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tokens: in the ionization balance in photoionization equilibrium (Ferland et al. 1998b ; Ferland 2003 ). In order to provide reliable DR rate coefficients for use in plasma modeling, the storage ring DR experiments combined with the new calculations with the AUTOSTRUCTURE code and the FAC code have made huge progress in the past few decades. Storage ring DR experiments of highly charged iron have been performed with almost all of the charge states for Fe 7+ to Fe 22+ (Savin 2007a ; Schippers 2012 ). Following the first paper to describe the calculation goals and methodology (Badnell et al. 2003 ), a series of the papers have been published based on the calculations by the AUTOSTRUCTURE code, and the calculated data are collected into the recent database CHIANTI (Dere et al. 2009 , 2019 ). By comparison of these calculations and the storage ring experiments, DR rate coefficients for carbon-like and oxygen-like ions have been identified as the most urgent needs for astrophysical applications, and therefore precision measurements of the DR rate coefficients of these ions that are abundant in astrophysical plasmas to benchmark the theories are required. Calcium is an abundant element in the solar corona and also in high-temperature astrophysical plasmas. Emission lines from highly charged calcium ions play an important role for diagnostics and modeling in astrophysical plasmas as well as fusion plasmas (Del Zanna Mason 2018 ; Träbert et al. 2018 ). For a C-like Ca 14+ ion, the previous observations were mostly from solar flare and solar wind based on the lines that are largely in the EUV range. These EUV lines are usually from the transitions within the 2 s 2 2 p 2 –2s 2 2 p 2 and 2 s 2 2 p 2 –2 s 2 p 3 configurations. The spectral lines of 555 Å and 1098 Å from 2 s 2 2 p 2 –2s 2 2 p 2 transitions of Ca xv were measured and identified by the Solar Ultraviolet Measurements of Emitted Radiation spectrometer on the Solar and Heliospheric Observatory (SOHO). With the Skylab observation, the ratios of the 181.90 or 215.37 Å lines versus the resonance EUV line at 200.98 Å are excellent density diagnostics at high densities of solar flares (Dere et al. 1979 ). This Skylab observation of Ca xv emission lines to derive the electron density was later on confirmed by a new data analysis and a measurement from the TEXT tokamak plasma (Keenan et al. 1988 , 2003 ). The Ca xv 181.90/200.98 Å ratio was also investigated by the observations of solar coronal emission lines with the Extreme-Ultraviolet Imaging Spectrometer (Culhane et al. 2007 ) on Hinode spacecraft (Kosugi et al. 2007 ), as discussed by Warren et al. ( 2008 ) and Del Zanna ( 2008 ), and several Ca xv emission lines at EUV range are formed at about 4MK and the electron density measurements at these temperatures are potentially important for understanding the coronal heating mechanism. In addition to the EUV emission lines, transitions of C-like Ca xv within the 2 s 2 2 p 2 –2 s 2 2 p 3 d configurations