G. M. Staebler

A gyro-Landau-fluid transport model

A physically comprehensive and theoretically based transport model tuned to three-dimensional (3-D) ballooning mode gyrokinetic instabilities and gyrofluid nonlinear turbulence simulations is formulated with global and local magnetic shear stabilization and E×B rotational shear stabilization. Taking no fit coefficients from experiment, the model is tested against a large transport profile database with good agreement. This model is capable of describing enhanced core confinement transport barriers in negative central shear discharges based on rotational shear stabilization. The model is used to make ignition projections from relative gyroradius scaling discharges.

A theory-based transport model with comprehensive physics

A new theory-based transport model with comprehensive physics (trapping, general toroidal geometry, fully electromagnetic, electron-ion collisions, impurity ions) has been developed. The core of the model is the new trapped-gyro-Landau-fluid (TGLF) equations, which provide a fast and accurate approximation to the linear eigenmodes for gyrokinetic drift-wave instabilities (trapped ion and electron modes, ion and electron temperature gradient modes, and kinetic ballooning modes). The new TGLF transport model is more accurate, and has an extended range of validity, compared to its predecessor GLF23. The TGLF model unifies trapped and passing particles in a single set of gyro-Landau-fluid equations. A model for the averaging of the Landau resonance by the trapped particles makes the equations work seamlessly over the whole drift-wave wave-number range from trapped ion modes to electron temperature gradient modes. A fast eigenmode solution method enables unrestricted magnetic geometry. The transport model uses...

Measurements of core electron temperature and density fluctuations in DIII-D and comparison to nonlinear gyrokinetic simulations

For the first time, profiles (0.3<ρ<0.9) of electron temperature and density fluctuations in a tokamak have been measured simultaneously and the results compared to nonlinear gyrokinetic simulations. Electron temperature and density fluctuations measured in neutral beam-heated, sawtooth-free low confinement mode (L-mode) plasmas in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] are found to be similar in frequency and normalized amplitude, with amplitude increasing with radius. The measured radial profile of two fluctuation fields allows for a new and rigorous comparison with gyrokinetic results. Nonlinear gyrokinetic flux-tube simulations predict that electron temperature and density fluctuations have similar normalized amplitudes in L-mode. At ρ=0.5, simulation results match experimental heat diffusivities and density fluctuation amplitude, but overestimate electron temperature fluctuation amplitude and particle diffusivity. In contrast, simulations at ρ=0.75 do not match either the experimentally de...

Gyro-Landau fluid equations for trapped and passing particles

A new system of gyro-Landau fluid (GLF) equations for tokamak plasmas is presented. The new equations include both trapped particles, which can average the Landau resonance, and passing particles which do have a Landau resonance. The trap GLF (TGLF) model is unrestricted in trapped fraction or perpendicular wave number of the electrostatic perturbation. The linearly unstable eigenmodes of the TGLF equations include low-frequency trapped ion modes all the way up to high-frequency electron temperature gradient driftwaves. Extensive benchmarking of the linear TGLF eigenmodes with a large database of gyrokinetic linear stability calculations verifies that the TGLF model is accurate over the full range of plasma parameters tested.

Improved core fueling with high field side pellet injection in the DIII-D tokamak

The capability to inject deuterium pellets from the magnetic high field side (HFS) has been added to the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. It is observed that pellets injected from the HFS lead to deeper mass deposition than identical pellets injected from the outside midplane, in spite of a factor of 4 lower pellet speed. HFS injected pellets have been used to generate peaked density profile plasmas [peaking factor (ne(0)/〈ne〉) in excess of 3] that develop internal transport barriers when centrally heated with neutral beam injection. The transport barriers are formed in conditions where Te∼Ti and q(0) is above unity. The peaked density profiles, characteristic of the internal transport barrier, persist for several energy confinement times. The pellets are also used to investigate transport barrier physics and modify plasma edge conditions. Transitions from L- to H-mode have been triggered by pellets, effectively lowering the H-mode threshold power by 2.4 MW. Pel...

ITER predictions using the GYRO verified and experimentally validated trapped gyro-Landau fluid transport model

The trapped gyro-Landau fluid (TGLF) transport model computes the quasilinear particle and energy driftwave fluxes in tokamaks with shaped geometry, finite aspect ratio and collisions. The TGLF particle and energy fluxes have been successfully verified against a large database of collisionless nonlinear gyrokinetic simulations using the GYRO code. Using a new collision model in TGLF, we find remarkable agreement between the TGLF quasilinear fluxes and 64 new GYRO nonlinear simulations with electron–ion collisions. In validating TGLF against DIII-D and JET H-mode and hybrid discharges we find the temperature and density profiles are in excellent agreement with the measured profiles. ITER projections using TGLF show that the fusion gains are somewhat more pessimistic than the previous GLF23 results primarily due to finite aspect ratio effects included only in TGLF. The synergistic effects of density peaking, finite β and E × B shear due to finite toroidal rotation lead to significant increases in fusion power above a reduced physics ITER base case. The TGLF results for ITER are confirmed using nonlinear GYRO simulations in place of TGLF to predict the temperature profiles within the TGYRO transport code. These results represent a snapshot of the ongoing effort to improve the TGLF model, validate it against experimental data, and make predictions for ITER.

L-mode validation studies of gyrokinetic turbulence simulations via multiscale and multifield turbulence measurements on the DIII-D tokamak

A series of carefully designed experiments on DIII-D have taken advantage of a broad set of turbulence and profile diagnostics to rigorously test gyrokinetic turbulence simulations. In this paper the goals, tools and experiments performed in these validation studies are reviewed and specific examples presented. It is found that predictions of transport and fluctuation levels in the mid-core region (0.4 < ρ < 0.75) are in better agreement with experiment than those in the outer region (ρ ≥ 0.75) where edge coupling effects may become increasingly important and multiscale simulations may also be necessary. Validation studies such as these are crucial in developing confidence in a first-principles based predictive capability for ITER.

The first transport code simulations using the trapped gyro-Landau-fluid model

The first transport code simulations using the newly developed trapped gyro-Landau-fluid (TGLF) theory-based transport model are presented. TGLF has comprehensive physics to approximate the turbulent transport due to drift-ballooning modes in tokamaks. The TGLF model is a next generation gyro-Landau-fluid model that improves the accuracy of the trapped particle response and the finite Larmor radius effects compared to its predecessor, GLF23. The model solves for the linear eigenmodes of trapped ion and electron modes, ion and electron temperature gradient modes, and electromagnetic kinetic ballooning modes in either shifted circle or shaped geometry. A database of over 400 nonlinear gyrokinetic simulations using the GYRO code has been created. A subset of 83 simulations with shaped geometry has been used to find a model for the saturation levels. Using a simple quasilinear (QL) saturation rule, remarkable agreement with the energy and particle fluxes from a wide variety of GYRO simulations is found for bo...

Non-dimensional scaling of turbulence characteristics and turbulent diffusivity

Plasma turbulence characteristics, including radial correlation lengths, decorrelation times, amplitude profile and flow velocity, have been measured during a ρ* scan on DIII-D while all other transport relevant dimensionless quantities (e.g., β, ν*, κ, q, Te/Ti) are held nearly constant. The turbulence is measured by examining the correlation properties of the local long wavelength (k⊥ρi ≤ 1) density fluctuations, measured with beam emission spectroscopy. The radial correlation length of the turbulence Lc,r is shown to scale with the local ion gyroradius, Lc,r ≈ 5ρi, while the decorrelation times scale with the local acoustic velocity as τc~a/cs. The turbulent diffusivity parameter, D~(Lc,r2/τc), scales in a roughly gyro-Bohm-like fashion, as predicted by the gyrokinetic equations governing turbulent transport. The experimental one fluid power balance heat diffusivity scaling and that from GLF23 modelling compare reasonably well.

Electron heat transport in improved confinement discharges in DIII-D

In DIII-D tokamak plasmas with an internal transport barrier (ITB), the comparison of gyrokinetic linear stability (GKS) predictions with experiments in both low and strong negative magnetic shear plasmas provide improved understanding for ion and electron thermal transport within much of the plasma. As previously reported, the region for improved ion transport seems well characterized by the condition OE~B>Y-, where SERB is the ExB flow shear, calculated from measured quantities, and y,, is the maximum linear growth rate for ion temperature gradient (ITG) modes in the absence of flow shear. Within a limited region just inside the ITB, the electron temperature gradient (ETG) modes appear to control the electron temperature gradient and, consequently, the electron thermal transport. The increase in electron temperature gradient with more strongly negative magnetic shear is consistent with the increase in the ETG mode marginal gradient. Closer to the magnetic axis the Te profile flattens and the ETG modes are predicted to be stable. With additional core electron heating, FIR scattering measurements near the axis show the presence of high k fluctuations (12 cm-l), rotating in the electron diamagnetic drift direction. This turbulence could impact electron transport and possibly also ion transport. Thermal diffusivities for electrons, and to a lesser degree ions, increase. The ETG mode can exist at this wavenumber, but it is computed to be robustly stable near the axis.