Modification to enhance the beam current of a simple ion gun

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 ∼10-7 Torr Ar by a helium cryopump. A milling rate of 3.8 Å/sec was obtained for SiO2 using 2 keV Ar ions. Sample to gun distance was 5 cm. This may be compared with the rate 0.8 Å/sec, obtained when operating in a static vacuum of 5×10-5 Torr Ar.