Measurement of the energy distribution of fast excited atoms by Doppler shift spectroscopy in an inertial-electrostatic confinement fusion device

Energy distributions of energetic neutral atoms resulted from charge-exchange reactions between accelerated ions and background atoms or molecules were measured by the Doppler shift spectroscopy in an inertial-electrostatic confinement fusion (IECF) device composed of a spherical vacuum chamber as an anode and a spherical hollow cathode grid concentrically placed in the chamber. Since ions generated between the cathode and the anode by a glow discharge are accelerated toward the spherical center by the electric field, and enter the hollow cathode to give rise to either beam-beam or beam-background colliding fusion, the energy distribution of such ions virtually determines the fusion reaction rate to great extent. The optical emissions from the center were measured in both hydrogen and helium IEC plasmas. The energy distributions in the radial direction were then evaluated from the broadening of the emissions, under an assumption of spherical symmetry. As a result, in both hydrogen and helium plasmas the maximum ion energies measured were found to be approximately 80% of the applied voltage to the cathode. In a hydrogen plasma, three energy peaks are found in the energy spectrum of fast neutrals, indicating almost the same birthplace of H₁⁺, H₂⁺, and H₃⁺ ions at approximately 80% energy of the applied voltage. In contrast, in a helium plasma, the energy peak was found to be much less down to 20% of the applied voltage.