EP0857352A1

EP0857352A1 - Ionic pump with perforated anode

Info

Publication number: EP0857352A1
Application number: EP96937347A
Authority: EP
Inventors: Michel Bolore; Bruno Delomez; Claude Henriot; Jérôme MARTIGNAC
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1995-10-27
Filing date: 1996-10-28
Publication date: 1998-08-12
Also published as: FR2740607B1; FR2740607A1; WO1997015943A1

Abstract

Said pump is comprised of at least a pumping unit comprising an anode (2) having a set of juxtaposed conductor cylinders (4) and two cathodes (6, 8) placed on either side of the anode and made of a metal which, when the pump is in operation, forms on the anode a deposit capable of adsorbing and absorbing the gas, a fluid-tight wall housing (18) wherein is placed each pumping unit and which is provided with an opening (20) through which the gas can penetrate and an internal region in the extension of said opening. In at least one pumping unit facing said region, the side-wall (5) of at least one portion of the anode cylinders is perforated, the side perforations of said cylinders are directly facing the region, and for each perforated cylinder, the perforated surface of the respective cylinder is lower than 30 % of the total side surface of that same cylinder. Application to accelerators of particles and to ultravacuum or high vacuum housings.

Description

IONIC PUMP WITH ADJUSTED ANODE

DESCRIPTION

TECHNICAL AREA

The present invention relates to an ion pump.

It applies in particular to obtaining very low pressures in particle accelerators and more generally in high vacuum or ultrahigh vacuum chambers.

STATE OF THE PRIOR ART

An example of a known ion pump is schematically represented in FIGS. 1 and 2.

Figure 1 is a schematic and partial perspective view of this known ion pump while Figure 2 is a schematic sectional view of this ion pump.

The ion pump of FIGS. 1 and 2 comprises a single pumping block comprising:

An anode 2 comprising a set of juxtaposed electrically conductive cylinders 4 whose respective axes such as the X axis are parallel to each other, and

• two flat cathodes 6 and 8, full or cut, parallel to each other, placed on either side of the anode 2, perpendicular to the axes of the cylinders of this anode, and made of a metal which , when the pump is in operation, forms on the cylinders of anode 2 a deposit capable of adsorbing and absorbing the gas that is to be trapped with this pump. Each of the cylinders 4 of the anode 2 is of course hollow and open at its two ends.

These cylinders 4 are held against each other by suitable means such as, for example, metal blades 10, 12, 14 and 16 which frame all of the cylinders 4.

Means not shown are provided to bring all of the cylinders 4 to a positive potential with respect to the cathodes 6 and 8. This creates an electric field between these cathodes 6 and 8 and the anode 2, parallel to the axes of the cylinders 4 .

As can be seen in FIG. 2, this ion pump also comprises an enclosure 18 with sealed walls in which the anode 2 and the two cathodes 6 and 8 are placed and which is provided with an opening 20 through which the gas which penetrates we want to trap with this pump.

The gas flow is symbolized by the arrows 22 in FIG. 2.

As can be seen in this FIG. 2, the enclosure 18 is provided with a flange 24 placed at the level of the opening 20 and making it possible to connect the ion pump to an enclosure or a pipe containing a gas which one wishes to trap. with the pump.

This pump also comprises means 26 external to the enclosure 18 and making it possible to create therein a magnetic field B parallel to the axes of the cylinders 4. The enclosure 18 is of course made of a non-magnetic material which allows the establishment of this magnetic field B inside this enclosure.

For example, an electric field of the order of 1 to 1.8 MV.πf ^{"is created} between each of the cathodes β and 8 and the anode 2. The magnetic field B, which is parallel to this electric field, is for example 0.14 T.

A spontaneous electrical discharge of electrons occurs between cathodes 6 and 8 and anode 2.

The trajectory of these electrons is lengthened by the spiral movement given to them by the magnetic field.

The cathodes 6 and 8 are made of a metallic material of the "getter" type which can be titanium or aluminum.

During the spiral trajectory of the electrons, numerous collisions take place with the molecules of the gas that we want to trap by means of the pump.

This results in ionization of these molecules.

The positive ions thus created are attracted to cathodes 6 and 8 and bombard them. Under the effect of this bombardment, atoms of the “getter” type material are torn from the cathodes 6 and 8 and are deposited on the cylinders 4 of the anode 2, thus creating thin layers of this material. These thin layers make it possible to adsorb and absorb the gas as in a sublimation pump.

An ion pump is thus a cathode sputtering pump of a "getter" type material in the presence of a magnetic field. It is specified that the conductive cylinders 4 are generally made of stainless steel.

It will be noted that all of these cylinders 4 have a structure of the "honeycomb" type, and that the bases of these cylinders can take various forms (circles, hexagons, squares, etc.). Cathodes 6 and 8 can be full or, on the contrary, perforated.

In the latter case, the enclosure of the ion pump is advantageously made of an electrically conductive and non-magnetic material and brought to the same potential as the anode of this ion pump.

Under these conditions, the "getter" type material is capable of being sprayed onto the internal walls of the enclosure of this pump and the pumping surface is increased.

The flow rate of an ion pump of the kind of that of FIGS. 1 and 2 is proportional to the space existing between the cathodes 6 and 8 and the anode 2.

The distance between this anode 2 and each of the cathodes 6 and 8 is for example equal to 3 mm.

To obtain a higher flow rate than that of the ion pump of FIGS. 1 and 2, it is known to use several pumping modules which are of the type of this pump and which are operated in parallel, in the same enclosure for a same pump.

This produces a set which is heavy, bulky and expensive.

It has also been proposed to shorten the peripheral cylinders, but the resulting increase in flow rate is not very noticeable.

STATEMENT OF THE INVENTION

The present invention relates to an ion pump whose flow rate is higher than that of known ion pumps and which does not have the drawbacks mentioned above.

According to the present invention, to increase the flow rate of the ion pump, an anode or a plurality of anodes are used, at least part of the cylinders are perforated: thus the gas is penetrated not only at the base of the anode, but also perpendicular to this anode.

Specifically, the subject of the present invention is an ion pump intended to trap a gas, this ion pump comprising at least one pumping block, each pumping block comprising:

An anode comprising a set of juxtaposed electrically conductive cylinders, the respective axes of which are parallel to each other, and

Two flat cathodes, parallel to each other, placed on either side of the anode, perpendicular to the axes of the cylinders of this anode, and made of a metal which, when the pump is in operation, forms on the anode a deposit capable of adsorbing and absorbing the gas, this ion pump also comprising an enclosure with a sealed wall in which is placed each pumping block and which is provided with an opening through which the gas penetrates and of an internal zone of the enclosure, called the pumping well, which is an extension of this opening, this ion pump being characterized in that, in at least one pumping block placed facing the pumping well, the wall lateral of at least part of the anode cylinders is perforated, in that the lateral additions of these cylinders are in direct view of the pumping well and in that, for each perforated cylinder, the perforated surface of this cylinder is less than 30 * of the lateral surface total ale of this cylinder.

According to a first particular embodiment of the invention, the side wall of these perforated cylinders of the anode is perforated in its central part, the median plane of the perforations passing through the pumping well. Indeed, the end parts of the cylinders are more efficient for pumping than the central parts of these cylinders.

According to a first variant corresponding to this first particular embodiment, the side wall of these perforated cylinders of the anode of this pump is provided with holes constituting the perforations.

According to a second variant, this side wall is provided with elongated openings which constitute the openings and extend parallel to the axis of the cylinder corresponding to this side wall.

According to a third variant, this side wall comprises a peripheral screen part in the vicinity of the median plane of cross section of the corresponding cylinder.

According to a fourth variant, this side wall is in two longituαmaiement parts spaced apart from one another.

According to a second particular embodiment of the invention, the openings have the form of two diametrically opposite longitudinal slots, the plane of which passes through the pumping well.

According to a first variant corresponding to this second particular embodiment, this side wall comprises mesh parts of elongated shape which extend parallel to the axis of the cylinder corresponding to this side wall.

According to a second variant corresponding to this second particular embodiment, the side walls of the cylinders of the same line of cylinders of the anode are produced in the form of two corrugated plates diametrically spaced from one another.

The openings can be formed in the first cylinders of the anode, arranged directly in front of the pumping well, allowing direct communication with this pumping well.

The ion pump can comprise a single pumping block which is placed opposite the opening of the enclosure, the lateral openings of said cylinders being in direct view of this opening.

In this case, the openings can be formed in the first cylinders of the anode, arranged directly in front of the inlet of the pump, allowing direct communication with said inlet.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limiting, with reference to the accompanying drawings in which:

FIG. 1 is a schematic and partial perspective view of a known ion pump and has already been described,

FIG. 2 is a schematic sectional view of the ion pump of FIG. 1,

FIG. 3 is a schematic sectional view of a particular embodiment of the ion pump object of the invention,

FIGS. 4A, 4B, 4C, 4D, 4E and 4F are schematic views of various particular embodiments of cylinders which can be used to form the anode of an ion pump in accordance with the invention, and

• Figure 5 is a schematic sectional view of another particular embodiment with a plurality of pumping blocks. DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The ion pump according to the invention, which is schematically shown in section in FIG. 3, has a single pumping block (anode 2 and cathodes 6) which is placed opposite the opening 20 of the enclosure 18, and this pump simply differs from the ion pump of FIG. 2 in that the cylinders 4, the assembly of which constitutes the anode 2 of the pump of FIG. 3, are perforated.

More precisely, it is the side wall 5 of each of these cylinders of the pump of FIG. 3 (which are hollow and open at their ends) which is perforated. For each cylinder, the open area is less than 30% of the total lateral surface of this cylinder.

In fact, the pumping phenomenon is linked to two causes:

- absorption of ions in the cathodes, especially in the axis of each of the cylinders constituting the anode;

- adsorption of gases on the walls of the anode where the getter material from the cathodes is deposited (sputtering phenomenon).

This last pumping mode is predominant and it is advisable not to decrease the surface of the anode in too large proportions. For example, simple metal wires would deprive the pump of great efficiency. This is why the openings made on the cylinders must not be excessive, hence the choice of the limit value of 30% mentioned above for the perforated surface. Preferably, for each cylinder, this perforated surface is less than a value of the order of 10% to 30% of the total lateral surface of this cylinder. In the case of Figure 2, the side wall of the cylinders has no opening.

The use of perforated cylinders makes it possible to increase the pumping acceptance area at the level of anode 2 of the ion pump of FIG. 3 compared to the ion pump of FIG. 2.

Various examples of perforated cylinders are schematically represented in FIGS. 4A to 4F which will be described later. In all cases, an anode is obtained which can be described as "transparent".

In known ion pumps, such as that of FIGS. 1 and 2, the gas molecules can only pass, to be pumped, between the small spaces separating the anode from the cathodes of these known pumps.

In an ion pump according to the invention like that of FIG. 3, there are, thanks to the perforated cylinders, additional openings which increase the flow rate of this pump compared to known ion pumps: the molecules of the gas to be pumped can pass from a cylinder of the anode of this pump according to the invention to another cylinder thereof. In addition, in the case of FIG. 3, the lateral openings of the cylinders 4 of the anode 2 are in direct view of the opening 20 of the enclosure of this ion pump.

Obviously, the metal retaining blades are removed from the places where they would obstruct the openings of the cylinders.

We can remove them completely, as shown in Figure 3, and secure all of the cylinders by welding. It is also possible to hold the cylinders using a U-shaped blade, the opening of the U being oriented towards the opening 20 of the enclosure 18.

One can also replace the blade 10 disposed opposite the opening 20 by two lateral tie rods.

Various examples of perforated cylinders, which can be used in the present invention, are schematically represented in perspective in FIGS. 4A to 4F.

The cylinder 2 of FIG. 4A is pierced with holes 30 around its entire periphery.

The cylinder 2 of FIG. 4B is pierced with openings 32 of elongated shape which extend parallel to the axis X of this cylinder and are distributed around the entire periphery thereof.

In the case of FIG. 4C, the side wall 5 of the cylinder 2 comprises a peripheral mesh part 34, which corresponds to the embodiment of FIG. 3.

In the case of FIG. 4D, the side wall of the cylinder 2 comprises meshed parts

36 and 38 of elongated shape, which extend parallel to the axis X of this cylinder over the entire length thereof.

In the case of FIG. 4E, the side wall of the cylinder 2 is formed of two parts 40 and 42 which are spaced from one another.

In the case of this FIG. 4E, the anode comprising such juxtaposed cylinders is formed of two parts and leaves a completely free passage in the middle.

These two parts of the anode are kept spaced from each other by a metal blade in U shape which was discussed above in the description of Figure 3.

In the case of FIG. 4F, the side walls of the cylinders of the same line of cylinders of the anode are produced in the form of two corrugated plates 44 and 46 diametrically spaced from one another.

The use of perforated cylinders in accordance with the invention does not disturb the electric and magnetic fields necessary for the operation of the ion pump.

Regarding the electrostatic potential, the equipotential lines are the same, whether the anode cylinders have solid or perforated side walls.

Thus, the electric field between the anode and the cathoαes retains its value.

As regards the magnetic field, the internal arrangement remains the same, the distribution of the magnetic induction depending only on the means 26 for creating the magnetic field.

These means 26 generally comprise two permanent magnets placed on either side of the enclosure 18 and formed of ferrite plates as well as a magnetic shielding (not shown) allowing the magnetic field generated by these magnets to close at the outside the enclosure 18.

With an ion pump in accordance with the invention, using cylinders of the type of that of FIG. 4D, gains have been obtained, compared to a conventional ion pump, which range from 10 s to 40 l, for use pressures of 10 ^"π Pa.

Note that we can distinguish two groups of ion pumps according to the invention in view of Figures 3 to 4F. In the first group, the openings are arranged in the central part of the cylinders, the median plane of the openings passing through the opening of the enclosure. This is the case in FIGS. 3, 4A, 4B, 4C and 4E. This median plane, which is perpendicular to the axes of the cylinders, has the reference P in FIG. 3.

In the second group, the openings form two diametrically opposite longitudinal slots. The plan of these slots passes through the opening of the enclosure. This is the case in FIGS. 4D and 4F where this plane of the slots has the reference PI.

It is specified that all the cylinders of an ion pump according to the invention, such as that of FIG. 3, do not have to be perforated. Only a part of them can be. In this case, it is preferable that the openings are formed in the cylinders placed directly in front of the inlet of the pump to allow direct communication with this inlet, that is to say with the opening of the enclosure of this pump.

Another ion pump according to the invention is schematically represented in section in FIG. 5 and comprises two pumping blocks 48 in an enclosure 18. We can also see in FIG. 5 the opening 20 of this enclosure and the flange 24 of which it is provided.

Each pumping block 48 is constituted like that of FIG. 3.

The area PP internal to the enclosure 18, which extends the opening 20 thereof and which is called the pumping well, has been represented by dotted lines.

Each pumping block 48 is placed opposite this pumping well PP. The axes of the cylinders of the anodes of the blocks are parallel to this well while the cathodes 8 are perpendicular thereto.

The lateral openings of the perforated cylinders are in direct view of this PP well. The means for creating an appropriate magnetic field in each block have been symbolized by an arrow B, the magnetic field here being parallel to the pumping well PP.

The flow of the pumped gas has the reference F. Of course, in the case of pumps according to the invention comprising a pumping well, two groups can also be distinguished as was done above.

In this case, for the first group, the side wall of each perforated cylinder is perforated in its central part and the median plane P of the perforations passes through the pumping well PP. For the second group, the openings have the form of two diametrically opposite longitudinal slots whose plane passes through this pumping well.

In addition, not all cylinders have to be perforated in each pumping block. It suffices that part of the cylinders of each pump block anode is. Preferably, these openings are formed in the cylinders disposed directly in front of the pumping well, which allows direct communication with this pumping well PP.

It should be noted that the notion of pumping well PP still has a meaning in the case where there is only one pumping block (FIG. 3). In this case, this block is still opposite this pumping well and the lateral openings are still in direct view of this well but, as can be seen in FIG. 3, the axes of the cylinders and the magnetic field are perpendicular to the PP well while the cathodes are arranged parallel thereto.

In a particular embodiment not shown, there is only one pumping block with perforated cylinder (s) facing the pumping well.

The other blocks are not placed opposite this well and have no perforated cylinder.

There can also be several pumping blocks facing this well, among which only a few have one or more perforated cylinders, the other blocks not being placed facing this well and having no perforated cylinder.

Claims

1. Ion pump intended to trap a gas, this ion pump comprising at least one pumping block, each pumping block comprising: • an anode (2) comprising a set of juxtaposed electrically conductive cylinders (4), the respective axes of which are parallel to each other, and • two cathodes (6, 8), parallel to each other, placed on either side of the anode, perpendicular to the axes of the cylinders of this anode, and made of a metal which, when the pump is in operation, forms on the anode a deposit capable of absorbing and absorbing the gas, this ionic pump also comprising a sealed wall enclosure (18) in which each pumping block is placed and which is provided with an opening (20) through which the gas penetrates and with an internal zone of the enclosure, called a pumping well, which is an extension of this opening, this ion pump being characterized in that, in at least one pumping block placed in re gard of the pumping well, the side wall (5) of at least part of the cylinders (4) of the anode (2) is perforated, in that the lateral openings of these cylinders are in direct view of the pumping well and in that, for each perforated cylinder, the perforated surface of this cylinder is less than 30% of the total lateral surface of this cylinder.

2. Ion pump according to claim 1, characterized in that this side wall is perforated in its central part, the median plane of the openings passing through the pumping well.

3. Ion pump according to claim 2, characterized in that this side wall is provided with holes (30) constituting the openings.

4. Ion pump according to claim 2, characterized in that this side wall is provided with elongated openings (32) which constitute the openings and extend parallel to the axis (X) of the cylinder corresponding to this side wall .

5. Ion pump according to claim 2, characterized in that this side wall comprises a peripheral screen portion (34) in the vicinity of the median plane of cross section of the corresponding cylinder.

6. Ion pump according to claim 2, characterized in that this side wall is in two parts longitudinally spaced from one another (40, 42).

7. Ion pump according to claim 1, characterized in that the openings have the form of two diametrically opposite longitudinal slots whose plane passes through the pumping well.

8. Ion pump according to claim 7, characterized in that this side wall comprises mesh parts of elongated shape (36, 38) which extend parallel to the axis of the cylinder corresponding to this side wall.

9. Ion pump according to claim 7, characterized in that the side walls of the cylinders of the same line of cylinders of the anode are produced in the form of two corrugated plates (44, 46), diametrically spaced one of the other.

10. Ion pump according to any one of claims 1 to 9, characterized in that the openings are formed in the first cylinders of the anode, arranged directly in front of the pumping well, allowing direct communication with this pumping well.

11. Ion pump according to any one of claims 1 to 10, characterized in that it comprises a single pumping block which is placed opposite the opening of the enclosure and in that the lateral openings of said cylinders are in direct view of this opening.

12. Ion pump according to claim 11, characterized in that the openings are formed in the first cylinders of the anode, arranged directly in front of the inlet of the pump, allowing direct communication with said inlet.