Classification, Structure, and Working Principle of Helium Mass Spectrometer Leak Detectors

The nitrogen mass spectrometer leak detector is a specialized instrument for leak detection using helium as the leak gas. It features stable performance and high sensitivity. It is the most commonly used leak detector in vacuum leak detection technology.

The helium mass spectrometer leak detector is a magnetic deflection type mass spectrometer. The sensitivity of a single-stage magnetic deflection instrument is 10^-9 to 10^-12 Pam3/s, widely used for leak detection in various vacuum systems and components. Compared to single-stage magnetic deflection instruments, the sensitivity of double-stage tandem magnetic deflection instruments can reach 10^-14 to 10^-15 Pam3/s, making them suitable for ultra-high vacuum systems, components, and devices. The reverse flow helium mass spectrometer leak detector changes the structural layout of conventional instruments, placing the tested item in the front stage of the main pump of the leak detector, thus allowing leak detection at high pressures, without the need for liquid nitrogen, and with minimal contamination of the mass spectrometer chamber. It is suitable for leak detection in vacuum systems with large leak rates and poor vacuum hygiene, with a sensitivity of up to 10^-12 Pam3/s.

The helium mass spectrometer leak detector consists of a mass spectrometer chamber made up of an ion source, analyzer, collector, cold cathode ionization gauge, and a vacuum system and electrical components.

　　1. Single-stage magnetic deflection helium mass spectrometer leak detector

Taking the HZJ-1 type instrument as an example, the structure of the single-stage magnetic deflection helium mass spectrometer leak detector is shown in Figure 2.

Figure: Single-stage magnetic deflection helium mass spectrometer leak detector

Figure: Ion deflection radius calculation formula

Inside the mass spectrometer chamber, there is: an ion source composed of a filament, ionization chamber, and ion acceleration electrode; an analyzer composed of an external uniform magnetic field, baffle, and exit slit; a collector composed of a suppression grid, collecting electrode, and high resistance; and a first-stage amplified electrostatic gauge tube and cold cathode ionization gauge. The working principle of the mass spectrometer chamber is shown in Figure 3.

In the ionization chamber N, the gas is ionized into positive ions, which are focused into a beam under the action of the electric field. Under the influence of the acceleration voltage, they pass through the slit of the acceleration electrode S1 at a certain speed into the analyzer. Under the action of a uniform magnetic field, ions with a certain speed will move in a circular trajectory, and their deflection radius can be calculated using formula (5).

It can be seen that when B and U are constant values, the deflection radius R of ion beams with different mass-to-charge ratios (me-1) is different. The B and R of the instrument are fixed, and the acceleration voltage U is adjusted so that the helium ion beam [(me-1)2] just passes through the exit slit S2, reaching the collector D, forming an ion flow that is amplified by the amplifier. This is reflected by the output meter and sound indicator; while ion beams with different mass-to-charge ratios [(me-1)1(me-1)3] cannot pass through the exit slit S2 due to their deflection radius being different from the instrument's R value, thus they are separated. (me-1)2=4, which is the mass-to-charge ratio of He+, and aside from He+, C is very rare and can be ignored.

　2. Double-stage tandem magnetic deflection helium mass spectrometer leak detector

Figure 4 shows the mass spectrometer chamber of the double-stage 900 shrinkage tandem magnetic deflection helium mass spectrometer leak detector. Due to the two analyses, the probability of non-helium ions reaching the collector is reduced. Additionally, an accelerating electric field is set between the two analyzers, specifically between the middle slit S2 in the figure and the adjacent baffle, allowing ions to be accelerated again before entering the second analyzer. Those non-helium ions with the same momentum as helium ions can pass through the first analyzer, but after being accelerated a second time into the second analyzer, they are separated due to their different momentum from that of helium ions. Due to the secondary separation, the instrument's background and background noise are significantly reduced, improving the instrument's sensitivity.

Figures 4-5: Double-stage tandem magnetic deflection helium mass spectrometer leak detector and reverse flow helium mass spectrometer leak detector

3. Reverse flow helium mass spectrometer leak detector

The structural characteristics of the reverse flow helium mass spectrometer leak detector are shown in Figure 5. This type of instrument is made based on the principle that the compression ratio of oil diffusion pumps or molecular pumps is related to the type of gas. For example, the compression ratio of a multi-stage oil diffusion pump for helium gas is 10^2; for other components in the air, the compression ratio is 10^4 to 10^6. During leak detection, helium gas entering the main pump's front stage through the leak hole in the tested item still has some backflow into the mass spectrometer chamber, and the leak signal is indicated by the instrument's output. This is the working principle of the reverse flow helium mass spectrometer leak detector.