Mode structure measurements of ion cyclotron emission and sub-cyclotron modes on DIII-D

We report mode structure measurements of coherent ion cyclotron emission (ICE) and sub-cyclotron modes on DIII-D. Through a dedicated experiment, we aimed to characterize a variety of modes via the upgraded ICE diagnostic in both L- and H-mode plasmas. In the L-mode plasmas, autopower spectrum peaks at harmonics of the ion cyclotron frequency fci were observed and are localized in the core of the plasma. Sub-cyclotron modes (f ∼ 0.5fci) were also observed in L-mode plasmas when the toroidal magnetic field strength was lowered from BT = 2.17 T. In H-mode plasmas, many ICE harmonics localized to the plasma edge were observed, with some exceeding the nominal ICE diagnostic bandwidth of f ∈ [0, 100] MHz. Polarization estimates made using an orthogonal pair of vertical and horizontal magnetic pickup loops on the outer wall of the machine indicate that ICE and sub-cyclotron modes have compressional polarization at the plasma edge, the latter being consistent with simulation efforts for comparable DIII-D plasmas. For all modes, the same harmonics are observed on both the centerpost and outer wall loops, indicating that ICE is poloidally extended. Finally, toroidal mode numbers were calculated using three outer wall loops for both core ICE and sub-cyclotron modes in L-mode plasmas. The sub-cyclotron case served as a benchmark for our calculation method, with measured numbers of n ∈ [−24, −18] roughly agreeing with heuristic estimates of n ∈ [−20, −13]. Core ICE mode numbers were measured to be n ∈ [−10, 5], which is comparable to measurements made on other machines.