Session CO5: Core and Edge Transport

Pedestal Turbulence: Microtearing in the Presence of RMPs

One of the key turbulence regimes in present-day tokamak pedestals is driven by the microtearing (MT) instability, allowing density and temperature profiles to evolve independently, thus distinguishing it from MHD-like regimes, where profiles are coupled. Based on the identification of quasi-coherent modes on DIII-D as signatures of MT turbulence, we investigate by means of nonlinear gyrokinetic simulations the impact of external resonant magnetic perturbations (RMPs) on this regime, applying a constant-in-time perturbation at the experimentally relevant length scale. We separate the dynamical impact on the MT from the island-induced flutter. Results are compared to scenarios where RMPs interact with electrostatic modes, in particular trapped-electron mode turbulence.

Additionally, we present progress on collisionless microtearing theory – where access to free energy is provided by the ∇B curvature resonance rather than collisionality – with a range of applications, including reversed-field pinches and the Pegasus spherical tokamak, where MT turbulence has been identified as an important player [D.R. Smith, TTF 2018].   

Gyrokinetic Landau collision operator in conservative form

A gyrokinetic linearized exact (not model) Landau collision operator is derived by transforming the symmetric and conservative Landau form. The formulation obtains the velocity space current density and preserves the conservative form as the divergence of this current density. The operator contains both test-particle and field-particle contributions, and finite gyroradius effects are evaluated in either Bessel function series or gyrophase integrals. While equivalent to the gyrokinetic Fokker-Planck form with the Rosenbluth potentials [B. Li and D. R. Ernst, Phys. Rev. Lett. 106, 195002 (2011)], the gyrokinetic Landau form explicitly preserves the symmetry between test-particle and field-particle contributions, thus automatically satisfies the conservation laws and H-theorem. The operator in conservative form is being implemented in the gyrokinetic GENE code with a finite-volume method that preserves the discrete conservation laws. The gyrophase integrals for finite gyroradius effects can be pre-computed with an adaptive integration program. While model field terms may conserve the first three velocity moments, their local effects in velocity space have little connection with the exact Landau operator, altering classical transport.   

Global gyrokinetic simulations of Alfvén modes in ITER plasmas.

The global gyrokinetic code ORB5 [S. Jolliet et al., Comp. Phys. Comm., 177, 409 (2007)] can simultaneously include electromagnetic perturbations, shaped MHD axisymmetric equilibria, zonal-flow preserving sources, collisions, and the ability to solve the full core plasma including the magnetic axis. Multiple ion species are described by gyrokinetic equations, while a drift-kinetic model is generally used for the electrons. Recently, the algorithm of  the electromagnetic solver has been rewritten, based on the so-called mixed-variable formulation of the gyrokinetic theory [A. Mishchenko et al., Phys. Plasmas 21, 052113 (2014)]. The resulting code  allows for significantly larger time steps even for experimentally relevant conditions. It has been applied to different Alfénic physics problems and verified against other  existing codes. In this work, we focus on the role of Alfvén instabilities, driven unstable  by an energetic particle population, in the standard ITER 15MA scenario (high-beta equilibrium)  with realistic density and temperature profiles for thermal ions, electrons  and energetic particles. The results obtained by those costly global simulations are particularly valuable for the validation of simpler analytical or hybrid-kinetic models.

  

Investigation of Multiscale Turbulence in DIII-D ITER Baseline Discharges

The role of ion-scale (ITG/TEM) and electron-scale (high-k TEM/ETG) turbulence and their coupling in reactor-relevant conditions has been investigated in DIII-D ITER baseline discharges. Dedicated experiments used localized ECH to stimulate conditions exhibiting cross-scale coupling and a suite of turbulence diagnostics (BES, DBS, PCI) were employed to provide an ideal test bed for understanding cross-scale coupling and for validating gyrokinetic simulation. Changes in profiles which favor cross-scale coupling and possible density fluctuation signatures were documented in the wavenumber spectrum of intermediate-k (k*rho_s ~ 2.5-5.0) fluctuations measured with DBS. Ion, electron, and multiscale gyrokinetic simulations using the CGYRO code were performed using all experimental inputs. Single scale simulations reveal that low-k turbulence cannot explain experimental electron heat fluxes, but that inclusion of high-k will likely resolve this discrepancy. Comparison of simulated and measured spectra, ion and electron heat fluxes, and impurity transport will be presented as part of an ongoing effort to validate the gyrokinetic model and to understand the nature of cross-scale coupling in reactor relevant conditions.   

On the Transport Physics of the Density Limit

Proximity to the density limit is characterized by degraded particle confinement, rise in edge turbulence and collapse of zonal flows. We present extensive theoretical work (Hajjar et al. 2018, Phys. Plasmas 25, 062306) which shows that the key parameter governing the flow collapse transition is electron adiabaticity. We show that zonal flow production drops precipitously in the hydrodynamic electron regime, as the non-diffusive vorticity flux (i.e. residual stress) declines in proportion to adiabaticity. A simple physical argument establishes that the familiar cartoon of zonal flow production by eddy tilting fails in the hydrodynamic regime, as there causality and Reynolds stress tend to decouple. We show that reduced zonal flow production is the mechanism for particle confinement degradation in L-mode, and present a feedback loop mechanism for the cooling front. Several associated experimental studies are suggested. We also discuss the density limit in H-mode and its relation to the H to L back transition. A key element here is the invasion of the edge by turbulence, which spreads from the SOL.   

Onset of anomalous-resistivity-facilitated magnetic reconnection in an evolving current sheet.

Reconnection in high-temperature plasmas is associated with the large values of current density.  The electron-ion friction in this context is traditionally modeled with the classical Spitzer resistivity, which is very small in most laboratory and astrophysical plasmas.  We present a theory of the onset of magnetic reconnection in a current sheet through the development of anomalous resistivity in a kinetically unstable current profile.  It is found that for sufficiently low aspect ratio current sheets (L/a not too large), kinetic instabilities are triggered prior to the system becoming plasmoid unstable.  The resulting reconnection rate is determined by  a marginal stability model of anomalous resistivity, balancing flux convection and diffusion.  For typical parameters in an Ohmically heated tokamak, we find a reconnection rate in a sawtooth crash which is consistent with observations.