Measurement of Electron Density near Plasma Grid of Large-scaled Negative Ion Source by Means of Millimeter-Wave Interferometer

Abstract

A millimeter‐wave interferometer with the frequency of 39 GHz (λ = 7.7 mm) was newly installed to a large‐scaled negative ion source. The measurable line‐integrated electron density (ne•l) is from 2×1016 to 7×1018 m−2, where ne and l represent an electron density and the plasma length along the millimeter‐wave path, respectively. Our interest in this study is behavior of negative ions and reduction of electron density in the beam extraction region near the plasma grid. The first results show the possibility of the electron density measurement by the millimeter‐wave interferometer in this region. The line‐averaged electron density increases proportional to the arc power under the condition without cesium seeding. The significant decrease of the electron density and significant increase of the negative ion density were observed just after the cesium seeding. The electron density measured with the interferometer agrees well with that observed with a Langmuir probe. The very high negative ion ratio of nH‐/(ne+nH‐) = 0.85 was achieved within 400 min. after the cesium seeding.