Plasma interferometry at high pressures

Akhtar, Khalid Pervaiz; Scharer, J.E.; Tysk, Shane; Kho, Enny

doi:10.1063/1.1533104

Cited by 66 publications

(48 citation statements)

References 7 publications

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“…For example, at ambient air temperature (approx. 300 K), electronneutral collision frequency is about 2.3 × 10 12 s −1 [34] which is far higher than that of the flames at 2000 K.…”

Section: Discussionmentioning

confidence: 99%

“…The theory of electromagnetic wave propagation in weakly ionised plasma given in Koretzky et al [35] and Akhtar et al [34] predicts attenuation index in the range of 35.7-33.6 dB m −1 for the HF-UHF frequency band when mean values of the measured collision frequency and electron density are used. This implies that a UHF signal caused to propagate along the fire-fuel interface of a very intense ionised fire may be strongly attenuated.…”

Section: Discussionmentioning

confidence: 99%

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Microwave attenuation in forest fuel flames

Mphale

Luhanga

Heron

2008

Combustion and Flame

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The flames of forest fuels form a weakly ionized gas. Assuming a Maxwellian velocity distribution of flame particle and collision frequencies much higher than plasma frequencies, the propagation of microwaves through forest fuel flames is predicted to have attenuation and phase shift. A controlled fire burner was constructed where various natural vegetation materials 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 phase and attenuation. The controlled fires had temperatures in the range of 500-1000 K and microwave attenuation of 1.0-4.5 dB m −1 was observed across the 0.5 m diameter cavity. Attenuations of this magnitude could affect active remote sensing systems signals at microwave frequencies in forest fire environments where flame depths of up to 50 m are possible. In the experiment, temperature was not the only controlling parameter for the ionisation; type of fuel burnt also influenced it. Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) analysis of the composition of the fuel confirmed that a higher content of alkali (with low ionization potential) lead to higher electron densities. Electron densities in the range of 0.32-3.21 × 10 16 m −3 and collision frequencies of 1.1-4.0 × 10 10 s −1 were observed for flames with temperature in the range of 730-1000 K.

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“…For example, at ambient air temperature (approx. 300 K), electronneutral collision frequency is about 2.3 × 10 12 s −1 [34] which is far higher than that of the flames at 2000 K.…”

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Microwave attenuation in forest fuel flames

Mphale

Luhanga

Heron

2008

Combustion and Flame

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“…According to Akthar [8], propagation constant (γ) is related to attenuation (α) and shift (β) coefficients in a medium by the relation; γ = α + i(β). Equation (1) can therefore be expressed as: …”

Section: Theoretical Considerationmentioning

confidence: 99%

Radio Wave Propagation Experiment in Sugarcane Fire Environments

Letsholathebe¹,

Mphale²,

Chimidza³

et al. 2016

JEMAA

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Large fires have an effect of suppressing Very or Ultra High Frequency (VHF/UHF) radio wave signals strength which consequently impact negatively on the efficiency of radio communications at the frequency ranges. Mobile hand-held radio operating at the frequency ranges is a major communication tool during fire suppression; therefore inefficient radio communication systems put lives of fire fighters at risk. One of the causes of signal attenuation in fire environment is plume ionization. Plume species which include graphitic carbon, alkalis and thermally excited radicals such as methyl are responsible for ionization. As atmospheric pressure ionized medium (combustion plasma), sugarcane fire has momentum transfer electron-neutral collision frequency much higher than plasma frequency, hence propagation of VHF/UHF radio waves through such a medium is predicted to suffer a significant attenuation and phase shift. Radiowave propagation measurements were carried out in a moderate intensity prescribed sugarcane fire at 151 MHz frequency over a 590 m path using a radiowave interferometer. The radio wave interferometer measured signal attenuation of 0.43 dB through the fire with maximum temperature and flame depth of 1154 K and 8.7 m, respectively.

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“…Phase shift and attenuation induced by weakly ionized gas such as a wildfire is expressed by Akhtar et al [1] as:…”

Section: Refractive Index Due To Unbounded Electronsmentioning

confidence: 99%

“…Refractive index due to ionization in the flame is related to the radio wave phase shift and attenuation coefficient by the equation [1];…”

Section: Refractive Index Due To Unbounded Electronsmentioning

confidence: 99%

Ray Tracing Radio Waves in Wildfire Environments

Mphale¹,

Heron²

2007

PIER

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Abstract-Wildfires are uncontrolled exothermic oxidation of vegetation. Flame combustion temperatures could be in excess of 1600 K. Under the high temperature environment, plants' organic structure crumbles to release omnipresent alkali nutrients into the combustion zone. The alkali based compounds thermally decomposed to constituent atoms which ultimately ionised to give ions and electrons. The presence of electrons in the flame lowers its refractive index, thereby creating a medium of spatially varying refractive index. In the medium, incident radio waves change speed and are consequently deflected from their original path. The refraction has an effect of decreasing signal intensity at a targeted receiver which is at the same height as a collimated beam transmitter which is at a considerable distance away from the former. A numerical experiment was set to investigate the sub refractive behaviour of a very high intensity eucalyptus wildfire (90 MWm −1 ) plume using two dimensional (2D) ray tracing scheme. The scheme traces radio rays as they traverse the plume. The ratio of number rays in a collimated beam reaching the targeted receiver to number of rays leaving the transmitter is used to calculate signal intensity loss in decibels (dB) at the receiver. Assuming an average natural plant alkali (potassium) content of 0.5%, attenuation (dB) was observed to be factor of both propagation frequency and temperature at the seat of the fire plume; and only of temperature at cooler parts of 154 Mphale and Heron the plume. The 2D ray tracing scheme predicted a maximum attenuations of 14.84 and 5.47 dB for 3000 and 150 MHz respectively at 0.8 m above canopy-flame interface over propagation path of 48.25 m. An attenuation of 0.85 dB was predicted for frequencies from 150-3000 MHz over the same propagation distance at plume height of 52.8 m above ground.

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Plasma interferometry at high pressures

Cited by 66 publications

References 7 publications

Microwave attenuation in forest fuel flames

Microwave attenuation in forest fuel flames

Radio Wave Propagation Experiment in Sugarcane Fire Environments

Ray Tracing Radio Waves in Wildfire Environments

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