  We report a novel measurement of the late time self-similar growth constant
$\alpha_B$ of the Rayleigh-Taylor instability (RTI) from controlled initial
perturbations. To this end, we have developed a laser-driven experiment,
fielded on the National Ignition Facility (NIF), to explore deeply non-linear,
multimode hydrodynamic growth of a planar interface. The measured value is
$\alpha_B=0.038\pm 0.008$, which is consistent with previously reported 2D
simulations and closer to previously reported 3D simulations. This resolved the
known discrepancy between experiments and simulations of RTI.
