US3224664A - Ion pump - Google Patents

Description

Dec. 21, 1965 N. w. ROBINSON ION PUMP Filed Sept. l5, 1963 FIC-3.2.

FIC-3.1.

INVENTOR. NORMAN WRIGHT ROBINSON AGEN United States Patent 3,224,664 ION PUMP Norman Wright Robinson, Reigate, England, assignor to No-(th American Philips Company Inc., New York, N.

Filed Sept. 13, 1963, Ser. No. 308,855 7 Claims. (Cl. 23d-69) This invention relates to vacuum ion pumps; more particularly to vacuum ion pumps of the type using the sputtering of a getter material for trapping ionized molecules of residual gases within an atmosphere to be pumped.

Such pumps are well known in the art and generally take the form of a structure comprising an anode, taking the form of a grid or mesh having passageways to a cathode surface made of a reactive material such as titanium. Such structures are subjected to a permanent magnetic field orientated so `as to provide a magnetic field coaxial to the passageways and imparting an oscillatory and helical motion to electrons emitted by the cathode in order to increase the path length and therefore enhance the probability of ionizing residual molecules.

To operate such pumps it is necessary to reduce the initial pressure by primary rough pumping after which the application of an electrical potential on the anode and cathode structures provides a glow discharge which ionizes residual gas molecules which when striking the cathode cause sputtering of the cathode material on and around the anode.

From a study of previously proposed pump structures it has been found that there is a disadvantageous tendency for the resultant erosion caused by sputtering of the cathode material to take place from a concentrated central spot and that this has the effect of limiting the life of the cathode surface.

It is the object of the present invention to provide an improved ion pump of the type described in which the electrodes are so arranged relative to one another and immersed in a permanent magnetic field which will provide enhanced pumping speeds and a substantial decrease in cathode erosion which increases the life of the cathode. The erosion patterns obtained from a pump constructed in accordance with this invention have displayed a larger, less concentrated eroded area in the form of an oval on the cathode surfaces which results in a considerably longer active life of the cathode material.

According to one embodiment of this invention there is provided a vacuum ion pump of the type described, characterised in that the lanode structure comprises a plurality of open sectors of a right annular cylinder arranged radially with an inner and outer cylindrical cathode structure, including permanent magnetic means having a radial magnetic eld substantially axial with said anode sectors.

According to a modification of an ion pump as described above there is provided a permanent magnet of annular form having a yoke and central pole piece and an outer hollow cylindrical pole piece. The cavity formed between the pole pieces providing a radial magnetic field into which the anode and cathode structures are immersed.

According to a further modification there is provided an annular permanent magnet having an integral yoke and central pole piece, the pole pieces being extended to embrace the lanode and cathode structures and the iuternal surfaces of the pole pieces being treated with a finish having good stable vacuum properties. Alternately, the inside surfaces of the pole pieces can be provided with stainless steel sleeves around which a cathode of a metal such as titanium, zirconium, tantalum or molybdenum, can be inserted. The stainless steel sleeves 3,224,664 Patented Dec. 2l, 1965 ICC can form, together with a yoke shaped disc, a separate continuous internal envelope for the pump and which will allow lengthy baking of the pump at high temperatures with the permanent magnet removed therefrom.

In the accompanying drawings, FIGURE l shows a diagrammatic drawing of an ion pump constructed in accordance with this invention. The pump structure proper is shown connected to a conical shaped neck for connection to an atmosphere to be pumped.

FIGURES 2 and 2a are diagrammatic representations of erosion patterns and electrostatic fields of known pump structures and FIGURES 3 and 3a show erosion patterns and electrostatic fields obtained from a pump structure according to this invention.

An embodiment of this invention will now be described with reference to FIGURE l of the drawings in which an outer cylindrical casing of soft iron, which is provided with an inner jacket 12 of stainless steel having a further inner jacket 13 of titanium forming a first cathode structure; an inner magnet structure 2 of soft iron is provided with a stainless steel jacket 11 around which a titanium sleeve 10 forms a second cathode structure.

An internal anode structure 8 comprising a sleeve constructed of a plurality of sectors of a right annular cylinder is made of titanium and positioned coaxially between the first and second cathode structures 13 and 10. Terminals 9 are provided for connection to the anode structure. The pump proper terminates at flange 5. In the drawing is shown an external mating flange 5A for reducing the open end of the pump structure through a conical neck 6 to a flange 7 for connection to the atmosphere to be pumped.

A ring shaped permanent magnet 3 is connected in series with a further internal magnet system 3A through a base yoke 4 arranged so that opposing magnetic poles set up a radial magnetic field across the outer soft iron casing 1 and the inner soft iron member 2. In order to enhance the desired spread in cathode erosion, the lower ratio limit of inner to outer diameters, of the pole pieces and/or cylindrical cathode structures, has been found to be not less than 1:1.2.

Referring now to FIGURES 2, 2a and 3, 3a, it will be seen that the erosion pattern obtained from the known pump structure, FIGURE 2, and its associated electrostatic field, shown in FIGURE 2a, results in a point of concentrated electrostatic field near the centre 16 of the cathode surface 15. The characteristic cushioned pattern on the cathode structure 15 in FIGURE 2 shows the erosion area at point 16 and further examination shows other faint diagonal lines of erosion extending to the corners of each pattern.

In FIGURES 3 and 3a, the anode structure 17 takes the novel form of a section of a right annular cylinder having curved cathode structures 1S and a radial magnetic field as indicated by the arrows. From a study of the lines of electrostatic field shown in FIGURE 3a, it will be seen that there is a concentration of maximum magnetic field and electrostatic field (for the same applied voltage) towards the narrow edge of the sector. The resultant erosion pattern shown in FIGURE 3 indicates an enlarged erosion area 20 on the cathode surface 1-8. This useful increase in erosion area enhances the life of the cathode and the spread in the point of maximum field intensity increases the pumping speed over a wider range 4of pressures.

A pump, constructed in the manner heretofore described in which the anode had a mean diameter of approximately 12.5 cms. and a length of 14 cms. with 96 sectors, was immersed in a magnetic field having an approximate field density of 900 gause at the outer cathode surface and approximately 1,200 gause at the inner cathode surface and an approximate anode to cathode spacing of 3 mm. has shown a measured average pumping speed of approximately 43 litres/sec. with an electric potential of 3 kv. over the range of 10-6 to 10-8 torr. With a potential of 4.5 kv. the pumping speed was approximately 57 litres/sec. and at 6 kv. the speed was increased to over 70 litres/sec. over the same range of pressures. These pumping speeds relate to nitrogen and measurements were obtained by use of the known expression:

(Pl-PZ) torr P2 where S is the speed of pumping in litre/sec.: C=the known impedance presented 4by an aperture interconnecting two vessels having measured pressure differences P1 and P2.

A similar pump having the same physical dimensions but having an anode which is divided into 384 sectors was shown to give an approximate 35% increase in pumping speed over the above mentioned speeds.

While this invention has been described with reference to a single anode structure having a central and outer cathode structure, further embodiments can be envisaged where a plurality of structures can be arranged coaxially with one another. Such a composite structure would give increased pumping speeds for a relatively small increase in physical bulk.

What I claim is:

1. A vacuum ion pump comprising inner and outer reactive cylindrical cathodes, a cylindrical anode structure having a plurality of open sectors of a right annular cylinder arranged radially between the inner and outer cylindrical cathodes, and means to produce a radial magnetic field substantially axial to said anode sectors.

2. A vacuum ion pump as claimed in claim 1, wherein said magnetic means comprises an annular permanent magnet, a central cylindrical pole piece and an outer S (litres/sec.) C (litres/sec.)

hollow cylindrical pole piece, said annular magnet being provided with a yoke at one end for completion of a magnetic circuit between the central and outer pole pieces.

3. A vacuum ion pump as claimed in claim 2 in which said yoke and central pole piece constitute an integral assembly.

4. A vacuum ion pump as claimed in claim 2 wherein the internal surfaces of the central and outer pole pieces are provided with a surface finish having good stable vacuum properties.

5. A vacuum ion pump as claimed in claim 2 wherein the inside surfaces of the central and outer pole pieces vare provided with stainless steel sleeves which together with an annular shaped member form a non-magnetic internal envelope for the pole pieces and yoke.

6. A vacuum ion pump as claimed in claim 2 wherein said inner and outer cathode structures comprise readily removable and replaceable open ended cylinders of a metal selected from the group consisting of titanium, zirconium, tantalum and molybdenum.

7. A vacuum ion pump as claimed in claim Z wherein the ratio of the diameters of the inner cylindrical cathode to the outer cylinder cathode is not less than 1:1.2.

References Cited by the Examiner UNITED STATES PATENTS 3,107,045 10/1963 Zaphiropoulos 230-69 FOREIGN PATENTS 1,179,458 12/1958 France. 1,198,854 6/1959 France. 1,281,403 12/1961 France.

883,189 11/1961 Great Britain.

DONLEY J. STOCKING, Primary Examiner.

WARREN E. COLEMAN, LAURENCE V. EFNER,

Examiners.

Claims (1)

1. A VACUUM ION PUMP COMPRISING INNER AND OUTER REACTIVE CYLINDRICAL CATHODES, A CYLINDRICAL ANODE STRUCTURE HAVING A PLURALITY OF OPEN SECTORS OF A RIGHT ANNULAR CYLINDER ARRANGED RADIALLY BETWEEN THE INNER AND OUTER CYLIN-

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