Development and operation of a backward wave oscillator based FIR scattering system for DIII‐D

The laser‐based far‐infrared scattering system (λ=1.22 mm) on DIII‐D is to be upgraded by the addition of a backward wave oscillator or carcinotron. By supplying up to 300 mW at λ=1.06 mm, the carcinotron is expected to increase the scattered power and signal‐to‐noise ratio by more than ten times. In addition, the laser has suffered from significant frequency pulling during high density discharges which has necessitated using undesirable geometry to obtain accuracy in the low‐frequency part of the fluctuation spectrum. However, the carcinotron‐based system will use an IF tracking system to simultaneously remove the excessive phase noise, frequency drift typical of high‐frequency tubes, and the effects of frequency pulling. This should allow more reliable measurements of low‐frequency asymmetric spectra under all discharge conditions. The local oscillator will be provided by a frequency doubled and locked klystron, to enable heterodyne detection of the scattered signal. Results of extensive laboratory test...