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The opacity information of a finite-temperature plasma is an important property and requires the population distribution of a given plasma condition. A population kinetic code for plasma spectroscopy, FLYCHK, has been widely used by researchers to study the spectroscopic properties of high-energy-density plasmas under a wide range of conditions. In this study, the FLYCHK calculation of the Planck and Rosseland mean opacities of low- to high-Z elements, such as aluminum (Z = 13), iron (Z = 26), and gold (Z = 79), under a wide temperature and density range (T = 10−3–102 keV, ρ = 10−6–102 g/cc) is reported. This study mainly focused on the quantitative comparisons of FLYCHK opacities with commonly used opacities: ATOMIC and PROPACEOS. Comparisons show that the FLYCHK mean opacities are comparable to other results over a wide range of plasma conditions. Aluminum opacities were analyzed in detail to understand the characteristics of FLYCHK opacity simulations.

The authors examine the performance of the so-called electron output scheme recently proposed by the Novosibirsk group. In this scheme, the key role of the FEL oscillator is to induce bunching, while an external undulator, called the radiator, then outcouples the bunched electron beam to optical energy via coherent emission. The level of the intracavity power in the oscillator is kept low by employing a transverse optical klystron (TOK) configuration, thus avoiding excessive thermal loading on the cavity mirrors. Time-dependent effects are important in the operation of the electron output scheme because high gain in the TOK oscillator leads to sideband instabilities and chaotic behavior. The authors have carried out an extensive simulation study by using 1D and 2D time-dependent codes and find that proper control of the oscillator cavity detuning and cavity loss results in high output bunching with a narrow spectral bandwidth. Large cavity detuning in the oscillator and tapering of the radiator undulator is necessary for the optimum output power.