Triple grating polychromator for Thomson scattering.

A high rejection, high transmission, triple grating polychromator with crossed dispersion has been designed and constructed for Thomson scattering plasma diagnostics. Identical gratings, collimating, and field lenses were used for all three stages. A mechanically convenient arrangement was made possible by using the field lenses to adjust the dispersion of the second stage to the required design value. The transmission in the passband for light polarized perpendicular to the rulings of the grating was measured at 33% for the instrument itself, and at 15% through the instrument and 1.8 m of attached fiber optics. With the 30-nm passband set 4-34-nm wavelengths away from the ruby laser line, the order of 10(-11) of 694-nm light incident in the input slit was present in each 3-nm wide output channel, giving a relative rejection factor of 10(-10).