We investigate the dynamics of a Bose-Einstein condensate held in an optical
lattice under the influence of a strong periodic driving potential. Studying
the mean-field version of the Bose-Hubbard model reveals that the condensate
becomes highly unstable when the effective intersite tunneling becomes
negative. We further show how controlling the sign of the tunneling can be used
as a powerful tool to manage the dispersion of an atomic wavepacket, and thus
to create a pulsed atomic soliton laser.Comment: 4 pages, 3 eps figure