Plasma Density Imaging using a 2-Dimensional, Second-Harmonic, Dispersion Interferometer

The Second-Harmonic Dispersion Interferometer (SHDI) has a single, common-axis, beam path, where the primary laser is frequency doubled before, and after, the plasma, allowing the dispersive-phase change between the two-color beams to be accurately measured. The system design is robust, low-cost, and low-maintenance, providing performance that is comparable to a microwave interferometer and second-harmonic CO₂ interferometer.[1] The 2D-SHDI is constructed using a pulsed, Nd:YAG laser, reflective-beam expanders, two-digital cameras, and image-processing software, providing: >10 mRad phase change, 100-μm spatial resolution, 1 ns sampling time,100 Hz frame rate.[2] The density is calculated from two phase image measurements, one for the background (BG) and a second one for the sample; digital subtraction removes the BG even in the presence of a high noise environment.[3] This paper presents a summary of the performance measured using a pulsed-plasma jet and pulsed-gas jet. The minimum instrument sensitivity is for a line-integrated plasma density of, ∫n·dl > 10¹⁴ cm^-2 , thus a low density measurement requires longer pathlenghs. The measured-density profile has a 3.5-cm diameter.