Stanford Research Systems RGA100, Operating Manual And Programming Reference

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6-32 Ionizer Control Commands
SRS Residual Gas Analyzer
Ionizer Control Commands
DGparam, param: 0 - 20,*
Description : Ionizer Degas command
Echo : STATUS error byte (unless command is stopped before completion, or
param=0).
DEGAS the ionizer by heating and electron stimulated desorption. The parameter
represents the desired DEGAS time in minutes and includes a one minute initial
ramping time.
Warning: Repeated degassing will considerably limit the lifetime of the
filament. Whenever possible replace a Degas procedure with a thorough probe
and vacuum system bakeout (See Probe Bakeout in RGA Maintenance Chapter).
Important: The Electron Multiplier is turned off at the beginning of a Degas
cycle, and left unbiased at the end, for protection.
During the DEGAS process, the electron energy is set to 400 eV, and the electron
emission current is ramped to 20 mA over a 1 minute period. The high ionizer
energy (c.a. 8W) combined with a large flux of highly energetic electrons
degasses the ionizer in a process known as electron stimulated desorption.
Degassing is done in a very controlled fashion. The electron emission current is
ramped slowly to avoid sudden bursts of pressure into the vacuum chamber, and
the filament is constantly checked to make sure it is always protected from a
harsh environment. If a problem is found in the filament operation at any time,
degassing is interrupted and the filament is left without any electron emission
current.
The degassing process involves several steps:
1. The FIL_ERR Byte is checked to make sure it is zero. If not zero, jump to 12.
2. The filament status is memorized so it can be restored at the end of the
command ( step 10).
3. The Degas_LED is turned on.
4. The FC and CDEM anode are connected to ground, and (if necessary) the
CDEM is turned off.
5. The electron energy is set to 400 eV.
6. The emission current is ramped from 1 to 20 mA over a period of one minute
(with the filament checked every 3 seconds)
7. The rest of the degas time is spent at full emission current and with the
filament checked once every second.