MASS SPECTROMETER FOR DETECTING LEAKAGES VIA A TRACER GAS

The present invention relates to a mass spectrometer (10) for detecting leakages via a tracer gas, said spectrometer (10) comprising:—an ionising means (3) intended to ionise said tracer gas;—at least one magnetic-field source (501) that generates a magnetic field (I) that is dependent on the electric current I supplied to said source (501) and that is intended to sort ionised elements;—a means (7) for detecting said tracer gas once ionised; characterized in that said spectrometer comprises a means (II) for adjusting the magnetic field, said adjusting means being configured to allow at least two separate adjustments, said adjustments having different sensitivities.

The present invention relates to the field of mass spectrometers, and more particularly to detecting leakages by tracer-gas mass spectrometry.

As a reminder, a mass spectrometer is a device that uses the movement of the ions in electrical and/or magnetic fields, in order to classify them according to their mass/charge ratios. As illustrated inFIG.1, a mass spectrometer1generally comprises an ionisation means3, an analyser5and a detection means7.

The ionisation means3is configured to ionise the chemical element or elements to be analysed. This ionisation also generates ions that will then be sorted and selected according to their mass/charge ratios.

This sorting can be obtained in various ways, but the concern is here, more particularly, with the analyses that proceed at least with a selection by dispersion of the ions by means of a magnetic field.

The ions thus sorted are next sent to the detection means7, such as a detector that converts the stream of ions received into an electric current. Said electric current output from the detector subsequently undergoes a processing of the signal making it possible to obtain more precise measurements relating to the ions.

This type of mass spectrometer can in particular be used for detecting leakages or monitoring the gastightness of objects, for example by measuring and quantifying a tracer gas, such as helium or dihydrogen. However, other types of tracer gas can be used, such as dioxygen, carbon dioxide, etc.

However, the mass spectrometers used for detecting a leakage are intended to be used in industrial environments in which numerous environmental factors may disturb the measurements or settings, in particular variations in temperature, shocks, vibrations, movements, maintenance of the equipment, etc.

Moreover, a mass spectrometer for detecting leakages must also be as multipurpose as possible, in particular by operating with various tracer gases, for example to adapt to the tracer gases available and/or to the leakage levels sought.

The present invention thus makes it possible to remedy one or more of the problems mentioned above, by proposing a mass spectrometer for detecting leakages by tracer gas, said spectrometer comprising:an ionisation means intended to ionise said tracer gas;at least one magnetic field source generating a magnetic field {right arrow over (B)} dependent on the electric current I supplying said source intended for sorting the ionised elements;a means for detecting said ionised tracer gas.

According to the invention, said spectrometer comprises a means for adjusting the magnetic field {right arrow over (B)} generated by said source, said adjustment means being configured to allow at least two distinct adjustments, said adjustments having different sensitivity.

According to a possible feature, said adjustment means comprises a pre-adjustment (or rough adjustment) and a fine adjustment.

According to another possible feature, one of the adjustments makes it possible to establish a nominal magnetic field {right arrow over (B0)}, while the other adjustment makes it possible to generate a variation in magnetic field Δ{right arrow over (B)} around the nominal value of the magnetic field {right arrow over (B0)}.

The electromagnetic field {right arrow over (B)} generated by the magnetic field source is therefore in this case the sum of the nominal magnetic field {right arrow over (B0)} and the magnetic field variation Δ{right arrow over (B)}.

According to another possible feature, said magnetic field source comprises an electromagnet.

It should be noted that the magnetic field source may also be a magnetic sector incorporating one or more electromagnets.

According to another possible feature, said adjustment means comprises at least two adjustment commands, a combinatorial circuit configured to combine the values of said at least two commands, and a circuit for controlling the current I circulating in said magnetic field source.

According to another possible feature, said at least two adjustment commands are electrical quantities, such as voltages.

According to another possible feature, the magnetic field {right arrow over (B)} depends on the values of the electrical quantities of said at least two adjustment commands.

According to another possible feature, the mass spectrometer comprises N adjustment commands and/or N magnetic field sources, where N is an integer greater than or equal to 3.

The multiplicity of the adjustment commands and/or of the magnetic field sources makes it possible to address a greater number of tracer gases and to facilitate the adjustments of said mass spectrometer according to the invention.

According to another possible feature, the combinatorial circuit comprises:a plurality of resistors R1, R2, R3and R4;an operational amplifier AO1associated with said resistors R1, R2, R3and R4to form a non-inverting summing circuit.

According to another possible feature, the control circuit comprises an operational amplifier AO2associated with a grounding resistor RSand with a transistor T1, the assembly forming a circuit of the voltage to current converter type.

The present invention also relates to a system for detecting leakages via tracer gas, characterised in that it comprises a mass spectrometer as defined above.

FIG.2is a highly schematic representation of an example of a leakage detection system100via tracer gas comprising a mass spectrometer10according to the invention. It should be noted that the mass spectrometer10comprises the same functional parts as the mass spectrometer1ofFIG.1, and thus the identical or similar elements will bear the same references and will not be detailed again.

Said system100thus comprises:a test chamber101configured to accommodate an object the gas tightness of which is to be tested;a leakage detection device102that is connected to the test chamber101and comprises the mass spectrometer10, a main vacuum pump107and an auxiliary vacuum pump109;a gas source103connected to the object and configured to fill said object with a tracer gas, such as hydrogen or helium.

The main vacuum pump107, such as a turbomolecular pump, has an inlet connected to the test chamber101, but is also connected to the mass spectrometer10. The auxiliary vacuum pump109is, for its part, connected to the outlet of the main vacuum pump107.

Said system100also comprises a plurality of valves111and113:a first valve111disposed on the pipe connecting the gas source103to the object being tested, which for its part is disposed in the test chamber101, said valve111making it possible to adjust the quantity of tracer gas injected into the object being tested;a second valve113disposed on the pipe connecting the test chamber101to the main vacuum pump107.

The main pump107generates a high vacuum by means of which the tracer gas, which is input into the test chamber101by a leakage of the object tested, is sucked. Subsequently, inside the main pump107, the tracer gas then flows mainly in the direction of the auxiliary pump109, but some of the tracer gas moves into the mass spectrometer10in order to be analysed therein.

10 The main vacuum pump107is for example a turbomolecular pump, a diffusion pump or any other type of molecular pump making it possible to achieve vacuum levels compatible with detecting leakages of the order of at least 10−3mbar.L/sec.

As illustrated inFIG.3, the mass spectrometer10thus comprises an ionisation means3, comprising for example an ion source301with a cathode301aand an anode301b. The ion source301is surrounded by a screen in which an opening (or diaphragm) is formed, enabling a beam of ions F to emerge towards an analyser5.

The analyser5is, for its part, configured to select the relevant ions, the sorting taking place in particular by means of a magnetic field {right arrow over (B)}. This is because the analyser5comprises in particular a magnetic-field source501configured to generate a magnetic field {right arrow over (B)} orthogonal to the plane of the path of the ions (i.e. orthogonal to the plane ofFIG.3, able to curve the path of the ions).

The magnetic-field source501is more particularly a source the magnetic field of which depends on the electric current that “supplies” said source.

Thus the magnetic-field source501is for example an electromagnet, i.e. a ferromagnetic material on which a winding is disposed, the magnetic field generated being dependent on the electric current circulating in said winding (in particular the direction of circulation and the intensity thereof).

It should be noted that said source501could also be a magnetic sector for example.

Thus, in the presence of a magnetic field {right arrow over (B)}, the beam of ions F is diverted. This is because a uniform magnetic field {right arrow over (B)} perpendicular to the plane of the path of the ions, because of the Lorentz force, will give said ions a curved path (the point of impact of the ion, and therefore the deviation thereof, making it possible to know the mass thereof from the charge).

The beam of ions F diverted by the magnetic field {right arrow over (B)} is then oriented towards one or more diaphragms after which a detection means7is disposed.

Said detection means7comprises for example one or more sensors703and/or705and an electronic circuit701connected to said sensors703and705to process the electrical signal coming from them.

It should be noted that the mass spectrometer10may also include the following elements (not shown), electrostatic lens or lenses for coupling, focusing, collecting, etc., accelerator plates, etc. These elements can be disposed at the ionisation source3, the analyser5or the detection means7, or between said means3,5and7.

Thus, when the magnetic field {right arrow over (B)} in the analyser5is adjusted so as to exactly address tracer gases having a determined mass M at the middle of at least one diaphragm501aor501b(serving as a selection slot), the other gases having the same electrical charge, but having masses different from M, will, in the analyser4, turn on different radii. The gases with a mass lower than M will turn on a smaller radius, while the gases with a mass greater than M will turn on a larger radius than the one associated with the gas of mass M.

To allow adjustment of the magnetic field {right arrow over (B)}, the mass spectrometer10comprises a means400for adjusting the magnetic field generated by said source501, for example by the electromagnet. This means is illustrated more particularly inFIG.4.

As illustrated on thisFIG.4, said adjustment means401is connected to the coil of the electromagnet501and is configured to vary the intensity of the current I circulating in said coil.

The adjustment means401thus comprises two distinct adjustment commands401and403, a circuit405for controlling the current of said electromagnet501, and a combinatorial circuit407for said commands401and403. Said combinatorial circuit407is configured to receive said commands401and403as an input and thus to generate as an output a function F dependent on the values of said commands401and403.

The resulting function F is thus sent to the control circuit405so that there is adjustment of the current I according to the values of said commands401and403.

Said commands401and403are for example digital to analogue converters that deliver, as an input of the combinatorial circuit407, respectively voltages V1and V2. It should be noted however that any electrical quantity could be adapted, by means of suitable arrangements.

Thus, said combinatorial circuit407will have at its output a voltage V5dependent on the input voltage values V1and V2, and resistors R1, R2, R3and R4, of the type:

F⁡(V1,V2)=VS=R4+R3R3⁢R2⁢V1+R1⁢V2R1+R2By judiciously choosing the value of said resistors R1, R2, R3and R4, for example by taking R1=R3=R4=R et R2=KR with k a constant greater than 1, and preferably very much greater than 1, the function F is simplified to give:

The two commands V1and V2will thus have a different weighting and will therefore influence the value of the output voltage V5distinctly. It should be noted that the influence of the voltage V2is K times less than the influence of the voltage V1on the output voltage V5.

Said control circuit405for its part comprises an operational amplifier AO2associated with a grounding resistor RSand with a transistor T1, the assembly forming a circuit of the voltage to current (or transconductance) converter type. The transistor T1is for example a bipolar transistor (or of the MOSFET type) the base (or gate) of which is connected to the output of the operational amplifier AO2, the collector (or source) of which is connected to the electromagnet501, and the emitter (or drain) of which is connected to the resistor RS.

The output voltage V5coming from the combinatorial circuit407is sent to the inverting input of the operational amplifier AO2, while the non-inverting input is connected to the grounding resistor RSand to the emitter of the transistor T1(more particularly connected to a node located between the emitter of the transistor T1 and the grounding resistor RS).

The output voltage V's coming from the operational amplifier AO2thus adjusts the value of the current I circulating through the transistor T1, but also through the electromagnet501. The current I depends on the voltages V1and V2of the commands401and403in accordance with the following formula:

The magnetic field {right arrow over (B)} is thus dependent (more particularly proportional for an electromagnet) on the intensity of the current I circulating in the coil of the electromagnet501, and consequently the field {right arrow over (B)} is here dependent on the commands401and403.

It can also be defined that the current I circulating in the electromagnet501in the following manner:

where I0is a nominal current, dependent on the voltage V1, which generates a nominal magnetic field {right arrow over (B0)}, and where ΔI is a small variation in current around the nominal current I0, dependent on V2, which generates a variation Δ{right arrow over (B)} in the magnetic field around the value of the nominal magnetic field {right arrow over (B0)}.

Thus, one of the adjustment commands401makes it possible to establish a nominal magnetic field {right arrow over (B0)}, while the other adjustment command makes it possible to generate a variation in magnetic field Δ{right arrow over (B)} around the value of the nominal magnetic field {right arrow over (B0)}.

It can also be defined that said adjustment means400comprises an adjustment command401equivalent to a preadjustment (or rough adjustment), while the adjustment command403is a fine adjustment.