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

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

The formation of a laser-driven shock in fused silica was observed experimentally. Fused silica slabs were irradiated with 0.53 /µm laser light in a pulse of 2 ns FWHM at intensities ranging from 10¹² W/cm² to 5 x 10¹³ W/cm², producing a pressure pulse ranging from < 30 GPa to 500 GPa. Shock trajectories were observed using streaked shadowgram and schlieren methods. The experiment was modelled with a one-dimensional Lagrangean laser-plasma hydrocode. Comparison of the simulation results with the experimental observations indicate that the high pressure shock develops anomalously slowly at intensities > 1 x 10¹³ W/cm². Furthermore the shock displayed non-steady propagation for a transient period following its formation. The non-steady propagation is interpreted to be due to a relaxation process in the phase transformation of the fused silica to the high pressure stishovite phase which occurs in the shock front. The slow formation of the shock at high intensities is consistent with a significant volume collapse (phase transition) possibly induced by isentropic compression; however, this interpretation is uncertain due to the complications introduced by non-equilibrium thermodynamics and the possibility of two-dimensional motion. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

No description available.

Previous tests of an explosion driven hydrodynamic shock tube revealed peak pressure data significantly lower than values predicted from the semiempirical scaling laws. It was hypothesized that part of the deviation was due to error in determining shock wave parameters and part might be due to measurement error caused by mechanical vibration of the tube. This investigation was conducted in two parts. In the first part, shock wave parameters were determined using a digital computer and curve fitting techniques to analyze digitized shock wave data. The second part involved determining the frequency components of the shock wave data noise content and comparing this to the dynamic characteristics of the tube which were investigated through an impulse testing technique. From these efforts higher values for the peak pressure were verified but no evidence was found that vibration of the tube caused significant degradation of shock wave test data.

In order to better simulate an explosive underwater environment, a new design of the existing explosion driven hydrodynamic conical shock tube has been proposed. This new concept calls for the removal of part of the old tube to accommodate a distributed breach plug. The distributed breach should enhance shock wave characteristics by minimizing the energy losses associated with plastic deformations which occur at detonation. This report makes use of a finite element program, SAP IV, to investigate the modal characteristics of the new distributed breach design. A dynamic response history analysis has also been performed in order to predict the response of the structure to loads characteristic of an ideal shock wave as it propagates along the tube axis. From these efforts some insight has been gained into the structural feasibility of the new design

An explosive driven, water filled, conical shock tube was designed and evaluated regarding its ability to amplify a charge weight and to produce hydrodynamic spherical shock waves. The results show that the shock waves in the tube are essentially spherical in nature--with an initial exponential shape, peak pressure attenuation as (1/R)1.13 and the time constant spreading roughly as (R).22. The charge weight was amplified by a factor of 3600 compared to a theoretical amplification of 7770. An estimate of the energy absorbed by the breach plug (which houses the charge) during an explosion was performed. The peak pressure data taken from the detonation of number 8 strength blasting caps were seen to satisfy the semiempirical scaling law. However, with the addition of plastic explosive to the blasting cap, peak pressure lower than that predicted by the scaling law was observed. At this time it is felt that a decreasing amplification factor with charge weight is the cause for the lower than predicted peak pressure. More data are needed to verify this hypothesis.

No description available.

No description available.