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Transpector MPS Operating Manual
The Transpector MPS filament is Dual Yttria-coated Iridium. See
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.
Centered inside the curved filament is the ion cage, which is mounted to the anode
plate. The cage has an open mesh structure to facilitate the flow of gas molecules
into the ionizing region. The potential (voltage) on the anode is positive with respect
to the electron repeller (also an open mesh structure); the potential on the filament
lies somewhere between these other two electrodes. 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.
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 hole in the source exit lens.
To attract positive ions, the focus lens is biased negatively with respect to the
anode.
The potential on the source exit lens is negative with respect to the anode, and (for
the particular design illustrated here) the focus lens as well. Part of the ion beam
passes through the hole in the exit lens and is injected into the mass filter.
The remaining portion of the beam 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 can therefore be used as a measure of the total pressure. When this
current exceeds a preset level, the voltages operating the sensor are turned off to
protect the sensor from damage due to an over-pressure condition.
NOTE:
This protection feature works only after the filament has been operating for
a short period of time. Therefore, the filament will not be protected from
excessive pressure if the sensor is turned on too early in a pump-down
cycle.
