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The scope of this research is to investigate experimentally electron (ne), negative ion (n-) and positive ion (n+) densities characterizing laboratory negative ion-rich plasmas, produced by electron attachment in N2O3, O2 and I2, and to find out the factors limiting the achievement of very low Ã (relative electron density Ã = ne/n+). These plasmas may be of great interest for the production of negative ion beams. It is shown experimentally that it is possible to produce plasmas with a high proportion of negative ions (n-/n+ Â¿ 90 %) and a low proportion of electrons, at densities n+ up to 1011 cm-3. The comparison of mass spectrometric data with kinetic calculations leads to the conclusion that the loss of negative ions by diffusion limits the lowest Ã achieved at low ion density (n+ < 109 cm-3). At higher ion density, mutual neutralization seems to control the Ã values. A general limitation seems to exist for the lowest Ã attainable in small plasmas produced by electron attachment : the confinement of negative ions in a plasma is due to the presence of electrons and therefore this confinement becomes inefficient when Ã drops to values as low as 10-3.