An ion gun has been modified to operate in a continuously pumped mode, in which gas is leaked directly into the ionization region, and the vacuum system is pumped to sim;10minus;7Torr Ar by a helium cryopump. A milling rate of 3.8 Aring;/sec was obtained for SiO2using 2 keV Ar ions. Sample to gun distance was 5 cm. This may be compared with the rate 0.8 Aring;/sec, obtained when operating in a static vacuum of 5times;10minus;5Torr Ar.
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