Electron Behavior in a Magnetically Expanding Plasma

Plasma expansion along diverging magnetic fields is a fundamental process for a variety of low-temperature plasmas with applications to plasma processing and spacecraft electric propulsion. Expansion coincides with the conversion of plasma thermal energy into directed ion kinetic energy. For plasmas in which the electron temperature greatly exceeds the ion temperature, ion acceleration is driven mainly by the formation of an ambipolar electric field. The gradient in electron pressure provides the source of the ambipolar electric field in addition to transferring the force of the accelerated plasma onto the magnetic circuit via diamagnetic drift currents. This talk will focus on two important topics related to the behavior of electrons in magnetically expanding plasmas that are critical to the thermal-to-kinetic energy conversion efficiency. The first topic, electron cooling, describes how electron energy is transported along the magnetic field lines. Experimental and theoretical results will be presented that highlight the importance of field-aligned electron heat conduction on electron cooling and the resulting energy conversion process. The second topic, electron demagnetization, describes how the electrons detach from the expanding magnetic field. Both experiment and theory will again be used to argue that electron detachment can result from finite Larmor radius effects in regions of low magnetic field and high magnetic field curvature.