The Effect of Microinstabilities on a Hot-Ion Plasma

Ion cyclotron instabilities occurring in an energetic ion plasma have been studied in the high-energy (540 keV H+2) ion injection machine DCX-2. A new microinstability, called the modified negative-mass instability, has been found to dominate the behaviour of the plasma. This instability limits the density of the injected plasma but produces strong ion cyclotron frequency fields which heat and trap cold ions to form a separate and denser group of highenergy ions. The optimum parameters of this group are: hot-ion density n+ ≈ 5 x 109/cm3; T⊥i ≈ 800 keV; plasma decay time, τ1/2 ≈ 0.5 seconds: length/Larmor radius, L/ai ≈6; plasma radius/Larmor radius, R/ai≈3; (T⊥/T‖)i≈103; and Te≈50 eV. The density of the cyclotron-heated plasma scales linearly with power input to the injected plasma up to the highest available power. In addition to this instability, Harris modes are observed in the injected plasma, the ion-cyclotron heated plasma and the molecular ion beam. In the latter case electron Landau damping of short wavelength modes has been observed. (author)