Density measurements in a DC arcjet using scanned beam deflection tomography

We have demonstrated a scanned beam deflection technique, and applied this technique to imaging the free stream of a dc arcjet plasma plume. An acousto-optic deflector sweeps a HeNe beam transverse to the jet flow direction. A transform lens and split photodiode measure angular beam deflections produced by refractive index gradients in the arcjet plume. Line scans of beam deflection angle are collected at a 1 kHz sweep rate. Assuming axial symmetry, tomographic reconstruction is used convert the beam deflection data to refractive index. Multiple one-dimensional scans are stacked to produce two-dimensional refractive index images. Index of refraction is directly related to density for measurements in pure argon. Good images are obtained at chamber pressures as low as 4 Torr. Measurements were performed using both pure argon and argon / hydrogen / methane mixtures in the arcjet reactor at a variety of reactor chamber pressures including conditions for diamond deposition. We found significant differences in the radial transport with chamber pressure and with feedstock composition. Comparison of index of refraction data with photographs of arcjet optical emission shows that the emission is not a good representation of the jet density. The simplicity and sensitivity of the scanned beam deflection technique may allow its use for process control when using arcjets for plasma deposition of material.