The mechanism for the formation and sustainment of a self-organized global profile and the 'E × B staircase' are investigated through simulations of a flux-driven ion temperature gradient (ITG) turbulence based on GKNET, a 5D global gyrokinetic code. The staircase is found to be initiated from the radially extended ITG mode structures with nearly up-down symmetry during the saturation phase, and is established as it evolves into a quasi-steady turbulence, leading to a self-organized global temperature profile and to meso-scale isomorphic profiles of the radial electric field and the temperature gradient. It is found that the quasi-regular E × B shear flow pattern is primarily originated from an even-symmetrical zonal flow produced by the extended ITG mode, which flow pattern exhibits an in-phase relation with the mean flow variation induced by the temperature relaxation. Consequently, the staircase is initiated through the profiles of total electric field and temperature gradient with a self-organized manner. Since the sign of E × B shear flow at the central part are opposite to that at both edges, it disintegrates the ITG mode into smaller scale eddies. Meanwhile, smaller scale eddies tend to be aligned radially by spontaneous phase matching, which can provide the growth of mode amplitude and the formation of radially extended mode structures, leading to the bursty heat transport. This process is repeated quasi-periodically, sustaining self-organized structures and the E × B staircase. Moreover, the equilibrium mean field is found to be of specific importance in causing the structures and dynamics from meso-to macro scales in toroidal plasmas.

The negative triangularity tokamak (NTT) is a unique reactor concept based on "powerhandling-first" philosophy with the heat exhaust problem as the leading concern. The present paper exposes a reactor concept using L-mode edge based on negative triangularity tokamak (NTT) configuration, providing merits of no (or very weak) ELMs, larger particle flux and large major radius for power handling. It is shown that a reasonably compact (R p from 9m to 7m) NTT reactor is possible by achieving higher confinement improvement (H H =1.5) and/or by utilizing reasonably higher magnetic field (B max =15.5T). Current physics basis and critical issues on its scientific and technical feasibility are discussed. 1. PHYSICS BACKGROUND OF NTT REACTOR

Nonlinear evolution of the kinetic ballooning mode (KBM) is investigated by extending the global toroidal gyrokinetic simulation code (GKNET) to an electromagnetic regime. It is found that the saturation process of KBM, which is unstable at high normalized pressure b, is significantly different from the ion temperature gradient (ITG) mode, which is unstable at low b. The KBMs get saturated by producing zonal flows and zonal magnetic fields. The production of zonal flow is weak in the initial saturation phase of KBM, which is in contrast to the ITG mode which produces strong zonal flows in the initial saturation phase. However, strong zonal flows are produced in the subsequent evolution of KBM, and a quasisteady state of KBM turbulence is established. In addition to the zonal flows, some low toroidal number modes, which are linearly stable against the KBM, dominate the KBM turbulence. The strong zonal magnetic field is also produced by the KBM. These zonal modes regulate the KBM turbulence.

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In order to realize high performance burning plasmas in magnetic-confinement fusion devices, such as tokamaks, both bulk plasma transport and that of energetic fusion alpha-particles, which result from different scale fluctuations with different free energy sources, have to be reduced simultaneously. Utilizing the advantage of global toroidal non-linear simulations covering a whole torus, here, we found a new coupling mechanism between the low-frequency micro-scale electromagnetic drift-wave fluctuations regulating the former, while the high-frequency macro-scale toroidal Alfven eigenmode (TAE) regulates the latter. This results from the dual spread of micro-scale turbulence due to the macro-scale TAE not only in wavenumber space representing local eddy size but also in configuration space with global profile variations. Consequently, a new class of turbulent state is found to be established, where the turbulence is homogenized on the poloidal cross-section with exhibiting large-scale structure, which increases fluctuation levels and then both transports, leading to deterioration in the fusion performance.

Flux-driven ion temperature gradient (ITG) turbulence and associated transport regulated by non-local and non-diffusive processes are investigated based on GKNET simulations in a global toroidal geometry. Among these processes, the instantaneous formation of radially extended quasi-coherent structure, which leads to the transport burst, is found to play an important role in causing global profile formation and relaxation. To elucidate the characteristics of such a transport process, we introduce the size probability distribution function (size-PDF) P (S) to analyze heat flux eddies in the real space, with S the eddy size, incorporated with Fourier-based approaches in spectral space. In the size-PDF to the quiescent phase, P (S) is found to be fitted by three piecewise power laws which transitions at two typical sizes, S a and S b , as P ∝ S −2/3 (S ⩽ S a ), P ∝ S −2 (S a ⩽ S ⩽ S b ), and P ∝ S −4 (S ⩾ S b ), where S a ∼ 50 and S b ∼ 200 in squared gyro-radius unit ρ 2 i for the system with a/ρ i ∼ 225 (a: the minor radius). On the other hand, the size-PDF in the bursting phase exhibits non-power-law irregular humps which corresponds to the quasi-coherent structures for S ⩾ S b reaching to S max ∼ 1500. Such a coherent structure is ascribed to the spontaneous alignment of smaller scale eddies through phase matching in radial direction, which is classified as a quasi-deterministic process. Resultantly, a large amount of free energy is extracted from the system due to subsequent growth of the event, by which a self-organized profile is established. The coherent structure is then readily disintegrated by self-generated zonal flows, followed by the energy transferred to smaller eddies. Finally, turbulent transport in the steady state of a flux-driven system is found to be regulated by the mixture of such quasi-deterministic process and probabilistic processes, which leads to stiffness and resilience in the profile formation and self-similarity in the relaxation.

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The formation mechanism of Internal Transport Barriers (ITBs) in flux-driven turbulence is studied by means of the full-f gyrokinetic code GKNET. In the adiabatic electron case with a weak magnetic shear configuration, toroidal momentum injection can change the radial mean electric field E_r through the radial force balance, leading to a kind of driven ITB formation in which the ion thermal diffusivity by Ion Temperature Gradient (ITG) turbulence decreases to the neoclassical transport level. Only co-current toroidal rotation in the outer region can benefit the ITB formation, which mechanism is identified to originate from a positive feedback loop between the radial E_r shear and resultant momentum flux. On the other hand, in the kinetic electron case with a reversed magnetic shear configuration, robust co-intrinsic rotation is driven near the q_min surface in ITG turbulence and sustain the E_r shear through the radial force balance, leading to the spontaneous reduction of ion turbulent thermal diffusivity, while it is not observed in the adiabatic electron case. In the presence of electron heating, counter-intrinsic rotation by Trapped Electron Mode (TEM) turbulence is selectively driven in the negative magnetic shear region, which provides steeper E_r shear formation and resultant larger reduction of ion turbulent thermal diffusivity. It indicates that the co-existence of different modes can trigger the discontinuity near q_min and then spontaneous ITB formation.