Modernizing Laser Absorption Models for ICF

Recent LLE experiments on underdense gas jets and directly-driven spheres [D Turnbull et al., PRL 2023 and PRE 2025] motivate us to revisit laser absorption by inverse bremsstrahlung. The data support the model presented here [M Sherlock et al., PRE 2024], based on Sommerfeld’s quantum-mechanical treatment “in vacuum.” Roughly, the Coulomb log L ≈ ln(b_max / b_min) with b_max = v_Te / ω, v_Te² = T_e/m_e and laser frequency ω. This contrasts with low-frequency models like Lee-More with b_max = v_Te / ω_pe and plasma frequency ω_pe. Our model accounts for non-Maxwellian electron distributions due to the Langdon effect [A B Langdon, PRL 1980], including in L. We compare to an older model in use at LLNL, with the Lee-More L and the Langdon effect included but not in L. Our b_max decreases absorption, while the Langon effect on L increases it. These two effects roughly cancel in Lasnex hohlraum simulations with the LHT common model [D Strozzi et al., PoP 2024]. A major open question is the role of the medium, namely dielectric screening. The model that best matches the LLE spheres data uses the electron Debye length as the screening length and neglects ion screening. This is not consistent with weakly-coupled plasma theory [J Dawson and C Oberman, PoF 1963] and recent molecular-dynamics simulations [J Kinney et al., PoP 2024]. Our current hypothesis is a theory with moderate coupling is needed, which slightly increases absorption and happens to be close to neglecting ion screening for the spheres data.