Direct measurement of ion-acoustic wave growth rates due to the return current instability

Return current instability (RCI), the process by which a cold current is driven to compensate for heat flux leading to ion-acoustic turbulence, has been proposed as a mechanism limiting the heat flux in inertial confinement fusion experiments. Here, we present measurements of the ion-acoustic growth rate driven by the return current instability and show it is inherently connected to non-local transport. Thomson scattering was used to measure a maximum growth rate of 5.1× 10 9 Hz, which was three times less than classical Spitzer-Harm theory predicts. The measured plasma conditions suggest that the electrons are non-local and Vlasov-Fokker-Plank (VFP) simulations that account for this non-locality reproduce the measured growth rates. Furthermore, the threshold for the return current instability was measured (δ T =0.017 ±0.002) to be in good agreement with previous theoretical models. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.