Plasma-Density Imaging using a Second-Harmonic, Dispersion Interferometer *

Typical plasma density interferometers are robustly stabilized, two-arm, high-cost/high-maintenance installations. The Second-Harmonic Dispersion Interferometer (SHDI) is a significant departure, utilizing a common-path laser beam, frequency doubled before, and after, the plasma sample, allowing the dispersive-phase shift between beams to be measured in a simple system. 

SHDI's configured with a CW laser, i.e., Nd:YAG, or CO₂ have been used for 1-D (line-of-sight) measurements and experiments have compared their performance with a conventional μ-wave interferometer, to demonstrate that a Nd:YAG SHDI is a low-cost, stable alternative (Ref.1). We have recently modified the SHDI for 2-dimensional, time-resolved imaging, using a pulsed Nd:YAG laser, beam-expansion optics, digital cameras, and image-processing s/w, to provide: ~10 mRad phase change, 100 µm resolution, 1 ns sampling, 100 fps, in a 6-mm diameter beam (Ref.2).

Further upgrades to the 2D-SHDI are underway to increase the beam diameter to 3.6 cm, and thereby demonstrate methods needed to enlarge the beam diameter even further. This system is presently being used to characterize a compact, pulsed-plasma source: n·dl > 10¹⁴ cm^-2 line-integrated plasma density, 50-us pulse duration, with a 2 x 3 cm transverse profile. This paper compares and summarizes the performance capabilities for the 1D- and 2D-SHDIs.