Estimates of the Electron Density Profile on LTX Using FMCW Reflectometry and mm-Wave Interferometry

An FMCW (frequency-modulated continuous-wave) reflectometer has been installed on the Lithium Tokamak Experiment (LTX) for electron density profile and fluctuation measurements. This diagnostic consists of two channels using bistatic antennas with a combined frequency coverage of 13.5$-$33 GHz, which corresponds to electron density measurements in the range of $0.2$$-$$1.3$$\times$$10^{13}$ cm$^{-3}$ (in O-mode). Initial measurements will utilize O-mode polarization, which will require modeling of the plasma edge. Reflections from the center stack (delayometry above the peak cutoff frequency), as well as line density measurements from a 296 GHz interferometer (single-chord, radial midplane), will provide constraints for the profile reconstruction/estimate. Typical chord-averaged line densities on LTX range from $2$$-$$6$$\times$$10^{12}$ cm$^{-3}$, which correspond to peak densities of $0.6$$-$$1.8$$\times$$10^{13}$ cm$^{-3}$ assuming a parabolic shape. If available, EFIT/LRDFIT results will provide additional constraints, as well as the possibility of utilizing data from measurements with X-mode or dual-mode (simultaneous O- and X-mode) polarization.