The instrument is capable of providing the three lowest spectral moments of the line emission encoded as DC, and amplitude and phase modulants of the electrooptically imposed carrier signal. These moments are related in a simple fashion to the line-integrated velocity moments of the (possibly inhomogeneous) velocity distribution function of the species under study. Some example experimental data illustrating the perfomrance of MOSS is available for view.
A number of experiments based on the MOSS spectrometer are underway at H-1NF. These include a number of direct and lens-coupled systems for Doppler and Zeeman spectroscopy, the TOMOSS tomographic spectroscopy apparatus for 2-d imaging of the plasma ion/atom temperatures and flow vorticity and the MOSS camera for multiple-spatial-channel fast, high-resolution spectral imaging of the H-1NF plasma. The camera, which utilizes a multi-anode phototube as detector, will be used for fast power modulation studies of transport in the heliac. Finally, we have developed a multiple-crystal extension of the MOSS which allows to study spectral lines at multiple simultaneous delays using spread spectrum modulation techniques. We call this the Spread-Spectrum Optical Fourier Transform or SOFT spectrometer.
Below left is a ray-trace visualization of the optical system used for 2-D spectral imaging. The photograph (right) shows the MOSS system being used for ion temperature and flow measurements at the H-1NF National Plasma Fusion Research Facility. Light is coupled to the instrument using high NA optical fibres, is collimated and then processed using the crystals inside the light tight enclosures. The units can be combined so as to measure simultaneously different spectral features along a common optical path Because the spectral information is carried in the temporal frequency domain, the camera offers the possibility to make truly two-dimensional images of the radiating source with high time-resolution.
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