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. In L-mode plasmas, harmonics of the ion cyclotron frequency f_ci were observed and are localized near the magnetic axis. Sub-cyclotron modes (f ~ 0.5f_ci) were also observed when B_T ≤ 1.25 T. ICE harmonics localized to the plasma edge were observed in H-mode plasmas, with some exceeding the nominal ICE diagnostic bandwidth of f ∈ [0, 100] MHz. Using an orthogonal loop pair, ICE and sub-cyclotron modes were found to be compressionally polarized at the plasma edge. The same ICE harmonics were observed by centerpost and outer wall loops, suggesting that the modes are poloidally extended. Finally, toroidal mode numbers were calculated for core ICE and sub-cyclotron modes. The sub-cyclotron case served as a benchmark, with the measured n ∈ [-24, -18] roughly agreeing with heuristic estimates of n ∈ [-20, -13]. Core ICE mode numbers n ∈ [-10, 5]​​​​​​​​​​​​​​ were measured, which is comparable to measurements made on other machines.