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Precision measurements of the electron temperature and the electron temperature fluctuations in the Tokamak Fusion Test Reactor (TFTR) (major radius 2.6 meters, minor radius 0.8 meters, ion temperature 200 million degrees, electron temperature 70 million degrees) are necessary to understand the details of plasma stability and energy confinement. These measurements must be made remotely since there will be high radiation levels ( 1019 neutrons/pulse, 100 rads/pulse) as well as high levels of stray radiofrequency energy (50 MHz) and time changing magnetic fields in the vicinity of the tokamak. Three different instruments are used for these studies: a fast scanning superheterodyne radiometer (0.002 sec temperature profile); a fast scanning Michelson Interferometer (0.01 sec scan); and a twenty channel grating polychrometer which monitors electron temperature at twenty locations in the plasma continuously. The scanning radiometer uses state of the art mixers, detectors and levelers and must be heavily shielded from stray magnetic and radiofrequency fields, but is insensitive to neutrons and x-ray radiation. The Michelson system is relatively insensitive to radiation or stray fields. The grating instrument is located outside the 1.54 meter concrete shield wall to avoid a subtle neutron-detector interaction. Because of the large n, 4He inelastic cross-section (7 barns at 1.25 MeV), the neutron flux from the tokamak can perturb the temperature of the liquid helium bath used to cool the detectors. A temperature rise of several millikelvin is equivalent to a significant fraction of the temperature signal at the edge of the plasma. All three instruments may be calibrated absolutely and are designed for reliability and ease of maintenance.
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