Microwave phase perturbation and ionisation measurement in vegetation fire plasma

Interaction of low temperature high collisional combustion plasma with electromagnetic wave energy results in a phase lag of the incident wave. If normalised collision frequency {(φ_eff)/2πf₀} is much greater than unity (1), the phase shift (Δφ) varies linearly with plasma frequency, and hence with ionisation in plasma. Ionisation in vegetation fires (highly collisional atmospheric pressure plasma) has implications on the safety of fire-fighters. During wildfire suppression, radio communication blackout at very/ultra high frequency has been experienced. A cylindrical fire burner was constructed in which various natural vegetation materials could be used as fuel. The burner was equipped with Type K thermocouples to measure fuel combustion temperature and was used as cavity for microwaves with laboratory quality network analyser to measure phase shift. In the experiment, microwaves were caused to propagate combustion zones of eucalyptus litter fires with maximum temperatures of 1142 and 1174 K, respectively. The measured phase shift ranged from 7.34 to 11.63°. Calculated ionisation from the phase shift measurements were from 1.9 to 2.0 × 10¹⁶ m^{- 3}.