INSTITUTE OF PHYSICS PUBLISHING PLASMA PHYSICS AND CONTROLLED FUSION Plasma Phys. Control. Fusion 43 (2001) 1535–1545 PII: S0741-3335(01)25063-5 Density profile measurements by AM reflectometry in TJ-II T Estrada 1 ,JS´ anchez 1 , B van Milligen 1 , L Cupido 2 , A Silva 2 , M E Manso 2 and V Zhuravlev 3 1 Laboratorio Nacional de Fusi ´ on por Confinamiento Magn´ etico, Asociaci ´ on Euratom-CIEMAT, 28040 Madrid, Spain 2 Associa¸ c˜ ao Euratom-IST, CFN, Instituto Superior T´ ecnico, 1096 Lisboa, Portugal 3 Institute Kurchatov, Institute of Nuclear Fusion, 123182 Moscow, Russia Received 21 May 2001 Published 18 October 2001 Online at stacks.iop.org/PPCF/43/1535 Abstract 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. 1. Introduction TJ-II [1] is a flexible helical device with a magnetic field of B 0 < 1.2 T, a major radius of 1.5 m and a minor radius of 0.22 m. A notable property of this device is its considerable flexibility with regard to the magnetic configuration. The rotational transform and the magnetic well can be varied over a wide range by changing the current fed into the coil structure consisting of two central coils (circular and helical), two vertical field coils and 32 toroidal field coils. Plasmas are produced and heated by two gyrotrons with frequency f = 53.2 GHz and total power P · 600 kW (second harmonic, extraordinary mode of polarization). A high-resolution Thomson scattering diagnostic measures the electron density profile at 160 spatial positions separated by 2.25 mm [2]. Per discharge, a single profile is obtained, 0741-3335/01/111535+11$30.00 © 2001 IOP Publishing Ltd Printed in the UK 1535