An electron string is a state of a one component electron plasma, which can
arise when multiply reflected electrons are accumulated in a strong soleno
idal magnetic field. Based on the Electron Beam Ion Source Krion-2 in the r
eflex 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.