US3582710A - Ultrahigh vacuum magnetron ionization gauge with ferromagnetic electrodes - Google Patents

Abstract

A magnetron ionization gauge having the capability of measuring ultrahigh vacuum of the order of 2 X 10 16 torr with high sensitivity and conversion constant is formed with a hollow cylindrical anode and an electron-containment ferromagnetic plate at either end of the anode. An ion-collector is centrally located within a cylindrical anode and extends the entire length thereof. An electron-emitting cathode is located off center between the ion-collector and the anode wall and parallel to the longitudinal axis. An ion-collector shield extends the entire length of the ion-collector and is interposed between the ion-collector and the electron-emitting filament. The device is biased well beyond cutoff to provide restricted curvilinear paths for ionizing the electrons circulating around inside the anode.

Description

United States Patent [72] Inventor Louis J. Favreau Elnora, N.Y. [21] Appl. No. 826,815 [22] Filed May 22, 1969 [45] Patented June 1,1971 [73] Assignee General Electric Company [54] ULTRAHIGH VACUUM MAGNETRON IONIZATION GAUGE WITH FERROMAGNETIC ELECTRODES 9 Claims, 1 Drawing Fig.

[52] U.S.Cl 315/108, 313/7, 313/156, 313/157, 324/33 [51] hit. Cl 1101i 7/16, HOlj 17/22, GOln 27/62 [50] Field ofSearch 324/33; 315/108,111;313/7,153,l56,157,162,231; 230/69 [56] References Cited UNITED STATES PATENTS 3,051,868 8/1962 Redhead 315/108 3,172,597 3/1965 Guyot 3l3/7X TO END PLATES AND SHIELD WA lee 3,387,175 6/1968 Lloyd et al 315/108 3,495,127 2/1970 Lafferty 315/108 3,505,554 4/1970 Vekshinsky eta 313/157 ABSTRACT: A magnetron ionization gauge having the capability of measuring ultrahigh vacuum of the order of 2 X 10 ton with high sensitivity and conversion constant is formed with a hollow cylindrical anode and an electron-containment ferromagnetic plate at either end of the anode. An ion-collector is centrally located within a cylindrical anode and extends the entire length thereof. An electron-emitting cathode is located off center between the ion-collector and the anode wall and parallel to the longitudinal axis. An ion-collector shield extends the entire length of the ion-collector and is interposed between the ion-collector and the electron-emitting filament. The device is biased well beyond cutoff to provide restricted curvilinear paths for ionizing the electrons circulating around inside the anode.

PATENTEUJUN 1 197+ TO END PLATES AND SHIELD VIA LEAD 42 IN VE NTOR:

LOU/8 .1. F4 VRE u y HIS ATTORNEY ULTRAIIIGI-I VACUUM MAGNIJTRON IDNIZATIQN GAUGE WITH FERROMAGNE'IIC ELECTRODES The present invention pertains to improvements in magnetron ionization gauges and, more particularly, to such devices which are capable of operating in the ultrahigh vacuum region and which are adapted to operate with a very high conversion factor to record extremely low pressures.

As is known in the art, ionization gauges are utilized to measure low gas pressures by measuring the number of positive ions created by the collision of electrons with gas molecules present within an anode-cathode space. The positive ions are attracted to a negatively biased ion-collector electrode and the current created thereby is a measure of the gas pressure within the device.

US. Pat. application Ser. No. 708,124, entitled Ultra-High Vacuum Magnetron Ionization Gauge with Ion-Collector Shield," filed Feb. 26, 1968 by J. M. Lafferty and of common assignee, discloses a magnetron ionization gauge with improved operation and the ability to measure pressures as low as torr. Devices in accord with that invention generally comprise a cylindrical anode, a pair of electron-containment end members, a longitudinally centered semicylindrical ioncollector electrode, an offcentered longitudinal electronemitting filament between the ion-collector electrode and the inner wall of the anode electrode and a shield electrode member interposed along the axis of the anode electrode between the ion-collector and the electron-emitting filament. Improved sensitivity and a higher conversion factor is achieved by operating the device well beyond cutoff and by applying a more positive potential to the shield electrode than that which is applied to the ion-collector to cause high energy electrons in the circulating space charge to be intercepted by the shield electrode, thereby reducing electron currents which would otherwise counteract the currents due to the attraction of positive ions and tendency to render the gauge inaccurate.

While the invention described in the aforementioned patent application provides a great improvement over prior art ionization gauges, the demands of an ever-expanding technology require the development of gauges with still further improved low pressure measuring abilities.

It is, therefore, an object of the present invention to provide a magnetron ionization gauge with increasedv sensitivity, reduced X-ray photoemission, and a higher conversion factor so that there are higher measurable currents per unit of pres.- sure measured.

Another object of the invention is to provide a magnetron ionization gauge having a more uniform magnetic field along the entire length of the ion-collector electrode.

Briefly stated, the present invention provides a magnetron ionization gauge including a cylindrical anode, a pair of ferromagnetic electron-containment end members therefor, a longitudinally centered, semicylindrical ion-collector electrode along the length dimension thereof, an offcentered, longitudinal electron-emitting filament between said ion-collector electrode and the inner wall of the anode electrode, and a shield electrode member interposed along the axis of the anode electrode between the ion-collector electrode and the electron-emitting filament. In operation, the gauge is biased with voltages and a longitudinal magnetic field such as to cause the device to operate well beyond cutoff, further minimizing the possibility of X-ray photoemission. Additionally, the ferromagnetic electron-containment end members provide a more uniform magnetic field along the axis of the ion-collector electrode thereby increasing the measurable current per unit of pressure measured. Additionally, by the introduction of ferromagnetic electron-containment end members, there is an attendant increase in the magnetic flux density which permits reduction in the magnitude of the longitudinal magnetic field required for the device.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the following detailed description, taken in connection with the appended drawing.

The FIGURE is a vertical cross-sectional view of a magnetron ionization gauge constructed in accord with the present invention.

In the FIGURE there is illustrated a magnetron ionization gauge, represented generally as 10, including an hermetically sealable envelope 11, comprising a flanged, cylindrical member 12, closed at one end by an apertured metal end plate 13 and at the other end by an apertured annular end plate 14 into which a tubulation 48, which connects gauge 10 to a system to be monitored, is fitted. Within envelope 11, the operative elements of the gauge include a hollow cylindrical anode member 15, a pair of electron-containment members or plates 16 and H7 juxtaposed at either end of, and not in contact with, anode 15, a thermionically emitting filament 18,0ffset from the longitudinal axis of anode 15, a semicylindrical ion-collector member 19 extending the entire length of the anode electrode and symmetrically located about the longitudinal axis thereof, and a shield electrode member 20, also of semicylindrical configuration, juxtaposed between ion-collector l9 and thermionic filament l3 and concentric with the longitudinal axis of the anode member 15.

Electron containment-end plates 16 and 17 are preferably circular discs of ferromagnetic material such as pure iron; however, it is understood that other configurations or materials are contemplated, all within the scope of this invention. For example, other ferromagnetic materials such as cobalt, nickel, gadolinium, or alloys of any of these materials includ ing iron could likewise be used. It is understood, of course, that the foregoing list of materials is merely for purposes of illustration and not by way of limitation. Obviously, other ferromagnetic materials could likewise be used. The end plates 16 and 17 are mechanically and electrically secured to shield electrode 20. Anode electrode 15 is supported upon a pair of anode support members 2i and 22. Support member 21 is connected to a lead member 23, which passes through a leadin bushing 24 and is hermetically sealed therethrough to the exterior of the device. Support member 22 is connected to a lead member 29 which passes through a lead-in bushing 30.

Lead-in bushing 24 comprises a longitudinally apertured ceramic member 25, having a bore therein sufficient to accommodate lead member 23 without making contact thereto. An inner, metallic flanged member 26 is hermetically sealed through an aperture in annular end plate M and surrounds insulator 25 and is sealed thereto by conventional metalceramic technique at flanged end 27 thereof. At the other end of insulator 25, an end cap member 28 is sealed in conventional ceramic-to-metal hermetic seal to insulator 25 and is hermetically sealed as by brazing, or otherwise, to lead member 23 as it exits from the bushing. Lead-in bushing 30 is similarly constructed.

loncollector electrode 19 is supported from a lead member 35 which passes through a lead-in bushing 36 including insulator 37, inner flanged seal 38, hen'netically sealed to insulator 37 at flanged end 39, and exterior flanged seal 40, which is in hermetic seal with the outer end of insulator 37 and is brazed or otherwise hermetically sealed to lead member 35 as it passes through the bushing. End members 16 and 17 are interconnected with shield electrode 20 and are supported on a lead support member 41 which is connected to a lead member 42, which is sealed through end member M- by a bushing 43 and by another diametrically opposed support, not shown. Filament I8 is electrically and mechanically connected to the upper anode end plate member l6 and passes through an aperture in the lower end plate member 17 and is supported upon a support member 44. Support member 44 is connected to a lead member 45 which is hermetically sealed through the envelope end member 14 through a bushing 46.

Each of the bushings 24, 3t), 36, 43, and 46 are so constructed that the lead wire passing therethrough makes no contact with the insulating member. Similarly, the inner portions of the bushing seals are so constructed that no contact is made with the inner portion of the insulator member and hermetic seal is made thereto only at the outer flanged end. Both of these measures greatly increase the insulating characteristic of the bushing, in that the bushing provides a long insulating surface path between the lead passing therethrough and the metallic member of the bushing exposed within the device, to prevent covering of the surface of the insulator and the short circuiting of the electrode leads passing therethrough.

The materials from which the ionization gauge is constructed are conventional; for example, they may be made of molybdenum, stainless steel or any other suitable nonmagnetic refractory metal, except end members 16 and 17 which, as previously described, are made of ferromagnetic material. The filament 18 is preferably a lanthanum boride coated rhcnium substrate such as is disclosed in US. Pat. No. 3,312,856.

Dimensionally, anode electrode may have an inner diameter of approximately one inch and a longitudinal length of approximately one and one-eighth inches. The circle described by the exterior configuration of the semicylindrical ion-collector 19 and shield electrode 20 may conveniently have an outside diameter of approximately 0.25 inch. Thermionic cathode 18 may conveniently be a 0.008 inch rhenium wire, lanthanum boride coated, and is geometrically located at approximately half the distance between shield electrode member 20 and the inner surface of anode electrode 15. Thus, for example, the center of the filament, in the dimensions mentioned above, would be three-sixteenths inch from either of shield electrode 20 and anode electrode 15. The electroncontainment end plates 16 and 17 may conveniently have a diameter of one inch and a thickness of one-sixteenth inch for the embodiment illustrated. However, the diameter of the end plates 16 and 17 is primarily dependent on the diameter of the anode electrode. The thickness of the end plates while not critical, may vary slightly with the type of material used. For example, slightly thicker end plates may be used if cobalt or nickel and their alloys are used instead of pure iron.

Voltage means are provided by a voltage supply, generally indicated as 50 and comprising, for example, a battery 51 and a voltage dividing resistor 52. Anode electrode 15 may conveniently be operated at a potential of approximately 300 volts positive, with end plate members 16 and 17 and shield electrode 20 operated at a potential of 0 volts, and ion-collector electrode 19 at a potential of approximately 100 volts negative. The upper end of filament 18 is connected to upper end plate member 16 at 0 voltage. The lower end of the filament is connected to battery 51 through a lead 45 causing a voltage of approximately 3 volts to be impressed on filament 18. A strong longitudinal magnetic field, represented by the arrow H, of approximately 600 oersteds, for example, is applied within the anode to cause magnetron action. This field, generated by either an electromagnetic or a permanent magnet appropriately positioned around the ionization gauge, together with the applied electric field is approximately four times that necessary to bias the device to cutoff. An ion-current measuring meter 53 is connected in the ion-collector circuit through a lead member 35 and is used as an indicator of collector ion current and, hence, is a measure ofgas pressure.

in operation, filament 18 is operated at a very low temperature of approximately 650 to 800 C. This is possible because of the lanthanum boride cathode which is an excellent electron emitter at relatively low temperatures. The electrons emitted from the electrode 18 initially tend to be accelerated to the anode 15; however, due to the crossed electric and magnetic fields existing within the interaction space therein, the electrons execute extended curvilinear paths, generally cycloidal or helicoidal in shape and circulate about the filament in a spiral path creating a negative space charge. Due to the very high magnetic field, electrons are precluded from approaching too close to the anode.

The device described in the aforementioned patent application by James M. Lafferty, adjusted the magnetic field and cathode-anode voltage so that the device operated far past cutoff to reduce the probability of electrons impinging upon the anode. In addition to operating beyond cutoff, the instant invention utilizes the ferromagnetic end plate members 16 and 17 to create a more uniform magnetic field along the length of the ion-collector electrode 19 thereby permitting operation at even lower magnetic field intensities. Whereas the magnetic field along the longitudinal axis of the ion-collector 19 tends to have a lower value at the vicinity of the ends thereof when the end members 16 and 17 are not ferromagnetic, by the use of ferromagnetic end members 16 and 17 the present invention insures a substantially constant magnetic field along the entire length of the ion-collector 19. Several important advantages are thereby obtained.

One important advantage gained by the use of ferromagnetic end members 16 and 17 is that the cutoff current to the anode is greatly decreased. Performance tests for an ionization gauge constructed in accord with the instant invention have demonstrated a decrease in incidence of electron bombardment of the anode and hence a reduction in cutoff current of approximately 25 percent when compared with the device described in the aforementioned Lafferty patent application. As a result, there is a decrease in X-ray photoemission at the ion-collector, thereby greatly decreasing erroneous indications of positive ion currents.

A second advantage which results directly from the decrease in cutoff current to the anode, is a substantial increase in the sensitivity of the gauge. Whereas the gauge described in the aforementioned Lafferty patent application is capable of measuring pressures of the order of 10' torr with a conversion constant of approximately 0.5 ampere per torr, gauges in accord with the present invention are capable of measuring pressures of the order of 2.0 l0" torr with a conversion constant of approximately 0.8 ampere per torr. Accordingly, in addition to the increased sensitivity, the ioniza tion gauge of the present invention also has an improved conversion constant. This results directly from the use of the ferromagnetic end members which provide a more uniform magnetic field along the longitudinal axis of the ioncollector, thereby increasing the efficiency of collecting ions at the ioncollector electrode 19.

In accord with another feature of the present invention, ionization gauges embodying the invention are more stable than prior art devices. It has been found that the electrons are used so efficiently in devices of the instant invention that an increase or a decrease in electrons occasioned by a change in emission from the filament does not affect the ion current appreeiably. Accordingly, the instant invention has converted a previously critical parameter of ionization gauges to one which no longer requires critical control.

In accord with another feature of the present invention, it has been found that the relative position of a permanent magnet or electromagnet used to create the magnetic field surrounding the ionization gauge is no longer as critical as in prior art devices.

In accord with still another feature of the present invention, it has been found that the ferromagnetic end plates may be used to achieve special magnetic field variations. This is readily accomplished by varying the thickness or shape of the ferromagnetic end plates so as to change the magnetic flux density along the axis of the ion-collector. This feature is particularly useful where it may be necessary to compensate for the effects of extraneous magnetic fields which may have a tendency to alter the desired magnetic field along the length of the ion-collector.

From the foregoing, it may be appreciated that there is described an improved magnetron ionization gauge having the capability of measuring ion pressures down to as low as 2X10 torr and having high conversion constants measured in amperes per torr of gas measured. This is achieved by utilizing a longitudinal ion-collector along the axis of a hollow cylindrical anode electrode with ferromagnetic electron-containment end members to provide a more uniform magnetic field along the entire length of the ion-collector electrode. ln further accord with the invention, there is provided a shield electrode for the ion-collector at a potential intermediate that of the ion-collector and anode to remove, from the circulating electron space charge, abnormally high energy electrons to preclude the bombardment of the ion-collector electrode thereby and the consequent diminution of the apparent positive ion current. Additionally, there is provided an offcenter, low temperature filament, shielded from the ion-collector to preclude erroneous ion currents and adapted to supply the necessary electrons to provide for ionizing electrons within the device.

While the invention has been set forth herein with respect to certain particular embodiments and examples, many modifications and changes will readily occur to those skilled in the art.

What I claim as new and desire to secure by Letters Patent of the U.S. is:

1. An ionization gauge adapted to measure ultrahigh vacua at high sensitivity and stability and comprising:

an hermetically sealable envelope adapted to be attached to a vacuum system;

a hollow cylindrical anode electrode within said envelope;

a pair of ferromagnetic end members disposed in spaced relationship to one another at opposite ends of and insulated from said anode electrode;

an ion-collector electrode disposed adjacent the longitudinal axis of said anode electrode extending substantially the entire length thereof and parallel thereto;

a thermionic filament disposed within said anode electrode space substantially parallel to the longitudinal axis thereof, extending substantially the entire length thereof, and offset from the longitudinal axis of said anode electrode;

a shield electrode disposed parallel to the longitudinal axis of said anode electrode, on the opposite side of said axis from said ion-collector electrode, interposed between said ion-collector electrode and said thermionic filament, and extending substantially the entire length of said anode electrode;

bias means for applying a positive potential to said anode electrode, a first negative potential to said ion-collector electrode, and a second substantially less negative potential to said shield electrode;

means for providing a longitudinal magnetic field to said device sufficient, in connection with the radial electric field existing between said ion-collector electrode and said anode electrode, to bias said device well beyond cutofi' so that the magnetic field applied is much greater than that just necessary to prevent electrons from reaching said anode electrode; and

said applied potentials being of a magnitude as to cause electrons within said anode space to describe elongated curvilinear paths about said ion collector and for electrons achieving abnormally high energies to be attracted to said shield electrode and removed from said space.

2. The gauge of claim 1 wherein said ferromagnetic end members are made of iron.

3. The gauge of claim 1 wherein said ferromagnetic end members are alloys of iron.

4. The gauge of claim 1 wherein said ferromagnetic end members are mechanically and electrically supported by said shield electrode.

5. The gauge of claim 2 wherein said ferromagnetic end members are mechanically and electrically supported by said shield electrode.

6. In combination with a magnetron ionization gauge of the type having a longitudinally extending ion-collector electrode centrally positioned within a cylindrical anode and an electron emitting cathode located offcenter between said ion-collector and said anode and a shield electrode interposed along the axis of the anode electrode between the ion-collector electrode and the electron-emitting electrode, the improvement comprising:

a ferromagnetic electron-containment member adjacent one end of said anode electrode. I 7. The combination recited in claim ll wherein said ferromagnetic electron-containment member is mechanically and electrically secured to said shield electrode.

8. The combination recited in claim ll comprising a second ferromagnetic electron-containment member adjacent the other end of said anode electrode.

9. The combination recited in claim 5 wherein said second electron-containment member is mechanically and electrically secured to said shield electrode.

Claims (9)

1. An ionization gauge adapted to measure ultrahigh vacua at high sensitivity and stability and comprising: an hermetically sealable envelope adapted to be attached to a vacuum system; a hollow cylindrical anode electrode within said envelope; a pair of ferromagnetic end members disposed in spaced relationship to one another at opposite ends of and insulated from said anode electrode; an ion-collector electrode disposed adjacent the longitudinal axis of said anode electrode extending substantially the entire length thereof and parallel thereto; a thermionic filament disposed within said anode electrode space substantially parallel to the longitudinal axis thereof, extending substantially the entire length thereof, and offset from the longitudinal axis of said anode electrode; a shield electrode disposed parallel to the longitudinal axis of said anode electrode, on the opposite side of said axis from said ion-collector electrode, interposed between said ioncollector electrode and said thermionic filament, and extending substantially the entire length of said anode electrode; bias means for applying a positive potential to said anode electrode, a first negative potential to said ion-collector electrode, and a second substantially less negative potential to said shield electrode; means for providing a longitudinal magnetic field to said device sufficient, in connection with the radial electric field existing between said ion-collector electrode and said anode electrode, to bias said device well beyond cutoff so that the magnetic field applied is much greater than that just necessary to prevent electrons from reaching said anode electrode; and said applied potentials being of a magnitude as to cause electrons within said anode space to describe elongated curvilinear paths about said ion collector and for electrons achieving abnormally high energies to be attracted to said shield electrode and removed from said space.

2. The gauge of claim 1 wherein said ferromagnetic end members are made of iron.

3. The gauge of claim 1 wherein said ferromagnetic end members are alloys of iron.

4. The gauge of claim 1 wherein said ferromagnetic end members are mechanically and electrically supported by said shield electrode.

5. The gauge of claim 2 wherein said ferromagnetic end members are mechanically and electrically supported by said shield electrode.

6. In combination with a magnetron ionization gauge of the type having a longitudinally extending ion-collector electrode centrally positioned within a cylindrical anode and an electron emitting cathode located offcenter between said ion-collector and said anode and a shield electrode interposed along the axis of the anode electrode between the ion-collector electrode and the electron-emitting electrode, the improvement comprising: a ferromagnetic electron-containment member adjacent one end of said anode electrode.

7. The combination recited in claim 1 wherein said ferromagnetic electron-containment member is mechanically and electrically secured to said shield electrode.

8. The combination recited in claim 1 comprising a second ferromagnetic electron-containment member adjacent the other end of said anode electrode.

9. The combination recited in claim 5 wherein said second electron-containment member is mechanically and electrically secured to said shield electrode.

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