Bob Walch

Electrostatic charging properties of Apollo 17 lunar dust

We report on our experimental studies of the electrostatic charging properties of an Apollo 17 soil sample and two lunar simulants, Minnesota Lunar Simulant (MLS-I) and Johnson Space Center (JSC-1). We have measured their charge after exposing individual grains to a beam of fast electrons with energies in the range of 20≤E≤90 eV. Our measurements indicate that the secondary electron emission yield of the Apollo 17 sample is intermediate between MLS-1 and JSC-1, closer to that of MLS-1.

Electrostatic charging properties of simulated lunar dust

The charge on grains of simulated lunar regoliths (MLS-1 and JSC-1) has been measured in a plasma containing thermal electrons with a few electron volts temperature and a second population of nearly monoenergetic fast electrons with energy variable from 15 to 100 eV. The measured charges are similar to that on glass microspheres of the same diameter. Both the glass and the simulated lunar samples showed multiple charge states in a finite electron energy range, that might help the theoretical models suggesting active electrostatic dust transport on the Moon. The measurements can be well described by a charging model which includes currents due the background thermal plasma, monoenergetic fast electrons and secondary electrons.

Rocket‐borne mesospheric measurement of heavy (m≫10 amu) charge carriers

A rocket-borne probe has been developed to detect charged atmospheric aerosols. The probe is a current collecting surface, shielded by a magnetic field to prevent the detection of electrons and ions below a threshold in mass. During the first test flight above White Sands, New Mexico, a narrow layer of positive charge carriers was detected at an altitude of 86.5 km with vertical extent of ≈500 m, most likely a sudden E-layer related to the deposition of meteoric metal ions. A wide layer of negative charge carriers was observed in the altitude range of 75–86 km, consistent with the density of negative ions in this altitude range.

Electron confinement in an annular Penning trap

Electron confinement in an annular version of a Malmberg–Penning trap [S. Robertson and B. Walch, Rev. Sci. Instrum. 70, 2993 (1999)] has been investigated for conditions in which mobility is the dominant source of transport. A non-neutral plasma of electrons is contained in the annular region between coaxial cylinders. An axial magnetic field provides radial confinement and electric bias potentials provide axial confinement. The electric field that drives transport is determined primarily by the potential difference applied to the cylinders. The measured density decay rates have the expected dependence upon electric field, collision frequency and magnetic field and are within a factor of 2 of calculated values. Experiments are performed with an axial field of 5–18 mT, a radial electric field of 11–38 V/cm, a helium pressure of 0.01–0.13 mTorr, an initial electron density of ∼106 cm−3, and the density decay times are 1–10 ms.

Electrostatic orrery for celestial mechanics

A device is described in which negatively charged submillimeter particles are trapped in orbits about a rod with a positive potential. Hollow glass microspheres about 50 μm in diameter are dropped past an emitting filament and accumulate a charge of approximately 5×105 electrons. The microspheres fall into a Kingdon trap (concentric cylinders with end caps) and are trapped by a rising potential of order 10 kV. The orbital decay time is determined by molecular drag and is about 40 min or 105 revolutions at 2×10−6 Torr. Orbiting particles are visible to the unaided eye and may be recorded photographically with a CCD camera. Possible applications from celestial mechanics include experiments on orbital resonances, Kirkwood gaps, and planetary ring phenomena.

Reduction of asymmetry transport in the annular Penning trap

In the Penning trap, there is transport of electrons in the limit of zero gas pressure that arises from asymmetric stray electric fields. In an annular version of the Penning trap, this asymmetry transport is shown to be greatly reduced when the plasma-facing surfaces are coated with colloidal graphite. In a separate device, an emissive probe is used to examine the space potential a few millimeters above coated and uncoated surfaces. It is found that the rms potential variation is approximately 250 mV for uncoated surfaces and 15 mV for coated surfaces. The characteristic length scale of the inhomogeneities is ∼1 cm. Glow-discharge cleaning, which is easily renewed, is shown to reduce the potential variation to the same level that is obtained with the colloidal graphite coating.

Neoclassical effects in the annular Penning trap

Neoclassical transport has been investigated with a modified Malmberg–Penning trap which has conductors along the axis to create an azimuthal magnetic field. The axial bounce motion of the electrons is accompanied by a radial drift which changes sign at the ends of the device causing drift orbits of finite radial extent. Analysis and numerical simulations show that the transport is neoclassical with mobility and diffusion coefficients depending upon the axial magnetic field alone rather than the absolute value of the magnetic field. Experiments with added helium gas to create electron-neutral collisions show that the electron mobility from an applied radial electric field and the Ware drift from an azimuthal electric field both have neoclassical values over a wide range of magnetic fields and collision frequencies.

Electrostatic charging of lunar dust

Transient dust clouds suspended above the lunar surface were indicated by the horizon glow observed by the Surveyor spacecrafts and the Lunar Ejecta and Meteorite Experiment (Apollo 17), for example. The theoretical models cannot fully explain these observations, but they all suggest that electrostatic charging of the lunar surface due to exposure to the solar wind plasma and UV radiation could result in levitation, transport and ejection of small grains. We report on our experimental studies of the electrostatic charging properties of an Apollo-17 soil sample and two lunar simulants MLS-1 and JSC-1. We have measured their charge after exposing individual grains to a beam of fast electrons with energies in the range of 20⩽E⩽90 eV. Our measurements indicate that the secondary electron emission yield of the Apollo-17 sample is intermediate between MLS-1 and JSC-1, closer to that of MLS-1. We will also discuss our plans to develop a laboratory lunar surface model, where time dependent illumination and plasma...

Annular Malmberg–Penning trap for studies of plasma confinement

An annular Malmberg–Penning trap is described for studies of plasma confinement. A plasma of electrons is contained in the annular region between coaxial cylindrical conductors and is confined radially by an axial magnetic field and axially by an electrostatic field. An azimuthal magnetic field created by a current-carrying center conductor causes gradient, curvature, and additional electric drifts thus allowing new types of transport studies. An initial electron density of 106 cm−3 is obtained with axial and azimuthal fields of ∼10 mT and fill pressures of 10−5 to 10−4 Torr of helium or argon. The electric mobility drift arising from electron collisions with neutral gas determines the density decay time of ∼2 ms.

Photoelectric Charging of Dust Particles

Publisher Summary This chapter discusses the experimental work on the photoelectric effect, focusing on the determination of the photoelectron energy distribution function through the use of a retarding potential and measurement of photoelectric yields. The chapter describes laboratory experiments that have been performed on the photoelectric charging of dust particles which are either isolated or adjacent to a surface that is also a photoemitter. It is found that zinc dust charges to a positive potential of a few volts when isolated in vacuum and that it charges to a negative potential of a few volts when passed by a photoemitting surface. The illumination is an arc lamp emitting wavelengths longer than 200 nm and the emitting surface is a zirconium foil.