ABSOLUTE CALIBRATION OF DENSITY PROFILES FROM BOTH O AND X MODE REFLECTOMETRY ON ASDEX UPGRADE

Reflectometry is a promising diagnostic for future fusion devices like ITER as it allows measuring density profiles with high temporal resolution, using either O or X mode operation. O mode has the advantage of simplicity, since the cut-off frequency depends only on plasma density, but it can only probe densities typically above 0.3 x 1019 m-3 because the wavelength needed to probe lower densities becomes too long compared to the density gradient scale length. X-mode sensitivity to both magnetic field and plasma density allows reflection from almost zero density where the cut-off frequency is the same as fce ( () e ce m reBf pi2 =). With combined O and X-mode operation (at the plasma edge) the complete profile can be probed. However, there are situations where X mode is not useful as occurs in some experiments i