Patent Application
20070286738
For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:
A schematic diagram of a prior art ion pump cell is shown in
A schematic diagram of a prior art vacuum ion-getter pump having multiple anode cells is shown in
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
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.
A simplified schematic diagram of a vacuum ion-getter pump in accordance with an embodiment of the invention is shown in
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.
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
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
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
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
In the vacuum ion-getter pump of
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:
Sievert's law is stated as:
P=A+2 log Q−B/T
Solving for concentration Q gives
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 10−2 torr, 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.
