The results of the probe and optical measurements of plasma parameters produced during the injection of the electron beam with the energy of 3-8 keV and current of 30-60 mA into a dielectric cavity at the argon pressure of 2-12 Pa are presented. The possibility of continuous injection of the electron beam into the cavity without current-collecting electrodes inside is demonstrated. The axial distribution of the values of the plasma parameters (density, electron temperature and floating potential) in the cavity is discussed. The plasma concentration in the cavity is shown to be approximately 20% higher, and the electron temperature 1.5-2 times higher, than in the case of generating the plasma in the free space of the vacuum chamber.