DIII-D research is addressing critical challenges in preparation for ITER and the next generation of fusion devices through focusing on plasma physics fundamentals that underpin key fusion goals, understanding the interaction of disparate core and boundary plasma physics, and developing integrated scenarios for achieving high performance fusion regimes. Fundamental investigations into fusion energy science find that anomalous dissipation of runaway electrons (RE) that arise following a disruption is likely due to interactions with RE-driven kinetic instabilities, some of which have been directly observed, opening a new avenue for RE energy dissipation using naturally excited waves. Dimensionless parameter scaling of intrinsic rotation and gyrokinetic simulations give a predicted ITER rotation profile with significant turbulence stabilization. Coherence imaging spectroscopy confirms near sonic flow throughout the divertor towards the target, which may account for the convection-dominated parallel heat flux. Core-boundary integration studies show that the small angle slot divertor achieves detachment at lower density and extends plasma cooling across the divertor target plate, which is essential for controlling heat flux and erosion. The Super H-mode regime has been extended to high plasma current (2.0 MA) and density to achieve very high pedestal pressures (similar to 30 kPa) and stored energy (3.2 MJ) with H-98y2 approximate to 1.6-2.4. In scenario work, the ITER baseline Q = 10 scenario with zero injected torque is found to have a fusion gain metric beta(TE) independent of current between q(95) = 2.8-3.7, and a lower limit of pedestal rotation for RMP ELM suppression has been found. In the wide pedestal QH-mode regime that exhibits improved performance and no ELMs, the start-up counter torque has been eliminated so that the entire discharge uses approximate to 0 injected torque and the operating space is more ITER-relevant. Finally, the high-beta(N) (<= 3.8) hybrid scenario has been extended to the high-density levels necessary for radiating divertor operation, achieving similar to 40% divertor heat flux reduction using either argon or neon with P-tot up to 15 MW.

An engineering upgrade to the neutral beam system at the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] enables time-dependent programming of the beam voltage and current. Initial application of this capability involves pre-programmed beam voltage and current injected into plasmas that are known to be susceptible to instabilities that are driven by energetic (E≥40 keV) beam ions. These instabilities, here all Alfvén eigenmodes (AEs), increase the transport of the beam ions beyond a classical expectation based on particle drifts and collisions. Injecting neutral beam power, Pbeam≥2 MW, at reduced voltage with increased current reduces the drive for Alfvénic instabilities and results in improved ion confinement. In lower-confinement plasmas, this technique is applied to eliminate the presence of AEs across the mid-radius of the plasmas. Simulations of those plasmas indicate that the mode drive is decreased and the radial extent of the remaining modes is reduced compared to a higher beam voltage case. In higher-confinement plasmas, this technique reduces AE activity in the far edge and results in an interesting scenario of beam current drive improving as the beam voltage reduces from 80 kV to 65 kV.

An upgrade to the Phase Contrast Imaging (PCI) diagnostic on the DIII-D tokamak has successfully combined PCI with density interferometry to provide electron density fluctuation measurements across an unprecedented range in wavenumber. The combined diagnostic uses a single laser and a single beam path through the tokamak, minimizing machine resources in anticipation of future reactor-scale devices where port access will be at a premium. The PCI multichannel detector provides low-noise, wavenumber-resolved detection of moderate-wavenumber turbulence, while the interferometer channel is sensitive to long-wavelength turbulence, including the peak of ion-scale turbulence and MHD. The overlap in wavenumber range between the two schemes allows for an in situ absolute calibration of the PCI.

A non-dimensional collisionality scan conducted on DIII-D confirms a model for ELM energy densities recently put forward by Eich (2017 Nucl. Mater. Energy 12 84-90), but also reveals key effects that may explain the large scatter typically observed about the scaling. Electron cyclotron heating (ECH) close to the plasma edge was used to raise electron temperatures at the pedestal top and lower collisionality to ITER level, while the power of neutral beam injection (NBI) was decreased during discharges to operate closer to the L-H transition threshold. The scan reveals no explicit pedestal pressure dependence of the ELM energy densities. While collisionality does not play a decisive role, the ratio of heating power to the H-mode threshold power is identified as parameter determining the agreement with the model, with discharges marginally above the threshold showing the largest scatter in the database and exceeding the predicted ELM energy up to twofold. Operation close to the L-H-threshold is accompanied by low ELM frequency and large ELM heat loads. Using linear stability calculations, ELM energy densities are shown to scale inversely with to the most unstable linear mode number before the ELM crash. There are indications that the scatter in the data when compared with the Eich model prediction is caused by including only a limited set out of all quantities considered by linear stability analysis. While further ELM studies near the LH threshold are of great priority, the overall agreement of DIII-D with the Eich model recommends its use in extrapolations towards ITER.

The first observation of increased electron temperature turbulence during edge localized mode (ELM) suppression by resonant magnetic perturbations (RMPs) is presented. These are long wavelength fluctuations (kθρs ≤ 0.2, where kθ = poloidal wavenumber and ρs = ion sound gyroradius) observed during H-mode plasmas on the DIII-D. This increase occurs only after ELMs are suppressed and are not observed during the initial RMP application. The T̃e/Te increases (>60%) are coincident with changes in normalized density and electron temperature gradients in the region from the top of the pedestal outward to the upper portion of the steep edge gradient. Density turbulence (kθρs ≤ 0.4) in this location was also observed to increase only after ELM suppression. These results are significant since they indicate that increased gradient-driven turbulent transport is one possible mechanism to regulate and maintain ELM-free H-mode operation. Investigation of linear stability of drift wave instabilities using the CGYRO code [Candy et al., J. Comput. Phys. 324, 73 (2016)] shows that the dominant mode moves closer to the electron mode branch from the ion mode branch only after ELMs are suppressed, correlated with the increased turbulence. The increased turbulence during ELM suppression, rather than with the initial RMP application, indicates that the often observed RMP induced “density pump-out” cannot be attributed to long wavelength edge turbulence level changes.

Negative magnetic shear has been demonstrated in DIII-D and JT-60U to mitigate the confinement degradation typically observed with increasing the electron to ion temperature ratio (Te/Ti). In recent experiments in DIII-D negative central magnetic shear (NCS) discharges, the thermal transport in the internal transport barrier formed around the radius of the minimum safety factor (qmin) remained almost constant and modestly increased in the region outside of qmin compared to the positive shear (PS) case, when Te/Ti increased from about 0.8 to 1.1 through electron cyclotron heating (ECH). The benefit of NCS extending into the region outside of qmin can be explained by the lower magnetic shear in the NCS plasma over the plasma radius relative to the PS plasma. Reduced confinement degradation at high Te/Ti with NCS plasmas was commonly observed in DIII-D and JT-60U. The mechanism of the different transport responses between the NCS and PS plasmas has been assessed in terms of fluctuation measurements and gyrokinetic simulations in DIII-D; NCS gave a smaller rise in the low-wavenumber broadband turbulent fluctuations with the increase in Te/Ti compared with the PS case. This is consistent with gyrokinetic simulations, which show a smaller rise in the growth rates of the ion temperature gradient mode in the NCS plasmas, with increasing Te/Ti. Gyrokinetic simulations also showed a change in the stability of the electron modes with ECH applied, consistent with higher-wavenumber fluctuation measurements, although more detailed simulations are needed to give a quantitative explanation for the experimental observations. Control of q-profile and magnetic shear will allow confinement improvement in future machines with dominant electron heating.

and Electron Scales in ITER Baseline Scenario Discharges on DIII-D A. Marinoni1, M. Porkolab1, J.C. Rost1, E.M. Davis1, R.I. Pinsker2, K.H. Burrell2, J. Candy2, G.M. Staebler2, B.A. Grierson3, G.R. McKee4, T. L. Rhodes5 and the DIII-D Team 1 MIT Plasma Science and Fusion Center, Cambridge MA 02139 USA 2 General Atomics, San Diego CA 92121 USA 3 Princeton Plasma Physics Laboratory, Princeton NJ 08540 USA 4 Univ. Wisconsin, Madison WI 53706 USA 5 Univ. California, Los Angeles, Los Angeles CA 90095 USA

Experimental results are presented where the macroscopic plasma conditions were manipulated by external actuators, such as injection of medium to low Zi impurity gases with ohmic heating (Alcator C-Mod) to dilute the main deuterium ion species, or deploy a different mix of NBI and ECH heating methods (DIII-D), thus enabling us to study the resulting changes in transport and turbulence. Subsequently we were able to carry out quantitative comparisons between gyrokinetic code predictions (GYRO and GS2) and compare them with measurements of the fluctuating density spectrum based on a calibrated Phase Contrast Imaging (PCI) technique. In Alcator C-Mod, dilution by nitrogen seeding was found to decrease the ion temperature gradient scale lengths in the outer regions of the plasma where ITG modes were dominant. GYRO simulations reproduced the observed change in the energy transport with the seeding as a reduction in ITG driven transport. The PCI measured density fluctuation amplitudes also decreased substantially with nitrogen seeding. Simulations of these plasmas with the nonlinear gyrokinetic code GYRO were performed and the density fluctuations from nonlinear GYRO simulations were found to agree with the experimental PCI measurements. On the DIII-D tokamak experiments simulating the NBI heated ITER Baseline Scenario showed that added electron cyclotron heating (ECH) affected turbulent fluctuations at different scales, as measured by Phase Contrast Imaging (PCI) diagnostic. After turning off the ECH power, the intensity of fluctuations at frequencies higher than 200 kHz increased within 20 ms, due to electron modes that were enhanced by the prompt response of the electron temperature inverse scale length in the outer third of the minor radius. Such modes were observed in non-linear gyro-kinetic simulations to generate a significant transient heat flux and an inward particle pinch. In contrast, the behavior of fluctuations at lower frequencies was dictated by the slower time evolution of other equilibrium quantities such as density and flow shear. In particular, the intensity of fluctuations was observed to decrease with the mean flow shear exceeding the maximum linear growth rate of ITG modes. 1.1. Turbulence and Transport Studies on Alcator C-Mod Ohmic Plasmas. Earlier studies of transport on Alcator C-Mod ohmic plasmas indicated that to get agreement with measured values of the ion thermal diffusivity, the main ion species deuterium charge density had to be reduced relative to that of electrons by 10-30 %, implying the presence of a few percent of intrinsic impurities with an effective ion charge of 8-10, corresponding to Oxygen like impurities [1]. The physics behind these observations was further investigated with the gyrokinetic codes GYRO [2] and TGLF [3] as well as additional experimental measurements by injecting known amounts of Nitrogen into ohmic plasmas [4]. All those measurements and simulations showed consistently that the dilution from the nitrogen seeding reduced turbulence and associated energy transport in the radial range of 0.7 < r/a < 0.9 where strong ITG turbulence dominated transport, well above gyrobohm levels [Fig.1a]. Measured turbulence levels with phase contrast imaging agreed with the gyrokinetic code predictions based on synthetic diagnostic method [Fig 1b]. In the gyrokinetic

Two new research tools – a Mirror Langmuir Probe (MLP) and a “Shoelace” antenna – have been employed to diagnose and actively perturb fluctuations in the Alcator C-Mod tokamak edge plasma. Both tools elucidate the physics associated with the Quasi-Coherent Mode (QCM, k⊥ ∼ 1.5 cm , f ∼ 50− 200 kHz), the edge fluctuation responsible for the increased particle flux which sustains the steady-state Enhanced Dα H-mode. In particular, the MLP has been used to characterize the QCM with unprecedented detail, showing it to be primarily a drift wave, with curvature also playing an important role. In addition, the Shoelace antenna coupled inductively to a fluctuation localized in the edge plasma and resonant at the QCM k⊥ and frequency, representing the first time a drift-wave-like fluctuation has been driven actively in the tokamak edge. PACS numbers: 52.35.Kt, 52.35.-g, 52.35.Ra, 52.70.-m

Heterodyne interferometry and phase contrast imaging (PCI) are robust, mature techniques for measuring low-k and high-k electron density fluctuations, respectively. This work describes the first-ever implementation of a combined PCI-interferometer. The combined system uses a single 10.6 mu m probe beam, two interference schemes, and two detectors to measure electron density fluctuations at large spatiotemporal bandwidth (10 kHz < f < 5 MHz and 0 cm(-1) <= k <= 20 cm(-1)), allowing simultaneous measurement of ion-and electron-scale instabilities. Further, correlating our interferometer's measurements with those from DIII-D's pre-existing, toroidally separated interferometer allows core-localized, low-n MHD studies that may otherwise be inaccessible via external magnetic measurements. The combined diagnostic's small port requirements and minimal access restrictions make it well-suited to the harsh neutron environments and limited port space expected in next-step devices. Published by AIP Publishing.

Performance predictions for future fusion devices rely on an accurate model of the pedestal structure. The candidate for predictive pedestal structure is EPED, and it is imperative to test the underlying hypotheses to further gain confidence for ITER projections. Here, we present experimental work testing one of the EPED hypotheses, namely the existence of a soft limit set by microinstabilities such as the kinetic ballooning mode. This work extends recent work on Alactor C-Mod (Diallo et al 2014 Phys. Rev. Lett. 112 115001), to include detailed measurements of the edge fluctuations and comparisons of edge simulation codes and experimental observations.