RU137653U1 - MASS SPECTROMETRIC ANALYZER OF GAS LEAK DETECTOR - Google Patents

Abstract

The proposed solution relates to vacuum technology, namely to static magnetic mass spectrometric analyzers with 180-degree rotation and double magnetic focusing and can be used in gas leak detectors, including helium ones, designed for leak testing of various systems and objects, allowing the pumping of the internal cavity to a deep vacuum or filling it with a helium-containing mixture, or other test gas, under excessive pressure. The problem to which the claimed technical solution is directed is to create a reliable, highly efficient mass spectrometric gas leak detector with improved technical and operational characteristics. The technical results achieved by the implementation of the claimed utility model are to increase reliability and increase the life of the mass spectrometer analyzer; reduced vacuum requirements; expanding the dynamic range of working pressures in the test object; elimination of cathode contamination; providing the ability to work with aggressive environments; reducing the time to complete technological operations; reducing the weight and size parameters of the mass spectrometric leak detector; reducing the cost of manufacturing a mass spectrometric leak detector. The mass spectrometric analyzer of a gas leak detector contains a vacuum chamber with connecting flanges, inside of which there are: - a source of test gas substance ions, consisting of an electron source and an ionization chamber; - a magnetic system that provides separation of ions by mass; - ion receiver. In this case, a plasma cathode made on the basis of a glow discharge plasma was used as an electron source. It is preferable that a Penning cell (PL) placed in an axial magnetic field be used as a plasma cathode with an electron emitter made in the form of a slit for forming a ribbon electron beam in the cell anticathode, from the side of the ionization chamber. It is also preferred that openings in the central part of the anode of the Penning cell are made for “pumping” residual gas from the vacuum chamber. It is also preferable that the mass spectrometric analyzer further comprises a reflection electrode mounted opposite the exit window of the ionization chamber, which is under the potential of the plasma cathode (virtual cathode), which provides reflection (oscillations) of the electrons supplied from the plasma cathode. In some cases, a metal plate, or a second plasma cathode, or a filament of a heated thermal cathode, installed symmetrically to the plasma cathode on the opposite side of the ionization chamber and creating an electric field reflecting electrons, can be used as a reflecting electrode. Preferably, the housing of the vacuum chamber was made in the form of a monoblock with connecting flanges, and the ion receiver and ion source were mounted on the connecting flanges on special brackets. Preferably, the ion source and receiver of the test gas substance are located in the inter-pole gap of permanent magnets from NeFeB with a magnetic field of 0.01 to 1.00 T. Preferably, helium ₄ He ^{+ is} used as the test gas. In some cases of execution, as a test gas substance, any gas can be used for which the magnitude of the magnetic field and the ion accelerating voltage are tuned. 1 independent claim, 9 dependent claims, 3 of FIG. drawing.

Description

The proposed technical solution relates to vacuum technology, namely to mass spectrometric devices, in particular, to static magnetic mass spectrometric analyzers with 180-degree rotation and double magnetic focusing.

The proposed technical solution can be used in gas leak detectors, including helium ones, designed for leak testing of various systems and objects that allow pumping out the internal cavity to a deep vacuum or filling it with a helium-containing mixture, or other test gas, under overpressure.

Mass analyzers are devices for spatial or temporal separation of ions with different values of m / z (mass to charge) in magnetic or electric fields, or combinations thereof. Distinguish between static and dynamic analyzers. In static ions are separated in constant or practically unchanged during their movement through the analyzer magnetic fields. Ions with different m / z values move in such an analyzer along circular paths of different radii and are focused sequentially onto the detector slit as a result of a smooth change in the electric and magnetic fields of the analyzer. In dynamic analyzers, ion separation occurs under the influence of pulsed or radio-frequency electric fields with a period of change less than or equal to the time of flight of the ions through the mass analyzer. The pressure in the analyzers should be low enough (<10 ^-1 Pa) to reduce the effect of ion scattering on residual gas molecules. The main characteristic of the mass analyzer is its resolving power R. It characterizes the ability of the analyzer to separate ions with slightly different masses and is determined by the ratio of the mass of the ion M to the width of its peak DM (expressed in atomic units of mass) at a certain level of peak height (usually 50 or 10%): R = M / DM. In single-focus mass analyzers, an ion beam formed in an ion source exits from a gap of width S1 in the form of a diverging ion beam and in a magnetic field is divided into ion beams with different values of m / z. In dual-focus mass analyzers, an ion beam is first passed through a deflecting electric field of a special shape in which the beam is focused by energy, and then through a magnetic field in which ions are focused in directions. There are several types of devices of this class: time-of-flight, quadrupole, magnetic etc. .. (http://www.xumuk.ru/encyklopedia/2448.html).

In general, mass spectrometric analyzers are a closed vacuum system in which there is a source of ions of a test substance, consisting of a source of electrons (cathode) ionizing the atoms of the test substance and an ionization chamber; mass separation devices; ion collector. At the same time, static magnetic mass spectrometers have the highest sensitivity.

For highly sensitive tightness control, mass spectrometric helium leak detectors are used - magnetic mass spectrometers tuned, as a rule, to a test gas substance ₄ He ⁺ . Any gas can be used as a gas test substance, with the appropriate setting of the magnetic field and the ion accelerating voltage. To separate the atoms of the test substance in crossed (mutually perpendicular) electric and magnetic fields, the ionization chamber of the device ionizes neutral helium (or other test gas) by an electron beam. The essence of the ionization method is to separate one (or two) electrons from a neutral helium atom by irradiating it with a beam of fast electrons, which in turn can be obtained by heating, for example, a metal ribbon (cathode).

Known mass spectrometer helium leak detector UL 1000 company Inficon, Switzerland, containing an ion source, 180 ° degree sector magnetic analyzer and ion receiver. A double iridium cathode coated with sodium oxide was used as a source of ions (Technical Reference Helium Leak Detectors, http://www.msht.ru/upload/iblock/ba9/ba9ae0709a8b30ec52c4a7a0dc11cf50.pdf.

Known mass spectrometric analyzer helium leak detector PTI-10, containing placed between the poles of a permanent magnet chamber with an ion source and an ion receiver. An incandescent tungsten cathode (http://www.accel.ru/vt/pdf/pti.pdf) was used as the electron source of the ion source.

Known mass spectrometric analyzer helium leak detector TI1-14M containing an ion source, drift space and an ion receiver. The mass spectrometric analyzer is enclosed in a non-magnetic steel casing and placed between the poles of a permanent magnet. The heated tungsten cathode of an ion source emits electrons that are accelerated by an electric field applied between the cathode and the ionizer body of the ion source. The cathode’s electronic current is stabilized by an emission stabilizer (HELIUM MASS-SPECTROMETRIC TI1-14M leak detector. Operation manual. Http://www.techeiscatel.ru/index.php/library/rukovodstva-po-ekspluatatsii/73-ti1-14-rukovodstvo- po-ekspluatatsii-techeiskatelya-gelievogo # 2)

Known for the closest to the essential features and selected as a prototype mass spectrometric analyzer of a gas leak detector MS-3, containing a vacuum chamber with connecting flanges, inside of which are located: a source of ions of a test gas substance, consisting of an electron source and an ionization chamber; a magnetic system that provides separation of ions by mass; ion receiver. An incandescent tungsten cathode is used as an electron source, emitting electrons accelerated by an electric field applied between the cathode and the ionizer body of the ion source. The cathode electron current is stabilized by an emission stabilizer. The mass spectrometric analyzer is made with 180-degree rotation and double magnetic focusing. A magnetic field acting along the direction of electron motion focuses the electron flux into a narrow beam passing into the ionizer. Further, an ion beam is formed from the ionizer chamber, moving perpendicular to the lines of force of the magnetic field. In a magnetic field, the ion beam emerging from the source is separated into individual beams containing ions with the same mass to charge ratio. Ions of a certain mass (in the general case of a certain ratio m / z - effective mass) emerging from an ion source by a diverging beam are again collected in a narrow converging beam in the plane of the input diaphragm of the electron receiver (double focusing). By changing the accelerating voltage, tuning to the “peak of helium” is carried out according to the maximum of helium ions directed to the ion receiver. A highly sensitive electrometric amplifier is connected to the ion receiver. The output signal proportional to the ion current of the test gas (helium) is fed to the processing device and displayed on the screen of the instrument control panel (Leak Detector Mass Spectrometric MS-3, Operation Manual SKSP 171011.011-01RE, LLC NPF PROGRESS, 2011 )

All of the above known mass spectrometric analyzers have common problems and disadvantages, namely:

- insufficiently high reliability and durability due to the small resource and unreliability of the thermionic electron source (solid-state heated cathode). The heated incandescent filament emitting electrons becomes so fragile during operation (even when working in a deep vacuum) that the slightest vibration destroys it, and when the vacuum conditions worsen, it simply burns out. Replacing the cathode without loss of vacuum is impossible, which leads to a decrease in the reliability of the device as a whole and an increase in the test time;

- a small dynamic range of working pressures in the test object, associated with the need to maintain the cathode under high vacuum and the limitations of high-vacuum pumping units;

- high manufacturing costs, due to the need to use expensive equipment;

- the presence of temporary losses during pumping of test objects to ensure a deep (<10 ^-1 Pa) vacuum;

- reducing the emission properties of the cathode, the sensitivity and resolution of mass spectrometric devices in the presence of organic contaminants in the pumped object;

- increase in noise of the electrometric channel, decrease in sensitivity, failure of cathodes and vacuum units when working with aggressive and radioactive substances;

- the complexity of electronic equipment and software that provide protection (blocking) of the cathodes of mass spectrometric analyzers in case of vacuum deterioration, and insufficiently high efficiency of their work. Devices of all manufacturers have vacuum locks, however, due to the inertia of actuators (solenoid valves), locks still do not provide complete protection of the cathodes from destruction;

- high labor costs for performing technological operations;

- significant weight and dimensions of the mass spectrometric leak detector.

The problem to which the claimed technical solution is directed is to create a reliable, highly efficient mass spectrometric gas leak detector with improved technical and operational characteristics.

The technical results achieved by the implementation of the claimed utility model are:

- improving reliability and increasing the life of the mass spectrometric analyzer;

- reduction of vacuum requirements;

- expanding the dynamic range of working pressures in the test object;

- elimination of cathode contamination;

- providing the ability to work with aggressive environments;

- reducing the time to complete technological operations;

- reducing the weight and size parameters of the mass spectrometric leak detector;

- reducing the cost of manufacturing a mass spectrometric leak detector.

The indicated technical results are achieved by the fact that the gas leak detector mass spectrometric analyzer contains a vacuum chamber with connecting flanges, inside which are located: - a source of test gas substance ions, consisting of an electron source and an ionization chamber; - a magnetic system that provides separation of ions by mass; - ion receiver. In this case, a plasma cathode made on the basis of a glow discharge plasma was used as an electron source.

It is preferable that a Penning cell (PL) placed in an axial magnetic field be used as a plasma cathode with an electron emitter made in the form of a slit for forming a ribbon electron beam in the cell anticathode, from the side of the ionization chamber.

It is also preferred that openings in the central part of the anode of the Penning cell are made for “pumping” residual gas from the vacuum chamber.

It is also preferable that the mass spectrometric analyzer further comprises a reflection electrode mounted opposite the exit window of the ionization chamber, which is under the potential of the plasma cathode (virtual cathode), which provides reflection (oscillations) of the electrons supplied from the plasma cathode.

In some cases, a metal plate, or a second plasma cathode, or a filament of a heated thermal cathode, installed symmetrically to the plasma cathode on the opposite side of the ionization chamber and creating an electric field reflecting electrons, can be used as a reflecting electrode.

Preferably, the housing of the vacuum chamber was made in the form of a monoblock with connecting flanges, and the ion receiver and ion source were mounted on the connecting flanges on special brackets.

Preferably, the ion source and receiver of the test gas substance are located in the inter-pole gap of permanent magnets from NeFeB with a magnetic field of 0.01 to 1.00 T.

Preferably, helium ₄ He ^{+ is} used as the test gas.

In some cases of execution, as a test gas substance, any gas can be used for which the magnitude of the magnetic field and the ion accelerating voltage are tuned.

A comparative analysis of the claimed utility model with the prototype showed that in all cases it is different from the known, closest technical solution by using a plasma cathode based on a glow discharge plasma as an electron source.

In preferred embodiments, the claimed utility model differs from the known, closest technical solution:

- the use of a Penning cell (PL) as a plasma cathode with an electron emitter made in the form of a slit for forming a ribbon electron beam in the cell anti-cathode, from the side of the ionization chamber;

- the implementation of a Penning cell placed in an axial magnetic field;

- the presence in the central part of the anode of the Penning cell of holes for “pumping” residual gas from the vacuum chamber;

- the presence of a reflecting electrode located on the opposite side of the ionization chamber under the potential of the plasma cathode and providing reflection of electrons supplied from the plasma cathode;

- the implementation of a reflective electrode mounted opposite the output window of the ionization chamber;

- using as a reflecting electrode either a metal plate, or a second plasma cathode, or a filament of a heated thermal cathode, mounted symmetrically to the plasma cathode on the opposite side of the ionization chamber;

- the implementation of the source and receiver of ions of the test gas substance, located in the pole gap of permanent magnets from NeFeB with a magnetic field of 0.01 to 1.00 T;

- the implementation of the ion receiver and ion source mounted on the connecting flanges, on special brackets;

- using helium ₄ He ⁺ as a test gas substance;

- using as a test gas substance any gas for which the magnitude of the magnetic field and the ion accelerating voltage are tuned.

The use of a plasma electron source, for example, a Penning cell, instead of a solid-state thermionic cathode, provides a high degree of reliability and durability of the mass spectrometric analyzer, a wide dynamic range of operating pressures and the ability to work with aggressive media in the test object, while eliminating cathode contamination. The practically unlimited resource of the plasma cathode provides a dynamic range of working pressures in the test object from 5 × 10 ⁴ Pa to 1 × 10 ^-3 Pa without additional technological operations. Self-cleaning of the cathode due to plasma etching of surfaces during operation of the device removes: a) the problem of contamination and b) the problem of creating protective interlocks. This allows you to reduce the number of vacuum valves, improve the overall dimensions of the mass spectrometric device, increasing the overall reliability of testing for tightness control and reducing the time required for technological operations. One of the main advantages of electron sources with a plasma cathode is their uncriticality at high pressures or in aggressive media, where the cathode resource decreases to a level shorter than the required electron-beam processing time. These advantages are significantly enhanced by the generation of electron beams under conditions of dynamically changing pressure inside the vacuum chamber, beams of large cross section, as well as when passing into the fore-vacuum pressure range. The high emissivity of the plasma provides the generation of electron beams, which in terms of brightness and power density are at the level of modern thermocathode electronic sources. The presence of holes in the central part of the anode provides compensation for ion pumping from the Penning cell (PL) by discharge and stable combustion of a glow discharge contracted by the axial magnetic field in the pressure range from 5 × 10 ⁴ Pa to 1 × 10 ^-3 Pa. Inside the cell of the anode of the nuclear material, due to the fact that the ions leave towards the cathode and anticathode, a deeper vacuum is obtained, which in turn worsens the combustion conditions of the glow discharge. This is the effect of ion pumping. At the same time, through the indicated openings into the anode, atoms of the residual gas fall from the vacuum chamber, the pressure equalizes.

The presence of a reflecting (“virtual”) cathode installed opposite the exit window of the ionization chamber allows increasing the ionization factor G = j _e .t _i , where: j _e is the electron current density in the ionization chamber, t _i is the ionization time (transit time, or residence time electron in the ionization chamber during oscillations). The reflecting electrode, which is under the potential of the plasma cathode, serves as a mirror for electrons supplied by the plasma, providing multiple reflection (oscillations) of the ionizing electrons emitted by the plasma cathode, which makes it possible to effectively ionize the test substance even at an electron emission current of Ie = 1 μA. Electron scattering during reflection is excluded by an axial magnetic field holding electrons on its lines of force (Larmor radius R _{l of an} electron with an energy of about 200 eV in a magnetic field of 0.17 T is much smaller than the aperture of the window of the ionization chamber). In this case, the reflected electrons partially pass through the slot emitter back into the plasma cell and stabilize the discharge from disruption in the entire pressure range. The design features of the inventive mass spectrometric analyzer and the use of permanent magnets from NeFeB provides a 180-degree angle of rotation of the ions and the necessary radius of their trajectory. The implementation of the receiver and the ion source mounted on special brackets on the connecting flanges of the vacuum chamber housing, made in the form of a monoblock, provides "collapsibility" of the design and simplifies the possibility of maintenance of the nodes. Using a thermal cathode as a reflecting electrode allows, if necessary, to carry out ignition of a discharge in the region of ultra-low pressures, to use it to create initial ionization, as a “igniting” electrode. Automatic adjustment during assembly allows preventive maintenance inside the camera without removing the magnet and without additional adjustment.

The proposed utility model is illustrated by schematic drawings of a gas leak detector mass spectrometric analyzer shown in FIG. 1-4.

Figure 1 presents a schematic drawing of a mass spectrometric analyzer of a gas leak detector, side view, longitudinal section.

FIG. 2 is a schematic drawing of a gas leak detector mass spectrometric analyzer, a cross-section along BB in FIG. one.

On Fig 3 presents a schematic drawing of the ion source mass spectrometric analyzer of a gas leak detector with a Penning cell, the leader node A in Fig 1, section along DG.

In FIG. 4 is a schematic drawing of an ion source of a mass leak detector of a gas leak detector with a Penning cell, view B in FIG. 3.

The mass spectrometric analyzer of a gas leak detector contains a vacuum chamber 1 made in the form of a monoblock with connecting flanges 2, 3. If necessary, there may be more connecting flanges. The internal volume of the vacuum chamber 1 contains: - a source of ions of a test gas substance 4, - a receiver of ions 5, - a magnetic system including permanent magnets 6, preferably from NeFeB, with a magnetic field of 0.01 to 1.00 T. The ion source 4 of the test gas substance contains an ionization chamber 7 and an electron source 8, which is a plasma cathode made on the basis of a glow discharge plasma, using a Penning cell (PL) with an electron emitter made in the form of a gap 9 placed in an axial magnetic field for the formation of a tape electron beam in the anticathode 10 of the Penning cell, from the side of the ionization chamber 7. In the central part of the anode 11 of the Penning cell, holes 12 are made for “pumping” the residual gas from the vacuum oh mass spectrometric analyzer chamber. Opposite the output window of the ionization chamber 7, a reflective electrode 13 is placed, mounted symmetrically to the plasma cathode 8, on the opposite side of the ionization chamber 7. The reflective electrode 13 is at the potential of the plasma cathode 8 and provides reflection of the electrons supplied from it. The drawings show a reflective electrode 13 made in the form of a tungsten filament, which, if necessary, can be used as a thermal cathode (for example, to ignite a discharge under ultra-low pressure in the analyzer chamber). In other applications, either a metal plate or a second plasma cathode can be used as the reflecting electrode 13. The ion source 4 is mounted on the connecting flange 2, and the ion receiver 5 is mounted on the connecting flange 3 on special brackets 19, 20, respectively. The source 4 and the receiver 5 of ions of the test gas substance are located in the pole gap of the permanent magnets 6, preferably of NeFeB with a magnetic field of 0.01 to 1.00 T. Helium ₄ He ⁺ can be used as a test gas substance, or any gas to which the magnetic field and ion-accelerating voltage are adjusted.

The inventive mass spectrometric analyzer operates as follows.

Electrons emitted by the plasma cathode 8 and accelerated to an energy exceeding the binding energy of the electron with the helium atom fall into the ionization chamber 7, where they ionize the atoms of the test gas and create a "cloud" of ions from which the ion beam is formed. The power supply of the ion source 4 is provided through a pressure seal 14. The accelerating voltage U _a and the magnetic induction B are selected so that the helium ions passing through the exit slit of the ion source 9, moving along a circular path 15, fall into the entrance slit of the ion receiver 5. Residual ions gas, moving along trajectories with a different turning radius, are discharged on the walls of the analyzer and are pumped out by a vacuum unit. The test object is connected to the input flange (not shown) of the vacuum chamber 1 of the magnetic analyzer. In the event of a leak in a connected object, blown by helium, helium molecules penetrate into the ionization chamber 7, are ionized in the ion source 4 and, after reaching ions 5, are fixed by an electrometric amplifier (not shown in the drawing) connected to the pressure lead 16. The signal of helium ions is detected as the electric current of the ion receiver 5 and, after appropriate processing by the measuring unit (not shown in the drawing), the result is output to a control device (not shown in the drawing). Operating pressure may range from 5 × 10 ⁴ Pa to 1 × 10 ^-3 Pa. The only drawback in the operation of the inventive mass spectrometric analyzer in the high-pressure region is a slight decrease in the sensitivity of the device due to scattering and recharging of helium ions on the atoms of the residual gas (decrease in the mean free path of helium ions). However, it was experimentally established that a decrease in sensitivity is not critical in this case, since an increase in the degree of ionization of the residual gas in the ionization chamber 7 at high pressures leads to an increase in the ion beam current, which is an alternative to the scattering process.

The algorithm of the device is as follows:

- After connecting the test object to the leak detector and reaching a pressure in the vacuum chamber 1 of the analyzer less than 5 × 10 ⁴ Pa, power is supplied to the analyzer devices.

- A glow discharge is excited in the plasma cathode cell 8.

- A band electron beam is formed through the emission gap 9 in the anticathode 10 of the plasma cell 8.

- In the gap between the anticathode 10 of the plasma cell 8 and the ionization chamber 7, the beam electrons accelerate to an energy exceeding the binding energy of the electron of the helium atom and the nucleus (the maximum of the ionization cross section does not fall on the incident electron energy of 130 eV and decreases very smoothly as it increases).

- Accelerated electrons enter the ionization chamber 7 and produce acts of collision with the atoms of the residual gas, producing their ionization. In this case, the excess energy is carried away by the knocked out electrons and the ions are not heated. The ionization reaction can be written as follows:

₄ He + e = ₄ He ⁺ + 2e,

where e is an electron, ₄ He is a neutral helium atom, ₄ He ⁺ is a helium ion.

moreover, the electrons undergoing collision leave the ionization process and partially go to the chamber walls, and partially, being in a strong magnetic field, create a region with a negative potential inside the ionization chamber, which is an electrostatic trap for positively charged ions, as a result of which the density of the latter and improved ion-optical characteristics of the ion source.

- Thus, a secondary, monoenergetic plasma is formed in the ionization chamber, from which an ion beam is already formed through the emission gap 9 by an extraction voltage U _a equal to the energy required to rotate helium ions to the ion receiver 5.

- Having performed the movement along a circular path 15, helium ions fall on the ion receiver 5 and are recorded.

- Helium atoms occur when helium is blown through a test object. In case of leakage of the object, helium through leaks enters the vacuum chamber 1 and is delivered by means of pumping units to the ionization chamber 7 of the analyzer.

- The appearance on the control device (not shown in the drawing) of a helium signal indicates a leak in the test object.

Claims (9)

1. Mass spectrometric analyzer of a gas leak detector containing a vacuum chamber with connecting flanges, inside of which are located: a source of test gas substance ions, consisting of an electron source and an ionization chamber; a magnetic system that provides separation of ions by mass; an ion detector, characterized in that a plasma cathode made on the basis of a glow discharge plasma is used as an electron source. 2. Mass spectrometric analyzer of claim 1, characterized in that a Penning cell placed in an axial magnetic field with an electron emitter made in the form of a slit for forming a tape electron beam in the cell anticathode, from the side of the ionization chamber, is used as a plasma cathode. 3. Mass spectrometric analyzer according to claim 2, characterized in that in the central part of the anode of the Penning cell holes are made for "pumping" of residual gas from the vacuum chamber. 4. Mass spectrometric analyzer according to claims 2 and 3, characterized in that it further comprises a reflection electrode mounted opposite the output window of the ionization chamber, which is under the potential of the plasma cathode (virtual cathode), which provides reflection (oscillations) of the electrons supplied from the plasma cathode. 5. The mass spectrometric analyzer according to claim 4 _, characterized in that a metal plate, or a second plasma cathode, or a filament of a heated thermal cathode are installed symmetrically to the plasma cathode on the opposite side of the ionization chamber and create reflective electrons as the reflecting electrode. 6. The mass spectrometric analyzer according to claim 1, characterized in that the housing of the vacuum chamber is made in the form of a monoblock with connecting flanges, and the ion receiver and ion source are mounted on the connecting flanges, on the brackets. 7. The mass spectrometric analyzer according to claim 1, characterized in that the source and receiver of ions of the test gas substance are located in the inter-pole gap of permanent magnets from NeFeB with a magnetic field of 0.01 to 1.00 T. 8. Mass spectrometric analyzer according to claim 1, characterized in that helium ₄ He ^{+ is} used as a test gas substance; 9. The mass spectrometric analyzer according to claim 1, characterized in that any gas for which the magnitude of the magnetic field and the ion accelerating voltage are tuned is used as a test gas.

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