The method of determination of the electron and H− densities in the photodetachment experiment, particularly when performed in plasma with a high n−/ne ratio, is discussed. Some essential peculiarities of this method arise from the creation of a potential well in plasma after photodetachment. The well captures the excess electrons, generated by the laser beam, and supports the quasineutrality in the plasma. The method is applied to the diagnostics of the extraction region of the negative ion source ‘‘Camembert III.’’ The dependencies of the negative ion and electron densities, and of the electron temperature upon the plasma electrode voltage, are obtained and discussed.

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