Density profile measurements with X-mode lower cut-off reflectometry in ASDEX Upgrade

Despite the fact that density profile measurements using X-mode lower cut-off reflectometry are foreseen to be used on ITER, little or no experience is available within the reflectometry community and to our knowledge no results on this subject have been published so far. In ASDEX Upgrade the multichannel broadband reflectometer is equipped with both O- and X-mode channels. While X-mode operation was designed for upper cut-off reflection, it is observed that for both high magnetic field and high density discharges the lower cut-off becomes accessible. Here we present reflectometry measurements obtained in ASDEX Upgrade using X-mode lower cut-off and compare both the resulting group delay and density profile with O-mode measurements performed simultaneously over the same plasma region. The possible use of this comparison to provide estimates of the magnetic field is briefly discussed.