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The emission‐line spectrum of an argon plasma in the soft X‐ray region 20–30 Å  is simulated using our calculated atomic data. The atomic data, including energy levels, spontaneous radiative decay rates and effective collision strengths, are obtained by the Flexible Atomic Code recently developed by Gu. Good agreement is found between the theoretical and experimental emission spectra. All strong lines observed in the experiment have been identified. Compared with previous theoretical results, the predicted intensity for the line located at 27.631 Å  from the 3d–2p transition of Ar xiv is dramatically reduced, resulting in a better agreement with experiment. Some weak lines are also identified in this work. In order to simulate the experimental spectra, the electron density and temperature are required. We choose density‐ and temperature‐sensitive lines to determine these quantities. Reliable results for the electron density around 8.7 × 1010 cm−3 and electron temperature around 2.4 × 106 K have been obtained. The present atomic data can also be used to identify the emission lines of astrophysical plasmas. As an example, some lines belonging to Ar ions are identified for the first time by comparing the spectra from Chandra LETG observation of Capella with the synthetic spectra. The line located at 27.018 Å, which was originally assigned to C vi (Lyγ), should be attributed to Ar xv from the present work. The line at 28.336 Å  is also identified for the first time as the 3s–2p transition of Ar xv.

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