A plasma sputter negative ion source was evaluated for its applicability for measuring the plasma potentials in fusion devices. Both the beam current density and the beam energy spread are key issues for this application. The energy spectra of self-extracted Au- beams from the source were measured under the condition of a constant work function of the production surface. The FWHM increases from 3 to 9 eV monotonically as the target voltage increases from 50 to 300 V, independently from the target surface work function of 2.2-3 eV.
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