Using the photodetachment technique we investigated the hydrogen negative ion density in the extraction region of a magnetically filtered multicusp ion source when argon is added to hydrogen. We found that the negative-ion density goes up at most by a factor of 1.5 when argon is added to low base hydrogen pressure (0.05 or 0.1 Pa), but goes down when argon is added to higher initial hydrogen pressure (0.5 or 1 Pa). Adding argon did not enhance the Werner and Lyman bands in the vacuum ultraviolet, which indicates that argon addition does not increase the production rate of vibrationally excited H2 molecules. The increase of the negative-ion density by adding argon into a low-pressure hydrogen discharge is probably due to the increase of the low-energy electron density.

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This topical review gathers the last updates concerning caesium-free negative ion sources presented during the 63rd Course of the International school of Quantum Electronics of the Ettore Majorana Foundation and European collaborative works related to these lectures. Hence, beyond the frame of this course this topical review addresses both theoretical and experimental work performed during these last few years and complexities represented by the conception of a negative ion source ranging from the creation of negative ions to their neutralization.

Various plasma parameters were measured for several hydrogen-argon mixtures in the multicusp H Ϫ ion source Camembert III, equipped with tantalum filaments. The density and the temperature of electrons were measured with the Langmuir probe situated in the center of the source and associated with the photodetachment diagnostics. The two-laser photodetachment diagnostics was used to obtain the temperatures of the two negative ion populations. At low hydrogen pressure ͑0.8 mTorr͒ a small concentration of argon additive enhances the hydrogen negative ion density ͑by approximately 60%͒, it also increases the electron density. The negative ion population fractions having the high-and low-temperature values behave differently when varying the total pressure in pure hydrogen and in hydrogen with argon additive. Several possible explanations of the drastic change of the relative ratio of negative ion populations when the argon fraction is increased, are proposed. Another interesting phenomenon observed during the experiments is the decrease with time of the H Ϫ density in the presence of argon. After adding the argon during the time interval of ϳ1 h the H Ϫ density goes down and finally establishes at a minimum value. The final H Ϫ density is lower than the H Ϫ density in pure hydrogen plasma before adding argon. This ''poisoning'' is discussed in terms of wall production of vibrationally excited H 2 molecules.

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