Plasma discharges with negative triangularity (δ = −0.4) shape have been created in the DIII-D tokamak with significant normalized beta (βN = 2.7) and confinement characteristic of the high confinement mode (H98y2 = 1.2) despite the absence of an edge pressure pedestal and no edge localized modes (ELMs). These inner-wall-limited plasmas have similar global performance as a positive triangularity (δ = +0.4) ELMing H-mode discharge with the same plasma current, elongation and cross-sectional area. For cases both of dominant electron cyclotron heating with Te/Ti > 1 and dominant neutral beam injection heating with Te/Ti = 1, turbulent fluctuations over radii 0.5 < ρ < 0.9 were reduced by 10-50% in the negative triangularity shape compared to the matching positive triangularity shape, depending on radius and conditions.

DIII-D experiments at low density (n_{e}∼10^{19}  m^{-3}) have directly measured whistler waves in the 100-200 MHz range excited by multi-MeV runaway electrons. Whistler activity is correlated with runaway intensity (hard x-ray emission level), occurs in novel discrete frequency bands, and exhibits nonlinear limit-cycle-like behavior. The measured frequencies scale with the magnetic field strength and electron density as expected from the whistler dispersion relation. The modes are stabilized with increasing magnetic field, which is consistent with wave-particle resonance mechanisms. The mode amplitudes show intermittent time variations correlated with changes in the electron cyclotron emission that follow predator-prey cycles. These can be interpreted as wave-induced pitch angle scattering of moderate energy runaways. The tokamak runaway-whistler mechanisms have parallels to whistler phenomena in ionospheric plasmas. The observations also open new directions for the modeling and active control of runaway electrons in tokamaks.

Plasmas with a negative triangularity shape have been created on the DIII-D tokamak that, despite maintaining standard L-mode edge radial profiles, reach volume averaged pressure levels typical of H-mode scenarios. Within the auxiliary power available for these experiments, plasmas exhibit near-zero power degradation while sustaining βN = 2.7 and H98,y2 = 1.2 for several energy confinement times. Detailed comparison with matched discharges at positive triangularity indicates that Trapped Electron Modes are weakened at negative triangularity, consistent with increased confinement and reduced intensity of fluctuations in electron density, electron temperature, and ion density. These results indicate that a negative triangularity plasma operating without an edge pedestal might provide an attractive scenario for operations in future reactors.

Collective ion cyclotron emission (ICE) at the ion cyclotron frequency and its harmonics is a potential passive diagnostic of the fast-ion distribution in fusion reactors. ICE is observed in most plasmas in the DIII-D tokamak and is most strongly excited by the fast ions from neutral beam injection. The conventional outboard-edge ICE is detected in H-mode plasmas. However, weaker centrally-localized ICE is measured in L-mode plasmas, including those with negative triangularity shapes. Similar ICE spectra are found with both ICE diagnostics systems, the dedicated magnetic probes and the instrumented antenna straps. Many differences in the behavior of this central ICE are generated by varying the deuterium beam injection angle into deuterium plasmas. The co-current ‘near-perpendicular’ beam excites the most central ICE from the co-current beams, with this emission detected from the fundamental to the fifth ICE harmonic. However, the counter-current ‘near-tangential’ beam destabilizes the highest amounts of centrally-localized ICE. This emission is spectrally broader than that driven by the co-current beams and is observed up to its seventh ICE harmonic. The central ICE excited by this co-current beam correlated strongly on the local electron density and related parameters (plasma current and neutron rate) and increased with deeper fast-ion loss boundaries towards the magnetic axis. This was also the case with second harmonic ICE driven by the counter-current beam but not with its stronger third harmonic emission. The central ICE harmonics destabilized by both beams are observed to have different temporal dynamics. The central ICE amplitude responds rapidly to transient MHD events; it dropped and recovered in less than a millisecond at each sawtooth event. ICE frequency splitting is triggered by both the co-current and counter-current ‘near-tangential’ beams. The data presented in this paper provide opportunities to test and validate models of excitation of ICE by energetic ions.

Kinetic instabilities in the MHz range driven by runaway electrons (REs) have been observed for the first time during the current quench (CQ) in disruptions triggered by massive injection of argon in DIII-D. These instabilities are well-correlated with intermittent RE losses in the beginning of RE current formation. The runaway current phase is not observed when the power of instabilities exceeds a threshold. Novel measurements of the RE distribution function during the CQ indicate that the instabilities appear when RE energy (E RE ) exceeds 2.5-3 MeV, the number of modes grows linearly with E RE , and their frequencies lie in the range 0.1-3 MHz, below the ion cyclotron frequency. Possible plasma waves exciting by REs in this region are proposed. Increase of the amount of injected argon decreases the E RE and increases the success rate of the runaway current formation, while increase of the pre-disruption plasma current acts in the opposite direction. No dependence on the pre-disruption core electron temperature is found.