US3510712A - Electron orbiting getter vacuum pump employing a time varying magnetic field - Google Patents

Description

. y 1970 M. RABINOWITZ E L 3,510,712

, ELECTRON ORBITINGr GETTER VACUUM PUMP EMPLOYING A TIME VARYING MAGNETIC FIELD Filed own-.20, 1967 FIG. I

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0 I00 200 30'0 BY M PEAK MAGNE |c FIELD (GAUSS) NEY AT P=4X IO' Torr 52 I20: h k; 80- AT P= I l0 T0" :1 Lu AT PZX IO' TW |NVENTQRS MARIO RABINOWITZ LAWRENCE T. LAMONT JR.

United States Patent O US. Cl. 313--7 Claims ABSTRACT OF THE DISCLOSURE An electron orbiting getter vacuum pump is disclosed. The pump includes a cylindrical cathode electrode structure surrounding a centrally disposed anode electrode structure. Electrons from a source of electrons are injected into the radial electric field between the anode and cathode structures with a substantial tangential velocity component such that the electrons go into spiral orbits around the anode electrode for ionizing gas in the region surrounding the anode. One or more slugs of getter material are carried on the anode structure for collecting a certain fraction of the orbiting electrons. The collected electrons heat the slugs of getter material, typically titanium, to sublimation temperatures causing the titanium getter material to be sublimed from the slugs and collected on the interior surfaces of the cathode structure. The condensed titanium getter material serves to getter the gases coming into contact therewith and thus to produce a vacuum within the pump envelope and within structures in gas communication with the pump envelope. A solenoid surrounds the pump envelope for producing an axially directed magnetic field in the region between the anode and cathode structures. The solenoid is energized with an alternating current to produce a time varying magnetic field which has a substantial component at a low audio frequency which is orthogonal to the electric field between the anode and cathode structures for periodically increasing the path length of the orbiting electrons and, thus, periodically increasing the ionization of gases within the pump. The periodic increase in the ionization of the gases within the pump substantially increases the pumping speed of the pump for insert gases such as argon which must be ionized in order to be pumped. It is also found that the periodic magnetic field produces a lesser increase in the pumping speed for active gases such as nitrogen. The peak amplitude of a periodic magnetic field is caused to have an intensity greater than a certain minimum field intensity B where B is the minimum magnetic field intensity required to cause an electron emitted from the cathode electrode structure to be sufficiently deflected such as to just miss the anode structure.

DESCRIPTION OF THE PRIOR ART Heretofore, electron orbiting getter vacuum pumps have been proposed which would employ a static axially directed magnetic field in the region between the anode and cathode structures for increasing the path length for the orbiting electrons. Increasing the electron path length increases the ionization of gases within the pump when compared to a similar pump utilizing no axially directed magnetic field. Such a prior pump is described in US. Pat. 3,244,969 issued Apr. 5, 1966. While such a static axial magnetic field improves the ionization of the gases within the pump, by increasing the path length of the orbiting electrons, it also decreases the electron bombardment of the titanium getter slugs forming a part of the anode structure. As a result, less getter material is sub- 3,510,712 Patented May 5, 1970 limed from the anode structure, thus, actually reducing the pumping speed of the pump for certain pressure ranges within which pumping speed of the pump is limited by the rate at which getter material is being sublimed. Such a getter limited region occurs for pumping nitrogen gas at pressure ranges higher than 5 times 10* torr.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved electron orbiting getter vacuum pump.

One feature of the present invention is the provision, in an electron orbiting getter vacuum pump of the type wherein the orbiting electrons bombard a slug of getter material for evaporating same, of means for producing a time varying axial magnetic field in the ionization region of the pump for periodically increasing the path length of the orbiting electrons to increase the ionization of gases as compared to a similar pump without the provision of the magnetic field.

Another feature of the present invention is the same as the preceding feature wherein the time verying magnetic field has a peak amplitude in excess of a certain minimum field intensity B where B is defined by the equation:

Where r is the radius of curvature of the circular path of an electron initially directed radially inward from the cathode and which will just miss the anode slug,

m is the mass of an electron,

e is the charge on an electron, and

V is the electric potential difference between the potential of the anode structure and the potential of the source of orbiting electrons.

Another feature of the present invention is the same as any one or more of the preceding features wherein the time varying magnetic field has a relatively low audio frequency fundamental component.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal schematic sectional view of an orbiting electron getter pump incorporating features of the present invention,

FIG. 2 is a schematic transverse section view of a portion of the pump of FIG. 1 taken along line 22 in the direction of the arrows,

FIG. 3 is a view similar to that of FIG. 2 depicting electron orbits under a different condition of axial magnetic field than that depicted in FIG. 2, and

FIG. 4 is a plot of argon pumping speed enhancement with an AC. magnetic field vs. peak magnetic field intensity in gauss at 3 different pressures within the vacuum pump.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown an electron orbiting getter vacuum pump 1 incorporating features of the present invention. The pump 1 includes a cylindrical vacuum envelope structure 2 which, in the embodiment shown in FIG. 1, also forms the cathode electrode of the pump. Alternatively, a separate cathode electrode structure, in the form of a cylindrical screen or helix may be disposed inside of the cylindrical envelope 2. In this latter case, the envelope 2 need not be cylindrical and, in addition, a plurality of such cylindrical cathode grid stuc- 3 tures may be employed within one envelope. In a typical example, the cathode envelope 2 is made of nonmagnetic stainless steel and has an inside diameter of approximately 6 inches and a length of approximately 19 inches.

An anode electrode structure 3 is concentrically disposed on the axis of the cylindrical cathode structure 2. The anode structure 3 includes a relatively thin refractory rod 4 as of 0.08 inch diameter tungsten rod having a plurality of cylindrical slugs of getter material as of titanium affixed thereto at a number of points along the length of the rod 4. In a typical example, the slugs 5 are each 1 inch long and have an outside diameter of approximately 0.4 inch. The anode rod 4, as of 20 inches in length, is supported at one end from the center of a discshaped feedthrough insulator structure 6 sealed over one end of the cylindrical envelope structure 2. The cathode envelope structure is grounded and a suitable positive potential as of 10 kilovolts is applied to the anode structure 3 from a suitable source of DC. potential 7.

A pair of thermionic filamentary electron emitters 8 are disposed at the upper end of the envelope 2 for injecting streams of electrons into the ionization region between the anode 3 and the cathode 2. A pair of ground plates 11, operating at cathode potential, are disposed adjacent the filamentary emitters 8 to serve as reflectors for reflecting the electrons into the ionization region 9. These ground plates 11 also serve to perturb the electric field in the region of the filamentary emitters 8 such that the electrons are emitted into the ionization region 9 with a substantial tangential velocity such that the electrons enter into spiral orbits about the anode structure 3. In this manner, the path lengths of the orbiting electrons are substantially increased since the electrons travel in spiral trajectories to and fro along the length of the anode structure 3 before being collected on one of the getter slugs 5. The long spiral path lengths of the orbiting electrons greatly increases the probability that such an orbiting electron will suffer an ionizing collision with a residual gas particle within the envelope 2. The filamentary emitters 8 are supported on leads 12 from the feedthrough insulator structure 6. The filamentary emitters are typically heated by AC. current supplied to the filamentary emitters by means of a suitable filament transformer, not shown. In addition, the filamentary emitters are operated at a DC. potential which is positive with respect to ground. The potential is applied to the filamentary emitters 8 from a source 13.

A cooling pipe 14 is spirally wound around the outside of the envelope 2 and a suitable coolant, as of water, is passed through the pipe for cooling the envelope 2. The bottom open end of the envelope 2 includes a flange 15 for connecting the pump envelope 2 to a structure to be evacuated. i

A solenoid 16 is coaxially disposed of and surrounds the vacuum envelope 2 for producing an alternating axially directed magnetic field in the ionization region 9. The solenoid 16 is energized from a source of alternating current 17 at a convenient low audio frequency as of 60 Hz.

In operation, the orbiting electrons spiral around the anode structure 3. In the process, residual gas particles within the envelope 2 are ionized by collision with the electrons and the ions are accelerated to and driven into the cathode electrode 2. A certain fraction of the orbiting electrons bombard the getter slugs 5 to heat the slugs to sublimation temperature. The sublimed getter material is collected on the inside surfaces of the cylindrical cathode structure 2. In the case of inert gases such as argon, the argon gas particles are ionized and driven into the cathode structure 2 where they are buried within a layer of getter material deposited upon the inside surfaces of the cathode. The buried ions are subsequently covered over by condensed getter material. In the case of active gases such as nitrogen the predominate pumping mechanism is one of active gettering of the neutral gas particles when they collide with the condensed layer of getter material deposited upon the inside surfaces of the cathode structure 2.

The time varying axial magnetic field B is caused to have an intensity sufiicient to substantially increase the path lengths of the orbiting electrons. An approximate value for the minimum magnetic field intensity B which will produce a substantial increase in the path length of the orbiting electrons is given by Equation 1 supra.

Equation 1 yields only the approximate minimum value of the magnetic field and is based on certain simplifying assumptions depicted in the model of FIG. 2. More specifically, it is assumed that the electron enters the pump in a radial direction in the absence of the radial electric field with a kinetic energy equal to the potential between the source of electrons and the anode and that the minimum magnetic field intensity B is that value of magnetic field which Will cause the radius of curvature of the electron to be such that the electron just misses the anode slug 5. For a case where the inside diameter of the cathode cylinder 2 is 6 inches and the diameter of the slug 5 is 1 inch, the required radius of the electron orbit is about 4.12 inches when the kinetic energy of the electron is 10 kv. Equation 1 with these parameters predicts that the minimum value of magnetic field intensity B should be equal to 32 gauss. Experimental results, depicted in FIG. 4, indicate that the threshold for i011 pumping enhancement for argon gas comes at a peak magnetic field intensity of about 55 to 65 gauss. Thus, Equation 1 is found to provide a relatively good approximation for the minimum magnetic field intensity required to produce enhancement of the ion pumping.

It is also expected that there will be a magnetic field intensity above which the ion pumping will not be substantially improved. This magnetic field intensity corresponds to a radius of curvature of the electron orbit which is equal to one-half of the radial spacing between the anode slug 5 and the cathode 2, r= /2(ca) as shown in the simplified model of FIG. 3. Substituting this value of r into Equation 1 the resulting equation is:

where ca is the spacing between the anode slug and the cathode, and the other symbols represent the same variables set forth for them in Equation 1.

It is found that the optimum value of magnetic field beyond which one would not expect to increase the electron path length and, hence, the pumping speed is, for the aforecited measurements, approximately 109 gauss. From the experimental results depicted in FIG. 4, it is seen that for a pressure of 1 times 10* torr the optimum magnetic field intensity is approximately 200 gauss. As seen from FIG. 4, a substantial enhancement in the pumping speed of the pump, for inert gases, can be obtained within certain operating pressure ranges by use of the alternating magnetic field.

Although the time varying magnetic field has been described as a 60 Hz. alternating field, this is not a requirement. It is only necessary that the field be time varying to produce a periodic increase ,in the path length of the orbiting electrons to periodically increase the ionization. The time varying magnetic field need not alternate as it may be merely a pulsating unidirectional magnetic field. However, the pulsations of the magnetic field should be characterized by a fundamental Fourier component having a frequency in the low audio range such that the period of the fundamental Fourier component is long compared to the typical lifetime of the orbiting electrons. Otherwise, the orbital path lengths are not substantially increased by the periodic magnetic field. It is also necessary that there be a substantial period of time wherein the axial magnetic field intensity is less than the minimum intensity B such that the path lengths are not substantially increased, thereby permitting the electrons to bombard the titanium getter slugs 5 to produce sufi'icient sublimation of getter material for gettering active gases and for covering over the inert gases buried in the walls of the cathode structure. With a constant axial magnetic field B, pumping speed for active gases like nitrogen is appreciably decreased at pressure above torr where the pump is operating titanium limited. However, use of the alternating magnetic field substantially increases the pumping speed for active gases as compared to the pumping speed obtained with a constant magnetic field. The alternating magnetic field, as compared with the constant magnetic field results in a small loss in pumping speed for inert gases.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In an electron orbiting getter vacuum pump, an anode electrode structure and a cathode electrode structure surrounding said anode electrode structure, means for injecting a stream of electrons into the region between said anode and cathode electrode structures with suflicient angular momentum to cause the injected electrons to go into spiral orbits adjacent said anode electrode structure for ionizing gas in the region surrounding said anode, said anode electrode structure including at least one target slug of getter material for collection of a certain fraction of the electrons to heat said getter slug by electron bombardment and to produce evaporation of said getter material for gettering gases within the pump, means for producing a time varying magnetic field in the region between said anode and cathode electrode structures with a component orthogonal to the electric field between said anode and cathode structures having a minimum amplitude less than, and a peak amplitude greater than, a field intensity B defined by the equation:

. V is the electric potential difference between the potential of the anode electrode structure and the potential of the means for injecting a stream of electrons mto the region between said anode and electrode structures;

said orthogonal component of said time varying magnetic field having a fundamental frequency component in the low audio range assuring that the period of said field is long compared to the typical lifetime of the orbiting electrons, and said orthogonal component being less than the field intensity B for a substantial period of time assuring sufficient electron bombardment of said target slug for sublimation thereof.

2. The apparatus of claim 1 wherein said means for producing a time varying magnetic field in the region between said anode and cathode electrode structures produces such a field having a component orthogonal to the electric field between said anode and cathode electrode structures with a peak amplitude substantially equal to a field intensity B defined by the equation;

L ae c-a e where c-a is the spacing between the anode target slug and the cathode electrode structure, and the other symbols represent the same variables set forth for them in claim 1. 3. The method of operating an electron orbiting getter vacuum pump of the type wherein orbiting electrons are utilized to ionize gases between cathode and anode electrode structures, and getter material is evaporated within the pump from an anode target by orbiting electron bombardment comprising the step of producing a time varying magnetic field in the ionization region surrounding the anode electrode structure having a component orthogonal to the electric field between the anode electrode structure and the cathode electrode structure of the pump of a minimum amplitude less than, and a peak amplitude greater than, a field intensity B defined by the equation:

where r is the radius of curvature in the absence of an electric field of a circular trajectory of an electron which leaves the cathode electrode structure in a radial direction and just grazes said target, In is the mass of an electron, e is the charge on an electron, and V is the electric potential dilference between the potential of the anode electrode structure and the potential of the means for injecting a stream of electrons into the region between said anode and electrode structures;

said orthogonal component of said time varying magnetic field having a fundamental frequency component in the low audio range assuring that the period of said field is long compared to the typical lifetime of the orbiting electrons, and said orthogonal component being less than the field intensity B for a substantial period of time assuring sufiicient electron bombardment of said anode target for sublimation thereof.

4. The method of claim 3 wherein the time varying magnetic field is a relatively low audio frequency alternating magnetic field.

5. The method of claim 3 wherein said time varying magnetic field is produced with said component orthogonal to the electric field between said anode and cathode electrode structure having a peak amplitude substantially equal to a field intensity B defined by the equation:

where c-a is the spacing between the anode target and the cathode electrode structure, and the other symbols represent the same variables set forth for them in claim 3.

References Cited UNITED STATES PATENTS 2,850,225 9/1958 Herb 23069 3,244,969 4/1966 Herb et a1. 313-7 X RAYMOND F. HOSSFELD, Primary Examiner U.S. Cl. X.R. 230-69; 3l3157

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