Development of Terahertz Rayleigh Scattering Diagnostics for a Solid Rocket Exhaust Plume
AIR FORCE RESEARCH LAB EDWARDS AFB CA PROPULSION DIRECTORATE
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This research proposes a new technique using terahertz radiation and Rayleigh scattering off of particulates in the plume. This technique is limited to firing the beam straight across the plume. Terahertz radiation wavelengths in relation to alumina particle sizes align with the Rayleigh criterion, which requires that the wavelength of light be at least ten times greater than the radius of the particle. Rayleigh scattering would allow for changing the location of the detector and even co-locating it with the source. Unfortunately, terahertz sources with the required power output are currently in production. Thus, a scaled experiment was setup using microwaves at 2.45 GHz with aluminum, brass, and stainless steel spheres of radii ranging from 4.76 mm to 9.53 mm. A magnetron power source was used to provide a power range of 200-2000 Watts into an anechoic chamber. The forward power was measured and then passed through a horn antenna, scattered off of particles, and collected in another horn antenna. The same test was performed for no spheres inside the chamber and the difference was considered the scattered power. These results were compared with predictions obtained from a Matlab code written for this research. From this experiment, it was shown that one can use Rayleighs equations to determine particle size based on scattered power detected if the number density is known. However, the uncertainties in index of refraction and scattering efficiency as well as a low signal to noise ratio led to large error bars and discrepancies between predictions and experimental data. Also, a notional terahertz diagnostic instrument was developed that would operate at a range of 20 km using a 1 kW source and a 3m diameter parabolic reflector antenna.
- Combustion and Ignition
- Rocket Engines