Effect of argon additive on H− density and temperature in volume negative ion source

Abstract

Various plasma parameters were measured for several hydrogen–argon mixtures in the multicusp H− 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 mTorr) 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− density in the presence of argon. After adding the argon during the time interval of ∼1 h the H− density goes down and finally establishes at a minimum value. The final H− density is lower than the H− density in pure hydrogen plasma before adding argon. This “poisoning” is discussed in terms of wall production of vibrationally excited H2 molecules.