5 – 3
Transpector SPS Operating Manual
The term
emission current
refers to the stream of electrons emitted by the filament.
Emission current is controlled by the temperature of the filament.
The filament is centered over a hole in the anode cylinder. The potential (voltage)
on the anode is positive with respect to the filament. The electron repeller, a flat
plate, is located behind the filament and is electrically connected to the negative
side of the filament. The potential difference between the filament and the anode
determines the kinetic energy (
electron energy
) of the emitted electrons. The
electron energy determines how gas molecules will ionize when struck by
electrons.
Electron energy can range from 70 eV down to 10 eV. Operation below 70 eV is
restricted to emission currents of no greater than 200 μA. At 70 eV or above, an
emission current of 2 mA will maximize sensitivity for background monitoring and
leak checking.
The ions formed within the anode cylinder are pulled away by the potential on the
focus lens and formed into an ion beam. (The focus lens is also called an extractor,
since it extracts the ions from the region in which they are created.) The focus lens
focuses 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 the
focus lens. The ion beam passes through the hole in the exit lens and is injected
into the mass filter.
The closed ion source pumping speed is approximately 0.7 L/s. This will create a
pressure differential between the ion source and the quadrupole region. The closed
ion source will operate with a maximum process pressure up to about 2 mTorr
(2.7E-3 mbar). This ion source is used with an orifice to reduce the high pressure
down to approximately 2E-4 Torr (2.7E-4 mbar). The pressure in the closed ion
source will be about 23 times higher than the pressure in the quadrupole (e.g.,
2.3E-4 Torr in the ion source will produce 1E-5 Torr in the quadrupole region).
See
section 6.5, Filament Control, on page 6-7
for more ion source information.
