Density profile measurements by AM reflectometry in TJ-II

An amplitude modulation reflectometry system is in operation at the stellar ator TJ-II. Recently, the first electron density profiles were obtained, sh owing good agreement with profiles measured by Thomson scattering and lithi um beam diagnostics. In order to measure density profiles from the plasma e dge, the extraordinary mode of polarization is used. A hyper-abrupt varacto r-tuned oscillator used in combination with active multipliers generate two frequency segments: 25-36 and 36-50 etc GHz sharing a unique common wave-g uide system (Ka band). The signal is amplitude modulated at 200 MHz and the phase demodulation is done at lower intermediate frequency. The time evolu tion of the electron density profile was measured under different experimen tal conditions. In this paper, we present the temporal evolution of the pro files obtained during a transition to an enhanced confinement mode and duri ng cold pulse propagation experiments. We also report the modification in t he shape of the density profile measured during a magnetic configuration sc an in which a low-order rational surface is moved from the scrape-off layer into the plasma confinement region.