Reflectometry on ITER

Abstract

Reflectometry will be used on ITER to measure the density profile in the main plasma and divertor regions, and to measure the plasma position and shape in order to provide a standby reference for the magnetic diagnostics in long pulse discharges. The high temperatures of the ITER core and the resultant significant relativistic downshift of the second-harmonic electron cyclotron absorption imply that both low-field side O-mode and high-field side lower cut-off (X−l mode) systems are required to access the full plasma profile. A low-field side upper cut-off (X−u mode) system will also be required for measurements of the scrape-off layer. For measurements of the plasma position and shape, an O-mode system is optimum due to the large range of magnetic field along the plasma periphery and the wide range of possible plasma configurations achievable on ITER. A robust real-time calibration technique of the whole transmission line is required. It is likely that an accurate estimate of the position of the plasma will require the simultaneous use of signals from the profile reflectometer. For the divertor, profiles with peak densities in the range 1019–1022/m3 are to be measured with a target resolution of 3 mm. The large density range will necessitate the use of more than one system. Installing these reflectometers on ITER incurs additional difficulties such as the routing of the millimetre wave radiation around the complicated first wall and divertor structures and design of antennas able to operate through the first wall and blanket.