Impact of ion spatial distribution on Inverse Bremsstrahlung Absorption

In the context of inertial confinement fusion (ICF), accurate modeling of inverse bremsstrahlung absorption (IBA) is critical for predicting laser energy deposition in plasma. A few years ago, we reported molecular dynamics (MD) simulation results (R. Devriendt and O. Poujade, Physics of Plasmas 29, 073301 (2022)) indicating that the IBA per unit length was roughly half the value predicted by standard theoretical models, though we lacked a theoretical justification at that time. In this work, we present a theoretical analysis demonstrating that the Coulomb logarithm, and thus the IBA per unit length, depends explicitly on the ion-ion radial distribution functions (RDFs) of the plasma. Conventional theoretical expressions for IBA assume a homogeneous plasma with randomly distributed ions (i.e., g_ii(r)=1). We show that MD simulations where we enforce g_ii(r)=1 agree well with these expressions. However, when the ions are allowed to relax to their natural RDFs, as they should in a realistic plasma, the IBA per unit length decreases significantly. These findings highlight the importance of ion correlations in determining laser absorption and suggest a path for refining theoretical models of IAB in ICF-relevant conditions.