A numerical prediction is presented for upper bounds on negative floating potentials of shaded spacecraft surfaces, using a local‐current‐balance formulation. Results include the following: (1) By influencing the velocity‐space cutoff boundaries for incident ion fluxes, the spacecraft geometry and sheath potential profile (particularly shaded‐sunlit asymmetries in the latter) have large influences on shaded‐surface potentials, which may exceed ‐ 20 kV in certain circumstances. (2) For electrically isolated surfaces in shaded cavities, negative floating potentials may exceed those on convex surfaces. (3) In some conditions, two distinct floating potentials are predicted. This implies the possibility of ‘bifurcation phenomena’ in which adjacent isolated surfaces made of the same material may follow different charging histories. It also implies that large and relatively sudden changes in surface potentials can be caused by gradual changes in either the external environment or beam emission currents.

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A multi-grid orifice probe and analysis technique are presented which permit the measurement of the following plasma parameters: the electron number density ne, the electron temperature Te, the space potential Vs, and an estimate of the metastable number density nm. Simultaneously, the analysis allows the quantitative determination of instrumental parameters such as the reflection coefficient for slow (<30 eV) electrons from the grids Re(G) and roughened (gold-black) collector Re(C), the secondary electron emission coefficient for fast (> 30 eV) electrons γe(G) from the grids, and the effective transparencies of the grids for electrons τe(G), and for metastables τm(G). Experimental ion, electron and total current characteristics are presented for a low-pressure (1·8×10−2 Torr) RF discharge in argon. The major part of the analysis method involves the use of ten `plateau values' obtained from the characteristics to determine values for most of these parameters. Te and Vs are determined separately. The values obtained are: ne=2·74×109 cm−3±18%, kTe=1·92 eV±5%, Vs=11·7±0·1 V, nm similar, equals 3·3×109 cm−3±50%, Re(G)=0·65±0·10, Re(C)=0·22±0·10, γe(G)=0·935±0·010, τe(G)=0·830±0·002, and τm(G)=0·850±0·002. The transparencies τe(G) and τm(G) are within 4% of the geometrical transparency τ(G)=0·860. Approximately equal numbers of atoms (1 in 2×105) are in metastable and ionized states.

For Pt. III see abstr. A24317 of 1974. The ion number density ni is determined by the orifice probe using a method independent of the electrons. The quantities measured experimentally through the orifice are the number of ions per unit time (ion saturation current ii) divided by the volume flow rate V of neutral gas. In the molecular flow region where V is constant, ni is simply obtained from a measurement of ii. This determination of ni is potentially adaptable to a wide pressure and density range. The method has been applied here to an RF plasma in argon with an input power of 25 W for pressures from 0.001-10 Torr and number densities from 108-1011 cm-3. For pressures between 0.02 and 0.2 Torr, ni obtained in this way is in good agreement with ne obtained from the probe characteristics. This determination of ni is much simpler and inherently more accurate (<15%) than that for ne using the orifice probe or Langmuir probes (>20%).