Innovative particle accelerators based on plasma technology allow a drastic reduction in size, thanks to the high acceleration field established inside plasmas, created and confined by particular devices. Plasma wake-field acceleration experiments are performed at the SPARC LAB test facility by using devices consisting in gas-filled capillaries, in which the plasma formation is achieved by ionizing the hydrogen gas through high voltage pulses. In this work, the experimental and theoretical characterization of a 400 mm long plasma source is presented, which represents the testing of the first plasma source for the EuPRAXIA@SPARC_LAB project. Its exceptional length of 400 mm makes it one of the world’s largest plasma sources for particle accelerators based on the high voltage ionization technique. The 3D-printed capillary is characterized by means of the spectroscopic technique based on the Stark broadening method, which allows to measure the evolution of the plasma density profile along the main plasma channel. In addition to the experimental analysis, the CFD software OpenFoam is used to simulate the dynamics of the neutral gas from its entrance inside the capillary to the ionization instant.