Analyzer Cell Operating Principle - Alcatel ASM 180 TD User Manual

Helium leak detector

Table of Contents

Analyzer cell operating principle

Cell principle

The mass spectrometry analyzer cell is used for helium

partial pressure measurements.

Magnetic deflection

The molecules of the gas being analyzed are bombarded

spectrometry

by an electron beam from a heated tungsten filament (1) in

an ionization chamber (3).

A large proportion of the molecules are transformed into

ions. These ionized particles are accelerated by an electrical

field: the acceleration voltage. A magnetic field deflects the

ion beam by a radius propotional to the mass of the ions.

The acceleration voltage directs the Helium ions to the

target at the entrance of an amplifier, an electron multiplier

based system, developed and patented by ALCATEL.

Leak flow rate

The stream of Helium ions is proportional to the partial

pressure of helium in the installation and its measurement is

used to find the value of the flow rate of the detected leak.

Vacuum operation

It is essential for the total pressure in the analyzer cell to be

less than 10

ions are not disturbed by residual molecules.

Separation of He ions

In order to separate the helium ions from the "noise" due to

from "noise"

"dispersed ions", a "braking electrode" (6), placed in front

of the target, eliminates secondary, low-energy ions.

Total pressure

The top of the cell contains an auxiliary electrode which

collects ions that have a higher mass than that of helium.

This electrode, the triode electrode (4), is used to measure

the total pressure inside the analyzer.

Alcatel Vacuum Technology France - ASM 180 TD/TD+ - ASM 181 TD+ User's Manual

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mbar so that the paths of the electrons and