No abstract

The Space Power Experiment Aboard Rockets I (SPEAR I) biased two 10‐cm radius spheres as high as 46,000 V positive with respect to an aluminum rocket body. The experiment measured the steady state current to the spheres and the floating potential of the rocket body. Three‐dimensional calculations performed using NASCAP/LEO and POLAR 2.0 show that both ion‐collecting and electron‐collecting sheaths were formed. The rocket body potential with respect to the ionospheric plasma adjusted to achieve a balance between the electron current collected by the spheres and the secondary electron‐enhanced ion current to the rocket body. This current balance was obtained with a large ion‐collecting sheath that enveloped most of the electron‐collecting sheath and reduced the area for collection of ionospheric electrons. The calculated current is in agreement with the flight measurement of a steady state current of less than 1/10 A. The calculations show that the rocket body was driven thousands of volts negative with respect to the ionospheric plasma. The calculated rocket potential is within the uncertainty of that inferred from ion spectrometer data. The current flowed through the space plasma. There was almost no direct charge transport between the spheres and the rocket body.

Spectral line broadening and radiative electron-ion recombination processes may make significant contributions to the total spectral background level when inductively coupled plasma excitation sources are observed with spectrometers having low stray light levels. These background contributions are more easily identified in inductively coupled plasma discharges because of their stable background level and by the fact that net analyte line intensities are affected to such a small degree by changes in the concentration of concomitants. The wings of collisionally broadened lines may produce significant background changes at wavelengths removed 10 nm from the parent line center. For some elements such as Mg, linear Stark-broadened lines produced spectral background at unexpectedly large displacements from the line center. The radiative Al ion-electron recombination continuum produces a greater than tenfold increase in the background from ∼210 down to 193 nm at Al solution concentration of 2500 μg/ml. Thus, a solution containing only 250 μg/ml of Al will cause an ∼100% increase in the background level below 210 nm over that measured when deionized water is nebulized into the plasma.

When atomic emission spectroscopy is employed for the determination of trace elements at concentrations near the detection limit, the spectral background will normally be a large fraction of the total measured signal. Precise background corrections are therefore required if accurate analyses are to be achieved. Experimental evidence is presented to document the fact that stray light may produce substantial shifts in the background signal measured with some spectrometers when the total composition of the sample changes. The data presented have been obtained with the inductively coupled plasma as an excitation source; similar effects can be expected with other excitation sources as well. Examples of various forms of stray light originating from grating defects (ghosts, near and far scatter) and defects in the design of spectrometers are presented. Various methods for the reduction, elimination or correction of stray light effects are also discussed.