US3258194A - Magnetically confined glow discharge apparatus - Google Patents

Description

June 28, 1966 s. L. RUTHERFORD 3,258,194

MAGNETIGALLY CONFINED GLOW DISCHARGE APPARATUS Filed July 29, 1963 fi oils r'ANT d 2.0? O. V m D I KPRESENT 0: INVENTION Y 0g 0 l 9 m I I .5 g m I I P(TORR)1 QPCOIH'ORR) O coz cm 10-" lolo- |om INVENTOR. SHERMAN L. RUTHERFORD Byway-2J6! (l TORNEY United States Patent Ofi ice 3.25am Patented June 218, 1966 3,258,194 MAGNETICALLY CONFINED GLOW DISCHARGE APPARATUS Sherman Lloyd Rutherford, Palo Alto, Calif, assignor to Varian Associates, Paio Alto, Calif., a corporation of California Filed July 29, 1963, Ser. No. 298,109 6 Claims. ((11. 230-69) This invention relates in general to magnetically confined glow discharge devices and more particularly to an improved anode for such devices, having critical proportions whereby ion current at low pressures is maintained to lower pressures than in existing devices. This critically proportioned anode extends to very low pressures, the operations of devices utilizing the magnetically confined glow discharge principle such as, for example, electrical vacuum pumps and vacuum gauges.

Heretofore, vacuum pumps and vacuum gauges have been built, having for their principle of operation, the establishment of a magnetically confined glow discharge within the interior of an open-ended tubular anode disposed between and spaced apart from two cathode plates and having a magnetic field threaded through the anode. Positive ions produced by the magnetically confined glow discharge are directed against the cathode plates. In the pump, the impinging ions produce sputtering of a reactive cathode material. The sputtered material is collected upon the interior surfaces of the pump where it serves to entrap molecules in the gaseous state coming in contact therewith. In this manner, the gas pressure within a vessel enclosing the cathode and anode elements is reduced. In the vacuum gauge, the impinging ions are collected by the cathode and the ion current is measured to give an indication of the gas pressure within the device to which the gauge is attached.

There is a relationship between pumping speed S, current I, and pressure P, S=K(I/P) where K is a parameter determined by pump geometry, operating conditions and the like. For prior art devices, discharge intensity (UP) is reduced at low pressures. Therefore, pumping speeds (for pumps) or sensitivity (for gauges) is also reduced. Maintaining the discharge intensity or ion current at low pressures extends operating performance by increasing the speed of the pump, or sensitivity of the vacuum gauge, at the low pressures.

According to the teachings of the US. Patent 3,028,071 dated April 3, 1962, and assigned to the same assignee as the present invention, there exists a critical minimum diameter d for the individual tubular anode compartments for each certain value of magnetic field intensity B. Ion currents for cells of diameter less than d for practical purposes, goes to zero. Furthermore, for a given magnetic field intensity B, there exists an optimum diameter d related to d and a product Bd which produces a maximum ion current for discharge intensity in the pressure range of 10- to'10 torr. The normal approach is to choose an economical permanent magnet that produces maximum magnetic field intensity B for minimum expense. The anode is then designed to fit the magnet and give the product Bd with a cell diameter d to obtain maximum ion current in the 10* to 10- torr pressure range. In a typical example, where a magnetic field intensity B of one kilogauss is utilized, a cell diameter of 0.5 inch will provide maximum ion current.

According to the teachings of copending application Serial No. 155,737, filed November 29, 1961, and assigned to the same assignee as the present invention, now US. Patent 3,174,069, issued March 16, 1965, if the available magnetic field is very large, for example, greater than three kilogauss, and the teachings of US. Patent 3,028,071 were followed, the anode cell diameter for maximum ion nearly constant.

current would be too small to fabricate economically, and power density such, that cooling of the anode would become a problem. Accordingly, Serial No. 155,737 teaches that for magnetic fields greater than three kilogauss, a most economical and easily fabricated anode structure is obtained in the l0 to 10- torr pressure range if the product of magnetic field intensity B and cell diameter d is approximately 2.5 kilogauss-inches. In a typical example, where a magnetic field intensity B of 17 kilogauss is utilized, a cell diameter of 0.15 inch will provide maximum ion current.

Measurements of the pumping speed of the magnetically confined glow discharge devices described in US. Patent 3,028,071 and Serial No. 155,737 were made at high pressures (high pressures in this context means l0='= to 10* torr), where the pumping speed, and the value of (UP), discharge intensity were found to be Since, with careful techniques, it is possible to reach 5X10 torr with these devices, it was assumed that pumping speed was maintained at these low pressures. It has been observed, however, that the pumping speed for the above magnetically confined glow discharge devices decreases at low pressures (be-low 10* torr).

In the present invention it was discovered that by increasing the Ed product, in particular d, the cutoff pressure (cutoff pressure is defined as that pressure where the UP ratio has fallen to one half its maximum value) is decreased, resulting in maintenance of pumping speed at the very low pressures.

Accordingly, it is the object of this invention to provide an improved magnetically confined glow discharge device having certain critical anode proportions whereby operating performance at low pressures is extended by maintaining the UP ratio, and thereby, pumping speeds at these pressures. One feature of the present invention is the provision of an anode cell for a magnetically confined glow discharge device wherein the product of the cell diameter d and magnetic field intensity B is greater than 1.0 kilogauss-inches, while the field intensity B lies within the 0.5 to 3.0 kilogauss range.

Another feature of the present invention is the pro vision of an anode cell for a magnetically confined glow discharge device wherein the cell diameter d lies within the 1 /2 to 2 /2 inch range while the magnetic field intensity B lies within the 1.0 to 2.0 kilogauss range.

These and other features and objects of the present invention and a further understanding may be had by referring to the following description and claims, taken in conjunction with the following drawing in which:

FIGURE 1 is a schematic block diagram depicting a typical evacuation system utilizing a novel magnetically confined glow discharge vacuum pump constructed in accordance with the teachings of the present invention;

FIGURE 2 is a plan view partly in cross-section of an improved magnetically confined glow discharge vacuum pump apparatus of the present invention;

FIGURE 3 is a cross-sectional view of the apparatus of FIGURE 2 taken along the lines 33 in the direction of the arrows;

FIGURE 4 is a graph of pumping speed versus pressure for high and low Ed; and,

FIGURE 5 is a graph of Ed versus cut-01f pressure, for a fixed anode cell diameter d and for a fixed magnetic field intensity B.

Referring now to FIGURE 1, there is shown in schematic block diagram form the improved magnetically confined glow discharge vacuum pump apparatus of the present invention as utilized for evacuating a given structure. More specifically, the vacuum pump 11 is connected by means of a conduit and through a first high vacuum joint 12 containing a pair ofultra-high vacuum flanges and a metal gasket (not shown) to a structure 13 which it is desired to evacuate. A vacuum sorption pump 14 is'also connected to the structure 13 to be evacuated by means of conduits and through a second high vacuum joint 15 and a high vacuum valve 16. Frequently, a mechanical pump is used instead of the vacuum sorption pump 14. To evacuate the structure, the vacuum sorption pump 14 is immersed in a refrigerating liquid 17, as, for example, liquid nitrogen, held in an open vessel 18. Gas molecules are sorbed by the pump 14 from the structure 13 serving to reduce the pressure within the structure 13 to between 5 and or lower microns at which point the valve 16 is closed and the vacuum pump 11 started.

, The pump 11 is supplied with operating potentials from a source 19 as, for example, a 60-cycle power line via a transformer 20. The secondary of the transformer 20 is provided with a rectifier 21 and a shunting smoothing capacitor 22 whereby 'a DC. potential may be applied between anodeand cathode members of the vacuum pump 11, which pump will be more fully described below.

Referring now to FIGURES 2 and 3, a shallow rectangular, cup-shaped member 23 as, for example, stainless steel is closed off at its flange open end by a rectangular closure plate 24 welded about is periphery to the flanged portion of the member 23, thereby forming a substantially rectangular vacuum-tight envelope 25.

A rectangular, cellular anode 26 as of, for example, titanium is carried upon the end of a conductive rod 27 as of, for example, stainless steel which extends outwardly of the rectangular vacuum envelope through an aperture in a short sidewall thereof. The conductive rod 27 is insulated from and carried by the vacuum envelope 25 through the intermediary of annular insulator frames 28, 29, 30 as of, for example, Kovar and cylindrical insulator 31 as of, for example, alumina ceramic. The free end of the rod 27 serves to provide a terminal for providing a positive anode voltage with respect to two substantially rectangular cathode plates 32. The cathode plates 32 are made of a reactive metal and are mechanically locked into position against the large fiat sidewalls of the vacuum envelope 25 via the intermediary of two cathode spacer plates 33. i

The cathode spacer plates 33, as of stainless steel, are provided with semi-cylindrical cars 34 struck therefrom for assuring proper spacing between the cathode plates 32. In a preferred embodiment, the anode-to-cathode spacing lies within the range of between /a and inch. The cathode plates 32 may be made of any one of a number of reactive cathode metals such as, for example, titanium, chromium, zirconium, gadolinium and iron. However, it is desirable, in order to prevent flaking of the condensed, sputtered layer of cathode material, to make the anode 26 and the cathode 32 of the same material.

Another sidewall 35 of the vacuum envelope 25 is apertured to receive the hollow conduit, which may be of any convenient inside diameter commensurate with the desired pumping speed. The hollow conduit communicates with the structure 13 which is desired to be evacuated and is provided with a suitable mounting flange.

A circular radial shield 36 as of, for example, molybdenum is carried transversely of the conductive rod and is disposed inside the first frame member 28 for shield ing the insulator 31 from sputtered cathode material which might otherwise coat the insulator 31 and produce unwanted voltage breakdown or current leakage thereacross. An annular spring 37 is positioned circumscribing the frame member 29 to provide a quick disconnection between the power connector, not shown, and the pump 11.

A horseshoe-shaped permanent magnet 38 is positioned with respect to the rectangular vacuum envelope 25 such that the magnetic field B is critically related to the diameter d of the individual cellular anode compartments in 4 accordance with a newly discovered principle which will be more fully described below. Although permanent magnets are shown, electromagnets may be used advantageously.

In operation, a positive potential of 2.0 kilovolts or more is applied to the anode 26 via the conductive rod 27. The vacuum envelope 25 and, therefore, the cathode plates 32 are preferably operated at ground potential to reduce hazard to operating personnel. With these potentials applied, a region of intense electric field is produced between the cellular anode 26 and cathode plates 32. This electric field produces a breakdown of gas within .the pump resulting in a glow discharge within the cellular anode 26 and between the anode 26 and the cathode plates 32. The glow discharge results in positive ions being driven into the cathode plates 32 to produce dislodgment of reactive cathode material which is thereby sputtered onto the nearby anode 26 to produce gettering ,of molecules in the gaseous stage coming in contact therewith. In this manner, the pressure within the vacuum envelope 25 and, therefore, the structure 13 communicating therewith, are evacuated.

Referring now to FIGURES 4 and 5, there is shown a graphical representation of the newly discovered principle of the present invention. More specifically, it has been discovered that by increasing the'Bd product, in particular d, discharge intensity is maintained at low pressures.

FIGURE 4 is a plot of pumping speed or discharge intensity versus pressure for vacuum pumps of different Bd products. Curve 1 is characteristic of the prior art devices having a low Bd productv Curve 2 is characteristic of vacuum pumps constructed in accordance with the teachings of the present invention having a high Bd product. The graph demonstrates the fact by increasing the Ed product, one sacrifices a certain amount of pumping speed at the higher pressures in exchange for a considerably lower cut-ofi? point at the lower pressures.

In one series of experiments, the cell diameter d was fixed and cut-off pressure observed for increasing Bd. Curve X of FIGURE '5 graphically depicts the results obtained. In another series of experiments, magnetic field intensity B was fixed and cut-off pressure observed for increasing Bd. Curve Y of FIGURE 5 graphically depicts the results obtained from this set of experiments. The most interesting fact demonstrated by these curves is that, while again cut-off pressure decreases with increase in the Ed product, increasing cell diameter d gives the most rapid reduction in cut-off pressure.

A design procedure for maintaining pumping speeds at low pressures by decreasing the cut-off pressure is suggested from the teachings of the above graphs. Ordinarily, strong magnetic fields are costly to produce, such that economics will normally dictate a certain maximum magnetic field intensity. Economical permanent magnets can be found today which produce a maximum magnetic field of up to 3 kilogauss for minimum expense. Fields below 0.5 kilogauss would not be useful, however, since the diameter d would have to be so large that pumping speeds per unit volume would be too low. The anode of the pump is then designed for the given cell sizes, such that the B0? product is greater than 1.0 kil-ogauss-inches. By additional experimentation it has been determined that keeping the Ed product greater than 1.0 kilogauss-inches assures a lower cut-off pressure than heretofore known. In the preferred embodiment d lies Within the 1 /2 to 2 /2 inches while B is between 1.0 and 2.0 kilogauss. The anode cell length I (see FIGURE 3) was selected to be 1 to 1.5 times the diameter d of the cell. Also, the shape of the cell is not limited to cells of square cross section,

but would apply equally as well to different configurations, for example, those of circular or hexagonal cross-section.

Thus, the diameter d refers to the smallest circle which can be circumscribed in the opening transversely to the axis of the shell, regardless of its cross-sectional configuration.

The teachings of the present invention have applicability to vacuum gauges. More specifically, it has been discovered that the principles set forth above with regard to critical proportions of anode dimensions for magnetically confined glow discharge vacuum pumps are equally applicable to anode proportions of magnetically confined glow discharge vacuum gauges of the type as taught by U.S. Patent 2,197,079 to F. M. Penning.

For gauges, it is important that the discharge intensity, I/P, be maintained constant over long ranges of pressure since current is used as a direct indication of pessure. Therefore, a vacuum gauge may be built having ion current and therefore sensitivity maintained to low pressures by applying a cellular anode having critically proportioned cellular compartments therein, according to the teachings of the present invention. However, in the vacuum gauge application the cathode plates are not made of a reactive material but instead, are made of a material which is either very difiicult to sputter or which, if sputtered, will not serve to entrap or getter gas coming in contact therewith if it is desired to minimize pumping action of the gauge.

The teachings of the present invention can also be used to advantage in triode pumps, those pumps which use a magnetically confined glow discharge and which include a third or collector electrode, such as, for example, U.S. application Serial No. 32,219, filed May 27, 1960, now abandoned, and assigned to the same assignee as the present invention. In addition, no auxiliary source of electrons is necessary to maintain a discharge at low pressures.

What is claimed is:

1. A magnetically confined glow discharge apparatus including an anode member sub-divided into a plurality of lesser hollow, open-ended compartments of diameter d in inches formed by holes extending into said anode member, a cathode member disposed opposite the open ends of said anode compartments and being slightly spaced apart therefrom and defining a path for glow discharge ion current between said anode and cathode members, means for applying a positive potential to said anode member with respect to said cathode member, means for producing and directing a magnetic field of intensity B in gauss substantially coaxially of the holes forming said lesser anode compartments for enhancing the glow discharge to pressures below torr, the Ed product being greater than one kilogauss-inch wile B lies within the range of 0.5 to 3.0 kilogauss.

2. The apparatus according to claim 1 wherein the length l in inches of said compartments is substantially equal to or greater than the diameter d thereof.

3. A magnetically confined glow discharge apparatus including, an anode member sub-divided into a plurality of lesser hollow, open-ended compartments of diameter d in inches formed by holes extending into said anode member, a cathode member disposed opposite the open ends of said anode compartments and being slightly spaced apart therefrom and defining a path for glow discharge ion current between said anode and cathode members, means for applying a positive potential to said anode member with respect to said cathode member, means for producing and directing a magnetic field of intensity B in gauss substantially coaxially of the holes forming said lesser anode compartments for enhancing the glow discharge ion current to pressures below l0' torr, d lying within the range of 1 /2 to 2 /2 inches 'While B lies within the range of 1.0 to 2.0 kilogauss.

4. The apparatus according to claim 3 wherein the length l in inches of said compartments is substantially equal to or greater than the diameter d thereof.

5. In a magnetically confined glow discharge apparatus including, an anode member divided into a plurality of hollow, open-ended compartments of a diameter d in inches formed by holes extending into said anode member, a cathode member disposed opposite the open ends of said anode compartments and being slightly spaced apart therefrom and defining a path for glow discharge ion current between said anode and cathode members, said anode member being adapted to be energized positively with respect to said cathode member, means for producing and directing a magnetic field of intensity. B in gauss substantially coaxially of the holes forming said anode compartments for enhancing the glow discharge to pressures below 10" torr, the improvement that the Ed product is greater than one kilogauss-inch while B lies within the range of 0.5 to 3.0 kilogauss.

6. In a magnetically confined glow discharge apparatus including, an anode member divided into a plurality of hollow, open-ended compartments of diameter d in inches formed by holes extending into said anode member, a cathode member disposed opposite the open ends of said anode compartments and being slightly spaced apart therefrom and defining a path for glow discharge ion current between said anode and cathode members, said anode member being adapted to be energized positively with respect to said cathode member, means for producing and directing a magnetic field of intensity B in gauss substantially coaxially of the holes forming said anode compartments for enhancing the glow discharge ion current to pressures below 10 torr, the improvement that d lies within the range of 1 /2 to 2 /2 inches while B lies within the range of 1.0 to 2.0 kilogauss.

References Cited by the Examiner UNITED STATES PATENTS 3,028,071 3/1959 Jepsen 230-69 MARK NEWMAN, Primary Examiner.

GEORGE N. WESTBY, Examiner.

F. A. ADAMS, Assistant Examiner.

Claims (1)

1. A MAGNETICALLY CONFINED GLOW DISCHARGE APPARATUS INCLUDING AN ANODE MEMBER SUB-DIVIDED INTO A PLURALITY OF LESSER HOLLOW, OPEN-ENDED COMPARTMENTS OF DIAMETER D IN INCHES FORMED BY HOLES EXTENDING INTO SAID ANODE MEMBER, A CATHODE MEMBER DISPOSED OPPOSITE THE OPEN ENDS OF SAID ANODE COMPARTMENTS AND BEING SLIGHTLY SPACED APART THEREFROM AND DEFINING A PATH FOR GLOW DISCHARGE ION CURRENT BETWEEN SAID ANODE AND CATHODE MEMBERS, MEANS FOR APPLYING A POSITIVE POTENTIAL TO SAID ANODE MEMBER WITH RESPECT TO SAID CATHODE MEMBER, MEANS FOR PRODUCING AND DIRECTING A MAGNETIC FIELD OF INTENSITY B IN GAUSS SUBSTANTIALLY COAXIALLY OF THE HOLES FORMING SAID LESSER ANODE COMPARTMENTS FOR ENHANCING THE GLOW DISCHARGE TO PRESSURES BELOW 10-7 TOOR, THE BD PRODUCT BEING GREATER THAN ONE KILOGAUSS-INCH WILE B LIES WITHIN THE RANGE OF 0.5 TO 3.0 KILOGAUSS.

Images