Phase Contrast Imaging

Phase contrast imaging measures absolute electron density fluctuations via changes of the refractive index, which act as a phase grid and cause scattering orders of the incident laser beam. A key component is the phase plate, which introduces a phase shift of the laser beam and its scattering orders by pi/2 and allows for direct detection of the plasma density fluctuations. The PCI system for W7-X uses a CO2 laser, emitting infrared radiation at a wavelength of 10.6 μm. The laser beam is expanded and collimated on an enclosed optics table, radiated through the plasma using ports AEZ50-AET50 and steered to a second table where the detection components are located. By filtering of the phase plate image, the interference pattern is dependent on the magnetic pitch angle and radial resolution is achieved. The detection is done by two (currentlty 1) detectors (32 channels each), which measure density fluctuations at two radial positions along the laser beam path independently.