  If a charged particle bunch propagates near a plasma-vacuum boundary, it
excites a surface wave and experiences a force caused by the boundary. For the
linearly responding plasma and ultra-relativistic bunch, the spatial
distribution of excited fields is calculated, and the force exerted on a short
and narrow bunch is approximated by elementary functions. The force attracts
the bunch to the boundary, if the bunch is in the vacuum, and repels otherwise.
There are also additional focusing and defocusing components of the force.
