Diocotron Instabilities in an Electron Column Induced by a Small Fraction of Transient Positive Ions

It is well known that a small fraction of positive ions can destabilize diocotron modes on electron plasmas. However, the historical (and recent) interpretation of experimental results in terms of 2D (or modified 2D) theories of ion-induced instabilities is apparently erroneous. Here, we experimentally characterize a strong exponential instability with no threshold, obtaining growth rates orders of magnitude larger than predicted. The positive ion population is maintained either by continuos external injection of ions or by ionization of the background gas within hot electron plasmas. In both cases, the observed exponential growth rate γm is directly proportional to the ion creation rate ν+, i.e., γm = κmν+, with κm ≈ (10 -10)/Ne for mθ = 1,2,3. Experimental results also suggest that non-2D effects, including end confinement fields, are important. This strong instability may have important implications for the anti-hydrogen creation technique of propelling anti-protons through trapped e clouds.