US2884550A - Ionization gauges and method of operation thereof - Google Patents

Description

April 28, 1959 J. M. LAFFERTY IONIZATION GAUGES AND METHOD OF' OPERATION THEREOF Filed Oct. 17, 1957 myn/Emana f77 Venter: dames /Vl La'efty, by 7Qv/ d United States Patent O IONFZATION GAUGES AND METHOD ,0F OPERATION THEREOF James M. Latferty, Schenectady, NX., assignor to General Electric Company, a corporation of New York Application October 17, 1957, Serial No. l690,848

8 Claims. (Cl. 3137) The present invention relates to improved ionization gauges and methods of operating the same to measure extremely low gas pressures.

ionization gauges are vacuum discharge devices generally including a thermionic cathods, an anode and a collector electrode. Thermally ejected electrons pass from the cathode to the anode. lf a gas is present, these electrons may undergo ionizing collisions with gas molecules. At low pressures, the probability of such collisions is proportional to the number of gas molecules present and, hence, to the gas pressure. Accordingly, in an ionization gauge, the positive ions created by such collisions are -collected by a negatively biased collector electrode. Collector electrode current is then a measure of gas pressure.

It has been found that if the electron paths are increased in length the number of collisions per electron may be increased, thus increasing the sensitivity of the gauge. In magnetron ionization gauges, van axial magnetic eld is imposed, causing electrons to spiral in the cathode-anode space, and at cut-Dif, just miss the anode and return to the cathode region. Magnetron ionization gauges operating upon this principle exhibit greatly irnproved sensitivity.

It has been found, however, that even `present-day magnetron ionization gauges may not be used to measure gas pressures lower than l-8 mm. This is due primarily to photoelectric emission by the collector electrode due to bombardment by soft X-rays. These soft X-rays are emitted at the anode when high velocity electrons are incident thereupon. Since photoelectric emission by the collector electrode is indistinguishable from positive ion collection, this phenomenon establishes an etfective lower limit to the pressure which may lbe measured by presentday ionization gauges.

Accordingly, it is an object of the present invention to provide improved ionization gauges and methods of operation capable of measuring, heretofore unobtainable, low pressures.

A further object is to provide a method of operating ionization gauges which greatly decreases background currents.

Another object of the present invention is to provide improved ionization gauges of such size and construction to enable them to function effectively at extremely high temperatures.

In accord with the present invention, pressures as low as approximately l013 mm. of mercury are measured utilizing a magnetron ionization gauge having a thermionic cathode, van anode and a negatively biased collector electrode. The value of applied axial magnetic eld and cathode-anode potential 'are adjusted so that the electrons emitted by the cathode just miss the anode and the tube operates under cut-soif conditions. To reduce background currents, the thermionic cathode is operated at a much lower temperature than heretofore utilized, and electron current is reduced a factor of 104 of that` convention'ally used in ionization gauges. At these 4low cur- 2,884,550 Patented Apr. 28, 1959 l2 rents the -amount of soft X-rays emitted at the `anode is markedly decreased, decreasing the background photoemission by the collector electrode and lowering the minimum pressures measurable by the device.

This method maybe practiced utilizing most magnetron ionization gauges. Most prior art gauges, however, -are not suitably protected against leakage currents which would prevent lowering of the measurable current minimum, as above. Accordingly, I further provide shielding means and high leakage resistances which reduce leakage currents to a level consistent with the vimprovements of the foregoing method.

In tubes operated and constructed as above, it is essential that anode and cathode be precisely located and so mounted as to maintain their position in spite of thermal and mechanical stress in order that high sensitivity be maintained. Accordingly, I provide anode and cathode structure ideally suited to the maintenance of high sensitivity in spite of thermal land mechanical stress.

The noval 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 by reference to the following detailed description taken in connection with the accompanying drawing in which:

Fig.l 1 illustrates, in vertical cross-section, Ia magnetron ionization gauge constructed in accord with one feature of the present invention,

Fig. 2 illustrates, in vertical cross-section, another magnen-on ionization gauge constructed in accord with another feature ofthe present invention, yand Fig. 3 is a set of curves illustrating electron Vand positive ion currents in magnetron ionization gauges.

The ionization gauge of Fig. l includes yan anode cylinder 1 supported in place by an integral annular concentric anode support member 1', a lamentary thermionic cathode 2 extending lalong the longitudinal aXis of yanode cylinder 1, `a disk-like collector electrode 3 and a mesh collector kelectrode 4, supported by annular collector electrode support member 4, at opposite ends of anode 1 and exterior thereof and perpendicular to the longitudinal axis thereof. Means for supplying an axial magnet lield comprises an annularcylindrical magnet 5 which may lbe either an electromagnetic coil or a permanent magnet.

The ionization gauge of Fig. 2 also includes a cylindrical anode 6, a doubled lamentary thermionic cathode 7 extending along the longitudinal axis of anode cylinder v6, a disk-like collector electrode 8 exterior of anode cylinder 6 at one vend thereof and Vperpendicular to the longitudinal `axial thereof. Means for supplying an axial magnetic eld comprises an annular cylindrical magnet f9 which may be either a permanent magnet or an ,electromagnetic coil.

In bothvof the magnetron ionization gauges as illustrated in Figs. 1 and 2, electrons are thermionically ejected from the cathode and migrate to the anode. During the passage from cathode to anode a fraction of the gaseous molecules present within the device are ionized by collisions with the migrating electrons. A fraction of the positive ions created by such collisions are collected at the negatively biased collector electrode causing a current to llow in the collector electrode circuit. At the `low pressures at which ionization gauges are utilized, this positive ion vcurrent is proportional to the pressure of the gas within the gauge. For increased sensitivity, that is, for amaximum positive ion current per unit electron current, .the length ofthepath ofthe electrons `between cathodeand anode should befa maximum. Electronspassing fromcathode yto anode lundergo a spiral motion'caused by the passage of .a Vcharged particle in a magnetic field. The spiral pathgtaken by the electronsis thus longer than the straight path would be in the absence of a magnetic field. Maximum sensitivity is obtained in a magnetron ionization gauge when the magnetic field is increased to a degree that electrons emitted from the cathode and accelerated under the electric field are curved to such an extent that they just fail to impinge upon the anode and curve back to the cathode region. When the magnetic field is of this order, the device is essentially cut ofi and substantially no electron current flows between cathode and anode. Under this condition, the path length of the electrons through the gas filled region between the cathode and the anode is a maximum and a maximum number of ionization collisions occur for each electron passing within this space.

In Fig. 3 of the drawing, the increase in ion current achieved by operating a magnetron ionization gauge at cut olf conditions is shown graphically. In Fig. 3, electron current (curve A) and positive ion current (curve B) are plotted as a function of the applied magnetic eld. The positive ion current is very low initially at low magnetic fields when the electrons ow radially from the filament to the cathode. As the magnetic eld is increased and electrons begin to miss the anode, the ion current rises sharply to a maximum yat cutoff. As the magnetic field is further increased, sensitivity drops and then decreases more slowly. The drop with increased magnetic field is due to a further constriction of the electron paths causing them to gyrate in smaller and smaller circular paths. Under these conditions the full space between cathode and yanode is not utilized and sensitivity is reduced. From the foregoing, it may readily be seen that for maximum sensitivity a magnetron ionization gauge should be operated `substantially at cut-off conditions. Cut-off conditions may be defined from Fig. 3 as the magnetic field, and cathode-anode voltage which cause the curve of electron curve versus magnetic field to decrease sharply from an initially high constant value. In Fig. 3 this value is achieved at approximately 380 oersteds.

From Fig. 3 it may further be seen that, at the point where maximum sensitivity is achieved, there is a substantial electron current flow. This means that a substantial number of electrons are impinging upon the anode at high velocity. This results in an inherent limitation in prior art magnetron ionization gauges. Most prior art magnetron ionization gauges will not measure pressures lower than approximately -8 mm. pressure. This is because the very small positive ion currents which flow at this level and below are masked by spurious background currents. One primary cause of these background currents is photoelectric emission from the collector electrode. The photoelectric emission from the collector electrode is caused primarily by bombardment of the collector electrode with soft X-ray photons which have their origin in the high energy collisions of electrons with the anode. A further source of photoelectric emission at the collector electrode is the direct impingement of photons from the incandescent filament onto the collector electrode.

In the prior art, attempts have been made to decrease the minimum measurable background current by decreasing the area of the collector electrode, thus decreasing the fraction of the total Isoft X-rays rand visible light striking the collector. This approach, however, has severe limitations, since decreasing the area of the collector electrode decreases the sensitivity of the device and there is a finite limit beyond which the area of the collector may not be decreased without sacrificing mechanical strength and causing microphonics.

In accord with the present invention I have discovered that it is possible to greatly decrease the minimum measurable gas pressures in magnetron ionization gauges by keeping the collector electrode at a substantially large area, operating the device substantially at cut-off for maximum sensitivity, and operating the cathode at la low the gauge.

temperature as compared with cathode temperatures conventionally utilized to thus greatly decrease electron current from cathode to anode. With the electron current between cathode and anode greatly decreased, the amount of soft X-rays emitted from the anode is also greatly decreased consequently reducing photoelectric emission from the collector. I have found that in operating ionization gauges in this manner, photoelectric emission by the collector ceases to 'be a limiting factor in the minimum pressure which may be measured by the gauge. I have further found that operating the cathode at a low temperature reduces the amount of incandescent light radiated from the cathode to the collector electrode, further decreasing photoelectric emission from the collector. I also have found that the ratio of electron current to the minimum measurable pressure in magnetron ionization gauges is substantially a constant and may be expressed by the relationship where Ie is the electron current from anode to cathode in amperes, and P is the pressure in mm.

By operating my ionization gauges at cathode-anode currents of 0.1 to 1.0 microampere current, as opposed to the 5 milliampere currents at which ionization gauges are conventionally operated, I am able to measure pressures 10-4 to 105 of the minimum pressures measurable by conventional ionization gauges. These low cathodeanode currents may be attained for example by operating a 0.008 diameter, 11/2 long cathode at a tempera ture of approximately 1325 C. to 1450 C. as compared with the normal temperature of l975 C. which would be conventionally utilized for this cathode. The ionization gauges of the present invention operated in this manner are operative to measure pressures as low as 5 X 10-13 mm. Minimum pressures measurable by most conventional ionization gauges are approximately of the order of l0-8 rnm. of mercury. The prior art modification discussed hereinbefore wherein the collector electrode is greatly reduced in area by making the collector a thin wire, thus reducing the sensitivity of the device, is only able to measure pressures as low as approximately 5x10-11 mm. of mercury. It is evident therefore, that by operating magnetron ionization gauges lat cut-off to obtain maximum sensitivity and at a greatly reduced electron current to reduce photoelectric emission, facilitates the measurement thereby of low pressures which have heretofore not been measurable with any other pressure measuring device.

Other Structural features of thc devices constructed in laccord with the present invention may further be observed by referring again to Figs. l and 2. .'n Fig. l, a further decrease in background currents which are a limiting factor upon the minimum measurable pressure is achieved by spacing ion collector electrodes 3 and 4 bctween annular metallic guard rings 11i-10' and 11-11 respectively. These guard rings are maintained electrically at the same potential as the ion collector electrodes. lon collector electrodes 3 and 4 are thus maintained free of leakage currents from anode cylinder 2 and cathode end plates 12 and 13. End walls for the device of Fig. l are provided by metallic disk-shaped end wall 12 and annular metallic end wall 13 which is integrally connected with a metallic tubulation 14 terminated in a flared end for easy connection to a device or system, the pressure of which is to be measured by Filamentary longitudinal cathode 2 is held in spring tension by a cathode spring 15 which bears between a cross-shape member 16, to which one end of cathode 2 is connected, and U-shaped ibracket 17, integrally connected with annular end wall member 13. The opposite end of cathode 2 is permanently fastened in tension as for instance by brazing or welding to a cathode terminal pin 18 which is press-fitted and brazed `into a central aperture in end wall 12. Because of this method of` support, cathode 2 may be located precisely at the axis of anode cylinder 1 and maintained in this position in spite of mechanical and thermal shock, greatly increasing the gauge sensitivity.

The respective electrodes of the device of Fig. 1 are all insulatingly separated from one another by annular ceramic insulating. members. Thus, annular insulating member 20 separates end wall 12 from guard ring 10', and insulating member 21 separates guard member 10 from collector electrode 3. Insulating member 22 separates collector electrode 3 from guard ring 10 and insulating member 23 separates guard ring 10 from anode support mem'ber 1'. Insulating member 24 separates anode support member 1 from guard ring 11, and insulating member 25 separates guard ring 11 from collector electrode 4. Insulating member 26 separates collector electrode 4 from guard ring 11' and insulating member 27 separates guard ring 11' from annular end wall 13.

While metallic members 3, 4', 10, 10', 11, 11', l2, 13 and 18 may be constructed of any highly electrically conductive material such as copper, they are preferably fabricated from titanium because of the unique get-tering characteristics thereof. When these elements are fabricated from titanium the Iionization gauge of Fig. 1 may be fabricated in accord with the method disclosed and claimed in my copending application Serial No. 590,849 tiled concurrently herewith and assigned to the present assignee. In accord with the invention of my copending application, electric discharge devices having titanium electrodes and titanium-matching ceramic bodies are fabrficated by ringthese materials at a temperature of from 700 C. to^1100 C'. in an inert gas atmosphere. One of the advantages of this process is that tiring in an inert gas atmosphere prevents the accumulation of metallic leakage paths along the surface of the ceramic members and sharply reduces leakage currents in devices fabricated in accord therewith. The luse of titanium for these members thereby results in a great advantage over the yuse of other metals.

Insulating ceramic members 20, 21, 22, 23, 24, 25, 26 and 27 are fabricated from an insulating ceramic which closely approximates the thermal coeicient of expansion of titanium, thus facilitating construction of the ydevices by tiring at high temperatures in 'accord with my aforementioned copending application. One such type of ceramic is denominated as Forst-erite and comprises a sintered agglomerate of silicon oxide, magnesium oxide and aluminum oxide. One s-uch Forsterite ceramic and the method of preparation thereof are disclosed and claimed in the copending application of A. G. Pincus, Serial No. 546,215, tiled November 10, 1955, and assigned to the present assignee. In forming the device of Fig. 1 the outside diameter of the metallic members and of the insulating ceramic members are chosen to be the same so that, upon firing, the device is formed into an integral cylindrical unit which is hermetically sealed and which has a smooth cylindrical outer surface. Annular cylindrical magnet means 5 then tits over the cylinder comprising the body of the gauge and is insulatingly spaced therefrom by a suitable insulating sleeve 28 which may for example Ibe Teflon or any dielectric insulating material which will withstand moderately high temperatures. Cathode 2, anode 1, and mesh collector 4 may conveniently Ibe made from tungsten, or molybdenum although other metals which do not react violently with titanium at 7001100 C. are suitable.

In the device of Fig. 2, as in that of Fig. 1, cylindrical anode 6 is supported by an annular anode support member 30 which lits concentrically over anode cylinder 6. Filamentary cathode 7 comprises a very closely bent V-shaped wire, conveniently of tungsten and conveniently approximately 0.008 to 0.01" in diameter. Cathode 7 extends substantially the entire length of anode cylinder 6 and is located at substantially the exact longitudinal center thereof. End4 walls for the device are provided 6 by annular metallic end wall members 31 and 32 respectively. Filament 7 is supported by a pair of substantially L-shape'd tungsten members 33 which are in turn -supported by pins 34 which extend through circular holes in annular end wall 31 and connect with contact pins 35. This method of support for cathode 7, together with the rigid manner in which anode 6 is supported keeps these electrodes concentric even with thermal and mechanical stress and increases the tube sensitivity. Contact pin 35 are spaced from end wall 31 by respective annular ceramic insulating members 36. Entrance into the gauge is attained through tubulation 37 which is fastened integrally with end wall 31 =by lbrazing. An annual metallic shield 38 is mounted upon support .pins 39 and prevents electrons from leaking out the end of anode 6. Collector electrode 8 is disk-shaped and is suspended 'at one end of anode cyly inder 6 perpendicular to the axis thereof from collector 5 field of approximately 160 oersteds.

support pin 40 which is suspended from collector terminal 41. Collector terminal 41 is in turn supported fby elongated collector bushing 42 which is in the form of a closed re-entrant annular cylinder having a reduced annular portion 43 which protrudes through an aperture in annular end wall 32 making the leakage path thereover doubly re-entrant, thus reducing to a minimum, leakage currents which constitute a limit on the low pressure performance limit of the gauge.

, Metallic members 30, 31, 32, 35 and 40 are preferably fabricated from titanium, while ceramic members 36, 42, 44 and 45 are preferably constructed of Forsterite ceramic which matches the thermal coelicient of expansion of titanium although other ceramics are suitable. With the metallic members of titanium and the ceramic members of Forsterite, the device of Fig. 2 may be constructed in accord with my aforementioned copending application so as to provide an hermetically sealed envelope havingH entrance only through tubulation 37 with a minimum of surface leakage over the interior parts thereof facilitating the attainment of low pressure measurements.

The exterior of the device formed by end walls 31 and 32, annular ceramic insulating side-wall members 44 and 45, and annular anode support member 30 is a smooth cylindrical surface over which annular cylindrical magnet means 9 is slidably movable and spaced therefrom by a suitable insulating sleeve 46 which may conveniently comprise any of the materials utilized to form insulating sleeve 28 of the device of Fig. 1.

ln both of the devices of Figs. 1 and 2, operating potentials may be supplied from a source of unidirectional potential as for example battery 50 and regulated by potentiometer 51. Electrical current through the cathode may be supplied by a source of potential, either unidirectional or alternating, represented generally by battery 52, and controlled by rheostat 53. In operation, the anode is maintained at a positive potential of, for example, approximately 300 volts with respect to the cathode, and the collector is maintained at a negative potential of, for example, approximately volts with respect to the cathode. The value of the magnetic field necessary for operating the devices at cutoff conditions may vary with the dimensions of the device utilized, however, with a. device as illustrated in Fig. 1 operating at the potentials indicated above wherein the anode cylinder is approximately 5/x" long and 1/2" in diameter, and the collector electrodes are approximately 5/1" in diameter, and spaced approximately y" away from the ends of anode cylinder 1, cutoff conditions are achieved with a magnetic field of approximately 300 oersteds. In a device as illustrated in Fig. 2 of the drawing wherein the anode cylinder is approximately ll/s" long and 1" in diameter, the cathode has a length of approximately 1% -and the collector electrode is approximately 1" in diameter and is spaced approximately 1A" away from the end of anode cylinder 6, the gauge operates at cutoff conditions with a magnetic Collector current,

measured by any conventional current indicating device, represented by meter 54 is a direct indication of the measured gas pressure.

In addition to operating to measure heretofore unobtainable low pressures, the devices of Figs. l and 2 possess the common features of maintaining extremely high leakage resistance between anode and collector electrodes. In both devices, one important factor in obtaining this high leakage resistance is the fabrication of the metal parts of the tube of titanium and the insulating ceramic members from a titanium-matching Forsterite ceramic, thus facilitating construction of the devices by tiring at a temperature of from 700 C. to 1100 C. in an inert gas atmosphere. When the devices are so constructed, the surface of the ceramic insulating materials remains completely uncontaminated with metallic deposits which are present when such devices are constructed by other methods or red in other atmospheres. Additional features which aid in maintaining low leakage currents within the tubes are, in the device of Fig. 1, the presence of guard rings -10' and 11-11' surrounding collector electrodes 3 and 4 respectively and operated at collector potential to maintain no voltage across the ceramics 21, 22, 25 and 26. In the device of Fig. 2 extremely high leakage resistance is maintained by the disclosed reentrant cylindrical collector electrode bushing 42 which presents an extremely long leakage path between anode and collector electrodes for the size of the device utilized.

Additionally, end-wall member 32 is connected at collector potential and functions as a guard ring between anode and collector, further decreasing leakage currents.

Another important feature common to the devices of Figs. l and 2 is the particular means which are utilized to maintain the filamentary cathode at the precise longitudinal axis of the cylindrical anode electrode, and to keep this configuration even though the devices are subjected to intense mechanical and thermal shock. This is necessary in order to maintain high sensitivity within the devices. As is mentioned hereinbefore with respect to the description of the curves of Fig. 3, when the magnetic field is increased beyond cutoff conditions, the paths of the circulating electrons become much shorter due to a smaller diameter of their helical paths. This shorter path results in fewer ionizing collisions and reduces the sensitivity of the device. If the means utilized in constructing the devices of Figs. l and 2 to maintain the lamentary electrode at the exact longitudinal axis of the cylindrical anode were not utilized, electrons circulating in the cathode-anode space would not be equidistant from the anode on both sides of the cathode while in their circulating path. Thus, in order to prevent excess cathodeto-anode current in accord with the present invention, the magnetic field would have to be increased to operate the device at cutoi, and the entire volume of the tube would not be utilized, thus causing the tube to operate at a reduced eiciency, lowering its sensitivity. In the devices of the present invention, the longitudinal cathode is maintained at the precise center of the cylindrical anode, by rigidly fixing these electrodes in place by ceramic-metal seals and pre-cut apertures, thus insuring maximum efficiency and sensitivity for the size of the device utilized.

An additional advantage of the devices` arises from the materials utilized. Since the devices are constructed entirely of ceramic and metal they may be used at extremely high temperatures up 'to 500 C. for example. Additionally, since the metal parts of the tube are 'constructed mainly of titanium, no additional getter need be provided.

While the invention has been set forth hereinbefore with respect to certain embodiments thereof, many modiiications and changes will immediately occur to those skilled in the art. Accordingly, by the appended claims I intend to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A magnetron ionization gauge electric discharge device for measuring gas pressures below l0*8 millimeters of mercury pressure comprising a therrnionic cathode, an anode, and a collector electrode disposed in close juxtaposition to one another electric bias means for operating said gauge under cutoff conditions; and means controlling the electrical current supplied through said cathode at a rate to maintain electron current between said cathode and said anode at a value of 0.1 to 1.0 microamperes.

2. A magnetron ionization gauge electric discharge device for measuring gas pressures below 10"8 millimeters of mercury pressure comprising a thermionic cathode, an anode and a collector electrode disposed in close juxtaposition in said cathode; electron bias means maintaining said anode at a positive potential with respect to said cathode; electric bias means maintaining said collector at a negative potential with respect to said cathode; means impressing an axial magnetic eld through said device perpendicular to the normal path of electrons from cathode to anode, the magnitude of said cathode-anode potential and of said magnetic eld being correlated so that electrons emitted from the cathode just fail to reach the anode and the device operates under cutoi conditions; and means controlling electrical current through the cathode at a rate to maintain the cathode-anode operating rcurrent at a value of 0.1 to 1.0 microampere.

3. A magnetron ionization gauge electric discharge device for measuring gas pressures below l0a millimeters of mercury pressure comprising a thermionic cathode,

' an anode, and a collector electrode in close juxtaposition to said cathode electric bias means maintaining said anode at a positive potential with respect to said cathode; maintaining said collector at a negative potential with respect to said cathode; means impressing an axial magnetic field through said device perpendicular to the normal path of electrons from cathode to anode, the magnitude of said cathode-anode potential and of said magnetic field being adjusted so that electrons emitted from the cathode just fail to reach the anode, and the device operates under cutoif conditions; and means controlling electrical current through the cathode so as to maintain the cathode temperature at a value of approximately 1325 C.-l450 C.

4. A magnetron ionization gauge capable of measuring gas pressures below l0*3 mm. of mercury comprising: an anode cylinder; an annular metallic anode support concentric with an external of a portion of said anode cylinder; a iilamentary cathode extending longitudinally along the axis of said anode, a disk-shaped metallic collector electrode insulatingly mounted within said device perpendicular to the axis of said anode cylinder and external of the volume thereof; a pair of metallic end wall members disposed in spaced relation at opposite ends of said gauge; a metallic tubulation integral with one of said end walls and forming means for attaching said gauge to a system, the pressure of which is to be measured; a plurality of annular ceramic insulating members interposed between said end walls and said anode; support member and hermetrically sealed thereto to formA an evacuable envelope having a smooth cylindrical lateral wall, an annular magnet means slidably insertable over said envelope, and an insulating sleeve interposed beforming an entrance to said device integral with one of said end wall members and substantially perpendicular thereto; a pair of metallic annular collector electrode guard rings in spaced relation with opposite sides of the peripheral portion of each of said collector electrode members; an axial lamentary cathode extending along the longitudinal axis of said cylinder; a plurality of annular ceramic insulating members interposed between and hermetically sealed to :said metallic members and forming therewith an evacuable envelope having a smooth cylindrical exterior Wall; annular magnet means substantially coextensive with said wall and exterior thereof; and an insulating sleeve interposed between said magnet means and said cylindrical walls.

6. The device of claim 5 wherein said metallic members are of titanium.

7. A magnetron ionization gauge electron discharge device comprising: a cylindrical anode; a metallic annular anode support member concentric with and external of a portion of said anode electrode; a lamentary electrode extending longitudinally along the axis of said anode cylinder; a pair of metallic end wall members in spaced relation with one another and substantially perpendicular to the axis of said anode cylinder, a rst one of end walls having a tubulation integral therewith and a pair of metallic cathode support pins extending therethrough and in sulated therefrom, the second of `said end Wall member having a central aperture therein; an elongated re-entrant ceramic collector electrode bushing extending through said aperture; a metallic collector terminal member extending concentrically within the exterior end of said bushing; a disk-shaped collector electrode suspended from said collector terminal and spaced at one end of said anode cylinder substantially perpendicular to the axis thereof; a plurality of annular insulating ceramic members spaced between and forming hermetic seals with said anode support member and said end wall members to form an evacuable envelope having a smooth surfaced cylindrical exterior wall; annular magnet means substantially coextensive with said wall and exterior thereof; and an insulating sleeve interposed between said magnet means and said cylindrical wall.

8. The device of claim 7 wherein said metallic mem4 bers are of titanium.

References Cited in the file of this patent UNITED STATES PATENTS 2,640,948 Burrill n June 2, 1953 2,745,059 Guager A May 8, 1956 2,750,560 Miles June 12, 1956 2,758,232 Fox Aug. 7, 1956 2,774,936 Beck et al Dec. 18, 1956 2,817,030 Beck et al. Dec. 17, 1957

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