Experimental investigation of late-time nonlinear evolution of ion-Weibel filaments*

The Weibel instability is a candidate mechanism for the generation of astrophysical seed fields: PIC simulations suggest that Weibel-generated fields might scale much more favorably for long length scales (𝐵∝𝐿⁰) than the Biermann-generated fields (𝐵∝𝐿^-1). This possibility, however, requires a mechanism to explain how the Weibel field structures might grow in size to large length scales. Existing theory and simulations describing various details of Weibel filament mergers provide models, whose predictions differ substantially depending on the included physics. For example, one model [C. Ruyer et al., Physics of Plasmas 22, 032102 (2015)] predicts filament wavelengths grow as 𝜆∝t², while another model [M. Zhou, et al., Phys. Rev. Res. 1, 012004 (2019)] predicts 𝜆∝t^1/2 (with time, t, representing asymptotically late times). Distinguishing these models requires linearly saturated ion Weibel filaments which were demonstrated in a large array of prior experiments at OMEGA; however these experiments did not cover later times where the largest difference between models exists. Presented in this work are new OMEGA experimental measurements at later times than previously observed. Proton radiography measurements are used to capture filament evolution and Thomson scattering records the plasma conditions, with the objective of determining which model best matches the late-time filament dynamics.