High-transmission 20-channel polychromator for observing non-Maxwellian electron velocity distributions in plasmas by Thomson scattering.
A high-transmission (~45%) twenty-channel polychromator equipped with near-infrared sensitive photomultipliers has been constructed to record Thomson scattering spectra at the TORTUR tokamak. The high transmission was achieved by the use of mirrors instead of fiber optics to guide the spectrally resolved light to a set of photomultipliers. Spectral analysis is performed with a holographically ruled concave grating. Acceptable dimensions of the wavelength selection mirrors were obtained by magnifying the spectral image by a factor of 5 with a Mangin mirror. Electron temperatures up to 1000 eV at a density of 5 x 10(19) m(-3) can be measured with an accuracy of approximately l%. Both high sensitivity and high resolution enable the detection of irregularities in the velocity distribution. For example, satellites corresponding to partial densities of (5 +/- 1) x 10(17) m(-3) were found at 23 nm from the laser wavelength.
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