Quantifying total inner beam Cross Beam Energy Transfer (CBET) at the National Ignition Facility

Cross-beam energy transfer (CBET) plays a pivotal role in controlling implosion symmetry across all current ignition designs at the National Ignition Facility. However, existing simulation models are unable to accurately predict low mode shape/symmetry. To better match experimental results, an empirical plasma wave amplitude limiter is often applied to the CBET model. This limiter requires adjustment for each specific design, laser drive, and wavelength configuration. It is not yet clear whether the limiter is compensating for shortcomings in the CBET physics itself or for other inaccuracies in the hohlraum or capsule models. This ambiguity increases the number of tuning shots needed and raises concerns about the reliability of implosion shape predictions.

To address this and to validate our CBET model, we have developed a novel experimental platform to directly measure CBET power transfer as a function of time between the inner and outer laser beams. In this talk, we present the first experimental results from this platform, examining CBET as a function of Δλ (the wavelength detuning between inner and outer cones) and varying hohlraum gas fill densities.