No abstract

Key words Electric probe, magnetized plasma, space potential and temperature oscillations. PACS 52.70.-m A baffled probe for distinguishing the oscillations of space potential and temperature in magnetized plasma is tested. The probe consists of a tungsten wire tip, oriented perpendicular to the magnetic field, that is partially shielded by ceramic baffles (masks). The probe works under the condition that the electron Larmor radius is much smaller than the probe radius, and the ion Larmor radius is comparable to or larger than the probe radius. The probe uses the same principles as Katsumata and plug probes, but has the advantage of convenient, realtime controllability of the ratio between electron and ion current while electrically floating, and insensitivity to uncertainties in the orientation of the probe tip relative to the direction of the magnetic field. By rotating the baffle configuration around the probe tip, the ratio between electron and ion probe current and consequently the relative sensitivity of the probe to oscillations of space potential and electron/ion temperature, can be adjusted. Thus, measurements of ac floating potential with different ratio of electron and ion currents allow us to distinguish oscillations of space potential from electron/ion temperature fluctuations. Experiments have been conducted in a fully ionized, barium plasma in the West Virginia University Q-machine.

A nonlinear, collisional, two-fluid model of uniform plasma convection across a field-aligned current (FAC) sheet, describing the stationary Alfvén (StA) wave, is presented. In a previous work, Knudsen showed that, for cold, collisionless plasma [D. J. Knudsen, J. Geophys. Res. 101, 10761 (1996)], the stationary inertial Alfvén (StIA) wave can accelerate electrons parallel to a background magnetic field and cause large, time-independent plasma-density variations having spatial periodicity in the direction of the convective flow over a broad range of spatial scales and energies. Knudsen suggested that these fundamental properties of the StIA wave may play a role in the formation of discrete auroral arcs. Here, Knudsen’s model has been generalized for warm, collisional plasma. From this generalization, it is shown that nonzero ion-neutral and electron-ion collisional resistivity significantly alters the perpendicular ac and dc structure of magnetic-field-aligned electron drift, and can either dissipate or enhance the field-aligned electron energy depending on the initial value of field-aligned electron drift velocity. It is also shown that nonzero values of plasma pressure increase the dominant Fourier component of perpendicular wavenumber.

A probe for measurements of space potential in magnetized plasma is tested in a fully ionized, barium, Q-machine plasma. The probe consists of a tungsten wire tip, situated perpendicular to the magnetic field, that is partially shielded by ceramic baffles (masks). The probe works under the condition that the electron Larmor radius is much smaller than the probe radius, and that the ion Larmor radius is comparable to or larger than the probe radius. By rotating the baffle configuration around the probe tip, the ratio between the electron and ion probe current can be adjusted. The probe uses the same principles as Katsumata and plug probes [V. I. Demidov et al., Rev. Sci. Instrum. 73, 3409 (2002)], but has the advantage of convenient control of the ratio between the electron and ion current, and is not sensitive to uncertainties in the orientation of the probe tip relative to the direction of the magnetic field. Measurements of potential are made while the probe floats electrostatically. When saturated electron and ion currents have comparable magnitudes, accurate, real-time measurements of space potential can be acquired.

Solutions of the collisional stationary Alfvén (StA) wave equations are presented for both laboratory and space plasma parameter regimes. The stationary inertial Alfvén (StIA) wave is a time-independent electromagnetic structure generated by plasma convection (V d = E/B 0 ) across field-aligned current sheets in a cold (T e = T i = 0) background plasma (Knudsen 1996 J. Geophys. Res. 101 10761). The StIA wave can accelerate electrons in the magnetic field-aligned direction to speeds in excess of the local Alfvén speed. Recently, the model of the StIA wave was generalized to include the effects of electron and ion collisional resistivity, as well as non-zero thermal pressure (Finnegan et al 2008 Phys. Plasmas at press). Here, the laboratory-relevant regime is evaluated. Solutions to the StA wave equations are presented for plasma parameters typical of the main-discharge and after-glow plasmas of the LArge Plasma Device at UCLA's Basic Plasma Science Facility. Cases for which the background plasma density is either enhanced or depleted by the StA wave are distinguished. Coulomb collisions are predicted to be important in damping the StA wave in the after-glow plasma and in supporting solitary density enhancements in the main-discharge plasma. The space regime is also evaluated. We present solutions to the StA wave equations for typical plasma parameters associated with the topside ionosphere and the solar corona. For typical plasma parameters in the topside ionosphere (Alt.∼1500 km), StA wave solutions are characterized by the parallel component of electric field E and plasma density, the spatial modulation of which are in good agreement with measurements associated with Alfvénic structures made previously by the Freja satellite. For plasma parameters typical of the solar corona, it is shown that the StA wave may provide a physical mechanism for generating small scale (∼10 m across), adjacent, magnetic-field-aligned, current channels of alternating polarity in the solar corona.