US3572972A

US3572972A - Self-starter device for penning-type ion pumps

Info

Publication number: US3572972A
Application number: US812620A
Authority: US
Inventors: Charles Biguenet; Jean Pontvianne
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1968-04-11
Filing date: 1969-04-02
Publication date: 1971-03-30
Anticipated expiration: 1988-03-30
Also published as: NL6905579A; FR1583456A; GB1242462A; DE1918496A1; JPS5010003B1

Abstract

An ion pump provided with an additional anode which is used for the self-starting of the pump, in particular when the pressure in the enclosure to be evacuated is too high for the penning electrodes to operate properly.

Description

United States Patent 1111 3,572,972

l 72] Inventors Charles Biguenet; [50] Field of Search 230/69; Jean Pontvianne, Paris, France 313/7, 174; 315/108; 324/33; 250/845; 417/49 [21 1 Appl. No. 812,620 [22] Filed Apr. 2, 1969 1 References C1ted [451 Patented Mar. 30, 1971 UNITED STATES PATENTS 1 1 s g Thomson-68F 3,176,907 4/1965 Redhead 230/69 1 Prwmy 11, 1968 3,377,495 4/1968 A1116 230/69X l g?! 3,381,890 5/1968 Hayashi 230/69 1 Primary Examiner-Robert M. Walker Attorney-Cushman, Darby & Cushman [54] SELF-STARTER DEVICE FOR PENNlNG-TYPE ION $3252 5 Dr in ABSTRACT: An ion pump provided with an additional anode aw g which is used for the self-starting of' the pump, in particular [52] US. Cl 417/49 when the pressure in the enclosure to be evacuatcdis too high [5 1 Int. Cl F04b 37/02 for the penning electrodes to operate properly.

SELF-STARTER DEVECE F01! PENNING-TYPE ION PUWS The present invention relates to self-starting arrangements in Penning-type ion pumps.

It is well known that the rate of pumping of Penning-type ion pumps decreases as a function of the rise in pressure and is virtually zero for pressures greater than l mm./Hg. If it is attempted to start up a Penning-type ion pump under these conditions, a high-current self-sustaining discharge is produced and the discharge current is limited only by series resistances in the circuit and the internal resistance of the voltage source. The available power of the discharge may be in the order of a kw. This power is distributed substantially equally between the electrodes of the pumping cells and causes them to be heated to a high degree.

This heating is even more marked when, as is frequently the case in thepreliminary pumping phase, the phenomenon is confined to a single pumping cell which then alone receives the whole of the discharge power.

The rise in temperature which results gives rise to massive liberation of gas from theelectrodes, and the pressure within the enclosure to be evacuated rises instead of diminishing.

' In order to avoid this phenomenon, thus far numerous arrangements have been proposed:

There is added to the ion pump a primary pump arrangement which makes it possible to achieve pressuresinside the vessel of less than In this case, if a mechanical pump is used, there is the risk that oil will penetrate into the system and .a large part of the advantage of the clean vacuum achieved by the ion pump, will be lost. It is also possible to employ an arrangement of zeolite pumps but it is then necessary to use at least three pumps to achieve pressures in the order quoted, and this means a high consumption of liquid nitrogen and expensive apparatus because of the necessity toprovide cocks havingan extremely low leakage rate. Frequently, too, a priming device is used which is constituted by titanium wire which is Sublimated on a cold wall. This apparatus is expensive, bulky and creates the risk of desorption during operation of the ion pump proper.

All these devices are nothing more than stages, separate from the pump itself, which are added into the pumping system.

It is an object of the invention to provide a device which ensures self-starting of the ion pumping (fromfor example l0 to 10" mm./Hg.), said device not being involved in the operation of pumping when the conditions of operation of the main pump (below 10 mmJl-lg.) are properly satisfied.

. According to the present invention, there is provided an ion pump comprising: achamber having means for connection to the enclosure to be evacuated; first cathode and anode members in regard of each other for ionizing the content of said enclosure; means for setting up an electric and a magnetic field between said members; at least one further cathode and one.

further anode; means for setting up an electricfield and a magnetic field between said further cathode and anode, said further cathode and anode being capable of operation at a temperature higher than said cathode and anode, members.

For a bettertunderstanding of the invention and to show how the same may be carried into effect reference will be made to the drawing accompanying the following description and wherein:

FIG. i shows a schematic section of a Penning-type ion pump to which the invention is applied;

HS. 1' is adetail of FIG. 1;

FIG. 2 shows an application of the invention to the structure of FIG. 3;

FIGS. 3 and 4 show two explanatory diagrams illustrating the operation of the pump of FIG. 2; and

FIG. 5 shows another embodiment of the invention.

In all the F IGS., similar reference numbers designate similar elements.

The principle of operation of a Penning-type ion pump will be first recalled.

The electrons emitted from a cold titanium cathode are accelerated towards an anode through a. space in which a magnetic field, substantially parallel to the electric field prevails. The effect of the magnetic field is to lengthen the electron trajectories. The electrons collide in the course of their trajectory with molecules of the gas which is to be evacuated and ionize them. The thus ionized molecules bombard the cathode and knock titanium atoms out of it which, thanks to the specific gettering nature of this metal, carry the molecules of the active gases present in the envelope towards the anode where they are deposited. As far as ionized atoms of neutral gases are concerned, these go towards the cathode where they are entrapped by the titanium atoms which they have entrained.

FIGS. 1 and 2 respectively illustrate, schematically and in section, a conventional Penning ion pump and a pump according to the invention.

The pump 1 FIG. I, 1' FIG. 2 is essentially formed by the elements schematically illustrated by the rectangle 10, which is not repeated in FIG. 2 in order not to overburden the latter. These elements are located in a sealed envelope 1! of revolution about the axis XY which in operation is clamped to the enclosure, not illustrated, to be evacuated, by means of the connector 12. The detail of the pumping electrodes is given in FIG. I, which shows in section and without their connections, the cathodes l0 and anodes 10" which are flat, mutually parallel and perpendicular to the axis XY, and may or may not contain holes, in accordance with the disclosure of the US. Pat. application Ser. No. 758,025, filed Sept. 6, I968.

The enclosures II is at the cathode potential and is connected by a connection 13 to the negative pole and the anodes to the positive pole via a connection 14, through a leadthrough I5.

Polepieces

2 and 3 are located at either side of the pump structure, the magnetic circuit being closed by an annature 4 which also acts as a screen. FIG. 2 illustrates the lines of mag netic force in the pump 1 as well as in the pump I which build up a volume of revolution. The magnetic field, between the

polepieces

2 and 3 is directed along lines M, and along lines m beyond the polepieces towards the axis XY.

According to the invention, the pump 1 shown in FIG. 2 comprises an additional electrode 5 or auxiliary anode, connected electrically in accordance with the invention to the anode I0" and consisting, for example, of a sphere located at the center ofthe structure and biased positively through the lead 14 coming through the leadthrough 15. In operation, a discharge develops between it and the portion 6 of the envelope ill.

Besides, the magnetic field m of

thepolepieces

2, 3, in combination with the electric field prevailing between the anode 5 and the envelope II, confines the self-starting discharge within the volume comprised between the electrode 5 and the portion 6 of the envelope ll 1.

The auxiliary anode 5 is thus heavily bombarded by ions generated between the wall 6, and anode 5, which converge towards the latter, due to the combination of electric and magnetic fields.

An experimental proof of this statement is to be found in the extremely high temperature reached by the auxiliary anode 5. It has been found that this electrode melts when made of stainless steel and evenwhen made of titanium whose melting point is relatively high. It has been necessary to use very high melting point metals such as tantalum or molybdenum to prevent melting.

In its turn, the wall portion 6, at cathode potential, is heavily bombarded by the ions in the discharge.

The

assembly

5, 6 acts during the self-starting phase as a true ion pump albeit not of the Penning-type, because of the different configuration of the lines of electric and magnetic fields.

It will therefore be advisable to employ for the portion a of the envelope 11, and possibly for the whole envelope llI, an active metal which atomizes very readily, for example titaniurn.

Moreover, the auxiliary anode 5 does not act merely as an element of the ion pump which it forms together with the wall portion 6, but also as a chemical pump, that is to say pumps by a mechanism of absorption of the gases by the metal of which it is constituted, when the latter is raised to a high temperature.

These various reasons have shown that the choice of the metal for the auxiliary anode 5 is not an arbitrary one. As already indicated, a high melting point metal should be used which has good chemical activity. it is for this reason that in accordance with the invention the choice is made for example between tantalum and molybdenum as already mentioned.

As far as the shape of the electrode 5 is concerned, it may be a sphere, as in the example of FIG. 2, or a wire, plate, cylinder, etc.

During the whole of the self-starting phase, the discharge remains localized in the space (S, 6) in the form of a selfsustaining discharge. The evolution of this discharge follows the known diagram an example of which is that of FIG. 3, where V is the voltage between cathode 6 and anode 5, and p the pressure and d the distance between the electrodes of the starter ion pump; the distance d during the course of the measurements was approximately that indicated in FIG. 2. Moving from A to B with reducing pressure, the discharge voltage remains substantially constant. From B to C the pressure falls progressively and the ignition voltage rises; the auxiliary anode 5 heats up and adds its own pumping effect.

From C to D the self-starting discharge extinguishes and the Penning discharge starts in the cells of the main pump. During this operation, pressure falls otT progressively in accordance with PEG. 4, the latter illustrating the variation of pressure p in mm./Hg., as a function of the time t in minutes (an experimental curve obtained in the case of a pumped volume of 25 litres).

it is practical but not of fundamental importance, in accordance with the invention to lodge the self-starter system in the body of the pump. However, it is equally possible to make it separate as that shown in FIG. 5, where 21, 22, 23 are toroidal magnets located around a nonmagnetic casing 24. On the axis of the structure the anode 5 is located, this anode being of an active refractory metal such as molybdenum or tantalum for example. The reference m schematically illustrates the lines of magnetic force between the anode 5 and the casing 24 which are respectively connected to the positive pole by the

insulated lead

26, 27 and to the negative pole through earth.

A device of this kind can be added to an installation containing a Penning ion pump, in order to ensure easy starting. It is tightly assembled by the connector 25 at some point on the enclosure 11 of the pump of HO. 2.

Of course, the invention is in no way limited to the examples described and illustratedwhich were given solely by way of examples, many other modifications obviously falling within .the scope of the appended claims.

We claim:

1. A sputter ion pump comprising:

a chamber having means for connection to the enclosure to be evacuated; first cold cathode and anode members in regard of each other for ionizing the content of said enclosure;

means for setting up an electric and a magnetic field between said members; and

at least one further cold cathode and one further anode; means for setting up substantially collinear electric field and magnetic field between said further cathode and anode, said further cathode and anode being capable of operation at a temperature higher than said cathode and anode members.

2. An ion pump as claimed in claim 1, wherein said further cathode is a part of said chamber.

3. An ion pump as claimed in claim 1, wherein means are provided for raising to the same potential said anode members and said further anode and to the same potential said further cathode and said cathode members, said ma netic field provided between said anode members and sat cathode members including lines of force extending between said further anode and cathode.

4. An ion pump as claimed in claim 1 comprising an additional chamber communicating with said enclosure to be evacuated, said further cathode being formed by at least a wall portion of said additional chamber, said further anode extending along the axis of the latter.

5. An ion pump as claimed in claim 2, wherein said further anode is a body placed axially of said chamber.

6. An ion pump as claimed in claim 5, wherein said body is a sphere.

'7. An ion pump as claimed in claim 1, wherein said further anode is of chemically active metal with a high melting point.

Claims

1. A sputter ion pump comprising: a chamber hAving means for connection to the enclosure to be evacuated; first cold cathode and anode members in regard of each other for ionizing the content of said enclosure; means for setting up an electric and a magnetic field between said members; and at least one further cold cathode and one further anode; means for setting up substantially collinear electric field and magnetic field between said further cathode and anode, said further cathode and anode being capable of operation at a temperature higher than said cathode and anode members.

3. An ion pump as claimed in claim 1, wherein means are provided for raising to the same potential said anode members and said further anode and to the same potential said further cathode and said cathode members, said magnetic field provided between said anode members and said cathode members including lines of force extending between said further anode and cathode.

6. An ion pump as claimed in claim 5, wherein said body is a sphere.

7. An ion pump as claimed in claim 1, wherein said further anode is of chemically active metal with a high melting point.