U.S. patent application number 11/451170 was filed with the patent office on 2007-12-13 for vacuum ion-getter pump with cryogenically cooled cathode.
This patent application is currently assigned to Varian, Inc.. Invention is credited to Peter Lukens.
Application Number | 20070286738 11/451170 |
Document ID | / |
Family ID | 38822200 |
Filed Date | 2007-12-13 |
United States Patent
Application |
20070286738 |
Kind Code |
A1 |
Lukens; Peter |
December 13, 2007 |
Vacuum ion-getter pump with cryogenically cooled cathode
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.
Inventors: |
Lukens; Peter; (Tucson,
AZ) |
Correspondence
Address: |
Varian Inc.;Legal Department
3120 Hansen Way D-102
Palo Alto
CA
94304
US
|
Assignee: |
Varian, Inc.
|
Family ID: |
38822200 |
Appl. No.: |
11/451170 |
Filed: |
June 12, 2006 |
Current U.S.
Class: |
417/49 |
Current CPC
Class: |
F04B 37/08 20130101;
F04B 37/02 20130101 |
Class at
Publication: |
417/49 |
International
Class: |
F04B 37/02 20060101
F04B037/02 |
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.
Description
FIELD OF THE INVENTION
[0001] 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
[0002] 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.
[0003] 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.sup.-2 to 10.sup.-3 torr in order to
begin operation.
[0004] 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.
[0005] Accordingly, there is a need for improved vacuum ion-getter
pumps and methods for operating vacuum ion-getter pumps.
SUMMARY OF THE INVENTION
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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
[0010] For a better understanding of the present invention,
reference is made to the accompanying drawings, which are
incorporated herein by reference and in which:
[0011] FIG. 1 is a schematic diagram of a prior art ion pump
cell;
[0012] FIG. 2 is a schematic diagram of a prior art vacuum
ion-getter pump; and
[0013] 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
[0014] 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.
[0015] 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 20a, 20b, . . . 20n located
between cathode plates 24 and 26. Power supply 30 is connected
between cathode plates 24, 26 and anode cells 20a, 20b, . . .
20n.
[0016] A magnet assembly 40 includes magnets 42 and 44 located on
opposite ends of anode cells 20a, 20b, . . . 20n. Magnet 42 may
have a north pole facing anode cells 20a, 20b, . . . 20n, and
magnet 44 may have a south pole facing anode cells 20a, 20b, . . .
20n. 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 20a, 20b, . . . 20n. The entire assembly
shown in FIG. 2 may be enclosed in a vacuum chamber.
[0017] The voltage between cathode plates 24, 26 and anode cells
20a, 20b, . . . 20n 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.
[0018] 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 120a and 120b 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 120a and 120b 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 120a, 120b, 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.
[0019] 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 120a, 120b. Cathode
plates 124 and 126 are electrically connected together, and anode
cells 120a and 120b are electrically connected together.
[0020] A magnet assembly 140 provides a static magnetic field 142
in the region of anode cells 120a, 120b 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.
[0021] Anode cells 120a and 120b, 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.
[0022] 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.
[0023] 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.
[0024] 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 120a
and 120b, as indicated schematically by dashed line 190.
[0025] 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.
[0026] 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.
[0027] Sievert's law describes the relationship between: [0028]
P=equilibrium pressure of hydrogen in torr; [0029] Q=concentration
of hydrogen in solid solution in the metal cathode in
torr-liters/gram; [0030] T=temperature in Kelvin; [0031] A,
B=coefficients related to the cathode metal.
[0032] Sievert's law is stated as:
P=A+2 log Q-B/T
[0033] Solving for concentration Q gives
Q = P 10 A 10 B 2 T ##EQU00001##
[0034] 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.
[0035] 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.sup.-2 torr, ion pumping begins and gases which do not
condense at higher temperatures are captured.
[0036] 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.
[0037] 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.
* * * * *