Isotope Effect of H^–/D^– Volume Production in Low‐Pressure H₂/D₂ Plasmas – Measurement of VUV Emissions and Negative Ion Densities

Abstract

Isotope effect on H− / D − volume production is studied by measuring both VUV emission and negative ion density in the source. In a double plasma type source, under some discharge conditions, extracted D − currents are nearly the same as H − currents, although VUV emission intensity (corresponding to production of vibrationally excited molecules) in D2 plasmas is slightly lower than that in H2 plasmas. Considering the factor √ 2 due to mass difference, D − ion density in the extraction region of the source is higher than H − ion density. In another experiment with a rectangular arc chamber, axial distributions of H − / D − ion densities in the source are measured directly using a laser photodetachment method. Relationship between H − / D − production and plasma parameter control with using a magnetic filter (MF) is discussed. Furthermore, relative intensities of extracted negative ion currents are discussed compared with the negative ion densities in the source. Production and control of D2 plasmas are well realized with the MF including good combination between the filament position and field intensity of the MF. Extracted H − and D − currents depend directly on negative ion densities in the source.

D− density distribution and its dependence on plasma parameters in volume negative ion sources

“…In the present study, B MF is variable from 60 G to 150 G on the center axis of the chamber. In this source, due to the external MF, the width of the half-maximum of magnetic field intensity is wider (about 16 cm in this case) than the case of a rod filter [4]. Namely, varying the intensity of the magnetic filter indicates also varying the strength of magnetic field distribution in both source region and extraction region.…”

“…We have studied relationships between H À [3] and D À [4,5] production and plasma parameters across the magnetic filter (MF). In this paper, plasma parameter control by varying the magnetic field intensity of the MF is presented.…”

“…Therefore, there are few studies on optimization of volume-produced D À ions. Then, we focus on understanding the negative ion production mechanism in the volume negative ion source [3][4][5].…”

Efficient negative ion production in rf plasmas using a mesh grid bias method

“…Therefore, spatial control of electron energy distribution ͑i.e., T e ͒ is necessary. 5,6 With the use of a magnetic filter, the electron energy distribution function in dc plasmas is well controlled for H − formation. Unfortunately, in rf discharge plasmas, plasma parameter control with magnetic filter is not so effective as one in dc plasmas.…”

Enhancement of pure volume negative ion production using a grid bias method or a magnetic filter method

“…Namely, H − ions produced by the following twostep process: Therefore, spatial control of electron energy distribution ͑i.e., T e ͒ is necessary. 2,3 So far, the magnetically filtered multicusp ion source has been shown to be a promising source of high-quality multiampere H − ions. To enhance H − ion production and extracted H − ion currents, a small amount of cesium vapor is usually seeded into the above volume ion source.…”