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Experimental investigations of plasmas in electromagnetic shock tubes Simpkinson, William Vaughan

Abstract

The plasmas produced in electromagnetic shock tubes have previously been studied in this laboratory and elsewhere. In general the temperatures and electron densities deduced from time-resolved spectra emitted by the plasma do not agree with the values calculated from shock theory. Photographs taken with a Kerr cell shutter revealed that luminous discharge gases with a very irregular front were driven down the tube and that no separate shock front could be seen ahead. The plasma behind the luminous front consisted of a mixture of rest gas and a considerable amount (~50%) of impurity from the driving discharge. In the work reported here, further attempts were made to produce shock heated plasmas. Various electrode configurations were tried but no improvement was observed. Some measure of success was attained with an electrodeless driver on the shock tube. Kerr cell photographs showed that with argon in the tube a shock wave appeared to be formed ahead of the discharge plasma. The shock speed was much slower than the speed of the advancing luminous front in the tubes with electrodes. However, no shock wave could be observed with helium. With argon in the electrodeless tube radiation could be observed from the gas ahead of the shock wave. Time resolved spectroscopic measurements on this radiation allowed rough determination of electron density and of the population of excited states of argon atoms and ions ahead of the shock front. This "preheating" of the gas is presumably due to ultraviolet light emitted from the discharge and the shock plasma. The values of electron density and temperature expected behind the shock front were calculated from shock theory, taking into account the preheating of the gas. The expected values agreed well with the electron density and temperature determined from spectroscopic measurements on the shock plasma. The study of the precursor radiation was continued in a shock tube with electrodes. In this tube the driving discharge was more luminous and the excitation and ionization of helium and argon ahead of the luminous front could be more readily observed than with the electrodeless tube. The number densities of helium atoms in various excited states were determined from the time resolved line intensities before and after the passage of the luminous front. The ratios of atoms in different levels differ from the expected ratios for thermal equilibrium conditions, both ahead of the luminous front and behind it. An estimate was made of the time required for the attainment of equilibrium by electron impact. The calculation indicates that ahead of the luminous front there is not sufficient time to attain equilibrium. On the other hand, for the high electron density found behind the luminous front, the equilibrium distribution is expected to be reached in times shorter than the observation times, in disagreement with the behaviour observed.

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