Photoelectric measurements were made of the shock-excited spectrum of a mixture of helium and argon. The electron density behind the shock wave and the temperatures of the plasma components were deduced from the spectroscopic measurements, assuming thermal equilibrium conditions in the shock plasma. The two temperatures were in fairly good agreement, supporting the equilibrium assumption. On the other hand, the temperature and electron density differ significantly from values expected for a one-dimensional shock wave. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

An experimental technique has been developed for the investigation of departures from thermodynamic equilibrium in plasmas. The relative populations of the upper and lower levels of a spectral line are measured by the reversal temperature method. The reversal temperatures themselves are determined by varying the relative exposure times of the background source and the plasma to be investigated. The relative population densities of the levels in the 2p⁵3s and 2p⁵3p configurations of Nel have been measured. A Neon glow discharge operated in the current region 1mA to 100mA served as the plasma. The results show that the excited gas is definitely not in thermal equilibrium. The relative intensities of the emission lines between the 2p⁵3s and 2p⁵3p configurations of NeI were measured photometrically and corrected for self-absorption. These intensities were then used in conjunction with the relative population densities to determine relative transition probabilities for the spectral lines concerned. The results are accurate to approximately 10% to 15% and are compared to the values obtained previously by other workers. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Spectroscopic measurements were made on plasmas in electromagnetic shock tubes to test the state of equilibrium in these plasmas. In an Argon-Oxygen mixture the relative intensities of AII and AIII lines and of OII lines indicated that complete equilibrium was not established. In a Helium plasma the observed line intensities showed that the populations of the Hel levels agreed with a Boltzmann equilibrium distribution. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A Flash Unit to supply a pulsed source of continuum radiation has been constructed to provide the light required for the study of absorption spectra of plasmas. The unit which contains the plasma has been designed to produce a gas of high purity. Several transitions in the excited neon were seen in absorption. At least two have not been reported previously. A preliminary determination of transition temperatures has been made. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The spectra emitted by a pulsed-arc discharge have been photographed with a medium resolution spectrograph to obtain Stark broadened line images measurable for both width and shift. Plasmas with densities near 10¹⁷cm⁻³ and temperatures of about 2.6 ev were produced by subjecting Argon - Nitrogen mixtures to a square current pulse. Light from the discharge was shuttered by a rotating mirror system so that the plasma was photographed in an interval during the current pulse when the plasma had optimal conditions for measurement. A technique in which the spectral lines from a standard source are photographed on the same plates as the plasma lines has been devised for calibrating the measurement routine and for facilitating shift measurements. Stark parameters were obtained by scanning the plates on a precision comparator. Nineteen ArII lines and six NII lines were studied. For Argon, the agreement with other experimental results is satisfactory but the theory is inadequate. Similarly, the NII theory does not predict the values measured here. On the other hand, some of the qualitative predictions by the Impact theory about the line shape and about the common widths and shifts of lines in the same multiplet have been confirmed. The experiment on the NII lines also reveals advantages of the present technique over other methods for obtaining Stark parameters. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A new experimental technique has been developed for the determination of absorption line profiles in steady-state plasmas. The method involves observing the total transmitted line intensity of one component of the longitudinal Zeeman pattern from a background source. The frequency shift of this line is determined from the known magnetic field and the Landé g-factors involved. The light from the background source is amplitude modulated by a chopping wheel, and the intensity of the transmitted light is measured with a phase-sensitive detector. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A significant improvement in magnetic probe measurements has been achieved by developing a 3-coil magnetic probe which partially corrects for the boundary error, i.e. for the error caused by an exclusion of the plasma current from the space occupied by the probe. The spatial resolution C of the three coil probe is roughly one-half of the probe radius, a. For current distributions which vary slowly with distance (scale length, ƛ ) it is shown that the fractional error in the magnetic field is 0.2 (C/ ƛ)². For a conventional probe the error is at least four times as large. Also it has been shown that spurious signals arising from poor probe geometry (and which often obscure the signals produced by the measured magnetic fields themselves) can be eliminated by making use of the symmetry of the discharge fields. Measurements have been made on a Z-pinch discharge which confirm the claimed 3-coil probe performance by revealing a "fine" structure of the current distribution in helium in the filling pressure range 0.5 to 4 Torr. The measurements are presented in the form of a catalogue of the spatial distribution of magnetic field and the current density for a Z-pinch discharge in He and Ar. Using the probe measurements a qualitative model of the collapse of the current sheet for 4 Torr in He is developed which differs from previous models. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The plasma-arc generator (see figure 1, 2) has been used for several years as an invaluable ground test facility in which the gas stagnation properties of hypervelocity flight may be reproduced. The plasma is produced by shorting an electric arc between two electrodes; then directing a gas stream through the arc. The gas stream is thus heated to a ''temperature " of forty to fifty thousand degrees Farenheit. At this temperature the gas is in a state of partial ionization termed "plasma". The plasma stream is then constricted and directed by a nozzle into a test chamber and over the surface of a model.

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