US20070286738A1

US20070286738A1 - Vacuum ion-getter pump with cryogenically cooled cathode

Info

Publication number: US20070286738A1
Application number: US11/451,170
Authority: US
Inventors: Peter Lukens
Original assignee: Varian Inc
Current assignee: Varian Inc
Priority date: 2006-06-12
Filing date: 2006-06-12
Publication date: 2007-12-13

Abstract

A vacuum ion-getter pump includes a vacuum chamber having a pumping port, an anode positioned in the vacuum chamber, a cathode positioned in the vacuum chamber in proximity to the anode, a voltage source coupled between the anode and the cathode, a magnet assembly to produce a magnetic field in the vacuum chamber, and a cooling device thermally coupled to the cathode. The cooling device may be a cryogenic cooling device.

Description

FIELD OF THE INVENTION

This invention relates to vacuum pumps known as vacuum ion-getter pumps and, more particularly, to vacuum ion-getter pumps having cooled cathodes for improved performance. Vacuum ion-getter pumps are sometimes referred to as sputter ion pumps.

BACKGROUND OF THE INVENTION

The basic structure of a vacuum ion-getter pump includes an anode, a cathode, and a magnet. The anode includes one or more pump cells, which may be cylindrical. Cathode plates, typically titanium, are positioned on opposite ends of the pump cells. A magnet assembly produces a magnetic field oriented along the axis of the anode. A voltage, typically 3 kV to 9 kV, applied between the cathode plates and the anode, produces an electric field which causes electrons to be emitted from the cathode. The magnetic field produces long, more or less helical electron trajectories. The relatively long trajectories of the electrons before reaching the anode improves the chances of collision with gas molecules inside the pump cells. When an electron collides with a gas molecule, it tends to liberate another electron from the molecule, forming a positive ion. The positive ions travel to the cathode due to the action of the electric field. The collision with the solid surface produces a phenomenon called sputtering, i.e., ejection of titanium atoms from the cathode surface. Some of the ionized molecules or atoms impact the cathode surface with sufficient force to penetrate the solid and to remain buried.
Prior art vacuum ion-getter pumps have generally satisfactory performance, but exhibit certain limitations. Such pumps have limited pumping capacity for light gases, such as hydrogen and helium. In addition, such pumps require a starting pressure on the order of 10⁻²to 10⁻³torr in order to begin operation.
U.S. Pat. No. 5,357,760, issued Oct. 25, 1994 to Higham, discloses a so-called hybrid cryogenic vacuum pump wherein a separate cryopump and a separate ion-getter pump are positioned in one vacuum chamber. The disclosed vacuum pump does not overcome the limitations described above.
Accordingly, there is a need for improved vacuum ion-getter pumps and methods for operating vacuum ion-getter pumps.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a vacuum ion-getter pump comprises a vacuum chamber having a pumping port, an anode positioned in the vacuum chamber, a cathode positioned in the vacuum chamber in proximity to the anode, a voltage source coupled between the anode and the cathode, a magnet assembly to produce a magnetic field in the vacuum chamber, and a cooling device thermally coupled to the cathode.
The cooling device may be a cryogenic cooling device, such as a closed cycle refrigerator. The closed cycle refrigerator may have a cold head in the thermal contact with the cathode. The anode may be operated at room temperature or may be cooled.
According to a second aspect of the invention, a method is provided for operating a vacuum ion-getter pump of the type including an anode and a cathode positioned in a vacuum chamber. The method comprises cooling the cathode. The cathode may be cryogenically cooled. The method may further comprise coupling the vacuum chamber to an enclosure to be evacuated, applying a voltage between the anode and the cathode and producing a magnetic field in the vacuum chamber.
According to a third aspect of the invention, a vacuum ion-getter pump comprises a vacuum chamber having a pumping port, an anode positioned in the vacuum chamber, a cathode positioned in the vacuum chamber, and a cryogenic cooling device thermally coupled to the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:

FIG. 1 is a schematic diagram of a prior art ion pump cell;

FIG. 2 is a schematic diagram of a prior art vacuum ion-getter pump; and

FIG. 3 is a simplified schematic diagram of a vacuum ion-getter pump in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A schematic diagram of a prior art ion pump cell is shown in FIG. 1. A cylindrical anode cell 20 has a cell axis 22. Anode cell may be fabricated of stainless steel, for example. Cathode plates 24 and 26 are positioned at opposite ends of anode cell 20 and may be perpendicular to cell axis 22. A power supply 30 applies a voltage, typically 3 kV to 9 kV, between the cathode plates 24, 26 and the anode cell 20. A magnet assembly (not shown in FIG. 1) produces a magnetic field 32 in anode cell 20 parallel to cell axis 22.
A schematic diagram of a prior art vacuum ion-getter pump having multiple anode cells is shown in FIG. 2. Like elements in FIGS. 1 and 2 have the same reference numerals. The ion-getter pump of FIG. 2 includes multiple anode cells 20 a, 20 b, . . . 20 n located between cathode plates 24 and 26. Power supply 30 is connected between cathode plates 24, 26 and anode cells 20 a, 20 b, . . . 20 n.
A magnet assembly 40 includes magnets 42 and 44 located on opposite ends of anode cells 20 a, 20 b, . . . 20 n. Magnet 42 may have a north pole facing anode cells 20 a, 20 b, . . . 20 n, and magnet 44 may have a south pole facing anode cells 20 a, 20 b, . . . 20 n. A magnet yoke 50 of magnetic material provides a return path for magnetic fields between magnets 42 and 44. In the configuration of FIG. 2, magnetic yoke 50 has a generally rectangular shape. In other prior art ion-getter pumps, the magnet yoke may be U-shaped, with an open side. Magnets 42 and 44 produce magnetic field 32 in the region of anode cells 20 a, 20 b, . . . 20 n. The entire assembly shown in FIG. 2 may be enclosed in a vacuum chamber.
The voltage between cathode plates 24, 26 and anode cells 20 a, 20 b, . . . 20 n results in the generation of free electrons in the anode cell volume. These free electrons ionize gas molecules that enter the anode cells. The ionized gas molecules are accelerated to the cathode plates, usually made of titanium or tantalum, resulting in sputtering of the cathode material onto surfaces of the anode cells. The sputtered cathode material readily pumps gas molecules and is the primary pumping mechanism in the ion pump. Secondary electrons produced from the ionization process sustain the plasma in the anode cells so that the pumping action is continuous. The magnetic field axial to the anode cells is required to maintain a long electron path and to sustain a stable plasma in the anode cells.
A simplified schematic diagram of a vacuum ion-getter pump in accordance with an embodiment of the invention is shown in FIG. 3. The pump includes an anode 120 and a cathode 122. Anode 120 includes anode cells 120 a and 120 b in the embodiment of FIG. 3. Cathode 122 includes cathode plates 124 and 126, and end plate 128 in the embodiment of FIG. 3. Anode cells 120 a and 120 b are located between and are spaced from cathode plates 124 and 126. End plate 128 is connected between cathode plates 124 and 126. The ion pump may include one or more anode cells. Each anode cell may have a cylindrical configuration and may be fabricated of stainless steel. The anode cells 120 a, 120 b, are oriented with their axes parallel to each other and perpendicular to cathode plates 124, 126. Cathode plates 124 and 126 and end plate 128 may be fabricated of titanium or tantalum, for example, or other suitable metals or alloys.
A power supply 130 applies a voltage, typically 3 kV to 9 kV, between cathode 122 and anode 120, and more particularly between cathode plates 124, 126 and anode cells 120 a, 120 b. Cathode plates 124 and 126 are electrically connected together, and anode cells 120 a and 120 b are electrically connected together.
A magnet assembly 140 provides a static magnetic field 142 in the region of anode cells 120 a, 120 b to facilitate vacuum ion pumping. In the embodiment of FIG. 3, magnet assembly 140 includes magnets 144, 146, 148 and 150, each of which may be a permanent magnet. It will be understood that different magnet arrangements may be utilized within the scope of the invention.
Anode cells 120 a and 120 b, cathode plates 124, 126 and end plate 128 are positioned with a vacuum chamber 160. Vacuum chamber 160 is sealed vacuum-tight, except for a pumping port 162 configured for attachment to an enclosure to be vacuum pumped. In the embodiment of FIG. 3, magnets 140, 146, 148 and 150 are located outside vacuum chamber 160. In other embodiments, the magnets may be located within vacuum chamber 160.
The cathode 122 is cooled, preferably cryogenically cooled, so as to capture gas molecules by a combination of condensation, sorption and physical burial of accelerated ions. As shown in FIG. 3, cathode 122 is thermally coupled to a cooling device 180. Cooling device 180 may be a cryogenic cooling device, such as a closed cycle refrigerator. Cathode 122 may be thermally anchored to a cold head 182 of a closed cycle refrigerator. Cooling lines and other connections between cooling device 180 and cold head 182 are isolated from the interior of vacuum chamber 160.
One suitable refrigerator is based on the Gifford-McMahon cycle. It will be understood that other refrigerator types, including other cryogenic refrigerators, may be used within the scope of the invention. The refrigerator preferably produces temperatures in the range used in cryogenic vacuum pumps, but cooled cathodes operating at temperatures above the range used in cryogenic vacuum pumps have a positive effect on pumping performance.
As described above, the cathode 122 is cooled and is preferably cryogenically cooled. In other embodiments, anode 120 is also cooled and may be cryogenically cooled. In the embodiment of FIG. 3, cold head 182 may be thermally coupled to anode cells 120 a and 120 b, as indicated schematically by dashed line 190.
In the vacuum ion-getter pump of FIG. 3, gas is pumped by capturing molecules through different mechanisms. One mechanism includes condensation of gas onto the cold cathode surfaces. Other mechanisms are based on creation of ions, confined by the magnetic field 142, that are accelerated into the cathode where they are captured by: (a) chemical combination on the cathode surface forming stable compounds (mainly oxides and nitrides); (b) burial and diffusion of small atoms, such as hydrogen, into the bulk of the cathode; (c) burial of noble gas atoms in the cathode; and (d) more complex molecules, such as water, carbon dioxide and methane, are dissociated in the high voltage discharge and their components are pumped by the above mechanisms.
Advantages of the disclosed pumping scheme include: (1) increased hydrogen pumping capacity due to the low temperature of the cathode, (2) the ability to pump from high starting pressures, and (3) the ability to pump light gases at temperatures well above those of a typical cryogenic pump operating at 20K.
Sievert's law describes the relationship between:

- P=equilibrium pressure of hydrogen in torr;
- Q=concentration of hydrogen in solid solution in the metal cathode in torr-liters/gram;
- T=temperature in Kelvin;
- A, B=coefficients related to the cathode metal.

Sievert's law is stated as:
P=A+2 log Q−B/T
Solving for concentration Q gives
$Q = \sqrt{\frac{P}{10^{A}}} 10^{\frac{B}{2 T}}$
As the temperature goes down, the equilibrium concentration of hydrogen at a given pressure goes up. This fact is well established and is utilized in getter pumps.
Cryocondensation of common gases, such as nitrogen, oxygen, carbon dioxide and water, onto the cryogenic cathode provides the ion pump of the present invention the ability to pump at pressures above the starting limit of the typical vacuum ion pump. When the total pressure is below the vacuum ion pump starting pressure, typically 10⁻²torr, ion pumping begins and gases which do not condense at higher temperatures are captured.
The vacuum ion-getter pump of the present invention can capture light gases, such as helium, hydrogen and neon, at a base temperature above that of a typical cryogenic pump. This reduces the thermal load on the closed cycle refrigerator and decreases the refrigerator's required capacity.
Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.

Claims

1. A vacuum ion-getter pump comprising:

a vacuum chamber having a pumping port;

an anode positioned in the vacuum chamber;

a cathode positioned in the vacuum chamber in proximity to the anode;

a voltage source coupled to between the anode and cathode;

a magnet assembly to produce a magnetic field in the vacuum chamber; and

a cooling device thermally coupled to the cathode.

2. The vacuum ion-getter pump as defined in claim 1, where in the cooling device comprises a cryogenic cooling device.

3. The vacuum ion-getter pump as defined in claim 2, wherein the cryogenic cooling device comprises a closed cycle refrigerator having a cold head in thermal contact with the cathode.

4. The vacuum ion-getter pump as defined in claim 2, wherein the cooling device operates at temperatures used in cryogenic vacuum pumps.

5. The vacuum ion-getter pump as defined in claim 3, wherein the cryogenic cooling device is based on the Gifford-McMahon cycle.

6. The vacuum ion-getter pump as defined in claim 1, wherein the cooling device comprises a cryogenic refrigerator.

7. The vacuum ion-getter pump as defined in claim 2, wherein the cathode comprises spaced-apart cathode plates and wherein the anode comprises a plurality of anode cells positioned between the cathode plates.

8. The vacuum ion-getter pump as defined in claim 2, wherein the magnet assembly comprises permanent magnets positioned outside the vacuum chamber.

9. The vacuum ion-getter pump as defined in claim 2, wherein the anode operates at or near room temperature.

10. The vacuum ion-getter pump as defined in claim 2, wherein the anode is thermally coupled to a cryogenic cooling device.

11. The vacuum ion-getter pump as defined in claim 2, wherein the voltage source maintains a voltage in a range of 3 to 9 kilovolts between the anode and the cathode.

12. A method for operating a vacuum ion-getter pump of the type including an anode and a cathode positioned in a vacuum chamber, the method comprising:

cooling the cathode.

13. The method as defined in claim 12, wherein cooling the cathode comprises cryogenically cooling the cathode.

14. The method as defined in claim 12, wherein cooling the cathode comprises operating the cathode at temperatures used in cryogenic vacuum pumps.

15. The method as defined in claim 13, further comprising operating the anode at room temperature.

16. The method as defined in claim 13, further comprising cooling the anode.

17. The method as defined in claim 12, further comprising:

coupling the vacuum chamber to an enclosure to be evacuated;

applying a voltage between the anode and the cathode; and

producing a magnetic field in the vacuum chamber.

18. A vacuum ion-getter pump comprising:

a vacuum chamber having a pumping port;

an anode positioned in the vacuum chamber;

a cathode positioned in the vacuum chamber; and

a cryogenic cooling device thermally coupled to the cathode.

19. The vacuum ion-getter pump as defined in claim 18, further comprising a magnet to produce a magnetic field in the vacuum chamber.