Low-wavenumber density fluctuations exhibit unique characteristics during the current ramp-up phase of ITER-like discharges that can partially explain the challenges of correctly modeling transport behavior and predicting global plasma parameters during this period. A strong interaction takes place between the evolving transport, safety factor (q) and kinetic profiles as well as the appearance and evolution of low-order rational surfaces. Density fluctuations from 0.75 < rho < 0.9 are transiently reduced to exceptionally low levels during early times and from 0.8 < rho < 0.9 at late times in the ramp-up in a manner that is different from behavior observed during steady-state plasma conditions with similar values of q(95). Turbulence is suppressed as low-order-rational q-surfaces enter the plasma; the local electron temperature likewise exhibits transient increases during these periods of reduced fluctuations indicating changes in transport that impact temperature and consequently the evolution of current density and plasma inductance. These observations can explain discrepancies between CORSICA modelling and the higher electron temperature found previously over the outer half radius. Comparison of turbulence properties with time-varying linear growth rates with GYRO and GENE demonstrate qualitative consistency with measured fluctuation levels, but calculations don't exhibit reduced growth rates near low-order rational surfaces, which is inconsistent with experimental observations. This indicates a mechanism that can contribute to reconciling observed turbulence behavior with transport models, allowing for the development of more accurate predictive tools.

Nonlinear gyrokinetic simulations using the GENE code have previously predicted a significant nonlinear enhanced electromagnetic stabilization in certain JET discharges with high neutral-beam power and low core magnetic shear (Citrin et al 2013 Phys. Rev. Lett. 111 155001, 2015 Plasma Phys. Control. Fusion 57 014032). This dominates over the impact of E x B flow shear in these discharges. Furthermore, fast ions were shown to be a major contributor to the electromagnetic stabilization. These conclusions were based on results from the GENE gyrokinetic turbulence code. In this work we verify these results using the GYRO code. Comparing results (linear frequencies, eigenfunctions, and nonlinear fluxes) from different gyrokinetic codes as a means of verification (benchmarking) is only convincing if the codes agree for more than one discharge. Otherwise, agreement may simply be fortuitous. Therefore, we analyze three discharges, all with a carbon wall: a simplified, two-species, circular geometry case based on an actual JET discharge; an L-mode discharge with a significant fast-ion pressure fraction; and a low-triangularity high-beta hybrid discharge. All discharges were analyzed at normalized toroidal flux coordinate rho = 0.33 where significant ion temperature peaking is observed. The GYRO simulations support the conclusion that electromagnetic stabilization is strong, and dominates E x B shear stabilization.

Quantitative comparisons of the calculated turbulence characteristics with fluctuation measurements in high performance H-mode plasmas on DIII-D show excellent agreement between the fluctuation amplitude and frequency spectrum (corresponding to poloidal wavenumber) for a moderate rotation discharge (Mo~0.2). The calculated ion heat flux also closely match the experimentally inferred value, while the electron heat flux was slightly underestimated, perhaps due to significant contributions from higher-k turbulence not included in these initial nonlinear simulations; linear growth rate calculations show significant growth rates for ETG-scale turbulence. Identifying and improving the fidelity with which simulations reproduce experiment will increase confidence in their predictive capability and provide computational tools for future performance optimization. To assess this, multi-scale spatiotemporal properties of turbulence were measured with multiple fluctuation diagnostics (Beam Emission Spectroscopy, Doppler-Backscattering, Phase Contrast Imaging) in highconfinement hybrid scenario H-Mode plasmas on DIII-D. Low-wavenumber density fluctuation amplitudes range from 0.2-1% over 0.5<ρ<0.85, while radial and poloidal correlation lengths are near 5 cm and 8 cm, respectively (Lc~10 ρi) at mid-radius. The nonlinear simulations used the measured kinetic profiles, calculated heat sources, and magnetic equilibria, and include kinetic electrons and electromagnetic fluctuations, both of which are essential to obtaining accurate comparisons with experiment. While the fluctuation amplitudes were similar between experiment and simulation, the low-k correlation lengths and decorrelation rates showed moderate differences, which could result from a variety of causes such as non-local or multiscale physics not included in the simulations, missing or incorrect descriptions of zonal flow physics and/or electromagnetic effects, or the influence of processes such as ELMs and core tearing modes not included in the gyrokinetic model. Complementary measurements were obtained in higher toroidal rotation plasmas (Mach Number, M~0.5) with otherwise similar kinetic profiles and dimensionless parameters (ρ*, q, β, Te/Ti, ν*) to test simulations at very high ExB shear. The measured turbulence amplitude and radial correlation lengths surprisingly stayed nearly constant as the ExB shear is increased by a factor of 2-3, while the poloidal correlation length increases with ExB shear, and the measured decorrelation rate scales closely with the local ExB shearing rate; the energy confinement time increases by nearly 50% at high average shear. Nonlinear simulations indicate that the turbulence in the high-rotation case is set by a delicate balance of strong ITG drive and ExB shear suppression, and obtaining converged nonlinear results with conventional ion-scale GYRO simulations has proved difficult. Progress in addressing these challenges with the newly-developed CGYRO nonlinear gyrokinetic code will be presented.

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 Te/Ti 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 that 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. Comput. Phys. 186, 545 (2003)] (and GENE [Jenko et al., Phys. Plasmas 7, 1904 (2000)]) 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 [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 q0>qmin>1.

In a continuing effort to validate turbulent transport models, the electron energy flux has been probed as a function of electron temperature gradient on the DIII-D tokamak. In the scan of gradient, a critical electron temperature gradient has been found in the electron heat fluxes and stiffness at various radii in L-mode plasmas. The TGLF reduced turbulent transport model (Staebler et al 2007 Phys. Plasmas 14 055909) and full gyrokinetic GYRO model (Candy and Waltz 2003 J. Comput. Phys. 186 545) recover the general trend of increasing electron energy flux with increasing electron temperature gradient scale length, but they do not predict the absolute level of transport at all radii and gradients. Comparing the experimental observations of incremental (heat pulse) diffusivity and stiffness to the models’ reveals that TGLF reproduces the trends in increasing diffusivity and stiffness with increasing electron temperature gradient scale length with a critical gradient behavior. The critical gradient of TGLF is found to have a dependence on q95, contrary to the independence of the experimental critical gradient from q95.

Recent EAST/DIII-D joint experiments on the high poloidal beta tokamak regime in DIII-D have demonstrated fully noninductive operation with an internal transport barrier (ITB) at large minor radius, at normalized fusion performance increased by. 30% relative to earlier work (Politzer et al 2005 Nucl. Fusion 45 417). The advancement was enabled by improved understanding of the 'relaxation oscillations', previously attributed to repetitive ITB collapses, and of the fast ion behavior in this regime. It was found that the 'relaxation oscillations' are coupled core-edge modes amenable to wall-stabilization, and that fast ion losses which previously dictated a large plasma-wall separation to avoid wall over-heating, can be reduced to classical levels with sufficient plasma density. By using optimized waveforms of the plasma-wall separation and plasma density, fully noninductive plasmas have been sustained for long durations with excellent energy confinement quality, bootstrap fraction >= 80%, beta(N) <= 4, beta(P) >= 3, and beta(T) >= 2%. These results bolster the applicability of the high poloidal beta tokamak regime toward the realization of a steady-state fusion reactor.

An electrostatic coherent mode near the electron diamagnetic frequency (20-90 kHz) is observed in the steep-gradient pedestal region of long pulse H-mode plasmas in the Experimental Advanced Superconducting Tokamak, using a newly developed dual gas-puff-imaging system and diamond-coated reciprocating probes. The mode propagates in the electron diamagnetic direction in the plasma frame with poloidal wavelength of ∼8  cm. The mode drives a significant outflow of particles and heat as measured directly with the probes, thus greatly facilitating long pulse H-mode sustainment. This mode shows the nature of dissipative trapped electron mode, as evidenced by gyrokinetic turbulence simulations.

A flux-tube model is implemented in the global turbulence code GEM [Y. Chen and S. E. Parker, J. Comput. Phys. 220, 839 (2007)] in order to facilitate benchmarking with Eulerian codes. The global GEM assumes the magnetic equilibrium to be completely given. The initial flux-tube implementation simply selects a radial location as the center of the flux-tube and a radial size of the flux-tube, sets all equilibrium quantities (B, ∇B, etc.) to be equal to the values at the center of the flux-tube, and retains only a linear radial profile of the safety factor needed for boundary conditions. This implementation shows disagreement with Eulerian codes in linear simulations. An alternative flux-tube model based on a complete local equilibrium solution of the Grad-Shafranov equation [J. Candy, Plasma Phys. Controlled Fusion 51, 105009 (2009)] is then implemented. This results in better agreement between Eulerian codes and the particle-in-cell (PIC) method. The PIC algorithm based on the v||-formalism [J. Reynders, Ph.D. dissertation, Princeton University, 1992] and the gyrokinetic ion/fluid electron hybrid model with kinetic electron closure [Y. Chan and S. E. Parker, Phys. Plasmas 18, 055703 (2011)] are also implemented in the flux-tube geometry and compared with the direct method for both the ion temperature gradient driven modes and the kinetic ballooning modes.

Global and local transport experiments in ohmic, L-mode and H-mode regimes on the Alcator C-Mod tokamak are summarized. For ohmic plasmas, earlier results derived for energy confinement scaling in the Alcator (linear) regime have been confirmed, and the saturated confinement regime has been shown to be equivalent to that of L-mode. For auxiliary heated regimes, C-Mod provided a unique laboratory to test the standard scaling laws that had been previously derived. C-Mod's L-mode performance matches the L-mode scaling laws quite well, but the confinement times in H-mode were about 50% above the existing H-mode scaling laws. This difference was significant and pointed up shortcomings in the range and conditioning of the existing database. H-mode studies emphasize quasi-steady regimes with good energy confinement, no impurity accumulation, and no large edge-localized modes. A new H-mode regime, where the pedestal is regulated by a continuous quasi-coherent mode, has been investigated extensively. The regime is most accessible at higher safety factor, triangularity, and collisionality and at low ion mass, suggesting that the mode is a form of resistive ballooning. Studies on C-Mod first showed the quantitative link between edge temperatures, core temperature gradients, and core confinement. This link unified L-mode and H-mode and established a strong connection between local and global transport. Further work on the role of critical gradient lengths and marginal stability lent quantitative support to the ion temperature gradient theories for ion transport and have helped elucidate nonlinear saturation mechanisms for the turbulence. Local transport studies demonstrated connections between transport channels, with energy, particle, and momentum transport varying across regimes in similar ways. Experiments carried out in collaboration with the DIII-D, ASDEX-U, and JET groups confirmed the dimensionless scaling approach over the widest available range in machine sizes. These studies suggest that plasma physics is the dominant influence on transport in the core and pedestal for standard L- and H-mode discharges. Dimensionless scaling experiments have shown a strong improvement in confinement with the normalized gyro size (1/rho*). Confinement was found to be Bohm-like in L-mode and gyro-Bohm-like in H-mode. These experiments also showed a strong degradation in confinement with collisionality.

S. Scott 1), A. Bader 2), M. Bakhtiari 3), N. Basse 4), W. Beck 2), T. Biewer 5), S. Bernabei 1), P. Bonoli 2), B. Bose 2), R. Bravenec 6), I. Bespamyatnov 6), R. Childs 2), I. Cziegler 2), R. Doerner 7), E. Edlund 2), D. Ernst 2), A. Fasoli 8), M. Ferrara 2), C. Fiore 2), T. Fredian 2), A. Graf 9), T. Graves 2), R. Granetz 2), N. Greenough 1), M. Greenwald 2), M. Grimes 2), O. Grulke 10), D. Gwinn 11), R. Harvey 12), S. Harrison 3), T. C. Hender 13), J. Hosea 1), D. F. Howell 13), A. E. Hubbard 2), J. W. Hughes 2), I. Hutchinson 2), A. Ince-Cushman 2), J. Irby 2), T. Jernigan 5), D. Johnson 2), J. Ko 2), P. Koert 2), B. LaBombard 2), A. Kanojia 2), L. Lin 2), Y. Lin 2), B. Lipschultz 2), J. Liptac 2), A. Lynn 14), P. MacGibbon 2), E. Marmar 2), K. Marr 2), M. May 15), D. R. Mikkelsen 1), R. McDermott 2), A. Parisot 2), R. Parker 2), C. K. Phillips 1), P. Phillips 6), M. Porkolab 2), M. Reinke 2), J. Rice 2), W. Rowan 6), M. Sampsell 6), G. Schilling 1), A. Schmidt 2), N. Smick 3), A. Smirnov 12), J. Snipes 2), D. Stotler 1), J. Stillerman 2), V. Tang 15), D. Terry 2), J. Terry 2), M. Ulrickson 16), R. Vieira 2), G. Wallace 2), D. Whyte 2), J. R. Wilson 1), G. Wright 3), J. Wright 2), S. Wolfe 2), S. Wukitch 2), G. Wurden 17), H. Yuh 18), K. Zhurovich 2), J. Zaks 2), S. Zweben 1)

This paper summarizes highlights of research results from the Alcator C-Mod tokamak covering the period 2006–2008. Active flow drive, using mode converted ion cyclotron waves, has been observed for the first time in a tokamak plasma, using a mix of D and 3He ion species; toroidal and poloidal flows are driven near the location of the mode conversion layer. ICRF induced edge sheaths are implicated in both the erosion of thin boron coatings and the generation of metallic impurities. Lower hybrid range of frequencies (LHRF) microwaves have been used for efficient current drive, current profile modification and toroidal flow drive. In addition, LHRF has been used to modify the H-mode pedestal, increasing temperature, decreasing density and lowering the pedestal collisionality. Studies of hydrogen isotope retention in solid metallic plasma facing components reveal significantly higher retention than expected from ex situ laboratory studies; a model to explain the results, based on plasma/neutral induced lattice damage, has been developed and tested. During gas-puff mitigation of disruptions, induced MHD instabilities cause the magnetic field to become stochastic, resulting in reduction of halo currents, spreading of plasma power loading and loss of runaway electrons before they cause damage. Detailed pedestal rotation profile measurements have been used to infer E r profiles, and correlation with global H-mode confinement. An improved L-mode regime, obtained at q 95 ⩽ 3 with ion drift away from the active X-point, shows very good energy confinement with a strong temperature pedestal, a weak density pedestal, and no evidence of particle or impurity accumulation, without the need for ELMs or any additional edge density regulation mechanism.

Global and local transport experiments in Ohmic, L-mode and H-mode regimes on the Alcator CMod tokamak are summarized. For Ohmic plasmas, earlier results derived for energy confinement scaling in the Alcator (linear) regime have been confirmed and the saturated confinement regime has been shown to be equivalent to L-mode. For auxiliary heated regimes, C-Mod provided a unique laboratory to test the standard scaling laws which had been previously derived. C-Mod’s L-mode performance matches the L-mode scaling laws quite well, but the confinement times in H-mode were about 50% above the existing H-mode scaling laws. This difference was significant and pointed up shortcomings in the range and conditioning of the existing database. H-mode studies emphasize quasi-steady regimes with good energy confinement, no impurity accumulation and no large ELMs. A new H-mode regime, where the pedestal is regulated by a continuous quasi-coherent mode has been investigated extensively. The regime is most accessible at higher safety factor, triangularity, collisionality and at low ion mass, suggesting that the mode is a form of resistive ballooning. Studies on C-Mod first showed the quantitative link between edge temperatures, core temperature gradients and core confinement. This link unified L-mode and H-mode and established a strong connection between local and global transport. Further work on the role of critical gradient lengths and marginal stability lent quantitative support to the ITG theories for ion transport and have helped elucidate nonlinear saturation mechanisms for the turbulence. Local transport studies demonstrated connections between transport channels, with energy, particle and momentum transport varying across regimes in similar ways. Experiments carried out in collaboration with the DIII-D, ASDEX-U and JET groups confirmed the dimensionless scaling approach over the widest available range in machine sizes. These studies suggest that plasma physics is the dominant influence on transport in the core and pedestal for standard Land H-mode discharges. Dimensionless scaling experiments have shown a strong improvement in confinement with the normalized gyro-size (1/ρ*). Confinement was found to be Bohm-like in L-mode and gyro-Bohm-like in H-mode. These experiments also showed a strong degradation in confinement with collisionality. Other articles in this issue discuss impurity transport, momentum transport, H-mode pedestal and threshold physics and internal transport barrier regimes.

Global and local transport experiments in Ohmic, L-mode and H-mode regimes on the Alcator CMod tokamak are summarized. For Ohmic plasmas, earlier results derived for energy confinement scaling in the Alcator (linear) regime have been confirmed and the saturated confinement regime has been shown to be equivalent to L-mode. For auxiliary heated regimes, C-Mod provided a unique laboratory to test the standard scaling laws which had been previously derived. C-Mod’s L-mode performance matches the L-mode scaling laws quite well, but the confinement times in H-mode were about 50% above the existing H-mode scaling laws. This difference was significant and pointed up shortcomings in the range and conditioning of the existing database. H-mode studies emphasize quasi-steady regimes with good energy confinement, no impurity accumulation and no large ELMs. A new H-mode regime, where the pedestal is regulated by a continuous quasi-coherent mode has been investigated extensively. The regime is most accessible at higher safety factor, triangularity, collisionality and at low ion mass, suggesting that the mode is a form of resistive ballooning. Studies on C-Mod first showed the quantitative link between edge temperatures, core temperature gradients and core confinement. This link unified L-mode and H-mode and established a strong connection between local and global transport. Further work on the role of critical gradient lengths and marginal stability lent quantitative support to the ITG theories for ion transport and have helped elucidate nonlinear saturation mechanisms for the turbulence. Local transport studies demonstrated connections between transport channels, with energy, particle and momentum transport varying across regimes in similar ways. Experiments carried out in collaboration with the DIII-D, ASDEX-U and JET groups confirmed the dimensionless scaling approach over the widest available range in machine sizes. These studies suggest that plasma physics is the dominant influence on transport in the core and pedestal for standard Land H-mode discharges. Dimensionless scaling experiments have shown a strong improvement in confinement with the normalized gyro-size (1/ρ*). Confinement was found to be Bohm-like in L-mode and gyro-Bohm-like in H-mode. These experiments also showed a strong degradation in confinement with collisionality. Other articles in this issue discuss impurity transport, momentum transport, H-mode pedestal and threshold physics and internal transport barrier regimes.

Research on the Alcator C-Mod tokamak has emphasized RF heating, self-generated flows, momentum transport, scrape-off layer turbulence and transport and the physics of transport barrier transitions, stability and control. The machine operates with PRF up to 6 MW corresponding to power densities on the antenna of 10 MW/m. Analysis of rotation profile evolution, produced in the absence of external drive, allows transport of angular momentum in the plasma core to be computed and compared between various operating regimes. Momentum is clearly seen diffusing and convecting from the plasma edge on time scales similar to the energy confinement time and much faster than neo-classical transport. Scrape-off layer (SOL) turbulence and transport have been studied with fast scanning electrostatic probes, situated at several poloidal locations and with gas puff imaging. Strong poloidal asymmetries are found in profiles and fluctuations, confirming the essential ballooning character of the turbulence and transport. Plasma topology has a dominant effect on the magnitude and direction of both core rotation and SOL flows. The correlation of self-generated plasma flows and topology has led to a novel explanation for the dependence of the H-mode power threshold on the ∇B drift direction. Research into internal transport barriers (ITB) has focused on control of the barrier strength, and location. The foot of the barrier could be moved to larger minor radius by lowering q or BT. The barriers, which are produced in C-Mod by off-axis RF heating, can be weakened by the application of on-axis power. Gyro-kinetic simulations suggest that the control mechanism is due to the temperature dependence of trapped electron modes (TEM) which are destabilized by the large density gradients. A set of non-axisymmetric coils was installed allowing intrinsic error fields to be measured and compensated. These also enabled the determination of the mode locking threshold and, by comparison with data from other machines, provided the first direct measurement of size scaling for the threshold. The installation of a new inboard limiter resulted in the reduction of halo currents following disruptions. This effect can be understood in terms of the change in plasma contact with the altered geometry during vertical displacement of the plasma column. Unstable Alfven eigenmodes (AE) were observed in low-density high-power ICRF heated plasmas. The damping rate of stable AEs was investigated with a pair of active MHD antennae.

A motional Stark effect (MSE) instrument to measure q(r) using the Torus Experiment for Technology (TEXT) neutral beam on the Alcator C-Mod tokamak has been designed and installed. The neutral beam is on the midplane, aimed radially, and designed to operate in hydrogen at 50 keV. The MSE optics view the outer half of the plasma −0.3<(R−R0)/a<1.05. This geometry results in a spatial resolution of 1 (edge)–3 (center) cm from a beam apertured horizontally to 2 cm. Estimates of the signal levels indicate for Hα that photoelectron statistical errors in the measurement of Bθ/Bφ can be less than 0.2% for ne0<2.0×1014 cm−3 similar to other MSE instruments. The collection optics reside within the vacuum chamber reflecting and imaging the neutral beam through a vacuum window in the neutral beam port. The optics are designed so that beam emission spectroscopy can be carried out simultaneously with MSE measurements. Optical signals are transmitted to remote detectors by fiber optics. Low Verdet glass and dielectric mirrors are used to minimize polarization changes caused by the ambient fields. Polarization is measured by conventional techniques using photoelastic modulators, optical filters, and photomultipliers.

Research on the Alcator C-Mod tokamak is focused on exploiting compact high density plasmas to understand core transport and heating, the physics of the H mode transport barrier, and the dynamics of the scrape-off layer and divertor. Rapid toroidal acceleration of the plasma core is observed during ohmic heated H modes and indicates a momentum pinch or similar transport mechanism. Core thermal transport observations support a critical gradient interpretation, but with gradients that disagree with present theoretical values. High resolution measurements of the H mode barrier have been obtained, including impurity and neutral densities, and the instability apparently responsible for the favourable 'enhanced D alpha' regime has been identified. Divertor bypass dynamic control experiments have directly addressed the important questions surrounding main chamber recycling and the effect of divertor closure on impurities and confinement. Future plans include quasi-steady-state advanced tokamak plasmas using lower hybrid current drive.

An analysis of tokamak density fluctuations data permits the determination of two characteristic exponents. The exponents correspond to the powers of a power-law dependence of the distributions of the long-lasting monotonic change (“flight”) of the density and the time length of these changes. Speculation based on these results leads to construction of the fractional kinetic equation for the distribution function of the flights. The asymptotic transport properties of the particle density distribution function are directly connected with the exponents obtained from the density fluctuations data.

Both the dynamic and equilibrium thermal responses of an L mode plasma to repetitive ECH heat pulses were measured and compared with predictions from several thermal transport models. While no model consistently agreed with all observations, the GLF23 model was most consistent with the perturbed electron and ion temperature responses for one of the cases studied, which may indicate a key role played by electron modes in the core of these discharges. Generally, the IIF and MM models performed well for the perturbed electron response while the GLF23 and IFS/PPPL models agreed with the perturbed ion response for all three cases studied. No single model agreed well with the equilibrium temperature profiles measured.

A motional Stark effect (MSE) instrument is being installed on the Alcator C-Mod tokamak at MIT. This MSE diagnostic will provide measurements of the spatial profile of the internal poloidal magnetic field. The MSE has its primary collection optics inside the vacuum vessel. The light collected by the internal optics passes through a vacuum window and is relayed to a fiber optic array. The MSE optics are shared by a beam emission spectroscopy (BES) diagnostic which measures electron density fluctuations and their spatial correlations. This optical system requires high throughput and spatial resolution of less than 1 cm at the focal plane in the plasma. The design requirements for the internal optics also include the effects associated with plasma impingement, plasma disruptions, and thermal excursions.