  The standard explanation for multiple filamentation (MF) of intense laser
beams has been that it is initiated by input beam noise (modulational
instability). In this study we provide the first experimental evidence that MF
can also be induced by input beam ellipticity. Unlike noise-induced beam
breakup, the MF pattern induced by ellipticity is reproducible shot to shot.
Moreover, our experiments show that ellipticity can dominate the effect of
noise, thus providing the first experimental methodology for controlling the MF
pattern of noisy beams. The results are explained using a theoretical model and
simulations.
