We report on the characteristics of the electronegative plasma in a large-scale hydrogen negative ion (H-) source. The measurement has been made with a time-resolved Langmuir probe installed in the beam extraction region. The H- density is monitored with a cavity ring-down system to identify the electrons in the negative charges. The electron-saturation current decreases rapidly after starting to seed Cs, and ion-ion plasma is observed in the extraction region. The H- density steps down during the beam extraction and the electron density jumps up correspondingly. The time integral of the decreasing H- charge density agrees well with the electron charge collected with the probe. The agreement of the charges is interpreted to indicate that the H- density decreasing at the beam extraction is compensated by the electrons diffusing from the driver region. In the plasmas with very low electron density, the pre-sheath of the extraction field penetrates deeply inside the plasmas. That is because the shielding length in those plasmas is longer than that in the usual electron-ion plasmas, and furthermore the electrons are suppressed to diffuse to the extraction region due to the strong magnetic field.
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