Microwave reflectometry for edge density profile measurements on TFTR

A new type of reflectometer not previously used on tokamaks has been installed on TFTR to measure edge electron density profiles. This reflectometer gives unambiguous edge profiles despite large edge density fluctuations and the presence of strong auxiliary heating. The effects of the large density fluctuations in the edge gradient region are overcome by reducing the multiplicity of fringes and eliminating phase excursions produced by scattering from density fluctuations at the reflecting layer. This is accomplished by using differential-reflectometry: the difference phase between two probing signals reflecting from cut-off layers separated by a distance much less than the correlation length of the density fluctuations is used. This system probes the TFTR plasma using the extraordinary mode (X-mode) with two signals swept from 90-118 GHz while maintaining a fixed difference frequency of 125 MHz between these signals. It has been used to obtain density profiles in the range of 1*10¹¹ to 3*10¹³ cm^-3 in high-field (4.5-4.9 T) full size (R₀=2.62 m, a=0.96 m) TFTR plasmas. The reflectometer launcher is located in an ion cyclotron range of frequencies (ICRF) antenna and views the plasma through a small penetration in the centre of the Faraday shield. Initial measurements demonstrated that this technique is an effective way to measure the electron density profile in the edge gradient region.