Y. Kosuga

An overview of intrinsic torque and momentum transport bifurcations in toroidal plasmas

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 electron cyclotron heating-driven counter torque are discussed in some detail. Symmetry breaking by lower single null versus upper single null asymmetry is related to the origin of intrinsic torque at the separatrix. (Some figures may appear in colour only in the online journal)

Momentum theorems and the structure of atmospheric jets and zonal flows in plasmas

The inviscid invariance of potential vorticity is used to derive momentum balance relations for zonal flows in drift wave turbulence. The relations are constructed by exploiting potential enstrophy balance and the Taylor identity, and link flow momentum to turbulence pseudomomentum, along with the driving flux, the dissipation and turbulence spreading. Applications to atmospheric jets and to zonal flows in plasmas are discussed.

Towards an emerging understanding of non-locality phenomena and non-local transport

In this paper, recent progress on experimental analysis and theoretical models for non-local transport (non-Fickian fluxes in real space) is reviewed. The non-locality in the heat and momentum transport observed in the plasma, the departures from linear flux-gradient proportionality, and externally triggered non-local transport phenomena are described in both L-mode and improved-mode plasmas. Ongoing evaluation of ‘fast front’ and ‘intrinsically non-local’ models, and their success in comparisons with experimental data, are discussed

On the efficiency of intrinsic rotation generation in tokamaks

A theory of the efficiency of the plasma flow generation process is presented. A measure of the efficiency of plasma self-acceleration of mesoscale and mean flows from the heat flux is introduced by analogy with engines, using the entropy budget defined by thermal relaxation and flow generation. The efficiency is defined as the ratio of the entropy destruction rate due to flow generation to the entropy production rate due to ∇T relaxation (i.e., related to turbulent heat flux). The efficiencies for two different cases, i.e., for the generation of turbulent driven E×B shear flow (zonal flow) and for toroidal intrinsic rotation, are considered for a stationary state, achieved by balancing entropy production rate and destruction rate order by order in O(k∥/k⊥), where k is the wave number. The efficiency of intrinsic toroidal rotation is derived and shown to be eIR∼(Mach)th2∼0.01. The scaling of the efficiency of intrinsic rotation generation is also derived and shown to be ρ∗2(q2/s2)(R2/LT2)=ρ∗2(Ls2/LT2), w...

A Concept of Cross-Ferroic Plasma Turbulence

The variety of scalar and vector fields in laboratory and nature plasmas is formed by plasma turbulence. Drift-wave fluctuations, driven by density gradients in magnetized plasmas, are known to relax the density gradient while they can generate flows. On the other hand, the sheared flow in the direction of magnetic fields causes Kelvin-Helmholtz type instabilities, which mix particle and momentum. These different types of fluctuations coexist in laboratory and nature, so that the multiple mechanisms for structural formation exist in extremely non-equilibrium plasmas. Here we report the discovery of a new order in plasma turbulence, in which chained structure formation is realized by cross-interaction between inhomogeneities of scalar and vector fields. The concept of cross-ferroic turbulence is developed, and the causal relation in the multiple mechanisms behind structural formation is identified, by measuring the relaxation rate and dissipation power caused by the complex turbulence-driven flux.

Spatio-temporal evolution of the L???H and H???L transitions

Understanding the L???H and H???L transitions is crucial to successful ITER operation. In this paper we present novel theoretical and modelling study results on the spatio-temporal dynamics of the transition. We place a special emphasis on the role of zonal flows and the micro???macro connection between dynamics and the power threshold (PT) dependences. The model studied evolves five coupled fields in time and one space dimension, in simplified geometry. The content of this paper is (a) the model fundamentals and the space?time evolution during the L???I???H transition, (b) the physics origin of the well-known ?B-drift asymmetry in PT, (c) the role of heat avalanches in the intrinsic variability of the L???H transition, (d) the dynamics of the H???L back transition and the physics of hysteresis, (e) conclusion and discussion, with a special emphasis on the implications of transition dynamics for the L???H power threshold scalings.

Nonlinear current-driven ion-acoustic instability driven by phase-space structures

The nonlinear stability of current-driven ion-acoustic waves in collisionless electron–ion plasmas is analyzed. Seminal simulations from the 1980s are revisited. Accurate numerical treatment shows that subcritical instabilities do not grow from an ensemble of waves, except very close to marginal stability and for large initial amplitudes. Further from marginal stability, one isolated phase-space structure can drive subcritical instabilities by stirring the phase-space in its wake. Phase-space turbulence, which includes many structures, is much more efficient than an ensemble of waves or an isolated hole for driving subcritically particle redistribution, turbulent heating and anomalous resistivity. Phase-space jets are observed in subcritical simulations.

On relaxation and transport in gyrokinetic drift wave turbulence with zonal flow

We present a theory for relaxation and transport in phase space for gyrokinetic drift wave turbulence with zonal flow. The interaction between phase space eddys and zonal flows is considered in two different limits, namely for K>>1 and K ≃ 1 where K is the Kubo number. For K>>1, the growth of an isolated coherent phase space structure is calculated, including the associated zonal flow dynamics. For K ≃ 1, mean field relaxation dynamics is considered in the presence of phase space granulations and zonal flows. In both limits, it is shown that the evolution equations for phase space structures are structurally similar to a corresponding Charney-Drazin theorem for zonal momentum balance in a potential vorticity conserving, quasi-geostrophic system. The transport flux in phase space is calculated in the presence of phase space density granulations and zonal flows. The zonal flow exerts a dynamical friction on ion phase space density evolution, which is a fundamentally new zonal flow effect.

Drift hole structure and dynamics with turbulence driven flows

The role of turbulence driven flows in describing drift hole structure and dynamics is discussed. Turbulence driven flows enter the plasma medium response and alter drift hole structures by changing the screening length of the drift hole potential. Specifically, turbulence driven flows shift the drift hole potential radially, and absorb drift hole energy via the hole-flow resonance. It is shown that the absorption shifts the phase of a momentum flux, and so enables the irreversible coupling of drift holes to turbulence driven flows. We show that drift holes and turbulence driven flows are dynamically coupled, and self-regulate each other, so that a stationary state can be achieved with non-zero turbulence driven flows. As an application, a bound on the fluctuation amplitude in the coupled system is derived. The bound is obtained by requiring that the resultant zonal flow velocity should be smaller than the critical flow velocity for the drift hole potential to be self-bound (i.e., the velocity that the sc...

A branch of energetic-particle driven geodesic acoustic modes due to magnetic drift resonance

Eigenmode analysis of geodesic acoustic modes (GAMs) driven by fast ions is performed, based on a set of gyrokinetic equations. Resonance to the magnetic drift of the fast ions can destabilize GAMs. A new branch is found in the family of GAMs, whose frequency is close to the magnetic drift frequency of the fast ions. The poloidal eigenfunction of this branch has bump structures in the poloidal direction where the resonance of the magnetic drift with the mode is strong. The ion heating rate by the GAMs is evaluated in the framework of quasi-linear theory. The heating is localized poloidally around the resonance locations. Owing to the bumps in the eigenfunction, the magnitude of the heating is much larger than that estimated without the magnetic drift resonance.