US3051868A

US3051868A - Ionization vacuum gauges

Info

Publication number: US3051868A
Application number: US52426A
Authority: US
Inventors: Redhead Paul Aveling
Original assignee: National Research Council of Canada
Current assignee: National Research Council of Canada
Priority date: 1960-08-29
Filing date: 1960-08-29
Publication date: 1962-08-28
Anticipated expiration: 1979-08-28
Also published as: GB931572A; GB914324A

Description

P. A. REDHEAD 3,051,868

IONIZATION VACUUM GAUGES Aug. 28, 1962 Filed Aug.29, 1960 2 Sheets-Sheet 1 ION CURRENT MEASUREMENT APPARATUS INVENTOR PAUL AVEL/NG REDHEAD PATENT AGENT United States Patent 3,051,868 IONIZATION VACUUM GAUGES Paul Aveliug Redhead, Ottawa, Ontario, Canada, assiguor to National Research Council, Ottawa, Ontario, Canada, a corporation of Canada Filed Aug. 29, 1960, Ser. No. 52,426 21 Claims. (Cl. 315108) This invention relates to cold cathode vacuum gauges of the type employing an axial magnetic field crossing a discharge path between coaxially mounted anode and cathode electrodes, and particularly concerns improvements in electrode configuration and arrangement with respect to an ionization space.

The present application is a continuation in part of my application Serial No. 741,367, filed June 11, 1958, for an invention in Vacuum Pump and Gauge for Measurement of High Vacuum, now abandoned.

The present invention relates to a class of cold-cathode ionization vacuum gauges having axial magnetic and radial electric fields such as are described in my copending application Serial No. 657,624, now Patent 2,937,295. In the specification of this patent there is described a structure including an elongate rod-like anode centered on the axis of a cylindric ion collector shell, the latter being shielded at its ends from the anode by a pair of auxiliary cathodes having the form of separate apertured discs or short cylinders coaxial with the anode. The arrangement resembles a typical magnetron except that the anode is set at the center and the electron trajectories have a radial inward component of motion. When a high electric field is produced in the space between the ends of the rod anode and the aperture of a cathode, field emission current is drawn from the margins of the aperture assisting in maintaining a discharge. The electrode arrangements described provide shielding for the ion collector so that field emission current is not drawn from the latter and hence does not mask the positive ion current to the ion collector. This provision has made possible the measurement of ultra-high vacuum down to the lower limits of existing current measuring apparatus.

The present invention seeks to greatly improve the sensitivity of a vacuum gauge of the type referred to by disposing the anode externally of an ion-collector cathode which has radially extending end flanges, and providing auxiliary cathodes in shielding relation with the end flanges to prevent field emission current from being drawn from the ion-collector cathode.

The present invention also seeks to improve the efliciency and accuracy of Penning type cold-cathode gauges having an anode and an ion-collecting cathode, by forming the portions of the cathode extending nearest the anode with highly polished rounded surfaces, so that in the absence of any shield electrode the electric field gradient at the cathode surfaces remains below the value required to draw out electrons from the metal by field emission.

According to the invention, a first expression thereof is embodied in a structure consisting in a tubular anode electrode and an electrically separate main cathode having a tubular portion with a common axis and forming an annular ionization space with said anode, the cathode having radial extensions spaced apart and bounding the ends of the space and extending closer to the anode than the tubular portion, and a pair of tubular auxiliary cold cathice ode bodies interposed as shields to reduce to a low value the electric field gradient at the margins of the radial extensions adjacent the anode when a potential difference of sufficient magnitude exists between the anode and the cold cathodes to draw current from the latter by field emission.

Also according to the invention, a vacuum gauge comprises .a tubular anode electrode, an electrically separate cathode having a tubular portion and an axis common with the anode to form an annular ionization space between them, the cathode having radial flanges integral with the tubular portion and axially spaced along it and extending toward the anode but being radially spaced therefrom to substantially close the ends of the chamber, the flange margins having toroidal form and being highly polished over the surfaces adjacent the anode to minimize field emission therefrom when a high potential difference exists between anode and cathode.

In either expression of the invention, when an axial magnetic field is applied through the ionization space conjointly with a radial electric field between cathode and anode, substantially no field emission current is drawn from the cathode, so that a current measuring device connected in series therewith to the negative terminal of a source of electrical supply measures substantially only positive ion current.

The invention in its several specific embodiments will be more particularly described hereinafter With reference to the accompanying figures of the drawing, in which,

FIG. 1 is a longitudinal axial cross-section of a gauge constructed according to the invention having a pair of magnet poles associated externally of the electrode structure;

FIG. 2 is a longitudinal cross-section of the device of FIG. 1, taken on the line 22;

FIG. 3 is a cut-away perspective view showing the electrode support arrangements for an electrode assembly prior to its enclosure in an envelope;

FIG. 4 is a schematic diagram showing the ionization gauge in circuit relation with a source of biasing potential and connected to measure positive ion current drawn by the ion-collector cathode, the gauge including a starting filament;

FIG. 5 is an axial cross-section of an alternate form of an ionization gauge having auxiliary shield cathodes;

FIG. 6 shows in axial cross-section a form of ionization gauge having polished cathode surfaces adjacent a rod anode to avoid electron emission current; and

FIG. 7 shows an ionization gauge similar to FIG. 6 but having the anode external to the cathode.

A magnetron type of vacuum gauge and ionization pump as represented in FIGURES 1, 2, 3, and 4 of the drawing comprises an electrode assembly supported on a press 28 which is mountable within an envelope 27 adjacent one end thereof. The envelope has two opposite walls or sides flattened and parallel, and an open end portion 24 adapted to be connected with a space to be evacuated. A unidirectional magnetic field is developed transversely of the flattened walls to pervade the space occupied by the electrode assembly, by means of the

opposed pole pieces

20 and 21 of a magnet.

The anode electrode has the form of a cylindric metal shell 10 having ends lying in parallel planes normal to its axis, and is preferably perforated or of mesh construction to facilitate movement of a gaseous substance through the cylindric surface. The shell is supported so as to reduce leakage currents along support structure when a high anode potential is applied. This structure comprises a suspension including a short stiff Wire 37 sealed into a vitreous extension 42 of the press 28, and a short stiff leadin wire 38 attached to the shell at a point approximately diametrically opposite the wire 37 and sealed into the tubulation 26. Lead-in 38 extends out of the envelope and serves as a conductor for the application of high positive potential to the shell anode.

An ion-collector cathode electrode includes a portion 11 having tubular form, and a diameter which is less than that of the shell 10, and is mounted within the latter in coaxial relation to provide an annular ionization space 22 between the electrodes. The ends of the cathode tubular portion extend axially beyond the planes of the ends of the anode shell, and support respective

parallel discs

12 and 13 coaxial with the common axis of the anode and cathode. The radii of the end discs are substantially equal to the radius of the anode shell, and the outer edges of the discs are preferably rounded.

A pair of apertured metal discs .14 and 15 serving as auxiliarly cold cathodes, having their outer peripheral edges formed as rounded flanges, are disposed to lie respectively between an end of the anode shell and a

cathode end disc

12 and 13, and coaxially therewith. The auxiliary cathodes have outer diameters greater than the diameter of .either the anode or the cathode end discs, and the diameters of their

circular apertures

18 and 19 are less than the diameter of the anode shell so that a major portion of the surface of

discs

12 and 13 is exposed to space 22.

End discs

12 and 13 are shielded from anode by virtue of their nested relation with respect to the auxiliary cathodes, which curve around the edges of the respective cathode discs. By the configuration of electrodes described, when a high positive potential is applied to the anode with respect to the auxiliary cathode, the electric field in the space between an auxiliary cathode such as 14 and a cathode end disc such as 12 remains at a low value. Accordingly, no electrons are drawn from the cold surfaces of the cathodes, while a current may be drawn by field emission from the auxiliary cathode margins. The latter are spaced relatively near to the ends of the anode shell, so that electrons pass across the

gaps

16 and 17 at opposite ends of the shell.

A number of slender metal or ceramic support rods 29 are suitably aflixed upon the outer peripheral edges of auxiliary cathodes 14 and to act as struts spacing them apart, and to assist in providing a rigid structure. Each of the auxiliary cathode discs is supported from respective stiff wire leads 39, -31, extending from the press 28.

The outwardly turned flanges of

auxiliary cathodes

14 and 15 may be disposed very close to the inner surface of envelope 27, and preferably are in conducting contact with a metallic deposit 43 applied to the flattened portions of the envelope wall over an area thereof at least coextensive with the lateral extent of the electrode assembly. Spring metal tabs 39 are welded by their one ends to the auxiliary cathodes and press their other ends in contact with the grounded layer 43. The latter is most conveniently realized as a thin film of platinum metal deposited upon the envelope wall by known techniques of reducing a platinum salt applied in a volatile vehicle, as by baking a coating of such substance. A suitable preparation is that sold under the trade name Liquid Bright Platinum, supplied by the Baker Platinum Company, containing a reducible platinum salt in oil-of-lavender.

The envelope 27 may be realized in any suitable material impervious to gaseous matter but pervious at least over those parts of the walls adjacent the cathodes to magnetic field. One material found to be satisfactory for the purpose is a borosilicate glass known as Pyrex 7740, made by the Coming Glass Company. The envelope may be fabricated in any suitable form, either as a scalable vessel or as a pump which may be connected by its tube extension 24 to a fore-pump or system to undergo evacuation. When very low pressures are to be attained, as for instance when a space is to be evacuated to residual pressures of the order of 10' mm. Hg, special care must be taken in constructing all seals and entries to minimize leaks. For example, clean tungsten wires are preferably employed as leads in press. 28, and as anode lead 38, these being sealed to intermediate glass bonding bodies of Nonex glass, as provided by the Corning Glass Company, these bodies being in turn fusion sealed into the Pyrex 7740 glass envelope.

While a number of metals and alloys might be used as electrodes, it is preferred when working at the highest vacuum to form them of a stainless nickel-chromium alloy, known as Nichrome U. As initially assembled, the electrode structures are inevitably contaminated and require de-gassing by the usual procedures of induction heating of the metal parts and baking of the assembly in vacuum.

In an alternative form of gauge shown in FIGURES 3 and 4, provision is made for the liberation of electrons within an ionization space 22 from a thermionic filamentary cathode 35 arranged to be heated by application of suitable voltage across its ends, so that when other electrodes are jointly connected with the positive terminal of a source of direct current of suitable voltage, a space discharge current may be drawn from the cathode. The filamentary cathode is supported by its ends from relatively

thicker wires

34, 36, which are secured by their ends as by welding to opposed inner faces of

auxiliary cathodes

14, 15. Filament 35 may be formed by reducing the diameter of a length of resistance -wire bent into U-shape, for example, a tungsten wire of 0.015 inch diameter having the intermediate part of the U reduced as by etching to about 0.003 inch diameter. The reduced portion is spaced closely to the tubular portion 11 of the cathode, for example about 0.050 inch therefrom. Struts 29 are preferably realized either as high resistance alloy metal wires or as ceramic rods, to avoid placing a short circuit across the ends of the filamentary cathode.

During the initial outgassing of the volume of envelope 27, a suitable vacuum pump or pumps are connected to the neck 24. Filament 35 is caused to be heated to electron emissive temperature by a current passed through it under low voltage, which is applied by connecting the lower auxiliary cathode 15 by means of switch S shown in FIGURE 4 to the conductor '41, which is at a selected potential above ground on the high voltage supply B. The anode shell 10 and the

cathode portions

11, 12, and 13 are held at a positive potential of about volts, by connecting

leads

32, 33, and 38 to a point on supply B. The magnet body is preferably removed during the outgassing steps.

Heating current need not be applied longer than is required to initiate space discharge current, since at pressures of about 10- mm. Hg the space current quickly builds up so that switch S may be moved back to ground the pair of auxiliary cathodes, the filament being kept hot by the diode current and ionization discharge across space 22.

When the gauge is employed in the measurement of low pressures, the electrodes are connected as shown in FIGURE 4, wherein an ion current measurement apparatus 23 is connected in series between the negative terminal of supply B and the cathode. A magnetic unidirectional field is applied in an axial direction throughout ionization space 22, as indicated by the array of arrows, and a radial electric field is provided by connecting a high positive potential with anode shell 10 and grounding the

auxiliary cathodes

14, 15. Electrons which appear Within space 22, however they may originate, are accelerated into orbital paths around the tubular cathode 11. The orbit radii tend to be constant, except when collisions with particles provide for deflection into an orbit of larger radius, so that as a consequence of a very large number of ionizing collisions some electrons eventually reach anode 10. Due to the relatively greater mass of positively charged particles produced in the ionization space 22, these ions move more readily inwards to give up their charges to the tubular cathode 11, which is substantially at the potential of the auxiliary cathodes. A positive ion current of low magnitude is measured by apparatus 23, giving an indication of the absolute pressure. Observations have revealed that -a remarkably linear relationship of pressure and current holds throughout a range from about mm. Hg to the limit of current measurement devices, at pressures estimated to be less than l0 mm. Hg.

The gauge is additionally useful as a pump, and has a sensitivity greater by about 20 times than that of the gauge described in my copending application. Using dry helium, in one instance a pumping rate was observed equivalent to about 0-.15 liter per second at l0 mm. Hg at 25 C.

One of the difficulties sometimes encountered with inverted magnetron forms of vacuum gauge is a long delay, often many minutes, which follows the application of operating voltages to the electrodes, before space 22 becomes ionized. In contrast therewith, the delay observed at equivalent pressure when using the device according to the present invention is remarkably reduced. Starting the gauge at still lower pressures may be still further improved by employing the filamentary cathode 35 in the circuit of FIG. 4, which is momentarily heated by operating switch S and promptly reconnecting auxiliary cathode with its mate 14. The release of thermally emitted electrons into space 22 serves to initiate the ionization phenomenon, which once begun is sustained by the continuing application of crossed electric and magnetic fields, so long as a sufficient population of gas molecules remains in the space 22.

The presence of the filamentary cathode does not detract from the pumping speed of the gauge or alter its accuracy as a generator of ion current proportional to pressure.

The lower limit of measurement of the gauge is substantially unaffected by any X-rays generated, despite the high potential applied to the anode, which in a typical instance will be about 6 kilovolts. Since the anode current and hence the X-ray intensity is a linear function of pressure, photon emission from the cathode does not amount to a significant fraction of the total ion current at any pressure.

In one embodiment constructed the applied field strength was of the order of 1200 gauss and the anode potential was 6 'kilovolts. The tubular portion of the cathode had a diameter of 3 mm, while the cathode end discs and the anode shell each had diameters of 30 mm. Such dimensions and their ratios are illustrative but not in any sense limiting, since various other dimensions may be chosen.

An alternative gauge may be constructed as shown in FIGURE 5, employing auxiliary cathodes having the form of

cylindric shell bodies

44 and 45, in shielding relation between the margins of

cathode end discs

12 and 13 respectively and the inner surface of anode shell 10'. The ionization space 22 is bounded at its ends almost entirely bythe cathode end discs, the projected areas of the shield bodies on a transverse plane being very small, so that ions may be collected with improved efliciency not only on the tubular cathode portion 11 but also through the

shield apertures

48 and 49 upon the exposed surfaces of

end discs

12 and 13.

The tubular portion 11 of the cathode is extended and joined with a stiff lead-in wire 33, the latter being sealed into an end wall 60 of envelope 27. Auxiliary cathode shell 45 is similarly sealed into the end wall coaxially with lead-in 33, and supports envelope 27 from its exterior cylindric face being joined with a radially inturned flange 66.

Envelope 27 comprises a tube having circular crosssection and is radially inturned at its other end to form flange 65. Auxiliary cathode shell 44 is sealed to flange 65 and also sealed on its inner face to out-turned flange 64 which extends from an axial neck 24 adapted to be connected to a vessel to be evacuated.

Shells

44 and 45 are rigidly supported from the envelope in spaced apart coaxial relation having cathode 11 symmetrically disposed between their opposed ends on a common axis.

An anode shell 10 having its ends lying in parallel planes normal to the common axis of the assembly is also coaxial with and uniformly spaced outwardly of the auxiliary cathodes, and has its ends extending in overlapping relation over the ends of the auxiliary cathodes. A leadin 58 is sealed through the envelope at the bead 37 intermediate the ends of the anode, which preferably is mounted in contact with the envelopes inner surface.

A permanent magnet 50 having the form of a cylindric shell having its ends oppositely poled is mounted in coaxial relation with the electrode assembly, to provide an axial magnetic field throughout ionization space 22, so that when positive voltage is connected with the anode and the auxiliary cathodes are grounded, an ion current may be measured by apparatus 23 connected between the cathode and ground.

The construction provides a very strong and rigid electrode support and a highly efficient ionization gauge in which electron trapping is efficient and ion collection is substantially complete.

The exclusion of field emission electron current from the cathode circuit may be eifected by means other than the interposition of a shield electrode between anode and cathode, and a vacuum gauge having useful accuracy for the measurement of pressures down to about 10 mm. Hg may be constructed essentially as shown in FIG. 6. In this embodiment the electrode assembly comprises a slender rod anode 10 coaxially located in a cylindric shell cathode 11 having

inturned end flanges

12 and 13 lying in planes perpendicular to the common axis. The flanges are uniformly spaced radially from the anode. The

inner margins

54 and 55 of the end flanges are formed to shape of a torus and that part of each of

surfaces

54 and 55 exposed to the anode is highly polished. The rod anode 10 is supported from inwardly formed press tubulation '57 which closes an end of an envelope 27. The latter comprises two separate tubular portions fixed respectively to the outer faces of

flanges

12 and 13 coaxially therewith. The tubular portion 56 is connectible by its open ended extension 24 with a space to be evacuated or whose vacuum is to be measured.

An axial magnetic field is applied by means of the cylindrical shell magnet body 50 coaxial with the cathode shell, and coextensive therewith. One end of the anode rod passes through the tubulation 57 and is connected in use with the positive terminal of a high voltage supply B having its negative terminal connected through current measuring apparatus 23 with cathode shell 11.

The radius of curvature of cathode surfaces 54, 55 is made large enough so that for a given radial spacing there is an operating potential difference between anode and cathode at which substantially no electron current is drawn from the cathode by field emission. Careful polishing, for example by known techniques of electropolishing of metal, is required to prepare the cathode surface so that its microstructure is suitable for the purpose, when an efiectively large voltage difference exists between anode and cathode. Such voltage should be at least as high as 2 kilovolts, and may be as high as 6 kilovolts or even higher. The radial spacing of the cathode end discs from the anode should not be so great that there is inefiicient trapping of electrons of the plasma generated in the ionization space 22, and a spacing of the order of 3 mm. is preferred, when the cathode shell has a tubular diameter of 3 mm.

The starting of the gauge, when crossed electric and magnetic fields have been applied, may be facilitated by applying to the tubular portion 11 at some point on its inner surface, an ionizing substance such as a beta ray emitter. The emitter may be located upon the end discs, but should generally not be placed anywhere upon the

curved surfaces

54, 55 to avoid roughening the electrode surface. However free electrons may occur in space 22, once they are free they are rapidly accelerated into orbits and cause a plasma to be generated by multiple collision processes. The

end discs

12, 13, being at the cathode potential, prevent migration of electrons out of the space and collect positive ions preferentially. Electrons in the plasma gradually migrate toward the anode while ions with their larger masses drift radially outwardly of the anode toward the cathode surfaces.

As a result, in an operating gauge, the current to the cathode is due almost entirely to collection of positive ions, such positive ion current component being much larger than any electron current component due to field emission, secondary liberation, and X-ray photons, except at extremely low pressures.

A diode type of cold cathode ionization gauge having the anode external to the cathode is shown in FIG. 7, wherein anode 10 is external to a coaxial tubular cathode 1-1. In this form of gauge the anode is supported between envolpe cap portions 60 and 6 1 whose respective

axial flanges

62 and 63 are sealed to the ends of the anode shell. The ends of the tubular portion 11 of the cathode are extended beyond the ends of the anode and are sealed coaxially therewith in the caps. The tubular portion of the cathode supports spaced

discs

12 and 13 adjacent the anode shell ends, to provide end closures for the ionization space 22 between the anode and the cathode, the discs being spaced relatively near to the inner surface of the anode shell. The radially

outer margins

54 and 55 of the discs are formed with toroidal ring surfaces, the radius of curvature being chosen in conjunction with the spacing between the anode and the nearest part of the disc, so that the electric field strength at the cathode surface is nowhere great enough to draw out electrons by field emission. To guard against field emission when anode 10 is connected .to a relatively high voltage, of the order of several kilovolts, the surfaces 54 and '55 must be polished carefully as has been described in connection with FIG. 6. The surface roughness which should not be exceded may be described as a profile whose irregularities do not have peak heights of more than about millionths of an inch. Scrupulous attention to cleanliness of the cathode surfaces is necessary in handling and assembling.

The tubular portion 11 has one end sealed through cap 61 'to provide a lead-in for couection with the negative terminal of the supply B, and connection is made with the positive terminal thereof to the outside of anode 10, a current-measuring apparatus 23 being connected in series with the negative terminal. The collection of positive ions at any part of the cathode while a plasma is maintained in space 22 serves as a measure of the density of gaseous matter. An axial magnetic field is provided by any suitable means, for example by the use of a cylindric shell magnet described for FIGS. 5 and 6, or of a U- magnet having spaced poles such as described for FIG. 1.

Anode is apertured over its surface so that the assembly may bodily be placed into a vessel or space whose degree of evacuation is to be determined. Alternatively, the assembly may be placed in an envelope of the form shown in FIG. 1 having a tubular neck adapted to be connected with a vessel. Any suitable tubular connection (not shown) may be led through a wall as convenient to connect with space 22.

I claim:

1. A high vacuum gauge comprising a tubular anode electrode and a cold cathode electrode having a tubular portion, said anode and said tubular cathode portion having unequal radii and being coaxially mounted to form an annular space, a pair of radial flanges extending from said cathode toward the anode and being spaced apart along said cold cathode and having marginal portions spaced uniformly nearest said anode in end closing relation with said annular space, means to apply an axial magnetic field throughout said space, bias means for said electrodes, current measuring means connected in series with said cold cathode and the negative terminal of said bias means, and means to limit the electric field strength adjacent the surfaces of said cold cathode to a value below that at which electrons may be drawn from said surfaces by field emission when said anode and said cathode are connected with said bias means.

2. A high vacum gauge comprising a tubular anode electrode and a cold cathode electrode having a tubular portion, said electrodes having a common axis and having an annular space between them, a pair of radial flanges extending from said cathode toward said anode and being axially spaced and parallel and having their margins spaced uniformly radially from said anode, means to apply an axial magnetic field through said space between said flanges, high voltage bias means for said electrodes, current measuring means connected in series between the negative terminal of said bias means and said cathode, and equipotential conductor means connected with said cathode adjacent said space for establishing an electric field strength at said flange margins which is below field emission strength when said anode and said cathode are connected to said bias means.

3. A high vacuum gauge as claimed in claim 2 wherein said equipotential conductor means comprises a polished curved metal surface layer integrally connected with said flanges, said layer having toroidal form.

4. A high vacuum gauge as claimed in claim 2 wherein said equipotential conductor means comprises a metal shield electrode disposed between a flange and said anode and connected with said negative terminal.

5. A high vacuum gauge comprising a tubular anode and a cathode having a tubular portion, said anode and said tubular portion having unequal radii and being coaxially mounted one within the other .to form an annular space, a pair of radial flanges extending from said cathode toward the anode and being spaced along said tubular portion and having formed margins spaced a constant radial distance from said anode, said flange margins nearest adjacent said anode being curved and polished to a toroidal surface of revolution, whereby to establish an electric field strength at said flange surfaces which lies below a value required to draw electrons therefrom by field emission when a radial electric field is developed between said anode and said cathode, said flanges trapping electrons gyrating in said annular space between them when an axial magnetic field is applied parallel to said axis through said space conjointly with said electric field, high voltage bias means for said electrodes, said bias means having a pair of polarized terminals, current measuring means, and circuit means connecting said cathode and said anode with respective bias terminals, said circuit including said current measuring means connected in series between said bias means and one of said anode and said cathode.

6. A high vacuum gauge comprising a cylindrical anode electrode and a cylindrical cold cathode electrode, said electrodes having a substantially common axis and being of unequal radius, means for applying a magnetic field parallel with said axis throughout an annular space between said electrodes, disc portio-ns of said cathodes spaced axially therealong and extending transversely of said axis toward the anode and having margins spaced equidistantly from the anode, high voltage bias means for applying a large dilference of electrical potential between 9 V 7 said cathode and said anode to produce a radial electric field in said space, field emission suppressor means for said cathode disc margins, said suppressor means comprising metal conductor bodies of revolution connected with the negative terminal of said bias means and held in spaced relation with said anode, whereby positive ion current flows to said cathode, and current measuring means for indicating positive ion current to said cathode.

7. A high vacuum gauge as in claim 6 wherein said disc portions have margins curved convexly toward said anode and said conductor bodies comprise polished surface layers of said curved margins.

8. A high vacuum gauge as in claim 6 wherein said metal conductor bodies comprise centrally apertured unitary disc shield cathodes spaced between the margins of said disc portions and said anode.

9. A high vacuum gauge as in claim 6 wherein said metal conductor bodies comprise cylindric shells spaced axially in said annular space and held coaxially with said anode, and being radially spaced between said anode and the margins of said disc portions.

10. A high vacuum gauge comprising a tubular anode and a tubular cathode, said anode and said cathode having unequal radii and being coaxially supported to forman annular space, a pair of integral radial extensions of said cathode spaced axially apart and spaced closer to said anode than the spacing of said tubular cathode, a pair of tubular auxiliary cathodes spaced axially apart and disposed coaxially for shielding said radial extensions from said anode and for supplying a field emission current to said anode when a suificiently high diiference of potential is applied therebetween, high voltage supply means for biasing said anode positively with respect to said cathodes, whereby when an axial magnetic field is applied through said annular space conjointly with the aplication of said potential positive ion current is drawn to said cathode, and means to measure said ion current connected between said cathode and the negative terminal of said supply.

11. A high vacum gauge comprising a cylindrical shell anode and a cylindrical shell cathode, said cylindrical shells having a substantially common axis and being radially spaced and electrically separate, an integral disc extension of said cathode being spaced radially nearest said anode, a shield electrode disposed in radially spaced relation between said anode and said disc extensions so that a major part of the surface of said cathode is unshielded with respect to said anode, whereby when a large difierence of potential is applied between said anode and said shield electrode a field emission current may be drawn to said shield and when a magnetic field is directed paanllel to said axis through said gauge a positive ion current may be drawn to said cathode from said anode as a measure of gas density, and means to measure said current.

12. A high vacuum gauge as claimed in claim 11 wherein a pair of disc extensions of said cathode are axially spaced apart to form axial boundaries for an ionization space between said anode and said cathode.

13. A magnetron device for the measurement of low pressures of gaseous matter comprising a cylindrical shell anode electrode having its edges lying in parallel planes normal to the cylinder axis, a cathode electrode of cylindrical form having a diameter less than the diameter of said anode supported coaxially therewith and having a pair of axially spaced radial extensions extending toward said anode and spaced from the anode, and a pair of cold auxiliary cathode electrodes formed as annular centrally apertured discs coaxial with and spaced in shielding relation between said extensions and said anode.

14. A magnetron device as claimed in claim 13 wherein said anode comprises a metal screen, said auxiliary cathodes comprise annular discs having their margins formed as flanges convexly curved toward said anode, and said cathode extensions comprise a pair of parallel end discs 10 having a diameter substantially equal to the diameter of said anode, each end disc being spaced from and nested in an adjacent auxiliary cathode.

15. A magnetron device as claimed in claim 14 further including a high voltage supply for biasing said anode positive with respect to said cathodes, means to apply a magnetic field through the device parallel with said axis, and current measurement means connected between said cathode and the negative terminal of said supply for indicating positive ion current.

16. An ionization vacuum gauge comprising an envelope pervious to magnetic field but impervious to gaseous matter adapted to be connected to a space to be evacuated and having an electrode support press adjacent one end, conducting wires sealed in said press and connected in supporting relation with a respective electrode of an electrode assembly therein, said assembly comprising a right cylindrical shell anode, a pair of cold auxiliary cathode electrodes comprising annular discs having their outer marginal edges flanged coaxially with and curved with respect to the edges of said anode to form therewith regions of high electric field gradient when a large diiference of potential exists between said anode and said auxiliary cathodes, a high voltage supply for biasing said anode positive with respect to said auxiliary cathodes, a cylindric cathode electrode centered coaxially in said anode and extending the length of said anode, said cathode having integral end discs spaced axially apart and having their marginal edges shielded from said anode within said flanges, and current measuring means connected between said cathode and the negative terminal of said supply 'for indicating positive ion current.

17. A gauge as claimed in claim 16 wherein said anode is supported jointly by a lead sealed in said press and by a further lead sealed into a wall of said envelope, and each auxiliary cathode disc has a separate electrode lead, and a filamentary conductor of refractory metal is connected between said auxiliary cathode discs and is spaced closely adjacent said cathode.

18. A device for use in measuring the pressure of a rarefied gas, in which a pair of field emission cold auxiliary cathodes, a cylindrical cathode electrode for collecting positive ions, and a cylindrical shell anode having a diameter different from the diameter of said cathode, are mounted coaxially with but separate from each other within an envelope to which gas may be admitted, in which the cathode has end disc extensions spaced axially apart and spaced from said anode, and in which the auxiliary cathodes comprise apertured disc structures spaced between the anode and said cathode extensions so that when a sufiiciently large potential is applied between said anode and said cathodes electrons are drawn by field emission from the auxiliary cathodes to the anode and so that when a magnetic field is applied parallel to the anode axis the electrons drift along and spiral within the anode between said cathode extensions, said cathode being arranged to collect positive ions produced by ionization from the gas but itself producing substantially no electrons by field emission processes, and current indicating means connected with said cathode to measure said collecting of positive ions.

19. A device as claimed in claim 18 wherein each auxiliary cathode disc has a separate electrode lead, and a filamentary conductor of refractory metal is connected between said auxiliary cathodes and has a portion of its length spaced closely adjacent said cathode.

20. A high vacuum gauge comprising an anode electrode defining an open ended space, a cold cathode electrode having spaced portions generally closing the open ends of said space, means for producing a magnetic field through said space in a direction generally transverse to said spaced cathode portions, bias means for said electrodes, current measuring means connected in series with said cold cathode and the negative terminal of said bias means, and means to limit the electric field strength and jacent the surfaces of said cold cathode to a value below that at which electrons may be drawn from said surfaces by field emission when said anode and said cathode are connected with said bias means.

21. A11 ionization vacuum gauge comprising a main cathode electrode and an anode electrode surrounding said m'ain cathode electrode and defining an ionization zone therebetween, means to apply a magnetic field throughout said zone, spaced portions of said main cathode extending transversely of said magnetic field toward said anode and defining boundaries of said zone, auxiliary cathode electrodes disposed nearest said anode in shielding relation with said main cathode portions, high voltage supply means biasing said anode positive with respect to said cathodes for impressing an electric field transversely of the magnetic field, the area of said main cathode exposed to said ionization zone being larger than the exposed area of said auxiliary cathodes, and means for indicating positive ion current flowing to the main cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,774,936 Beck et al Dec. 18, 1956