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.
We have investigated the transient behavior of the ion sheath around a spherical metallic probe in a laboratory plasma. The transients were initiated by step-function changes in the probe potential; only negative potentials were used in order to minimize the disturbance to the ambient plasma. The ion sheath was detected through its effect on the complex admittance of the probe at two frequencies, 5 and 15 MHz. The plasma frequency was varied over the range 5–32 MHz. As expected, the admittance transients were quite rapid (<50 μsec except close to the plasma frequency) following the application of a large negative probe bias; under this condition the electrons are quickly repelled to form a new thicker ion sheath about the probe, and further slow changes are too small to be measured. The return to a smaller negative bias also has an initial rapid variation due to the return of the electrons, but this is followed by a much slower part due to the ambipolar diffusion back to the steady-state thin sheath configuration. Undershoots and overshoots were observed near the plasma frequency. An approximate theory for the transient sheath properties is described and numerical results of the theory for a typical plasma condition are presented. A few discrepancies are noted, but in general there is agreement between the numerical results and the experiment.
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