein said deflector electrode includes a cylindrical inner surface and an inner surface in the shape of a truncated cone, said cylindrical and truncated cone surfaces both being coaxial with said collector electrode opposing one another and converging away from said aperture to form a reflection trap.

3. An ion gauge as defined in claim 2 wherein said collector electrode protrudes from the apical end of said truncated cone surface and said cylindrical surface surrounds said cone surface.

4. An ion gauge as defined in claim 3 wherein said cylindrical surface has a larger diameter than said aperture and at the end thereof adjacent said aperture tapers inward to a diameter substantially the same as said aperture.

5. An ion gauge as defined in claim 1 wherein the soft x-ray reflecting surfaces of said deflector electrode have an x-ray reflection coefficient of less than 0.5.

6. An ion gauge as defined in claim 1 wherein said annular extractor electrode has an inside diameter smaller than said aperture and smaller than the diameter of said hollow deflector electrode adjacent thereto.

7. An ion gauge as defined in claim 6 wherein said annular extractor electrode has a v-shaped cross-section with the apex thereof directed inward.

8. An ion gauge as defined in claim 1 wherein said wall has a truncated conical region surrounding said aperture directed away from said cage.