Optical imaging diagnostics for fusion plasmas

Imaging diagnostics are used for spatially - and temporally - resolved quantitative measurements of plasma properties such as the ionization particle source, particle and energy loss, and impurity radiation in magnetically confined fusion plasmas. Diagnostics equipped with multi-element solid-state detectors (often with image intensifiers) are well suited to the environment of large fusion machines with high magnetic fields and x-ray and neutron fluxes. The authors have used both conventional (16 ms/frame) and high-speed video cameras to measure neutral deuterium Hα (6563 A) emissions from fusion plasmas. Continuous high-speed measurements are made with video cameras operating at 0.1-0.5 ms/frame; gated cameras provide snapshots of 10-100 μs during each 16-ms video frame. Digital data acquisition and absolute intensity calibrations of the cameras enable detailed quantitative source measurements; these are extremely important in determining the particle balance of the plasma. In a linear confinement device, radial transport can be determined from the total particle balance. In a toroidal confinement device, the details of a particle recycling can be determined. Optical imaging in other regions of the spectrum are also important, particularly for the divertor region of large tokamaks. Absolutely calibrated infrared cameras have been used to image the temperature changes in the walls and thereby determine the heat flux. Absolutely calibrated imaging ultraviolet spectrometers measure impurity concentrations; both spatial and spectral imaging instruments are employed. Representative data from each of these diagnostic systems is presented