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Transpector XPR 3+ Operating Manual
3.4.1 The Ion Source
The Transpector XPR 3+ sensor’s ion source, optimized for detecting residual
gases in a vacuum system, has a fairly open construction that facilitates the flow of
gas molecules into the ionizing region.
The ion source of Transpector XPR 3+ operates on the same principles as the
larger ion sources of standard open ion source sensors. However,
Transpector XPR 3+ is built with a dual ion source which supplies one ion stream
to the quadrupole filter and a second ion stream to a total pressure collector. This
design allows the total pressure collector to be well isolated from other electrodes
in the ion source so that the small ion currents from the Transpector XPR 3+
source can be measured accurately.
Inside the ion source, a heated filament emits electrons, which bombard the gas
molecules, giving them an electrical charge. While this charge may be either
positive or negative, Transpector XPR 3+ detects only positive ions. Once a
molecule is charged, or ionized, electric fields can be used to manipulate it.
The filament is an iridium wire with yttrium-oxide coating. The Transpector XPR 3+
filament can be protected by the Pirani Interlock, which controls emission within
safe operating parameters.
The term “emission current” refers to the stream of electrons emitted by the
filament. The filament is heated with a DC current from the emission regulator
circuit, with the resulting temperature of the filament used as the means of
controlling the emission current.
The potential (voltage) on the anode is positive with respect to the potential on the
filament. The potential difference between the filament and the anode determines
the kinetic energy (usually called the electron energy) of the emitted electrons. The
electron energy in turn determines how gas molecules will ionize when struck by
the electrons.
A three-sided repeller is centered around the filament and is connected to the low
voltage side of the filament. This geometry and potential focuses the electrons
through the partial pressure region and on into the total pressure ion region as
shown in
. The ions formed within the cage on the anode are pulled away
by the potential on the focus lens and formed into a beam. (The focus lens is
sometimes called an extractor, since it extracts the ions from the region in which
they are created.) The focus lens also serves to focus the ion beam into the
quadrupole. To attract positive ions, the focus lens is biased negatively with respect
to the anode.
The ion beam generated in the partial pressure chamber passes through the hole
in the focus lens and is injected into the mass filter. The ion beam generated in the
total pressure chamber strikes the exit lens and is neutralized, resulting in a current
flow. The magnitude of this current is related to the pressure in the ion source, and
