  Multi-filamentation opens new degrees of freedom for manipulating
electromagnetic waves in air. However, without control, multiple filament
interactions, including attraction, repulsion or fusion often result in
formation of complex disordered filament distributions. Moreover, high power
beams conventionally used in multi-filament formation experiments often cause
significant surface damage. The growing number of applications for laser
filaments requires fine control of their formation and propagation. We
demonstrate, experimentally and theoretically, that the attraction and fusion
of ultrashort beams with initial powers below the critical value enable the
eventual formation of a filament downstream. Filament formation is delayed to a
predetermined distance in space, avoiding optical damage to external beam
optics while still enabling robust filaments with controllable properties as if
formed from a single high power beam. This paradigm introduces new
opportunities for filament engineering eliminating the need to use high peak
power lasers sources.
