On collisionless ion and electron populations in the magnetic nozzle experiment (MNX)

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 Delta phi(ps) = T-mr(e), where T-mr(e) is the electron temperature in the main region, and length similar to 3 cm, comparable to the ion-neutral mean-free-path; immediately downstream of the aperture is an electrostatic double layer of strength Delta phi(DL) = 3-10 T-mr(e) and length 0.3-0.6 cm, 30-600 lambda(D). The existence of a small, ca. 0.1%, superthermal electron population with average energy similar to 10 T-mr(e) is inferred from considerations of spectroscopic line ratios, floating potentials, and Langmuir probe data. The superthermal electrons are suggested to be the source for the large Delta phi(DL).