High-speed imaging of the plasma has allowed observation of the transition to turbulent flow during interaction with a mechanical target. CTIX can create large Reynolds number flows, which allows us to measure the physics of nonlinear interactions and resulting fluctuations. Future work will use these techniques to explore a variety of other interaction geometries.
The top graph in the figure below is the 2-D Fourier transform of the plasma luminosity in the region before it has interacted with the target. The bottom graph is the FT of the plasma after it has interacted with the target. The color scale represents the magnitude of the FT for any given wavevector (kx, ky), and the units are arbitrary, but the same for both top and bottom graphs. A significant increase in fluctuation amplitude is observed, as well as wider distribution of waves in the ky direction. Also the k-spectrum is noticeably more choppy and irregular for the turbulent flow than for the incoming laminar flow. The dominant wave modes in the turbulent spectrum are localized near k = (2.5, 0) (1/cm), where k = 2(pi)/wavelength. A custom-made discrete Fourier transform algorithm was used instead of a prepackaged FFT algorithm in order to provide more precise control of the resulting resolution in k space.