Transition Of Thermodynamic Property Of Electron In A Magnetically Expanding Plasma

Abstract

Transition of thermodynamic behavior for the electrons in a magnetically expanding collisionless plasma is experimentally observed by local measurement using an axially movable cylindrical Langmuir probe. The polytropic indexes which are deduced from the correlations among the plasma potential, effective electron temperature, and electron density seem to have a quite different value of $3 \pm 0.2$ and $1.17 \pm 0.03$ near the nozzle throat and far from the nozzle, respectively. The transition of polytropic index can be explained by a theoretical calculation of effective polytropic index depending on the double-adiabatic laws and the thermal energies for magnetized anisotropic plasmas1. The results show that the significant electron cooling near the nozzle throat is caused by nearly one-dimensional adiabatic expansion along almost parallel strong magnetic field. With increasing the distance from the nozzle throat, the magnetic field strength decreases and the electron cooling rate is also decreased. The present study demonstrates that a sufficiently high and nearly longitudinal magnetic field configuration can enhance the electron cooling near the nozzle throat significantly. It also means that the ion acceleration can be enhanced through high potential drop generated by high electron cooling rate. Ion demagnetization is correlated with the change in the thermodynamic property of electrons along the distance from the magnetic nozzle.