Measurements of diverging ion motion in an inertial electrostatic confinement device using Doppler spectroscopy

With the aim of determining the ion direction, Doppler spectroscopy is carried out in a Farnsworth fusor with a spherical cathode grid during star mode operation. The Doppler shift of Balmer-α radiation from excited deuterium atoms (D^∗) is analyzed to find the kinetic energy spectrum. Diverging D^∗ is measured at a cathode voltage of -20 kV, with currents between 0.5 mA and 3.4 mA, and in the pressure regime of 1.6 Pa-2.6 Pa. D^∗ is produced during a charge exchange reaction of fast deuterium ions with the background gas. The measured spectra can only be formed via diverging ions due to momentum conservation during this charge exchange reaction. Dominant blue shifts coming from measurement locations inside microchannels moving toward the observer and red shifted spectra coming from microchannels moving away from the observer prove this diverging ion motion. A kinetic energy distribution of D^∗ over different longitudinal positions inside the microchannel is measured at a pressure of (1.7 ± 0.1) Pa. The results indicate an increase in D^∗ velocity from the center of the cathode toward the cathode edge. This can be explained by a virtual anode, which accelerates ions created in the center of the cathode outward. These results disagree with the converging ion model but could be explained by the hollow cathode model.