On the thermodynamics of diamagnetic plasma expansions

Particle heating in a diamagnetic plasma expansion is studied by means of well‐diagnosed simulation with an axisymmetric particle‐in‐cell code. Moments of the particle distribution function are obtained for spatially distinct subsets of the particles to examine temperature and density histories for different regions of the expanding plasma. The simulation is followed through one expansion‐contraction cycle. While adiabatic behavior is observed during much of the cycle, significant deviations from the adiabatic result in strong particle heating. Anomalous ion heating occurs throughout the plasma during the expansion phase. This is manifested earliest in the ion parallel temperature, which increases first in the expanding plasma's outer reaches and last in the center. This heating originates from the entropy that is generated at the barrel ends of the plasma as the initial expansion along the ambient field is essentially free. Later, after a diamagnetic cavity is formed, ions within the cavity are reflected by the magnetic mirrors at the necks and transport some of the generated entropy back into the center of the cavity. The ion heating that occurs can easily raise the bulk plasma temperature by an order of magnitude over the initial adiabat.