Prediction and Measurement of Electron Density and Collision Frequency in a Weakly Ionised Pine Fire |
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Authors: | Kgakgamatso Mphale Mohan Jacob and Mal Heron |
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Institution: | (1) Physics Department, University of Botswana, P/Bag 0022, Gaborone, Botswana;(2) Electrical and Electronic Engineering Department, James Cook Univeristy, Townsville, QLD 4811, Australia;(3) Marine Geophysical Laboratory, James Cook Univeristy, Townsville, QLD 4811, Australia |
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Abstract: | Pine litter flame is a weakly ionised medium. Electron-neutral collisions are a dominant form of particle interaction in the
flame. Assuming flame electrons to be in thermal equilibrium with neutrals and average electron-neutral collision frequency
to be much higher than the plasma frequency, the propagation of microwaves through the flame is predicted to suffer signal
intensity loss. A controlled fire burner was constructed where various natural vegetation species could be used as fuel. The
burner was equipped with thermocouples and used as a cavity for microwaves with a laboratory quality network analyzer to measure
wave attenuation. Electron density and collision frequency were then calculated from the measured attenuation. The parameters
are important for numerical prediction of electromagnetic wave propagation in wildfire environments. A controlled pine litter
fire with a maximum flame temperature of 1080 K was set in the burner and microwaves (8–10.5 GHz) were caused to propagate
through the flame. A microwave signal loss of 1.6–5.8 dB was measured within the frequency range. Based on the measured attenuation,
electron density and electron-neutral collision frequency in pine fire were calculated to range from 0.51–1.35 × 1016 m−3 and 3.43–5.97 × 1010 s−1 respectively. |
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Keywords: | Wildfire Microwave attenuation Weakly ionised gas Chemi-ionisation Thermal ionization |
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