B. Van Zyl

THE LOW-ENERGY NEUTRAL ATOM IMAGER FOR IMAGE

The ‘Imager for Magnetosphere-to-Aurora Global Exploration’ (IMAGE) will be launched early in the year 2000. It will be the first mission dedicated to imaging, with the capability to determine how the magnetosphere changes globally in response to solar storm effects in the solar wind, on time scales as short as a few minutes. The low energy neutral atom (LENA) imager uses a new atom-to-negative ion surface conversion technology to image the neutral atom flux and measure its composition (H and O) and energy distribution (10 to 750 eV). LENA uses electrostatic optics techniques for energy (per charge) discrimination and carbon foil time-of-flight techniques for mass discrimination. It has a 90° x 8° field-of-view in 12 pixels, each nominally 8° x 8°. Spacecraft spin provides a total field-of-view of 90° x 360°, comprised of 12 x 45 pixels. LENA is designed to image fast neutral atom fluxes in its energy range, emitted by auroral ionospheres or the sun, or penetrating from the interstellar medium. It will thereby determine how superthermal plasma heating is distributed in space, how and why it varies on short time scales, and how this heating is driven by solar activity as reflected in solar wind conditions.

Absolute measurement of low‐energy H0 fluxes by a secondary emission detector

Secondary negative and positive charge emission coefficients for bombardment of a gas‐covered Cu surface by H+, H0, and H− have been measured for projectile energies of ∼25–2500 eV. The secondary negative charge yield for H0 impact was found to be 1.15±0.08 times that for H+ impact. For H− impact, the secondary negative charge yield decreased less rapidly with projectile energy than that of H+ and H0 impact, being about an order of magnitude larger at the lowest energies investigated. The secondary positive charge yields were found to be independent of the projectile charge state and were about an order of magnitude smaller than the negative charge yields. The measurement techniques are described, and the results are compared with the data of other investigators.

N2+(X), N2+(A), and N2+(B) production in e− + N2 collisions

A variety of cross-section data for electron-impact ionization of N2 have been analyzed, including those for total N2+ production and for the Meinel and first-negative emissions from excited N2+ ions. These results are used to infer the relative production rates of the N2+ (X), N2+ (A), and N2+ (B) product ions during the reactions, yielding branching-ratio fractions of 0.320±0.147, 0.535±0.112, and 0.145±0.019, respectively, for 100-eV electrons. The data are compared with the relative formation rates of these ions recently obtained using electron-energy spectroscopy coincidence measurements. The significant discrepancy between these two procedures is discussed, and the results obtained from our data analysis are recommended for auroral modeling at the present time.

New molecular‐dissociation furnace for H and O atom sources

A new type of molecular‐dissociation furnace has been developed for producing thermal beams of H and O atoms to be used as targets for fast atomic projectiles in various scattering experiments. Target‐particle densities in the beam from the furnace of about 1011 cm−3 have been achieved with molecular‐dissociation fractions in excess of 0.7 for each species. The techniques used are described and compared with more traditional furnace designs.

Cross sections for electron capture and loss. II. H impact on H and H/sub 2/

The electron-capture and electron-loss reactions for H impact on H and H/sub 2/ have been examined for projectile energies between 2.0 and about 0.1 keV. Relative cross sections for these reactions were measured directly, and the data for H targets were placed on an absolute scale by normalizing to the available results for H/sub 2/ targets. For the electron-capture (ion-pair formation) reaction, some new data are also reported here for H/sub 2/ targets. The crossed-beam techniques used to accomplish the measurements are described, and the results are compared with other experimental and theoretical data where possible.

Charged‐particle production in low‐energy H+H2 and H+He collisions

Absolute cross sections for producing H+, H−, H+2, He+, and e− have been measured for fast hydrogen atom impact on H2 and He targets. The hydrogen atom energy ranged between 50 eV and 3.0 keV. For the H+H2 reaction, the dominant ion‐formation process for hydrogen atom energies below 250 eV was found to be H−+H+2 production. For He targets, production of H+ dominated over the entire hydrogen atom energy range. The results are compared, where possible, with the data of other investigators and are discussed in terms of possible reaction mechanisms.

Generation of a fast atomic hydrogen beam

Apparatus has been developed for producing a beam of fast hydrogen atoms having energies between 10 and 3000 eV. The procedure involves generation of a beam of hydrogen ions (H−), appropriate ion acceleration and trajectory definition, and ion neutralization by a photodetachment process. The magnitude of the resulting neutral atom flux can be determined to within an uncertainty of less than ±3% by a technique described. An atom beam intensity in excess of 1011 atoms/sec at 1000 eV is readily obtainable, with the intensities at other energies scaling approximately inversely with the primary ion velocity. The advantages of the method over other neutral beam formation procedures are discussed, and techniques allowing considerable enhancement of the beam intensity over that presently achieved are suggested.

Degradation of vacuum-exposed SiO2 laser windows

Observations of a damage phenomenon at the surface of fused silica and crystalline quartz windows are presented. Uncoated windows were mounted at Brewsters angle to facilitate the introduction of a vacuum chamber directly into the cavity of an Ar-ion laser (488 - 514 nm). The transmission of these windows, prior to evacuating the chamber to less than 1 Torr, approaches the theoretical value of > 99.9%, remaining constant indefinitely. However in our normal usage, the chamber is evacuated (P < 10-7 Torr), exposing the windows to high vacuum as well as UV borelight from the laser discharge. After several hours of operation, the intracavity power is observed to decrease monotonically (by approximately 15% per hour) accompanied by the development of a red fluorescence on the inside window surface where exposed to the visible laser radiation. Partial rejuvenation of the windows can be accomplished by reintroduction of gas into the vacuum chamber. Possible damage mechanisms are presented.

Excited hydrogen and argon atom production by charge transfer of metastable Ar+ ions in H2 molecules

Observations of excited products from charge‐transfering collisional between Ar ions and molecular hydrogen are reported.(AIP)

GENERATION OF A FAST ATOMIC-OXYGEN BEAM FROM O- IONS BY RESONANT CAVITY RADIATION

An apparatus has been developed for producing a beam of ground‐electronic‐state oxygen atoms with energies variable from 4 to 1000 eV with a 1.5 eV FWHM energy distribution. The technique involves extraction of negative ions from a low‐voltage gas‐discharge source, mass selection of the extracted O− with a Wien‐type velocity filter, O− acceleration or deceleration and focusing by electrostatic ion optics, and electron detachment from O− by intracavity laser radiation. A 25 W argon‐ion‐laser cavity has been extended to include the ion‐beam vacuum chamber so that the intracavity radiation intersects the O− ion trajectories normally. Depending on the laser configuration in use, ion‐neutralization efficiencies between 5% and 25% have been achieved at 5 eV O− energy. Thus, 5 eV O‐atom fluxes of ∼1011 atoms/s (∼1012 atoms/cm2 s) have been achieved for O− currents of ∼10−7 A. The advantages and limitations of the technique are discussed, and preliminary measurements of the secondary‐negative‐charge production fr...