A series of DIII-D [J. L. Luxon, Nucl. Fusion 42 614 (2002)] low torque quiescent H-mode experiments show that density gradient driven trapped electron mode (DGTEM) turbulence dominates the inner core of H-Mode plasmas during strong electron cyclotron heating (ECH). Adding 3.4 MW ECH doubles T e /T i from 0.5 to 1.0, which halves the linear DGTEM critical density gradient, locally reducing density peaking, while transport in all channels displays extreme stiffness in the density gradient. This suggests fusion α-heating may degrade inner core confinement in H-Mode plasmas with moderate density peaking and low collisionality, with equal electron and ion temperatures, key conditions expected in burning plasmas. Gyrokinetic simulations using GYRO [J. Candy and R. E. Waltz, J. Comp. Phys. 186 545 (2003)] (and GENE [F. Jenko et al., Phys. Plasmas 7, 1904]) closely match not only particle, energy, and momentum fluxes, but also density fluctuation spectra from Doppler Backscattering (DBS), with and without ECH. Inner core DBS density fluctuations display discrete frequencies with adjacent toroidal mode numbers, which we identify as DGTEMs. GS2 [W. Dorland et al., Phys. Rev. Lett. 85 5579 (2000)] predictions show the DGTEM can be suppressed, to avoid degradation with electron heating, by broadening the current density profile to attain q 0 > q min > 1.

Millimeter-wave imaging diagnostics identify phase-locking and the satisfaction of 3-wave coupling selection criteria among multiple magnetic island chains by providing a localized, internal measurement of the 2D power spectral density, S(ω, kpol). In high-confinement tokamak discharges, these interactions impact both plasma rotation and tearing stability. Nonlinear coupling among neoclassical tearing modes of different n-number, with islands not satisfying the poloidal mode number selection criterion ⟨m, m('), m - m(')⟩, contributes to a reduction in core rotation and flow shear in the vicinity of the modes.

Observation of modes consistent with the trapped-electron mode (TEM) has been made using the electron cyclotron emission imaging (ECEI) diagnostic on the DIII-D tokamak. The new measurements enable the extraction of spectral properties, including poloidal dispersion relations. The spatially correlated radial structure shows qualitative consistency with radially global linear gyrokinetic simulations, using the poloidal wavenumber selected in a narrow frequency band in the ECEI data. Simulations of trapped-electron modes (TEM) driven by the electron temperature gradient yield phase velocities in close agreement with measurements. As found previously in the outer core of DIII-D L-Mode plasmas, the electron temperature fluctuation levels from nonlinear gyrokinetic simulations fall below experiment.

Microwave Imaging Reflectometry (MIR) couples a multi-frequency illumination source to a shaped, 2D array of miniature, quasi-optical substrate lens antennas in order to provide a 2D image of the fluctuating electron density within a confined plasma. The successful implementation of this technique on the DIII-D tokamak is now entering its second year of operation and has obtained a wealth of previously unobtainable data pertaining to coherent modes and broadband turbulence in core and edge regions. The first measurements of 2D Alfvén eigenmode structure using this technique are presented. The poloidally spaced measurements provide the poloidal wavenumber and define a dispersion relationship for a spectrum of modes. This capability is shown to readily distinguish different eigenmodes and branches of instability, even when Doppler shift due to plasma rotation causes the observed frequencies to overlap. Furthermore, direct measurement of the poloidal propagation of unstable eigenmodes allows for comparison to the locally fitted plasma fluid rotation. This technique has been used to understand new measurements of edge harmonic oscillation (EHOs) thought to contribute to the formation of the so-called quiescent high-confinement mode, or QH-mode, on DIII-D. The incidence of diagnostic artifacts in the characterization of these long poloidal wavelengths is being explored in detail with synthetic diagnostic forward modeling techniques and the full-wave reflectometer codes FWR2D and FWR3D, revealing a correlation with non-idealities of mode structure. Planned diagnostic upgrades that will improve the quality of this data and allow for further investigation of ELM suppression and naturally ELM-free operating scenarios are also discussed.