Abstract
The millimeter-wave frequency-modulated continuous-wave (FM-CW) reflectometer on NSTX is a multichannel system providing electron density profile measurements with a frequency coverage of 13–53GHz [corresponding O-mode density range of (0.21–3.5)×1013cm−3]. Recently, this system has been modified to allow ultrafast full-band sweeps for repetition intervals down to 10μs. For this system to function as a fluctuation diagnostic it is crucial to eliminate artifacts in the phase derivative caused by nonlinearities in the frequency sweep; we introduce a simple hardware technique for reducing these artifacts to ≃0.3%. For NSTX, the additional bandwidth (⩽100kHz) greatly enhances the capability of the FM-CW reflectometer as a diagnostic for low frequency magnetohydrodynamics instabilities (e.g., internal kinks, resistive wall modes, neoclassical tearing modes, as well as fast-particle driven fishbones and low frequency toroidal Alfvén eigenmodes).
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