Since Nebiki, et al. reported that the tapered glass capillary having
an outlet diameter less than 1 lm works as a focusing device
for MeV ions [1], it has been applied to micro beam production
in various fields such as nuclear reaction analysis [2], high-contrast
X-ray imaging [3], and cell surgery [4]. Thanks to low conductance
of gas through the capillary outlet aperture, it can work also as an
orifice for differential pumping, enabling us to extract micro beams
into air without any thin-foil window [5]. However, micro beam
current available from the tapered capillary is small compared
with conventional micro beam systems using electromagnets because
ion transmission through the capillary is several percent at
the most. The mechanism of ion transport through the capillary
is not fully understood, but small angle scattering on the capillary
inner wall has so far been considered to be responsible for it [6].