Deceleration-Phase Rayleigh–Taylor Growth Effects on Inferred Ion Temperatures in Room-Temperature, Direct-Drive Implosions*

Performance degradation in direct-drive inertial confinement fusion implosions can be caused by several effects, one of which is Rayleigh–Taylor (RT) instability growth during the deceleration phase. In room-temperature, plastic target implosions, this deceleration-phase RT growth is enhanced by the density discontinuity and finite Atwood numbers at the fuel–pusher interface. For the first time, experiments at the Omega Facility systematically varied the ratio of deuterium-to-tritium within the DT gas fill to change the Atwood number. Ion-temperature variation (ΔTi), as measured by different detectors along different lines of sight during implosions, was smaller in shots that had RT-unstable Atwood numbers (increased RT growth) than those with RT-stable Atwood numbers (and less expected RT growth). Increased levels of short-scale RT growth are suspected to be the cause of reduced ion-temperature variation. Simulations with low-mode-only asymmetries show increased ΔT_i whereas the addition of high modes also shows a reduction in this variation, similar to what is believed to occur in experiments.