Plasma-Density Imaging using a 2-Dimensional, SecondHarmonic, Dispersion Interferometer

Optical interferometers used for plasma-density measurements are typically robustly mounted, two-arm, highcost, high-maintenance installations. The Second-Harmonic Dispersion Interferometer (SHDI) is an important departure, utilizing a common-path laser beam that is frequency doubled before, and after, the plasma sample. This configuration allows the dispersive-phase shift between the SH beams to be measured in a simple, low-cost, low-maintenance system.

Present SHDI's provide a 1-D (line-of-sight) measurement, configured using a CW laser, i.e., Nd:YAG, or CO2 . Recent experiments compared the performance of these two designs to that of a conventional μ-wave interferometer, demonstrating that the Nd:YAG SHDI is the least complex, most stable system design (Ref.1).

We have recently modified the SHDI for 2-Dimensional, time-resolved imaging, using a pulsed Nd:YAG laser, beamexpansion optics, digital cameras, and image-processing s/w, providing: >10 mRad phase change, 100 µm resolution, 1 ns sampling time, 100 Hz frame rate, and 6-mm diameter beam (Ref.2). Further upgrades provide a 3.6-cm beam diameter.

The 2D-SHDI is now being used to characterize a compact, pulsed-plasma source, n·dl > 10 14 cm -2 line-integrated plasma density, 50-us pulse duration, and roughly a 2 x 3 cm transverse profile. This paper will provide a summary of the 1D- and 2D-SHDI performance and capabilities.