The ATRAP apparatus combines positrons (which enter from the top) with
anti-protons (which enter from below) and meet about one-third of the
way up from the bottom to make neutral anti-hydrogen atoms. To do this
the positrons pass through a special rotatable electrode (the element
with the circular hole near the bottom of the wide part of the apparatus).
Reported by: Gabrielse
et al., Physical Review Letters, 18 November 2002.