Development of Real-Time FPGA-Based Signal Processing for Chirped-Frequency Interferometry on LTX-β

The UCLA 288 GHz interferometer provides line density measurements along a radial midplane chord on LTX-β. For electron density fluctuations, interferometry (k_⊥<1 cm^-1) and far-forward scattering (FFS, k_⊥≈0.7–3 cm^-1) have contrasting sensitivities to different scale lengths. The diagnostic uses a narrow-band chirped-frequency source with a direct-conversion detector to generate a 750 kHz intermediate frequency (IF) signal. The frequency dependencies of the source and detector combine to distort the IF signal, causing cross-contamination between the interferometry and FFS components and increasing the susceptibility of line density tracking to fringe skips. In order to increase the measurement reliability, a signal processing system based on the Digilent Eclypse Z7 platform has been designed to correct for distortion effects in real time. This hardware performs multiple functions: 1) digitization of the raw data at 140 MSa/s, 2) real-time correction of the IF signal to remove distortion and scattering effects, and 3) output of a phase comparator (line density) signal. Further details of the design and implementation will be presented at the conference.