Continuity of electrostatic potential around magnetic mirror throat

The electrostatic potential profile around a magnetic mirror throat is studied. Bohm's criterion expresses the necessary condition for the formation of a stationary sheath in front of the absorbing wall. This condition, when applied to the mirror throat of a magnetic mirror, requires that the ion drift velocity flowing into the mirror throat from the confining region must exceed the ion sound velocity for the continuity of the potential. It is found, however, that the ionization around the mirror throat renders the potential continuous even with zero ion drift velocity. (author)