DE2013106A1 - Ion getter vacuum pump - Google Patents

Description

Applicant: United States Atomic Energy Commission Washington D. G., USA

Ion generator vacuum pump

The invention "relates to those provided with a control grid Ion getter vacuum pumps, in which the in ole is vacuum-tight Housing located gas molecules ionized by circulating electrons and by sublimated getter material be covered.

In ion getter vacuum pumps, electrons are introduced into the pump introduced with such energy «that those in the pump» Gas molecules are ionized by collision "

The gas molecules generated in this way are caused by electric fields in the pump accelerates so that they are on appropriate Impinge on the pump surfaces and penetrate into them.

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2013 109

getter material (e.g. Ti or Ta) is sublimated in time or deposited in some other suitable manner on these surfaces so that the ions are further covered. That freshly deposited getter material also reacts with chemically active gases without previously required ionization. The pumping action is based on the transfer of molecules from the gas phase to the solid phase.

The constant supply of getter material is z. B. by bombardment with the also used to ionize the gas Electrons secured for the purpose of heating to the sublimation temperature. This has compared to the also common Resistance heating, the undesired thermal degassing of the electrical connections and adjacent parts such. B. Insulators and supports has the advantage of thermal insulation of the getter material. In the case of sublimation However, due to electron bombardment, there are mutually exclusive requirements for the respective strength of the electric field used in the pump. On the one hand, the electrostatic field should be as low as possible so that the electron path is as long as possible before being captured at an electrode. A long electron orbit increases the Probability of the electron collision with the gas molecules. A low field also allows an average

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before kinetic energy of the orbiting electrons, which the corresponds to the maximum ionization cross-section of the gas, so that a greater probability of ionization of the gas molecules "is established in the collision. On the other hand, and inconsistent with this, a strong den is for heating the getter material to the sublimation temperature Electrostatic field to impart the required initial energy to electrons. Such a strong field but shortens the electron path, reduces the ionization and gives the electrons a higher than the optimal one average kinetic energy.

The replacement of burned-out hot cathodes is also unfavorable and difficult, since this interrupts the vacuum in the pump chamber must become. . .

The invention has an ion getter takuumpump for the task with which an optimal ionization of the gas molecules and an optimal sublimation of the getter material independently of one another is possible.

This object is achieved by the ion getter vacuum pump of the invention, in which the gas molecules located in a vacuum-tight housing are ionized by circulating electrons and are covered by sublimed getter material

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solved by that a grid is arranged between the housing and an anode provided with a getter material, an electron gun or the like pushes electrons into the space between the grid and the housing, which there collide with the gas molecules and ionize them by suitable means between the grid and the housing a first electrostatic field is built up, which brings the electrons to the grid and the ionized gas molecules pulls to the housing, and by other suitable means between the grid and the anode a second electrostatic Field is built up that accelerates the injected electrons and shoots at the getter material, so that this sublimates on the inner surface of the pump housing will.

In this design, the greater part of the electron orbit is under the influence of a relatively low one electrostatic field between the grid and the pump housing. After passing through the grid, the electrons arrive under the influence of a relatively high electrostatic field between the grid and the anode. This gives the electrons such a high energy that when they hit the getter material they hit the Heat the sublimation temperature. As a result, it is sublimated on the inner surface of the pump housing and covers the Gas molecules.

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Because the electrons are through two independent electrostatic Fields can be influenced, each field in the With a view to optimizing in its area without adversely affecting the area of the other field will. Thus, the field between the pump housing and the grille can be independent in terms of the maximum Ionization of the gas molecules can be adjusted while the field between the grid and the anode is independent can be adjusted with regard to the optimal sublimation speed.

According to a further embodiment of the invention, the electron gun or the like for injecting the electrons into the pump housing so arranged, and with such, Means described below provided that the filament cathode can be replaced without the vacuum in the Interrupt the pump housing.

Further details and favorable configurations of the invention can be found in the subclaims and the following description can be taken from the drawings «

Show in the drawings

FIG. 1 shows the ion getter valmumpump in cross section Invention;

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FIG. 2 shows this in section along the section line 2-2 of FIG. 1;

FIG. 3 shows a further embodiment in cross section two hot cathodes mounted inside the pump.

As FIG. 1 shows in cross section, the ion generator vacuum pump contains a for connection to a not shown, Vacuum chamber to be pumped out at one end 11 of the open housing 12. The open end 11 contributes to the production of this Connection to the vacuum chamber has a flange 14 ·. By the electron gun built into one end of the cylindrical housing 31 electrons are injected into the interior of the housing. The electron spinner forms with the central axis of the housing an angle so that the electron injection is tangential with an axial component he follows. This is particularly favorable for achieving a long spiral path of a large number of electrons, however, as explained below, the axial component is not an absolute prerequisite for this. The figure As an example, FIG. 1 shows a typical angle, while FIG. 2 shows the tangential position of the electron gun to the housing illustrated. The electron centrifuge 31 consists of one which is removably fastened in the centrifuge housing 34 Hot cathode which is connected to the voltage source 36. The electrons emitted from the hot cathode 33

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are focused by the negatively biased focusing cup. At the entry end of the electron gun is the one with a central opening that is adjustable and thus the voltage source 38 which supplies the desired electron current in each case is attached to the accelerating anode 37 connected to it. The electrons are in an initially between the housing wall of the housing 12 and a cylindrical grid 22 extending path shot into the housing. Now, however, an electrostatic field is generated by a variable direct current voltage source 28 between the housing 12 and the grid 22 which is positively biased towards it and maintaining that at both ends of the housing an axial direction towards the opposite end of the housing Has component. The electrons injected into one end of the housing therefore become one at the opposite end Directional moment of motion imposed at the end, so that it is continuously along a spiral circular path 32 around the grid 22 and along it until its angular moment is caused by field asymmetries or by collision with Gas molecules is consumed.

A grid 16 electrically connected to the housing 12 is placed over the open end 11 of the housing 12. This grid 16 allows the passage of neutral gas particles from the vacuum pump into the pump housing. Since the grid 16 in electrical

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Respect a continuous surface with the pump housing "forms, it reflects the electrons in the same way as the housing wall when approaching this. In various, suitable ways, e.g. B. by adjusting the hot cathode bias voltage, The electrons can have an energy between the potential energy of the housing and that of the cylindrical grid 22 lying total energy are injected. At this medium energy level, the electrons can themselves in the absence of the grid 16 neither reach the housing nor get out of the field area of the pump, although they do something extend higher than with the existing grid, before they reach the equipotential at which the entire axial kinetic Energy is converted into potential energy so that the electrons are now reflected back down into the pump will.

An anode electrode 18 is insulated from the housing 12 along the center axis of the latter by the insulating bush 19 appropriate. A suitable getter material, for example in the form of cylindrical pieces made of titanium 20, is attached along the anode. The cylindrical grid 22 is concentric to the housing 12 and to the electrode 18 on one of the grid against the remaining pump parts electrically insulating attachment 24 attached. A shield 25 surrounds this insulating attachment 24 and prevents the passage of vaporized material

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termaterial on the fastening. One over the insulating bushing Connection 26 passed through the pump housing 23 connects the grid 22 to the positive terminal of a variable DC voltage source 28, the negative terminal of which mib the Pump housing 12 is connected. A cylindrical shield shields both insulators 19 and 23 from evaporated getter material and at the same time serves as a support for the insulating fastening 24. A second adjustable direct current voltage source 29 is placed with its positive terminal on the lower end of the anode electrode 18, while its negative Connection is connected to the pump housing.

During operation of the pump 10 is by the voltage source 28 between the cylindrical grid 22 and the cylindrical Housing 12 a first electrostatic field with radial and axial, but essentially no azimuthal components built up. The pump injects the electrons with sufficient initial angular momentum that the The radius of their path does not become too small and they cannot be captured by the grid 22. The angular moment of the um the lattice of spirally circling electrons is retained because the field does not give the electrons any rotational force Can impose pump axis. The electrons therefore continue on their spiral-shaped path until they reach their angular moment by gas collisions or by disturbances of the cylindrical

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Lose field symmetry. Those existing at the ends of the pump Axial components of the field reflect the electrons back to the center of the pump. The electrons therefore run on a spiral path around the grid from one to the other end of the pump and back again.

After losing the angular moment, most of the electrons are so weakened that they fall through the grid 22 and in the area of influence of the built up by the direct current voltage source 29 between the grid 22 and the anode 18, relatively strong electrostatic field, which accelerates them, so that they hit the getter material 20 hit the anode 18 and heat it. Through appropriate Setting the acceleration voltage 29 can set the sublimation independently to the respective target value will.

For the highest possible ion generation, the average kinetic energy of the electrons correspond to the maximum ionization of the inert gases present. The ionization cross-section the gas begins at a certain threshold value of kinetic electron energy, increases rapidly to a maximum value and then slowly decreases with increasing kinetic electron energy. Therefore lies the average kinetic electron energy above or below

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L. corresponding to the maximum ionization cross-section, the average cross-section can be considerably below the maximum. In known ion getter pumps, one and the same electrons fulfill the double function of ionization as well as sublimation.Since a high voltage has to be applied to the anode for the required sublimation temperature, the average kinetic electron energy here is considerably higher than the value corresponding to the maximum ionization cross section. These pumps therefore generate much less ionization than the possible maximum. If one tries to reduce the anode voltage in these known pumps to such an extent that the electrons receive the optimal average kinetic energy, then, as a result of the lower sublimation, there is less getter material for the connection with. the active gas available, so that the pumping speed of the active gas is correspondingly lower. But the pumping speed of the inert gas is also lower, despite the higher ionization, since the gases must be both ionized and covered by the sublimated getter material for permanent removal.

In contrast, according to the invention, the cylindrical grid 22 is on a suitable average, the electrons their optimal kinetic energy is kept corresponding to the maximum ionization-imparting voltage Vp, the correct one

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BATH ORIGINAL

The grid tension can be determined experimentally. In practice the pump may contain more than one inert gas, e.g. B. helium, argon and methane. In this case it is entirely possible, and in some cases favorable, to optimize the pump performance for each gas separately and in a specific order. As a guideline for varying the various The following approximate equation can be used for the parameters:

~ T - Ε *

where V ~ is the grid voltage of the grid 22, R is the radius of the pump housing 12, r is the radius of the grid 22, T is the average nominal value of the kinetic electron energy, E. is the total electron energy and β is the electronic charge.

The anode 18 is placed on the for the desired rate of sublimation required variable voltage maintained. Due to the electrostatic field between the housing and the Grid 22 a decoupling of grid and anode is achieved, so that the sublimation process of the for the maximum Ionization required electron orbit separated, although the electrons here also perform the double function of ionization and meet the sublimation. The use of separate

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electric fields also makes it possible on the anode to attach a large amount of getter material without as is known in ' Ion getter pumps with only one electric field to increase the diameter of the anode and thus to shorten the path length of the circulating electrons. Another shortening of the orbit in known pumps will be caused by the amplification of the electric field, which in turn leads to the output of the increased power for heating the larger amount of getter is required. Since, according to the invention, the conditions of long electron paths and sublimation are separated, a large amount of getter can be attached to the anode and thus the service life of the pump is considerable can be extended without affecting the pumping speed.

For the exchange of the hot cathode 33 of the electron centrifuge 31 there is a valve through the wall of the centrifuge housing 34- guided, and fixed in the same. The valve 41 includes one aligned with the central axis of the centrifugal housing Passage 40 through which the cathode 33 »of the focusing cup 35 and the anode 37 are guided and through the adjusting bellows 46 can be brought into any position relative to the inlet opening 39 of the pump. The cathode 33, the focusing cup and the anode 37 are arranged on the insulating sleeve 47, which in turn is attached to the bellows 46. Through the

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Socket, the connections are also led to the appropriate voltage source. If the hot cathode 33 burns out, the bellows 46 is withdrawn, the valve 41 is closed and the pump housing is thus sealed, the valve seat covering the opening 40. The cathode can now be removed and replaced without affecting the vacuum. The arrangement also enables the distance between the cathode 33 and anode 37 to be adjusted. After the cathode 33 'has been replaced , the centrifugal housing can be pumped out through the opening 42 before the valve 41 is opened and the bellows 46 is pulled out. The valve 42 is either a very small valve or a pinch tube. The arrangement means a further extension of the pump.

The housing is so strongly cooled by the cooling coils 45 that that there is no significant gas desorption, especially on the pump housing itself.

In the arrangement explained, the electrons run in one long spiral path and the electron slingshot gives them an axial component; further is the electron slingshot attached outside the housing 12 for better insulation. Satisfactory pump operation is also possible with the Formation of Figure 3 possible. Here two hot cathodes 50 are attached and sealed in the lower end of the pump housing; their bias is negative to the space potential »that

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otherwise it would prevail at the cathode point and would be positive the housing 10. Also in this training, the electrons forced a spiral path as the cathodes 50 near the end of the housing where the field between the grid and the housing has an axial component and therefore imposes an axial moment of motion on the electrons.

In a further embodiment, for. B. the elarfcrone bullet in the arrangement of FIG. 1 in the manner shown in FIG. 3; for this purpose, the anode 37 and the focusing cup 35 are omitted, while the cathode 33 is arranged at a point i »GeJiätise 12 w:? .rd _? at which the positive potential of the first, electrostatic field acts as an anode and removes the electrons from the cathode 33.

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Claims (9)

Jt Claims l ^ ion getter vacuum pump, in which the in a vacuum-tight Housing "located gas molecules ionized by circulating electrons and covered by sublimated getter material are, characterized in that between the housing (12) and one provided with a getter material (20) Anode (18) a grid (22) is arranged, an electron gun or the like electrons in the space between ψ the grid and the housing, which collide there with the gas molecules and ionize them, a first electrostatic field is built up by suitable means between the grid and the housing, which the electrons to the
Grid and pulls the ionized gas molecules to the housing, and by further suitable means between the grid and the anode a second electrostatic field is built up, which accelerates the injected electrons and shoots at the getter material, so that this is on the inner surface fc of the pump housing is sublimated. Q098Z »1/1 205 2. ion getter vacuum pump according to claim 1, characterized by means of adjusting the initial kinetic energy of the electrons and means of adjusting the first electrostatic field to a level that adjusts the kinetic energy of the electrons to a height corresponding to the maximum ionization cross-section of the gas molecules. 3 ion getter vacuum pump according to claim 1, characterized in that that the first electrostatic field is on a potential gradient below the second electrostatic field is held and means for adjusting the second field to that of the optimal sublimation of the getter material appropriate height are provided by the electrons. 4. ion getter vacuum pump according to claim 1, characterized in that that the first electrostatic field with a cylindrical symmetry around the grid with an elongated, radial, the electrons to the grid and the ionized gas molecules to the housing wall pulling center zone and one the electrons attracting this central zone, at this adjacent end zone with radial and axial components is being built. 009841/1205 5. ion getter vacuum pump according to claim 1, characterized in that that the housing is cylindrical and the electron gun is arranged so that the electrons be shot in tangential and partially axial direction into the interior of the housing. 6. ion getter vacuum pump according to claims 1-4, characterized in that the electron centrifuge outside of Housing (12) is arranged, 7. ion getter vacuum pump according to claims 1-5 or 6, characterized in that the electron centrifuge a contains removable hot cathode (33) and means for sealing the pump housing for removing the hot cathode. 8. ion getter vacuum pump according to claim 7 »characterized in that that the seal consists of a valve (41). 9. ion getter vacuum pump according to claim 7 or 8, characterized in that the housing containing the hot cathode can be pumped out via a valve opening (42). 009841/1205