Pressure optimization for H− ion production in an electron cyclotron resonance-driven and a filamented source

Abstract

The negative ion density as a function of the hydrogen pressure (1–8mTorr) in the electron cyclotron resonance-driven version of the magnetic multipole volume source “Camembert III” is measured by means of the photodetachment technique. An optimum value is observed between 4 and 5mTorr, yielding a H− ion density of about 1.5×109cm−3 in the center of the source. The electron density monotonously increases in the range ∼(0.5–2.5)×1010cm−3 and the electron temperature decreases (∼1.25–0.5eV). The optimum pressure for H− production is equally reported for a conventional filamented multipole source, in which the influence of rovibrationally excited hydrogen molecules in the electronic ground state on the formation of H− is analyzed. The physical mechanism which determines the existence of this ion density maximum is discussed.