No abstract

The role of the plasma electrode (PE) bias in the extraction process in a large-volume hybrid multicusp negativeion source, which is driven by 2.45-GHz microwaves, is studied. Spatially resolved negative-ion and electron density measurements close to the extractor aperture were performed under various pressures (1-4 mtorr) by means of the electrostatic probe and photodetachment technique. As the low positive voltage applied to the PE is slightly increased (from 4 to 7 V), the electron temperature passes through a minimum (0.2-0.6 eV), while simultaneously, both the negative-ion density and the H − extracted current reach a maximum (∼1−2 × 10 9 cm −3 and ∼ 0.5 mA/cm 2 , respectively). Optimum pressure values for the extracted negativeion current and the negative-ion density are found between ∼1.5 and 3 mtorr. It is deduced that the negative-ion density measured in the center of the source cannot be directly correlated with the ion extracted current. The electron density and the associated extracted electron current linearly decrease as a function of the PE bias. The physical mechanisms explaining the experimental results are discussed.

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.