An electron string is a state of a one component electron plasma, which can arise when multiply reflected electrons are accumulated in a strong solenoidal magnetic field. Based on the Electron Beam Ion Source Krion-2 in the reflex mode of operation electron strings were used for production of highly charged Argon ions. Evolution of K X-ray spectra during the confinement of Ar ions was obtained. Dielectronic recombination of Ar16+ ions with string electrons was detected. The possibility of K X-ray diagnostic for studies of electron strings is discussed.