We have tried to shed more light on the possible mechanisms of the beneficial effect of gas mixing for highly charged ion production, in particular on the presence of an ion cooling effect. For the first time a method was applied to deduct the ion temperature from all measured ionic currents. The ion temperature values derived showed a clear decreasing trend in conjunction with mass changes of the gas mixture, consisting of “pure” argon (no mixing gas), argon plus natural oxygen, argon plus isotopic O18, or argon plus Ne22. Each of the applied mixing gases gives a higher charged ion (HCI) output (highest for O18) as well as a lower ion current for the singly charged beam-particle output. The relative high particle fraction (about 40%) of singly charged nonbeam particles is an indication that effective plasma ion cooling is possible. Although the differences in ion temperature calculated are small, the effect is likely substantial, since the ion confinement has a dependency with (Ti)−5/2. Hydrogen currents (residual) are observed to be smallest for the best HCI-confined plasmas; so far no explanation for this can be given.
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Drentje et al. (2000) studied this question.