Microwave Interferometer for the Levitated Dipole Experiment

Measuring and understanding the evolution of the plasma density is an important goal for the Levitated Dipole Experiment (LDX). Theoretical considerations suggest that the density profile may naturally evolve to a highly peaked profile with $\delta (n V) \sim 0$, or $n \sim 1/r^4$. Knowledge of the density profile is also necessary for the reconstruction of the overall equilibrium parameters of the confined plasma. In LDX we have built and tested a 4-cord interferometer diagnostic using heterodyne receivers at 60 GHz. Using the multi-cord interferometer, we have measured the radial density profile and its evolution over time in a plasma confined by a supported dipole field. Initial measurments show a range of profiles---from peaked to flat---have been created in the supported-mode of LDX and that the maximum density is usually around $4$x$10^{10}$ cm$^{-3}$.