Abstract
Some properties of a decaying helium plasma have been studied using time resolved spectroscopy and probe diagnostics. The plasma was produced in a pulsed, repetitive, hot cathode discharge in helium at a pressure 11 torr , and the light emitted in the afterglow of the discharge was measured by means of a spectrometer-photomultiplier combination. Single photoelectrons were counted on a scaler during a preset gate time of each discharge cycle, and after a preset number of cycles recorded on punched tape. The spectrometer was calibrated for absolute intensity measurements of the spectral lines of atomic helium. The overall conductance of the positive column was determined by measuring the voltage difference between two probes inserted into the plasma, passing a very small current pulse between the anode and cathode in the afterglow. Heavier current pulses were used to heat the free electrons selectively, thus providing so-called 'afterglow quenching'. From the measured absolute intensities of the helium lines, the number densities of the excited states of helium were calculated. All levels with principal quantum number n {>=} 8 were found to be in near Saha equilibrium with the free electrons at a temperature 1,275 deg K in the early afterglow (15-35 {mu}s
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Citation Formats
Stevefelt, J.
Spectroscopic Study of Recombination in the Early Afterglow of a Helium Plasma.
Sweden: N. p.,
1968.
Web.
Stevefelt, J.
Spectroscopic Study of Recombination in the Early Afterglow of a Helium Plasma.
Sweden.
Stevefelt, J.
1968.
"Spectroscopic Study of Recombination in the Early Afterglow of a Helium Plasma."
Sweden.
@misc{etde_20956285,
title = {Spectroscopic Study of Recombination in the Early Afterglow of a Helium Plasma}
author = {Stevefelt, J}
abstractNote = {Some properties of a decaying helium plasma have been studied using time resolved spectroscopy and probe diagnostics. The plasma was produced in a pulsed, repetitive, hot cathode discharge in helium at a pressure 11 torr , and the light emitted in the afterglow of the discharge was measured by means of a spectrometer-photomultiplier combination. Single photoelectrons were counted on a scaler during a preset gate time of each discharge cycle, and after a preset number of cycles recorded on punched tape. The spectrometer was calibrated for absolute intensity measurements of the spectral lines of atomic helium. The overall conductance of the positive column was determined by measuring the voltage difference between two probes inserted into the plasma, passing a very small current pulse between the anode and cathode in the afterglow. Heavier current pulses were used to heat the free electrons selectively, thus providing so-called 'afterglow quenching'. From the measured absolute intensities of the helium lines, the number densities of the excited states of helium were calculated. All levels with principal quantum number n {>=} 8 were found to be in near Saha equilibrium with the free electrons at a temperature 1,275 deg K in the early afterglow (15-35 {mu}s after end of the discharge). By measuring the absolute intensities of some of the molecular helium bands, an estimate of the rate of conversion of atomic helium ions into molecular helium ions was obtained. The atomic line radiation, as well as the molecular band radiation, was assumed to result from collisional-radiative recombination of atomic and molecular helium ions, respectively. The rate of recombination down to the metastable level n = 2 was obtained from the measured line intensities. By adding the rate of ambipolar diffusion, calculated from known literature data, quite good agreement with the measured decay rate for the electron density was found. The measured line intensities were also used to calculate the collisional deexcitation rate constants for the excited states of helium. The measured plasma conductance was found to be in fair agreement with kinetic theory, taking into account collisions of electrons with both neutral atoms and with charged particles. When a current pulse is applied in the early afterglow, the spectroscopically determined electron temperature is, however, smaller than predicted by theory. Also, the dependence of the recombination coefficient upon electron temperature was found to be somewhat different from what the collisional-radiative model predicts.}
place = {Sweden}
year = {1968}
month = {Feb}
}
title = {Spectroscopic Study of Recombination in the Early Afterglow of a Helium Plasma}
author = {Stevefelt, J}
abstractNote = {Some properties of a decaying helium plasma have been studied using time resolved spectroscopy and probe diagnostics. The plasma was produced in a pulsed, repetitive, hot cathode discharge in helium at a pressure 11 torr , and the light emitted in the afterglow of the discharge was measured by means of a spectrometer-photomultiplier combination. Single photoelectrons were counted on a scaler during a preset gate time of each discharge cycle, and after a preset number of cycles recorded on punched tape. The spectrometer was calibrated for absolute intensity measurements of the spectral lines of atomic helium. The overall conductance of the positive column was determined by measuring the voltage difference between two probes inserted into the plasma, passing a very small current pulse between the anode and cathode in the afterglow. Heavier current pulses were used to heat the free electrons selectively, thus providing so-called 'afterglow quenching'. From the measured absolute intensities of the helium lines, the number densities of the excited states of helium were calculated. All levels with principal quantum number n {>=} 8 were found to be in near Saha equilibrium with the free electrons at a temperature 1,275 deg K in the early afterglow (15-35 {mu}s after end of the discharge). By measuring the absolute intensities of some of the molecular helium bands, an estimate of the rate of conversion of atomic helium ions into molecular helium ions was obtained. The atomic line radiation, as well as the molecular band radiation, was assumed to result from collisional-radiative recombination of atomic and molecular helium ions, respectively. The rate of recombination down to the metastable level n = 2 was obtained from the measured line intensities. By adding the rate of ambipolar diffusion, calculated from known literature data, quite good agreement with the measured decay rate for the electron density was found. The measured line intensities were also used to calculate the collisional deexcitation rate constants for the excited states of helium. The measured plasma conductance was found to be in fair agreement with kinetic theory, taking into account collisions of electrons with both neutral atoms and with charged particles. When a current pulse is applied in the early afterglow, the spectroscopically determined electron temperature is, however, smaller than predicted by theory. Also, the dependence of the recombination coefficient upon electron temperature was found to be somewhat different from what the collisional-radiative model predicts.}
place = {Sweden}
year = {1968}
month = {Feb}
}