US3496399A

US3496399A - Ion gauge with collector plates anf anodes perpendicular to each other

Info

Publication number: US3496399A
Application number: US689436A
Authority: US
Inventors: John Doble Buckingham; Thomas Chapman; John Stuart Cleaver; Peter Neville Thorne
Original assignee: Edwards High Vacuum International Ltd
Current assignee: Edwards High Vacuum International Ltd
Priority date: 1966-12-12
Filing date: 1967-12-11
Publication date: 1970-02-17
Anticipated expiration: 1987-02-17
Also published as: DE1648463B1; GB1159494A

Description

F 1970 J. D. BUCKINGHAM ET L, 3,496,399

10 GAUGE WITH COLLECTOR PLATES ANF ANODES PERPENDICULAR To EACH OTHER Filed Dec. 11. 19s? 2 Sheets-Sheet 1 ATTORNEY Feb. 17, 1970 J. n. BUCKINGHAM ETA!- 3,495,399

108 GAUGE WITH COLLECTOR PLATES ANF ANODES PERPENDICULAR To EACH OTHER Filed Dec. 11, 1967 2 Sheets-Sheet 2 INVENTOR BY fd' flW ATTORNEY United States Patent "ice US. Cl. 3137 8 Claims ABSTRACT OF THE DISCLOSURE A vacu-um device comprising the electrode arrangement of, for instance, a Bayard Alpert ionisation gauge in which there are a pair of parallel electron collector anodes, the electron emitting filamentary cathode passing between them, and a pair of parallel ion collector electrodes. The ion collector electrodes and the anodes are at right angles to each other so that the chance of ions striking the filament is minimised.

This invention relates to electric discharge vacuum devices and is particularly, although not exclusively, concerned with ionisation gauges.

It is well known that the conventional Bayard Alpert type of ionisation gauge has an upper pressure limit of approximately torr. More recently it has been shown that, by making certain changes in the geometry of the gauge, the operating limit can be extended to about 1 torr. In one such arrangement an electron-emitting filament is disposed between closely spaced parallel electrodes, one of which serves as the electron collecting anode and the other as an ion collector. However, as electrons travel in straight lines normal to the plane of the electron collecting anode, ions are formed which in travelling to the ion collector tend to hit the filament since they tend to travel in a direction essentially at right angles to the ion collector. This bombardment of the filament causes damage to the filament, particularly if it is coated with a low temperature emitting material, and also results in a premature deviation from linearity in the pressure/ current characteristics of the gauge at pressures of the order of 1 torr.

In order that the linearity of an ionisation gauge be extended to such high pressures, the following requirements are necessary:

(a) The geometry must be such that the electron paths do not change appreciably with pressure.

(b) The gauge sensitivity must be small compared with the reciprocal of the highest pressure to be measured. This is accomplished by reducing the distance between filament and electron collector.

(c) The ion collector must be positioned so that the efiiciency of ion collection remains constant over the entire pressure gauge. Since the probability that an ion will be collected increases with pressure, the geometry of the device must be such that all positive ions are collected. In practice this means that the ion collector must be large with respect to the filament.

According to the present invention an electric discharge vacuum device having a filamentary electron emitting cathode is provided with a pair of parallel electron col lector anodes and a pair of parallel ion collector electrodes, the said two pairs being disposed at right angles to each other so as to define a box-like region.

The filamentary cathode is preferably in the form of a 3,496,399 Patented Feb. 17, 1970 straight wire passing symmetrically through the said boxlike region.

It will be seen that such an arrangement can considerably lessen, and substantially eliminate, any bombardment of the filament by ions.

With advantage the filamentary cathode is constituted by a platinum/rhodium alloy coated with a rare earth oxide of low work function.

The invention will now be further described, by way of example, with reference to accompanying drawings in which:

FIGURE 1 shows a cross-section of the various electrodes of a triode ionisation gauge constructed in accordance with the invention;

FIGURE 2 shows a central cross-section of the discharge components of the triode ionisation gauge of FIG- URE 1, the section passing through the filament axis; and

FIGURE 3 shows an exploded view of a preferred electrode arrangement similar to that shown in FIGURES 1 and 2.

Referring now to the drawings, FIGURE 1 shows a filamentary electron emitting cathode 1 positioned to lie axially within the space defined by a pair of parallel ion collectors 2 and a pair of parallel electron collector anodes 3. The anodes 3 are each in the form of L-shaped strips attached to an apertured metal plate 4 which passes across the space at the ends of the electrode structure as is indicated in FIGURE 1 by the non-sectioned portion. The ion collectors 2 are electrically connected by wire 5.

FIGURE 2 shows a central cross-section as mentioned above and includes the electrode arrangement shown diagrammatically in FIGURE 1, the section being in the direction of arrows A-A in that figure. The structure is supported partly from arm 6 by means of bolt 7 and a number of ceramic insulators 8 and partly by means of a second arm 9 shown only in FIGURE 1 by means of a similar bolt 10. Bolts 7 and 10 support a main apertured base plate 11 and the two ion collectors 2. The remaining pair of electrodes (anodes 3) and the filament 1 are respectively supported by the plate 4 and a bracket 12 each carried by bolts 13 on ceramic spacers 14. The anodes 3 are welded to plate 4 as already mentioned. The filament 1 is spacially tensioned by the form of the bracket 12 which includes the sprung terminal portion 15 to which the filament is welded. The

arms

6 and 9 also serve as the filament supply lines,

angled portions

16 and 17 being respectively welded to an extending portion of the corresponding bracket 12. The electron collector anodes 3 are maintained at a constant potential by a wire 18 welded to plate 4 and the ion collectors 2 by a wire 19. Base plate 11 is earthed by means of wires 20. The two

arms

6, 9 terminate in a glass mounting plate (not shown) having the required supply terminals.

The relative distances used are as follows. The anodes 3 are 0.125 inch apart, the ion collectors 2 are 0.100 inch apart and the gap between each adjacent ion and electron collector is 0.010 inch. The ion collectors 2 are maintained at earth potential, although they are insulated by the ceramic washers 8 from plate 11, and the ion current is measured'by amplifying the current carried by wire 19. The filament 1 may be maintained at 35 volts and the anodes 3 at 250 volts. While the above arrangement is suitable, the essential requirement guiding the relative electrode dimensions chosen and the relative applied voltages is that they give rise to an electro-static field distribution within the electrode structure such that:

(a) The ions will be drawn to the ion collector rather than towards the filament, and

(b) The potential on the ion collector does not exert a controlling influence on the free passage of electrons from the filament to the electron collectors.

For a gauge with geometrical spacings and electrode potentials as given above it has been found that efiicient operation is obtained, a filament heating current of approximately 2 amps being required to generate the necessary levels of stabilised electron emission current. It will be seen from the above values that the ratio between the voltage levels on the anodes and the filament is approximately 7:1. While this ratio may vary from slightly different geometrical arrangements, for any particular arrangement it should be kept very close to its optimum value. If it is not realised, the filament heating current can rise to at least double the desired 2 amp value in order to provide the same level of electron emission current. If the ratio is varied by too great an extent the electron current is eventually completely cut-off even if the filament current is raised to a value at which it burns out.

It has been found that the lowest pressure at which the gauge current/ pressure characteristic is linear depends upon the filament voltage. The reason for this is that relatively high energy electrons bombarding the anodes result in the generation of X-rays which then fall on the ion collectors and cause ejection of photo-electrons. This effect becomes noticeable at pressures below torr. However, it can be combated by reducing the anode voltage to 50 volts and the filament voltage 7 volts. In this Way the voltage ratio is retained so the field distribution is unchanged, and the gauge is found to operate linearly at pressure of the order of 10- torr. These voltages are particularly suitable as the 50 volt anode potential is a voltage below which the sensitivity of the gauge decreases rapidly. With the anodes at 50 volts the gauge sensitivity is still 30% of that at 250 volts while the potential is only of its original value.

Thus the pressure range of the gauge described lies approximately between 1 and 10- torr. At such high pressures the performance of cathodes presently available deteriorates if they are operated in the presence, for example, of water vapour or oxygen. Consequently the application of high pressure ionisation gauges has generally been limited to their use in inert or inactive atmospheres. However, the provision of a cathode able to withstand oxidising or other deleterious atmospheres Without loss of emission by poisoning of the coating or by damage to the filament substrate by oxidation is most desirable. In an attempt to overcome these problems it has been found that a filament of platinum 10% rhodium alloy coated with yttrium oxide is most suitable. This combination gives a cathode of low work-function applied to a substrate of high melting point. The low temperature of operation made possible by the low work function ensures that evaporation from the filament substrate is sufiiciently low for the life of the system to be many times that of cathode systems previously used. Furthermore, by using an alloy of noble metals a so-called non-burn out filament has been provided. In addition the cathode appears to be much less susceptible to poisoning than other cathodes currently in use.

While the gauge described with reference to FIGURES 1 and 2 is found to be suitable, a preferred electrode arrangement is shown in exploded form in FIGURE 3 of the accompanying drawings. Basically FIGURE 3 shows the same components as the previous figures whereby 1' represents the filaments, 2' the ion collectors, 3 the anodes, 4' the apertured anode carrier plate, 11' the base plate and 14' the ceramic spacers. However the ion collectors 2' are shown to be flat as opposed to the cranked shape illustrated in FIGURE 1. This is possible as the ceramic spacers 14' are less deep than the spacers 14. It will also be seen that the filament tensioning arrangement now comprises two fiat filament end terminal plates 21 and a single S-shaped tensioning spring 22 which keeps the filament aligned during operation. The spring 22 comprises a high working temperature nickel alloy which can withstand both the required operating currents and the bake-out temperatures used in the vacuum apparatus with which the gauge is to be associated. An additional feature of the FIGURE 3 arrangement is the provision of expansion relief spring washers 23 which are provided on each of the four sets of ceramics.

Such a gauge as have been described, While having small electrode spacings, are easily reproducible from gauge to gauge which is essential for reliable and reproducible pressure/ current characteristics. :It is also suitable for being baked, as necessary, to about 450 C. and out gassed by eddy current heating to around 850 C., no loss of geometrical accuracy being suffered. In other words, a robust, accurate and geometrically easily reproducible gauge is obtained.

We claim:

1. An electrical discharge device including in combination:

(a) a pair of parallel electron collector anodes;

(b) a pair of parallel ion collector electrodes, said pair of anodes and said pair of ion collector electrodes being disposed at right angles to each other; and

(c) a filamentary electron emitting cathode in the form of a Wire passing between said pair of anodes and said pair of ion collector anodes.

2. A device according to claim 1 in which said filamentary cathode is provided at an end with a tensioning device.

3. A device according to claim 1 in which said filamentary cathode comprises a platinum/rhodium alloy wire coated with a rare earth oxide of low work function.

4. A device according to claim -1 in which said electron collector anodes comprises metal strips upstanding from an apertured plate, said .aper-tured plate being parallel to, and disposed between said ion collector electrodes.

5. An electrical discharge vacuum device having an electrode arangement including in combination:

(a) abase plate;

(b) support means connected to said base plate and adapted to support said base plate within vacuum apparatus;

(c) a pair of parallel ion collector electrodes supported by said base plate and separated therefrom and from each other by insulating spacers, said pair of ion collector electrodes being connected electrically;

'(d) first electrical connection means connected to said ion collector electrodes and adapted for connect-ion to current measuring means;

(e) an apertured electrically conductive plate su ported by said base plate and separated and insulated therefrom by further insulating spacers, said apertured plate being parallel to, and disposed between said ion collector electrodes;

(f) a pair of parallel anodes provided on said apertured plate at either side of said aperture and at right angles to said collector electrodes, said anodes being electrically connected by means of said apertured plate; and

(g) second electrical connection means connected to said anodes and adapted for connection to a voltage supply to maintain said anodes at a constant potential;

(h) a filamentary electron emitting cathode supported by said base plate and separated and insulated therefrom by said further spacers, said cathode extending symmetrical-1y between said pairs of ion collector electrodes and anodes; and

(i) third electrical connection means connected to said cathode and adapted for connection to a volt-age source whereby current may fiow along said cathode.

6. A device according to claim 5 in which said filamentary cathode comprises a platinum/rhodium alloy wire coated with a rare earth oxide of low work function.

7. A device according to claim 5 further including a tensioning device connected to said filamentary cathode.

8. A device according to claim 7 in which said tcn- 3,306,112 2/1967 Icnckel 324-33 X sioning dcvicecomprises an S-shapc'd spring. 3,394,286 7/1968 Brock 3137 X References Cited JAMES W. LAWRENCE, Primary Examiner UNITED STATES PATENTS 5 C. R. CAMPBELL, Assistant Examiner 2,820,946 1/1958 Robinson 324-33 2,836,790 5/1958 Hickam et a1. 32 9-33 3,193,724 7/1965 Klopfer 313-7X 31518;32433 3,254,256 5/1966 Mel-ling et .al 3137 X