Radiant energy – Ionic separation or analysis – Static field-type ion path-bending selecting means
Patent
1991-08-08
1993-05-11
Berman, Jack I.
Radiant energy
Ionic separation or analysis
Static field-type ion path-bending selecting means
250294, 250281, H01J 4930
Patent
active
052104133
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a partial pressure gauge based on the principle of mass spectrometry, and utilizes a magnetically confined, modified cold-cathode discharge configuration as its ion source.
BACKGROUND OF THE INVENTION
In general, partial pressure measuring instruments are used in conjunction with a test gas in order to locate leaks, measure leak rates and examine the tightness of vacuum systems. They usually operate on the principle of mass spectrometry. In recent years methods have been developed with the aim of simplifying the spectrometers and producing more compact gauges (cf. e.g. UK Patent Application, GB 2191334 A), yet the overall complexity, especially with respect to the generation of ions, has remained considerable.
A particularly compact gauge having a large sensitivity coupled with a cost-effective design could be realized by a combination of magnetic mass separation preceded by an efficacious cold-cathode ion source. Such a gauge would be only slightly more complicated than the customary cold-cathode gauges used for vacuum measurements. Fundamentally, the latter consist of a discharge chamber in which a combination of electric and magnetic fields form a trap for electrons which, in the ideal case, can only leave the discharge after losing energy by collisions with gas molecules; in other words, the electrons cannot reach the anode. The trapped electrons correspond to a large electron current which efficiently ionizes the residual gas in the vacuum. To a first approximation, its magnitude depends on the strengths of the electric and magnetic fields and the geometry of the discharge region and is virtually constant over several decades of the residual gas pressure. Hence the ionization current is proportional to the pressure of the residual gas and corresponds to the discharge current in the gauge. Vacuum gauges based on this principle are well known both in the literature and in practice and are designated as Penning gauges, magnetron gauges or inverted magnetron gauges (Ref: Manfred v. Ardenne, Tabellen zur angewandten Physik, Vol. II (1975) 169-172).
This type of cold-cathode discharge also represents an excellent ion source. However, the energy spread of the ions is so large that if applied to a mass spectrometer, the required resolution, which is proportional to the square root of the relative energy spread .DELTA.E/E, can be attained only if the ions are further accelerated to a higher energy. In turn, higher energetic ions require higher field strengths for mass separation. This is the reason why the mass spectrometers currently employed for leak detection or residual gas analysis utilize essentially sophisticated hot filament ion sources coupled with additional extractions and focussing potentials.
SUMMARY OF THE INVENTION
The invention described herewith relates to a partial pressure gauge based on the principle of mass spectrometry, comprising a mass spectrometer whose ion source is characterized by a magnetically confined cold-cathode discharge which is modified by an inhomogeneous magnetic field in such a manner that ions having a low energy and small energy spread pass through one or several apertures in the cathode, and that without any further acceleration or focussing, these ions are separated and measured according to their momentum in a magnetic field. The fact that the density of electrons in a magnetically confined discharge is determined by both the electric and the magnetic fields underlies the realization of the present invention. By replacing the customary, to a first approximation homogeneous magnetic field in the discharge volume by an inhomogeneous magnetic field having a well-defined peak in the field strength, a locally high density of electrons is obtained which severely restricts the position of ionization of the residual gas. Ions originating in this zone are accelerated by the electric field of the discharge region towards the cathode and can leave the discharge space via apertures therein. As long as the ionization
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patent: 3580052 (1971-05-01), Young
patent: 3803481 (1974-04-01), Compton et al.
patent: 4270091 (1981-05-01), Mann
patent: 4761553 (1988-08-01), Juravic, Jr.
Berman Jack I.
Beyer James
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