US3387175A

US3387175A - Vacuum gauge having separate electron collecting and electron accelerating electrodes

Info

Publication number: US3387175A
Application number: US437456A
Authority: US
Inventors: William A Lloyd; Robert L Jepsen
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1965-03-05
Filing date: 1965-03-05
Publication date: 1968-06-04
Anticipated expiration: 1985-06-04
Also published as: GB1112796A; DE1573923A1

Description

VACUUM GAUGE HAVING SEPARATE ELECTRON COLLECTING AND J 968- w. A. LLOYD ETAL 3,387,175

ELECTRON ACCELERATING ELECTRODES Filed March 5, 1965 INVENTORS WILLIAM A. LLOYD ROB RT JEPSEN BY a4...

v ORNEY United States Patent 3,387,175 VACUUM GAUGE HAVING SEPARATE ELECTRON COLLECTING AND ELECTRON ACCELERATING ELECTRODES William A. Lloyd, San Jose, and Robert L. Jepsen, Los Altos, Calif., assignors to Varian Associates, Palo Alto, Calif., a corporation of California Filed Mar. 5, 1965, Ser. No. 437,456 11 Claims. (Cl. 315-108) ABSTRACT OF THE DISCLOSURE The effective X-ray limit in low pressure ion gauges is substantially reduced if not entirely eliminated in a gauge in which electron collection is effected at potentials too low for any appreciable X-ray emission caused by electron bombardment. In the disclosed gauge, electrons are emitted from a ribbon filament coaxially disposed about the lower end of an elongated high voltage accelerating anode. As the electrons accelerate toward the high voltage anode, they are caused to miss and spiral about the high voltage anode by a magnetic field of about 600 gauss until they reach and are collected by a low voltage electron collector surrounding and coaxial with the upper end of the high voltage anode. The potential on said low voltage electron collector is insufficient for appreciable X-ray emission by electron bombardment. In

v addition an ion collector is so arranged that if a few X-rays are emitted, the secondary electrons they liberate from the ion collector will be returned to the ion collector by the magnetic field.

With the development of the Bayard-Alpert gauge (21 Review of Scientific Instruments 571, 1950), scientists were presented with a laboratory instrument capable of pressure measurement two orders of magnitude lower than theretofore known. Attempts at achieving still lower pressures have been hampered through failure to construct a gauge whose X-ray limit is less than 2 l0- torr, the accepted limit of the Bayard-Alpert gauge today. This invention relates to an improved ionization gauge whose effective X-ray limit has been substantially lowered, and in fact has not yet been determined.

Present day ionization gauges normally include: a relatively small surface area ion collector, for example, a thin wire; a highly positively charged grid surrounding the collector for both accelerating and eventually collecting the electrons, and a filamentary cathode disposed outside the grid for emitting the electrons. It has been known that the electrons will eventually strike the grid and with sufiicient energy to give rise to X-ray production. The ion collector, despite its small size, will still intercept a portion of these X-r-ays, which leads to photoelectric emission of electrons from the ion collector surface. As is well known in the art, the arrival of positive ions at the ion collector surface is indistinguishable from the departure of photoelectrons therefrom in the ion collector measuring circuit, thereby establishing an effective lower limit to the pressure measurable. By measuring the X-ray current in the gauge, pressures somewhat below the X-ray limit can be inferred (by subtracting the X-ray cur rent from the total current), but the precision with which the pressure can be determined is seriously impaired by the presence of this X-ray current.

It is the principal object of the present invention to provide an improved ionization gauge having a substantially reduced X-ray limit.

Briefly stated, in accordance with one teaching of the present invention there is disclosed an ionization gauge in which the functions of accelerating and collecting the 3,387,175 Patented June 4, 1968 electrons are, in effect, separated by the provision of a high voltage electrode for accelerating electrons and a low voltage electrode for collecting electrons, thereby substantially reducing the production of X-rays. In accordance with another aspect of the present invention, magnetic means are provided, being so arranged with respect to the electrodes of the gauge that electrons are compelled to move in such a way as to miss the electron accelerator electrode, but have a high probability of striking and being collected at the low voltage electron collecting electrode after having pursued a greatly lengthened electron path. Because of the low energies with which the electrons are collected, the rate of X-ray generation is much reduced. At the same time most of the photoelectrons which may be produced by any residual X-rays striking the ion collector surface may be constrained by the magnetic field in such a way as to follow paths returning to the ion collector, consequently reducing further the X-ray effect.

One feature of the present invention is the provision in an ionization gauge of a high voltage electrode for accelerating electrons, a low voltage electrode for collecting electrons, and magnetic means so arranged that electrons are compelled to move in such a way as to just miss the electron accelerator electrode, but have a high probability of striking the electron collecting electrode after having pursued a greatly lengthened electron path.

Another feature of the present invention is the provision of an ionization gauge of the above type wherein the ion collector is so arranged that most photoelectrons which may be produced by any residual X-rays striking the ion collector surface are constrained by the magnetic field in such a way as to follow paths returning to the ion collector.

These and other objects and features 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 drawings in which:

FIG. 1 is a fragmentary cross-sectional view of a novel gauge constructed in accordance with the teachings of the present invention, shown connected to a system whose pressure is being measured, and including the associated circuitry in schematic;

FIG. 2 is a cross-sectional view taken along the lines 2--2 of FIG. 1; and

FIG. 3 is a fragmentary cross-sectional view of another embodiment of the electron accelerator of the present invention.

Referring now to FIGS. 1 and 2, there is shown an ionization gauge 11 constructed in accordance with the teachings of the present invention. The gauge 11 includes a primary source of electrons 12, an electron accelerator 13, an electron collector 14, an ion collector 15 and magnetic means 16.

The source of electrons 12 may be of the filamentary thermionic type made of, for example, thoria-coated iridium in the shape of a thin, cylindrical ring coaxially supported at the base of gauge 11 on heater lead-in

wires

17, 18 about the lower end of the electron accelerator 13. Electrode 12 acts as a source of primary electrons which collide with gas molecules to form positive ions.

Electron accelerator 13 is an elongated metal member made of, for example, stainless steel in the shape of a thin rod of relatively small surface area supported on a lead-in wire 19 along the axis of ion collector 15. Electrode 13 is maintained at a high positive potential so as to accelerate electrons but is so positioned, as will be explained further, that it intercepts practically none of these electrons.

The electron collector 14 is a short cylindrical member as of stainless steel disposed about one end of the electron accelerator 13 and supported on a lead-in wire 20. Electrode 14 is so arranged as to intercept and collect nearly all of the electrons and maintained at a low enough positive potential that striking electrons will not cause X- rays to be emitted from the electron collector surface.

The ion collector 15 is an elongated, cylindrical member made of, for example, stainless steel and forms a portion of the outer wall of the gauge 11. The collector 15 is typically at or near ground potential and serves to collect atoms or molecules ionized positively as a result of collisions with electrons.

A glass or ceramic header assembly 21 through which the lead-in wires pass in electrically isolated and vacuumtight manner is sealed to the base of collector 15. A glass or ceramic ring 22 sandwiched between the top of collector 15 and a metallic ring 23 serves to maintain vacuum integrity of gauge 11 and electrically isolates collector 15 from ring 23. Ring 23 is welded to a vacuum flange 24 and may be removably connected in vacuum-tight manner to a mating flange 25 welded to the wall 26 of a system whose pressure is being measured upon compression of a soft metal gasket 27 as, for example, by tightening circumferentially arranged bolts 28. Thus, gauge 11 is illustrated as an evacuable appendage to the wall 26 of the system whose pressure is being measured.

The magnet means 16 comprises an annular cylindrical member which may be either a permanent magnet or an electromagnetic coil slidably mounted about and insulated from, by means of an insulating sleeve 29, the ion collector 15. Ceramic header 21, ring 22 and sleeve 29 must be of very high resistance so as to prevent development of leakage currents thereacross.

In accordance with one aspect of the present invention the magnetic means 16 is so arranged with respect to the electrodes of the gauge 11 that electrons emitted from the source 12 are compelled to move in modified cycloidal paths just missing the electron accelerator 13 as they proceed axially upward toward the electron collector 14.

It has been observed that there exists a residual current to the ion collector 15 caused by ion emission from the filament 12. Ion collector 15 is electrostatically shielded from the filament 12 by the provision of ion suppressor means comprising a cylindrical member'30, preferably stainless steel, supported on lead-in wire 31.

In operation, the ion collector 15 is essentially at ground, the ion current being measured by any suitable means 32 capable of measuring very small currents. The filament 12 is maintained at +l-40 volts with respect to ground by means of a power supply 33, the electron accelerator 13 is maintained at +175-500 volts with respect to ground by means of a power supply 34, the electron collector 14 at +-30 volts with respect to the filament 12 by means of a power supply 35, and the ion suppressor means 30 at 20 volts with respect to the filament 12 by means of a power supply 36. Power is supplied for heating filament 12 by means of a power supply 37, typically 2-3 volts and 3-4 amperes. The magnetic means 1 6 provides a magnetic field intensity of 600-800 gauss within the electrode region of the gauge 11.

Electrons are emitted from the filament 12 and accelerated toward the electron collector 14 principally due to the potential on the electron accelerator 13. In other embodiments, as illustrated in FIG. 3, for example, the electron accelerator 13 may be so constructed as to further aid in drawing olf electrons from the filament 12, for example, by the provision of an outwardly extending disc-like member 38. Before collection, however, a certain number of the electrons will collide With gas atoms and molecules, thereby forming positive ions. These positive ions are collected at the ion collector 15, causing current flow through means 32, the number collected being an index to molecular density, i.e., pressure. The magnetic means 16 is so arranged with respect to the electrodes of the gauge that electrons are compelled to move in modified cycloidal paths just missing the electron accelerator electrode 13 as they proceed axially upward, but have a high probability of striking and being collected at the electrode 14, after having pursued a greatly lengthened electron path. At the same time any photoelectrons which may be produced by X-rays striking the ion collector surface are constrained to return to the ion collector, consequently greatly reducing X-ray effect.

In an actual embodiment and using potentials in the above specified ranges, an Alnico VIII magnet providing a field intensity of 670 gauss, a collector 1% inches in diameter and 1% inches long, an electron accelerator with a rod /s inch in diameter and a filament inch in diameter set in 0.4 inch from the end of the electron accelerator rod and an electron collector of one inch diameter and 0.25 inch long were used. Several advantages were realized from the above construction. For example, there was increased sensitivity due to the proximity of the electron cloud to the ion collector and the increased electron path length.

Since many changes can be made in the above construction and many apparently widely dilferent 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 drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An ionization vacuum gauge for determining pressure within a given space comprising: an elongated relatively small surface area electron accelerator; an electron source positioned adjacent one end of said elongated electron accelerator; an electron collector positioned adjacent the opposite end of said elongated electron accelerator and insulated therefrom; an ion collector spaced apart from said electron accelerator, and magnet means to produce a magnetic field having lines of force parallel with the axis of said electron accelerator.

2. A gauge according to claim 1 wherein said magnet means is so arranged that photoelectrons produced by X-rays striking said ion collector are constrained to return to said ion collector.

3. The gauge according to claim 2 wherein said electron accelerator and said magnet means cause the electrons to pursue modified cycloidal paths about said accelerator as they proceed in the direction of said electron collector.

4. A gauge according to claim 1 wherein said electron source and said electron collector are each cylindrically shaped electrodes coaxial with said elongated electron accelerator.

5. A gauge according to claim 4 including means for applying a first potential to said electron accelerator; and means for applying a second potential to said electron collector, said second potential being less than said first potential and insufficient to cause appreciable X-ray emission by electron bombardment.

6. The gauge according to claim 4 including means coaxial with said source of electrons for electrostatically shielding said source of electrons from said ion collector.

7. An ionization vacuum gauge for determining pressure within a given space comprising: four spaced-apart electrodes forming an electrode region including, a tubular ion collector, an elongated, relatively small surface area electron accelerator extending coaxially within said ion collector, a source of electrons positioned within said ion collector and about one end of said electron accelerator and a cylindrical electron collector disposed within said ion collector and about the opposite end of said electron accelerator; and, means to produce a magnetic field having an axis coinciding with the axes of said ion collector and said electron accelerator.

8. The gauge according to claim 7 including a discshaped member extending outwardly from said electron accelerator near said source of electrons.

9. An ionization vacuum gauge for determining pressure within a given space comprising: an electron source, an electron accelerator, an electron collector insulated from said electron accelerator, and an ion collector; means applying a high accelerating potential to said electron accelerator, and means applying to said electron collector a collecting potential substantially lower than said accelerating potential.

10. A gauge as claimed in claim 9 in which said accelerating potential is 175 to 500 volts above ground and said collecting potential is 25-70 volts above ground.

11. A gauge as claimed in claim 10 further comprising means applying 15-40 volts above ground to said electron source.

References Cited UNITED STATES PATENTS Re. 25,369 4/1963 Redhead 3137 X 3,193,724 7/1965 Klopfer 315-108 X 3,319,117 5/1967 Wheeler 315-108 X 10 JAMES W. LAWRENCE, Primary Examiner.

S. A. SCHNEEBERGER, Assistant Examiner.