Development of a Cross-Beam Energy Transfer–Mitigation Demonstration Platform*

The efficient and uniform absorption of laser energy is essential for successful laser-direct-drive inertial confinement fusion implosions. Cross-beam energy transfer (CBET) is a major challenge to this goal. CBET significantly reduces the total laser absorption by tens of percent and increases the nonuniformity of the absorption above the 1% rms required for successful implosions. Modeling has shown that adding large bandwidth to the lasers will significantly mitigate the effects of CBET. Absorption fractions and uniformities near no-CBET levels can be achieved with large bandwidth. To this end, the large-bandwidth FLUX laser is being deployed as a proof-of-concept laser on the OMEGA Laser System. Demonstrating CBET mitigation during a multibeam implosion, however, is not straightforward since a single large-bandwidth beam will not significantly affect the overall implosion performance. To measure a noticeable CBET mitigation effect, we have developed a beamlet diagnostic that images unabsorbed implosion light. A beamlet of light from each OMEGA beam is captured as a discreet spot in the image. The intensities of a few of the beamlets will be strongly dependent on CBET with the FLUX laser, as determined by the beam/diagnostic geometry. With the FLUX laser and the beamlet diagnostic, the mitigation of CBET using bandwidth will be demonstrated.