The soft particle spectrometer uses electrostatic deflection for simultaneous measurement of the differential energy spectra of electrons and protons. The energy range is from 10 eV to 10 keV per unit charge with ±40% energy spread. Twelve stage BeCu electron multipliers are used as detectors in the counting mode over a five decade dynamic range. A swept mode of energy selection provides a spectrum in 0.5 sec for high fluxes, while a stepped mode suitable for lower fluxes takes 40 sec. A typical electron spectrogram for a transpolar pass is included as an example of the data reduction procedure used.
An experimental investigation of the impedance characteristic of a spherical plasma probe has been carried out for a quantitative assessment of theoretical work, and of hydrodynamic or fluid theory in particular. A stable, low temperature (T ~ 500øK) plasma was produced in nitrogen by a cold cathode discharge, with a plasma frequency of the order of 15 MHz and with an electron collision frequency in the range of 100 to 107 sec-•. The probe consisted of two hemispheres, one serving as a guard to eliminate the effects of the, connecting leads; this geometry provided an almost purely radial electric field over the test hemisphere. Probe diameters ranged from 7 to 20 mm ( 10 to 30 Debye lengths). Hydrodynamic probe theory was tested against the experimental results for the specific cases corresponding to the probe at floating potential and at space potential. Good agreement was obtained for both the real and imaginary parts. The real part of the impedance shows a peak near the plasma frequency, a small shift being explicable in terms of realistic sheath profiles. Electron density can thus be deduced rather accurately, and electron temperature approximately, on the basis of hydrodynamic theory. Accurate values of the electron-neutral collision frequency were also obtained for plasmas where the collision frequency was larger than 0.4 times the radian plasma frequency. At lower pressures the presence of collisionless or Landau damping was clearly established, and the more accurate kinetic theory is necessary to explain the experimental results. 493 494 TARSTRUP AND HEIKKILA SPHERICAL PROBE IMPEDANCE 495
Pulse-height spectra have been obtained for electron multipliers with discrete dynodes of CuBe. A peak in the spectrum shows up at low pulse heights (≈ 0.1 pC for a multiplier with a dc gain of about 106), but a long tail extends out to very large pulses. These observations can be understood in terms of secondary emission processes. They have provided a good basis for the design and calibration of a soft particle spectrometer for the ISIS-2 satellite. The detection efficiency for 300-eV electrons, with a 14-stage venetian blind multiplier having a total gain of 106, is about 80%. Below 100 eV the detection efficiency falls off, but in practice this can be overcome by postacceleration. At higher energies it also decreases to about 30% at 4 keV. This efficiency is reduced at very high counting rates, above 107 sec−1, owing partly to amplifier bandwidth limitations related to noise immunity requirements, and partly to the finite rise time of the multiplier pulses.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.