  Propagation of Langmuir solitons in inhomogeneous plasmas is investigated
numerically. Through numerical simulation solving Zakharov equations, the
solitons are accelerated toward the low density side. As a consequence,
isolated cavities moving at ion sound velocities are emitted. When the
acceleration is further increased, solitons collapse and the cavities separate
into two lumps released at ion sound velocities. The threshold is estimated by
an analogy between the soliton and a particle overcoming the self-generated
potential well.
