By applying an after acceleration electric field, the detection and registration of the ion species and spectral distribution of low energy ion beams(<1 keV)have been achieved by the Thomson spectrometer with CR-39 nuclear track detector as detection plate. With the use of this modified spectrometer, the low energy and high energy density plasma ion beams generated by a coaxial plasma gun have been detected and investigated. The ion beams were mainlyN+ions and were distributed in the energy range fromE∼0.12to 0.4 keV. ©1997 American Institute of Physics.