Density measurements in a DC arcjet using scanned beam deflection tomography

Area of Science:

• Plasma Physics
• Optical Diagnostics
• Materials Science

Background:

• Arcjet plasma plumes are vital for material deposition but their internal dynamics are complex.
• Accurate density measurements are essential for understanding and controlling plasma processes.
• Existing diagnostic methods may not fully capture the spatial and temporal characteristics of arcjet plumes.

Purpose of the Study:

• To demonstrate and apply a scanned beam deflection technique for imaging dc arcjet plasma plumes.
• To investigate the relationship between refractive index, density, and plasma composition.
• To evaluate the technique's suitability for process control in plasma material deposition.

Main Methods:

• Utilized an acousto-optic deflector to sweep a HeNe laser beam across the arcjet plume.
• Measured angular beam deflections using a transform lens and split photodiode.
• Employed tomographic reconstruction to convert deflection data into two-dimensional refractive index images.

Main Results:

• Successfully generated 2D refractive index images of arcjet plumes at chamber pressures as low as 4 Torr.
• Observed significant differences in radial transport based on chamber pressure and feedstock composition (argon vs. argon/hydrogen/methane).
• Found that optical emission is an unreliable indicator of plasma jet density.

Conclusions:

• The scanned beam deflection technique provides sensitive and direct measurements of plasma density.
• The technique's simplicity and effectiveness suggest its potential for real-time process control in arcjet-based material deposition.
• Understanding radial transport variations is key for optimizing deposition parameters.