US2499288A - Vacuum analyzer - Google Patents

Description

.J. G. BACKUS VACUUM ANALYZER 2 Sheets-Sheet 1 Filed July 2, 194.7

f- SPEC TROME' TEE BOX CA THODE lO/V SOURCE POWER w M m H m m SUPPLY COMPONENTS W/ TH/N MAGNET/C F/ELD COLLECTOR PLATES AMPL /F/E/? ION SOURCE POWER SUPPL Y INVENTOR.

JOHN G BAG/(U5 BY ATTORNEY.

Feb. 28, 1950 J. G. BACKUS 7 9 VACUUM ANALYZER Filed July 2, 1947 2 SheetsSheet 2 INvENToR; JOHN G BACKUS BY Win-M ATTORNEY Patented Feb. 28 1950 NITED STATES P VACUUM ANALYZER Application July 2, 1947, Serial No. 758,509

3-Claims. (Cl. 25041.9)

This invention relates to a vacuum analyzer adapted for detecting gas leaks in a highly evacuated vessel and for analyzing the gases present in said vessel as to their type and their relative concentrations within said vessel, and more particularly to a vacuum analyzer having an improved ion generating source.

In vacuum systems such as the tank portion of the calutron, information relative to the qualitative composition of the residual gases present is of great importance in order that ultimate vacuums can be quickly and efficiently obtained. By use of the vacuum analyzer progress of the bake-- out processes and information on other outgassing phenomena can be simultaneously monitored.

In order to properly analyze the gases present in a vessel undergoing evacuation it is necessary for the analyzer ion source to be capable of operating over a Wide range of gas pressures; Dilliculty has been experienced with conventional ion sources in this respect, particularly at low pres sures wherein they are incapable of producing a sufficient supply of ions and of providing an ion beam having sufiicient homogeneity of energy to insure proper resolution of the beam. This invention includes an improved ion generating source which when used in a vacuum analyzer is capable of detecting one part of helium to 50 000 parts of air at pressures as low as 1 10-- mm. oi mercury contrasted to a sensitivity of one part of helium to 5,000 parts of air at the same pressure when using a conventional type of ion generating source.

A Vacuum analyzer having an ion source slit together with a single slit positioned on the Ion-"i1 tudinal baflle plate opposite an ion collector plate will portray for a given nominal ion accelerating voltage, indications of only such ions having comparable mass.

Let us now consider a vacuum analyzer having an ion source slit together with two slits in the longitudinal baflle plate spaced successively apart from the ion source slit. Then fora nominal ion accelerating voltage, ions of greater mass traverse longer arcuate paths than do ions of a lesser mass. Consequently in order to admit ions having the lesser mass to a collector plate, a slit. in the longitudinal baiile plate must be located nearest the ion source slit. Conversely;in order to admit ions having the greater mass to a collector plate, a slit in the longitudinal. baffle plate must be located further from the ion source-slit than in. the case i or ions having lesser mass.

By incorporating a multiple slit-bafile plate the,

usefulness of the device as a vacuum analyzer is extended to include for a given nominal accelerating Voltage the indication of ions of a wide range of masses.

It is therefore an object of this invention to provide a vacuum analyzer whereby various gases present in a highly evacuated vessel can be detected and the relative concentrations of. said gases indicated.

A. further object of this invention is to provide an improved ion generating source capable of generating from the gas molecules present in a vessel evacuated to extremely low pressure, a sufficiently abundant supply of ions to produce satisfactory ion beams for use in the vacuum analyzer.

A further object of this invention is to provide means for portraying on the screen of a cathode ray oscilloscope evidences of the relative mass and concentrations of ions of gases present in a highly evacuated vessel, said ions varying widely in relative mass over a range of approximately 1 to 50.

A further object of this invention is to provide selective means of portraying on th screen of a cathode ra oscilloscope evidences of ions having relatively low mass, together concurrently with ions of relatively higher mass; or, of ions of relatively higher mass singularly;

A further object of the invention is to provide electrical circuits necessary to complete the overall operation of the device a a vacuum analyzer comprising an ion source voltage supply. an ion accelerator voltage supply; a pre-amplifier, an amplifier, and a cathode-ray tube.

Other objects and advantages of the invention will be apparent from the following descript on and claims considered together with the accompanying drawings in which:

Figure 1 is a block diagram of the functional elements comprising this invention.

Fig. 2 is a partial schematic representation of certain elements of the mass spectrometer as indicated by section line 22 of Fig. 1 showing the direction of the electromagnetic field, the ion source, and the spectrometer box.

Fig. 3 is a perspective view. partly in section of the mass spectrometer, together with an ion generator; mounted on a face plate.

Referring to Figs. 1 and 3, my improved vacuum analyzer generally indicated at I is suitably mounted on a face plate 2, the. latter being posiitioned on the wall of a vessel whose vacuum analysis is desired such as a calutron tank. and thereby projecting'the vacuum analyzer i into the interior of the vessel, and hermeticallysealing the wall thereof. Further, face plate 2 has a gas valve 22 mounted thereon for test purposes. The vacuum analyzer I comprises essentially the following, all being within the vessel: an electron oscillator type of ion source 3, an ion beam chamher 4, and ion current collector plates I3 and I4.

Referring now to Fig. 2, the vacuum analyzer I is positioned in a substantially uniform magnetic field denoted by the arrows, which is parallel to the longitudinal axis of the ion source 3 and normal to the large face of the spectrometer box 4. The ion source 3 consists of an anode cylinder 6 having a pair of electrically connected cathode plates Ia and 1b spaced from the open ends of the anode cylinder 6.

Referring again to Figs. 1 and 3, a slit 8 is provided in the wall of anode cylinder 6 through which ions may be withdrawn and projected into the box 4 through an accelerating slit 9. There is also provided in the box 4 a longitudinal baffle A 10 provided with slots II and I2 through which the ion beams may pass and be collected by collector plates I3 and I4 respectively. Moreover, a pair of beam-defining vanes 5a and 5b are positioned along the 90 radius of the ion beam of greater mass and are so spaced as to trim the beam to an angular divergence of Considering now the electrical connections. the cathode plates Ia and lb. and the anode 6 are respectively connected to the negative and positive terminals of a hi h voltage ion source power supply I8 of the order of 2000 volts. Further, the anode 6 and the chamber 4 are respectively connected to the terminals of a sinusoidally varying high volta e accelerator sup ly I5 which varies from zero to a few thousand volts, the output of the vol age su ply I5 being also applied to the horizontal pla es of an oscillosco e I6. Conside ing the manner of o eration of recti-- fier I5 it becomes apparent that after a few cvcles the condenser becomes charged to a posi ive D. C. potential nominally equal to the secondary voltage of the transformer. The conden er once charged remains substantially at a D. C. potential due to the R. C. circuit having a comparatively long time con tant by virtue of t e l rge load resistance. Consequentlv, on negative eaks of secondary A. C. vol a e. t e voltage develo ed in the secondarv winding of the ransformer will be opposed by the equal volta e of the condenser, the resultant output vol a e at such instant being zero: further. on positive peaks of secondary A. C. voltage, the voltage developed in the secondary windin of the transformer will be additive to the equal voltage of the condenser, the resultant voltage cutout at such instants being double he A. C. peak value and o i ive in polarity. Interpolation by this analvsis indicates the output voltage to be sinusoidal and ranging from zero to a posit ve potential substantially twice the peak A. C. voltage of the secondary.

Collector plates I3 and I4 are connected as shown (Fig. 1) to a terminal strip in the preamplifier stage II. Provision is made in said stage I'I, whereby the input thereto can be electrically connected by means of a switch 2I to collector plate I3 in addition to its connection to collector plate I4. The output of the preamplifier stage I! is connected to ground through a resistor I9, the voltage drop across resistor I9 being amplified by the amplifier and applied to the vertical deflection plates of the oscilloscope I6.

In operation, the vacuum analyzer is mounted in the vessel to be tested and the gas in the anode cylinder 6 is of substantially the same composition and concentration as the gas in the vessel. When voltage from I3 is applied to the electrodes Ia, lb and 6 of the ion source 3, electrons will oscillate between the cathodes 1a and lb through the cavity in the anode cylinder 6 breaking up and ionizing the gases in the cavity, the kind of ion formed being dependent on the gases present, their quantity being dependent on the gas pressure. The ions are withdrawn through slit 9 into box 4 by the accelerating voltage impressed between the box 4 and the anode 6. The varying accelerating voltage results in the different ion beams sweeping across slots II and I2 and impinging onthe respective collector plates I3 and I4 causing voltages to be impressed on the vertical plates of the oscilloscope I6 proportional to the current density of the particular ion beam. Moreover, the ions traverse circular paths, the radii depending on the impressed accelerating voltage, the mass and charge of the individual ions, and the magnetic field, in accordance with the formula:

H=strength of the magnetic field in gausses; V=the accelerating VOltage in practical volts to which the ions are subjected; r=the radius of curvature in inches of the paths along which the ions are projected;

and

=the mass-to-charge ratio of the positive ions projectcd along arcuate paths and passing transversely through magnetic field H.

The ion egress slots I I and I2 in the baffle surface I0 have been spaced from the ion source slot 9 so that for a given accelerating voltage V and a given magnetic field H, ions of mass two and ten on the scale of O+=16 will pass through their respective ion egress slots, whereby the ration of their radii of curvature becomes This arrangement precludes ambiguity in differentiating between the Hz ion and the HzO+ ion which otherwise would occur were the slots II and I2 positioned in accordance with ion masses two and eighteen. More particularly, slots II and I2 are so spaced from slot 9 that when an ion beam passes through slot II no ion beam passes through slot I2, thus avoiding overlapping peaks on the oscilloscope I6. Inasmuch as the oscilloscope sweep is controlled by the accelerating voltage, the oscilloscope trace is an indication of the various ion densities, the abscissa identifying the individual ion beam and the ordinate the density thereof, the former being an indication of the gases present and the latter the concentrations thereof.

Thus, after the system has been practically degassed, the presence of a high HO+ peak is an indication of water vapor pointing to a water leak while a high N+ peak would indicate an air leak. By directing a stream of hydrogen at a suspected surface region of the vessel a rise in the H peak would indicate a leak in this region.

Although this invention has been described with reference to a particular embodiment thereof, it is not limited to this embodiment nor otherwise except bythe terms of the following claims.

What is claimed is: v i Y 1. Inamass spectrometer anion generating source comprising a pair of spaced cathodes having a gaseous region therebetween, an apertured tubular anode axially aligned with, spaced from, and intermediate said cathodes, said anode being positioned within the magnetic field of said spectrometer and axially aligned therewith.

2. A vacuum analyzer comprising a mass spectrometer having an ion source, a spectrometer box, and a receiver, said ion source comprising a tubular apertured anode disposed in and parallel t0 the magnetic field of said mass spectrometer and a pair of cathodes disposed adjacent the ends of said anode, said spectrometer box being positioned adjacent said ion source and having an ion access slot formed therein, and being further provided with at least two ion egress slots, said receiver being disposed exterior to said spectrometer box and adjacent said ion egress slots, and electrical elements cooperatively associated with said mass spectrometer providing a visual indication of the kind and amount of ions formed at said ion source and received at said receiver.

3. A vacuum analyzer comprising a 180 degree mass spectrometer having an ion source, an ion \2 receiver, and an ion beam therebetwee'n, means supplying a sweep voltage to said mass spectrometer whereby the trajectory of said ion beam is varied, and an ion baflie plate interposed between said receiver and said ion beam situated on the diameter of said ion beam path and having two ion egress apertures formed therein, the ratio of distances from said ion source to said apertures being substantially JOHN G. BACKUS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,146,025 Penning Feb. '7, 1939 2,331,189 Hipple Oct. 5, 1943 2,341,551 Hoover Feb. 15, 1944 2,355,658 Lawlor Aug. 15, 1944 2,370,673 Langmuir Mar. 6, 1945

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