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UBC Theses and Dissertations

Precursor ionization Whelan, Patrick James Thomas Aquinas


The preionization of a shock tube's gas before the shock passes through it is called the precursor effect. An experimental and theoretical study has been carried out on precursor ionization in an electromagnetic shock tube. The precursor ionization was detected with different types of electric probes and also with photoraaltipliers. Extensive experiments indicated that the ionization was not due to diffusion of particles from the discharge in the shock tube driver. The ionization is primarily caused by radiation from the discharge of wavelengths less than 2000 Å. Radiation from the shock front makes a negligible contribution to the ionization. Langmuir double probe measurements indicated that the gas was about 0.1% ionized and that the electrons in the precursor were not in thermal equilibrium with the gas atoms and ions. The time interval between detection of ionization at two stations was independent of the shock tube gas (air, argon, helium), and corresponded to a propagation speed greater than 1/20 the speed of light. The precursor had a main component lasting about 50 microseconds with ionization proportional to the square of the discharge current. There was also a weaker component which lasted for about 500 microseconds. The experimental results can be understood in terms of a theoretical model based on black body radiation. Considering the driver to act as an infinite slab radiator, whose temperature is a function of the discharge parameters, an expression is derived for the number of photons emitted in some frequency interval. Assuming the electron density to be proportional to radiation absorption from such a radiator, the electron density variation with distance from the driver can be adequately understood. When the shock tube is considered to behave as a transmission line, whose resistance per unit length is proportional to the electron density, one can explain the variation of the shock tube's gas potential both with distance from the driver and with time.

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