Cooperative Effects in a Tenuous Energetic Plasma Contained by a Magnetic Mirror Field

Abstract

The formation and characteristics of a steady-state hydrogen plasma contained in a magnetic mirror field are described. The mean ion energy is 20 keV. The plasma is formed by ionizing and trapping a portion of a beam of energetic hydrogen atoms passing through the confining field. The methods of measurement used to determine the plasma properties are described. Measurements of the radial and azimuthal trapped-ion distributions, the average ion and electron densities, and the plasma potential are compared with the predictions of simple theory, neglecting cooperative plasma effects. The observed deviations from these simple predictions show that the plasma properties are dominated by cooperative phenomena. The plasma density is found to be limited to a low value (∼4 × 107 ions/cm3) by a flute or drift instability. This instability is characterized by a low frequency rotation of the plasma at a frequency typically close to the ▿B precession frequency of a 20-keV proton in the nonuniform mirror field. The plasma density at the unstable limit is observed to be related to the value of plasma potential. This relationship can be understood quantitatively by an extension of an earlier theoretical treatment of the drift instability to the case of unequal ion and electron densities. The flute oscillation frequency is also derived from this theory. Oscillations at the ion gyrofrequency are observed, marking the presence of an electrostatic resonance type of instability. Coupling of these modes with the flute oscillations is described, but detailed identification of the electrostatic resonance modes cannot yet be made.