Density profile measurements by AM reflectometry in TJ-II

An amplitude modulation reflectometry system is in operation at the stellarator TJ-II. Recently, the first electron density profiles were obtained, showing good agreement with profiles measured by Thomson scattering and lithium beam diagnostics. In order to measure density profiles from the plasma edge, the extraordinary mode of polarization is used. A hyper-abrupt varactor-tuned oscillator used in combination with active multipliers generate two frequency segments: 25-36 and 36-50 etc GHz sharing a unique common wave-guide system (Ka band). The signal is amplitude modulated at 200 MHz and the phase demodulation is done at lower intermediate frequency. The time evolution of the electron density profile was measured under different experimental conditions. In this paper, we present the temporal evolution of the profiles obtained during a transition to an enhanced confinement mode and during cold pulse propagation experiments. We also report the modification in the shape of the density profile measured during a magnetic configuration scan in which a low-order rational surface is moved from the scrape-off layer into the plasma confinement region.