GB2256310A

GB2256310A - A vacuum gauge

Info

Publication number: GB2256310A
Application number: GB9111746A
Authority: GB
Inventors: Paul Graham Lethbridge
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1991-05-31
Filing date: 1991-05-31
Publication date: 1992-12-02
Also published as: GB9111746D0

Abstract

A vacuum gauge head comprises an outer cathode sleeve 1 having an inlet 2 at one end for communication with the vacuum to be measured. Positioned within the cathode sleeve are two cathode discs 5, 6 both having central apertures through which an anode 7 is positioned. A circular, flat, metal ring 11 is fixed to one or both of the discs 5, 6 symmetrically about the aperture. The edge of the internal surface of the ring 11 is to be made sharp. This can be achieved by fabricating the ring from a pure nickel sheet using a photo-etching technique, producing a sheet of thickness of 0.1 mm. This sharp point will, thus, decrease the long strike (start up) times associated with initiation of cold discharge of low pressure gauges. Details of seals 8, 9, 10 etc are also disclosed. <IMAGE>

Description

IHPROVEMENTS RELATING TO VACUUH GAUGES This invention relate to vacuum gauges and, more particularly, to improvements to cold cathode ionisation vacuum gauges, especially those of the inverted magnetron type.

Cold cathode ionisation gauges for measuring vacuum, sometimes referred to as "Penning" gauges, are well known. They generally comprise an anode and one (or more) cathodes with a large potential difference between the anode and the cathode(s) and a substantial magnetic field applied by a permanent magnet in the area between the electrodes. In these gauges, the anode and cathode are held in a predetermined configuration relative to each other by means of a vacuum feed through which isolates the electrodes within the gauge from the atmosphere outside.

With the gauge subjected to the vacuum to be measured, electrons emitted by the cathode are accelerated towards the anode by the electric field; however, the action of the magnetic field causes the electrons to adopt a very long, non-linear trajectory, for example helical, before striking the anode; as such, the probability of ionisation of gas molecules present in the vacuum by collision with the electrons is much higher even at low pressures. Positive ions formed by the collisions are attracted by the cathode to produce an ion current in an external circuit, the size of which is related to the molecular weight and density of the gas at a given temperature and therefore to the level of vacuum.

In gauges of the inverted magnetron type, the cathode commonly has associated with it means to define a region within it in which the ion collection, and hence ion discharge current to the external circuit takes place and is measured.

In such inverted magnetron gauges, however, problems are sometimes encountered in the initiation of cold discharge (strike or start up) of the gauge at very low pressures, for example less than 1 x 10- mbar. In particular, the gauge may take several minutes or even hours to strike because the low probability of an ionising event occurring is reduced due to low gas density.

The present invention is concerned with an improved gauge of this type in which the strike time is generally faster, typically of the order of a few hundred milli-seconds.

In accordance with the invention, there is provided a vacuum gauge of the ionisation type having a gauge head which comprises: - an outer cathode sleeve having an inlet at one end for communication with the vacuum - a pair of cathode discs positioned substantially radially within the cathode sleeve and defining therebetween an ion collection region, the discs both having central apertures therein - an anode positioned within the cathode sleeve and within the central apertures of the discs wherein the surface of at least one of the apertures presents a sharp point or edge to the anode.

The cathode discs must be in electrical contact with the cathode sleeve itself. The discs may be positioned within the sleeve in any convenient manner. Each disc may form the end wall of a cylinder positioned concentrically within the cathode sleeve. They may be made from any suitable material including stainless steel.

The anode will generally be solid and be made of metal, for example, stainless steel.

For a better understanding of the invention, reference will now be made, by way of exemplification only, to the accompanying drawings in which: Figure 1 shows a sectional view through a gauge head of the invention.

Figure 2 shows an enlarged view of part of the head shown in Figure 1.

With reference to the drawings, Figure 1 shows a gauge head which comprises a substantially cylindrical cathode sleeve 1 having an inlet 2 at one end for communication with the vacuum to be measured. Within the sleeve 1 are positioned concentrically first and second cylinders 3,4 each having a circular end portion 5,6 respectively which form the cathode discs; both discs have circular central apertures. The cylinders 3,4 and hence the discs 5,6, are in electrical contact with the cathode sleeve 1.

Also positioned centrally within the cathode sleeve 1 and passing through central apertures of discs 5,6 is an anode 7 in the form of a solid rod made from stainless steel.

The anode 7 is held in position by means of a fused glass compression seal 8 placed within the aperture of a ring 9. The anode 7 is also held in position by use of a plastic collar 10 fitted which arrangement effects electrical insulation between the cathode sleeve 1 and the anode 7.

In accordance with the invention, either or both of discs 5 and 6 possesses a sharp point or edge which points in the direction of the anode.

In preferred embodiments of the invention the apertures of one or both discs is partially closed by fixing to the disc a component providing the sharp point within the aperture. Ideally the aperture is circular and a flat metal ring is fixed to a disc surface symmetrically about the aperture; the internal surface of this ring can usefully provide the required sharp edge which is presented to the anode leaving, for example, a uniform gap between the edge and the outer surface of the anode.

Advantageously the edge of the internal surface of the ring can be made microscopically sharp by means of fabricating the ring from a pure nickel sheet using a photo-etching technique. The sheet may have a thickness of, for example, O.lmm.

More advantageously, the metal ring is made from a very pure metal such as platinum or nickel which can, in use, automatically provide for self cleaning of the sharp edge.

Figure 2 shows an enlarged view of the anode 7 and the disc 6. The wall of the disc 6 has symmetrically affixed about the central aperture by spot welding a flat ring 11 of very pure nickel so that the aperture is partially closed by the ring. The ring 11 as a whole, and in particular the edge presented to the anode 7 is microscopically sharp which arises from the manufacture of the ring by a photo-etch technique.

In use of the gauge, the anode is held at a potential difference of, for example, 4 Kv relative to the cathode sleeve 1. The gauge operates by measuring the current flowing between the anode and the cathode ID when a "cold cathode discharge" is formed within the region defined between discs 5 and 6 by the ionisation of gas molecules by electrons emitted from the cathode.

The presence of the ring 11 acts in use as a 1,striker" to cause the gauge instantaneously to start under high vacuum conditions. It is thought that the intense field formed by the presence of the sharp end causes instantaneous ionisation of particles in the immediate vicinity of this field in a manner known as "field ionisation".

The action of the striker can be further enhanced by arranging for a momentary, for example a few milli-seconds, sharp rise in the potential difference between the anode and the cathode, for example by an extra 500 V above the normal operating 4 KV. During the sharp rise in potential difference, the electric field strength between the striker and the anode increases proportionally, thereby enhancing the ionising power of the gauge head.

Claims

CLAIHS

1. A vacuum gauge of the ionisation type having a gauge head which comprises: - an outer cathode sleeve having an inlet at one end for communication with the vacuum - a pair of cathode discs positioned substantially radially within the cathode sleeve and defining therebetween an ion collection region, the discs both having central apertures therein - an anode positioned within the cathode sleeve and within the central apertures of the discs wherein the surface of at least one of the apertures presents a sharp point or edge to the anode.