On Collisionless Ion and Electron Populations in the Magnetic Nozzle Experiment (MNX)
Authors: S.A. Cohen, X. Sun, N.M. Ferraro, E.E. Scime, M. Miah, S. Stange, N. Siefert, and R.F. Boivin
Date of PPPL Report: January 2006
The Magnetic Nozzle Experiment (MNX) is a linear magnetized helicon-heated plasma device, with applications to advanced spacecraft-propulsion methods and solar-corona physics. This paper reviews ion and electron energy distributions measured in MNX with laser-induced fluorescence (LIF) and probes, respectively. Ions, cold and highly collisional in the main MNX region, are accelerated along a uniform magnetic field to sonic then supersonic speeds as they exit the main region through either mechanical or magnetic apertures. A sharp decrease in density downstream of the aperture(s) helps effect a transition from collisional to collisionless plasma. The electrons in the downstream region have an average energy somewhat higher than that in the main region. From LIF ion-velocity measurements, we find upstream of the aperture a presheath of strength Δφps = mrTe, where mrTe is the electron temperature in the main region, and length ~ 3 cm, comparable to the ion-neutral mean-free-path; immediately downstream of the aperture is an electrostatic double layer of strength ΔφDL = 3 – 10 mrTe and length 0.3 – 0.6 cm, 30 – 600λD. The existence of a small, ca. 0.1%, super-thermal electron population with average energy ~10 mrTe is inferred from considerations of spectroscopic line ratios, floating potentials, and Langmuir probe data. The super-thermal electrons are suggested to be the source for the large ΔφDL.