High-power laser–matter interaction and the related high-energy density physics (HEDP) are two important topics in modern physics. However, for decades, a very restrictive limitation prevents researchers from studying these topics efficiently: nanosecond-kilojoule class lasers are typically characterized by a low repetition rate (1 shot h−1) that restricts the data acquisition and limits the available statistics for analysis. Here, we describe the first results obtained using the high-repetition rate nanosecond-kilojoule (L4n laser beamline) experimental platform commissioned at the Extreme Light Infrastructure—Beamlines. We proved the capability to deliver hundreds of joules shots every three min with a very good repeatability. Using high-resolution x-ray spectroscopy, we studied highly resolved spectra of H-like and He-like Cl lines emitted from polyvinyl chloride targets and demonstrated that the plasma parameters (electron temperature and density) derived from the measured spectra are consistent with hydro-simulations. These results demonstrate the fulfillment of the designed platform performance which should become one of the most important assets for the HEDP community in the following decades.