Illustration of crossed-beam energy transfer, a scattering process that is known to lower the efficiency of laser heating in certain inertial confinement fusion experiments. In the geometry shown here, two laser beams are directed at a spherical target (containing a mixture of deuterium and tritium.) Central rays from beam 0 can intersect outer rays from beam 1 where the expansion velocity of the plasma is equal to the speed of sound (the Mach 1 surface). The beat wave between these rays (insert) imprints a Bragg diffraction grating in the plasma. Because the grating moves at the speed of sound, it acts like a Bragg cell, scattering light moving in the direction of beam 0 (k0) to the scattered direction (k0scat), which is exactly in the direction of beam 1 (k1). The energy that would otherwise have been deposited in the target hence “turns around,” which can lead to a significant reduction in coupling efficiency.