  The two-plasmon-decay instability (TPD) is a critical target preheating risk
in direct-drive inertial confinement fusion. In this paper, TPD collectively
driven by a normal-incidence laser beam (Beam-N) and a large-angle-incidence
laser beam (Beam-L) is investigated via particle-in-cell simulations.
Significant TPD growth is found able to develop in this regime at previously
unexpected low laser intensities if the intensity of Beam-L exceeds the
large-angle-incidence threshold. Both beams contribute to the growth of TPD in
a "seed-amplification" manner where the absolute instability driven by Beam-L
provides the seeds that get convectively amplified by Beam-N, making TPD
energetically important and causing significant pump depletion and hot electron
generation.
