Extension of the Density Range for Electron Cyclotron Heating (ECH) on DIII-D With 2nd Harmonic O-mode

Studies of high-density plasma regimes that use second harmonic X-mode (X2) ECH are hampered by excessive wave refraction and loss of accessibility to the absorption layer. The upper density limit can be extended by a factor of two by using the O-mode polarization, with the price being < 100% single-pass absorption for O2. Investigation of this option both experimentally and theoretically is underway. An initial experiment in the Super H-mode regime was performed in which 110 GHz EC power was injected in O-mode from the launchers above the outboard midplane in which single-pass absorption of 80-90% was achieved, yielding a slight measured increase in local Te. In this configuration power not absorbed on the first pass through the resonance layer is expected to reflect from the center post and be dissipated in the lower part of the vacuum vessel, which does not contain microwave-sensitive equipment or windows. Comparison of predicted damping with the TORAY and GENRAY codes, as well as with analytic estimates, is being carried out. The dependence of the damping on aiming and on plasma parameters is under study, with the goal of optimizing the scenario so O2 can be applied to core "impurity-chasing" in high-density regimes.