A systematic, constructive and self-consistent procedure to quantify nonlocal, nondiffusive action at a distance in plasma turbulence is exposed and applied to turbulent heat fluxes computed from the state-of-the-art full- f, flux-driven gyrokinetic GYSELA and XGC1 codes. A striking commonality is found: heat transport below a dynamically selected mesoscale has the structure of a Lévy distribution, is strongly nonlocal, nondiffusive, scale-free, and avalanche mediated; at larger scales, we report the observation of a self-organized flow structure which we call the " E × B staircase" after its planetary analog.

The fundamental properties of steep neoclassical plasma pedestals in a quiescent tokamak plasma have been investigated with a new guiding center particle code XGC: an X-point included Guiding Center code. It is shown that the width of the steepest neoclassical pedestals is similar to an experimentally observed edge pedestal width, and that a steep pedestal must be accompanied by a self-consistent negative radial electric field well. It is also shown that a steep neoclassical pedestal can form naturally at a quiescent diverted edge as the particle source from the neutral penetration (and heat flux from the core plasma) is balanced by the sharply increasing convective ion loss toward the separatrix. The steep neoclassical pedestal and the strong radial electric field well are suppressed by an anomalous diffusion coefficient of a strength appropriate to an L-mode state; nonetheless, the E×B shearing rate increases rapidly with pedestal temperature. Additionally, the present study shows that a steep pedestal at the diverted edge acts as a cocurrent parallel momentum source.

The XGC1 edge gyrokinetic code is used to study the width of the heat-flux to divertor plates in attached plasma condition. The flux-driven simulation is performed until an approximate power balance is achieved between the heat-flux across the steep pedestal pressure gradient and the heat-flux on the divertor plates. The simulation results compare well against the empirical scaling λ q ∝1/B P γ obtained from present tokamak devices, where λ q is the divertor heat-flux width mapped to the outboard midplane, γ=1.19 as found by T. Eich et al. [Nucl. Fusion 53 (2013) 093031], and B P is the magnitude of the poloidal magnetic field at the outboard midplane separatrix surface. This empirical scaling predicts λ q ≲1mm when extrapolated to ITER, which would require operation with very high separatrix densities (n sep /n Greenwald > 0.6) [Kukushkin, A. et al., Jour. Nucl. Mat. 438 (2013) S203] in the Q=10 scenario to achieve semi-detached plasma operation and high radiative fractions for acceptable divertor power fluxes. Using the same simulation code and technique, however, the projected λ q for ITER's model plasma is 5.9 mm, which could be suggesting that operation in the ITER Q=10 scenario with acceptable divertor power loads may be obtained over a wider range of plasma separatrix densities and radiative fractions. The physics reason behind this difference is, according to the XGC1 results, that while the ion magnetic drift contribution to the divertor heat-flux width is wider in the present tokamaks, the turbulent electron contribution is wider in ITER. Study will continue to verify further this important projection. A high current C-Mod discharge is found to be in a mixed regime: While the heat-flux width by the ion neoclassical magnetic drift is still wider than the turbulent electron heat-flux width, the heatflux magnitude is dominated by the narrower electron heat-flux.

An overview of the physics of intrinsic torque is presented, with special emphasis on the phenomenology of intrinsic toroidal rotation in tokamaks, its theoretical understanding, and the variety of momentum transport bifurcation dynamics. Ohmic reversals and ECH-driven counter torque are discussed in some detail. Symmetry breaking by LSN vs. USN asymmetry is related to the origin of intrinsic torque at the separatrix.

Global electrostatic ITG turbulence physics, together with background dynamics, has been simulated in realistic tokamak core geometry using XGC1, a full-function 5D gyrokinetic particle code. Adiabatic electron model has been used. Some verification exercises of XGC1 have been presented. The simulation volume extends from the magnetic axis to the pedestal top inside the magnetic separatrix. Central heating is applied, and a number, momentum, and energy conserving linearized Monte-Carlo Coulomb collision is used. In the turbulent region, the ion temperature gradient profile self-organizes globally around R/L T = (Rd log T /dr = major radius on magnetic axis/temperature gradient length) ≃ 6.5 − 7, which is somewhat above the conventional nonlinear criticality of ≃ 6. The self-organized ion temperature gradient profile is approximately stiff against variation of heat source magnitude. Results indicate that the relaxation to a self-organized state proceeds in two phases,namely a transient phase of excessively bursty transport followed by a 1/f avalanching phase. The bursty behaviors are allowed by the quasi-periodic collapse of local E × B shearing barriers.

Article:Ku, S., Chang, C. S., Hager, R. et al. (9 more authors) (2018) A fast low-to-high confinement mode bifurcation dynamics in the boundary-plasma gyrokinetic code XGC1. Physics of Plasmas. 056107. ISSN 1089-7674 https://doi.org/10.1063/1.5020792 eprints@whiterose.ac.uk https://eprints.whiterose.ac.uk/ Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request.A fast low-to-high confinement mode bifurcation dynamics in the boundary-plasma gyrokinetic code XGC1 A fast low-to-high confinement mode bifurcation dynamics in the boundary-plasma gyrokinetic code XGC1 A fast edge turbulence suppression event has been simulated in the electrostatic version of the gyrokinetic particle-in-cell code XGC1 in a realistic diverted tokamak edge geometry under neutral particle recycling. The results show that the sequence of turbulent Reynolds stress followed by neoclassical ion orbit-loss driven together conspire to form the sustaining radial electric field shear and to quench turbulent transport just inside the last closed magnetic flux surface. The main suppression action is located in a thin radial layer around w N ' 0:96-0:98, where w N is the normalized poloidal flux, with the time scale $0:1ms.Published by AIP Publishing. https://doi

A new nonambipolar neoclassical transport (X-transport) mechanism has been identified which can be an irreducible baseline source of a strong radial electric field and edge pedestal formation immediately inside the separatrix in a diverted tokamak. Due to the vanishingly small poloidal magnetic field in the vicinity of a divertor X-point, there exists an ion velocity space hole at the thermal energy level. This becomes a source of a nonambipolar, collisional, convective radial transport of plasma ions, as the ions scatter into and out of the loss hole in the vicinity of an X-point. The widths of the Er and edge pedestal layers are somewhat smaller than the ion poloidal gyroradius, measured at the midplane. A simple estimate shows that the X-transport rate can be significant enough to influence a high mode transition and edge pedestal formation.

Spontaneous rotation sources in a quiescent tokamak edge plasma are studied without an external momentum source, such as, beam injected or wall-born neutrals. Discussions are based upon example neoclassical solutions from an edge gyrokinetic particle code. The main study is performed in a DIII-D plasma [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] with the ion Grad-B drift directed toward the single-null divertor. Comparison with a reversed Grad-B drift case is also shown. It is found that there is a robust spontaneous co-current toroidal plasma rotation source in the far scrape-off plasma due to the wall sheath effect. As the edge pedestal width becomes narrower, the co-current rotation in the far scrape-off becomes weaker, but there appears a stronger co-current rotation in the pedestal top/shoulder from the X-point orbit loss effect, possibly providing a co-rotation boundary condition to the core plasma. Reversal of the magnetic field and plasma current brings down the overall co-rotation, especially in the far scrape-off plasma.