Microinstability Limitations of the DCX-1 Energetic Plasma

Abstract

A steady state plasma with an energetic (≈ 300 keV) proton component is formed by dissociation of part of a 600-keV H2+ beam that makes a single pass in the midplane of a 2: 1 magnetic mirror field with a central field value of 10 kG. Both gas dissociation and Lorentz dissociation have been employed. The pontentially accessible density (density expected in the absence of instabilities) with Lorentz dissociation is the larger by more than an order of magnitude. The plasma is stable against flute instabilities, but microinstabilities (ion-electron or negative mass instabilities) are evidenced by radio frequency signals at ω ≳ ωci. The lowest density threshold for observation of these signals is 4 × 105 fast protons cm−3. With Lorentz dissociation the microinstabilities eventually eject protons, as observed by detection of these direct losses correlated with microinstability signals and by failure of charge-exchange accountability. The losses are first observed at densities of 5−10 × 107 cm−3; they amount to as much as 0.9 of the proton input current to the plasma; and, along with instability-driven scattering to larger plasma volume, they limit the proton density to 1−2 × 108 cm−3, as much as a factor of 15 below the density expected in the absence of such effects.