Plasma Electrode Bias Effect on the $\hbox{H}^{-}$ Negative-Ion Density in an Electron Cyclotron Resonance Volume Source

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.

Study of the Plasma near the Plasma Electrode by Probes and Photodetachment in ECR-driven Negative Ion Source

“…In conclusion the explanation of the H -ion density peak can be summarized as follows 4 : the depletion of the electron population, when the positive bias of the PE is enhanced, disturbs the local plasma neutrality and new negatively charged particles should replace the electrons collected by the PE. In a collisionless situation, the electrons present in the bulk plasma are unable to cross the transverse magnetic field in front of the PE.…”

“…It is generally accepted that a positive PE bias (V PE ) reduces the extracted electron current. In some experiments, an optimum V PE for negative ion extraction was found [2][3][4] . A possible cause of these effects was suggested in 3 : electrons are trapped in the weak magnetic field and lost along the field lines, whereas positive and negative ions are not trapped because of their large Larmor radius.…”

Roles of a plasma grid in a negative hydrogen ion source

“…Figure 8 presents a typical example of the variation of the current collected by the PE as a function of the PE bias with respect to the wall, along with the associated variation of the extracted electron and negative ion currents. 11 This experiment was effected in the source Camembert III in which the plasma was produced by seven dipolar sources operated by microwaves (2.45 GHz). The first important result in Fig.…”

Multi-dipolar microwave plasmas and their application to negative ion production

“…The plasma parameters (electron density, electron temperature, and negative ion density) were studied in the center of the chamber 22,23 and in the vicinity of the PE, 12,24 using electrostatic probes and laser photodetachment technique. 25 In the central field-free region of Camembert III, the EEPF exhibits a bi-Maxwellian structure.…”