Masatoshi Yagi

Physics of collapses: probabilistic occurrence of ELMs and crashes

A statistical picture for the collapse is proposed. The physics picture of the crash phenomenon, which is based on the turbulence-turbulence transition, is extended to include the statistical variance of observables. The dynamics of the plasma gradient and the turbulence level are examined, with the hysteresis nature in the flux-gradient relation. Probabilistic excitation is predicted. The critical condition is described by the statistical probability.

A theoretical model of H-mode transition triggered by condensed neutrals near X-point

The effects of neutrals near the X-point of tokamaks on the L/H transition are examined. Four mechanisms for bipolar losses, i.e. the loss cone loss, the bulk viscosity loss, the charge-exchange (CX) loss between ions and neutrals and the anomalous loss, occur simultaneously in high-temperature plasmas. Neutral particles near the X-point are found to trigger the H-mode transition. They cause an additional ion loss cone loss via collisions. The lower limit of the neutral density near the X-point for the onset of the H-mode transition is estimated. Neutral particles at the main plasma are shown to prevent the H-mode transition because of CX loss.

Transport Dynamics of Collapse in a Giant ELM

Dynamics of collapse in a giant ELM (Edge Localized Mode) is presented. A model of transport bifurcation between the H-mode and the magnetic braiding mode (M-mode) is applied to the simulational study of edge plasma dynamics. The bifurcation contains the hysteresis characteristics. The pressure profile development, the propagation of the bifurcation (transition) fronts and the resultant bursting fluxes are obtained. The appearance of a pivot point in the pressure profile, an avalanche followed to the transition front propagation, and the structure of the burst are shown.

Transition to an enhanced internal transport barrier

The role of rotation shear in the internal transport barrier formation is examined by means of a one-dimensional transport simulation based on the current-diffusive ballooning mode turbulence model. Delayed transition to the enhanced improved transport was reproduced in the medium heating power range. The threshold power is strongly reduced by the effect of rotation shear. Simulation including particle transport was also applied to NBI-heated plasmas and the steepness of density and temperature gradients is discussed.

1D simulation of sawtooth crash based on transport bifurcation

The crash of the temperature profile associated with a propagating transport bifurcation between the L-mode and the magnetic braiding mode (M-mode) in a tokamak plasma is presented. The magnetic braiding mode is induced by the magnetic stochasticity. If the pressure gradient exceeds a threshold value a magnetic stochasticity sets in and the heat conductivity is much enhanced (M-mode). The back transition to the L-mode occurs when the pressure gradient decreases to another threshold value. The model of the transport bifurcation is included in the 1D transport code. The crash of the temperature profile and the propagation of the crash front (avalanche) are realized by this model. The collapse without a precursor oscillation is revealed.

Comparison Analysis of Lorenz Model and Five Components Model

The Lorenz model for Rayleigh-Benard convection is extended to the five components model taking an autonomous shear flow effect into account. The five components model is numerically solved and analyzed in detail. Based on the Lyapunov exponent analysis , how the introduction of the new degrees of the freedom (the shear flow) changes the chaotic behavior of the solution is examined. The attractors, the time evolutions, the power spectra, the Nusselt number of the five components model are compared with those of the Lorenz model. It is found that the solutions of both models converge to the same one when the Rayleigh number is small. When the Rayleigh number exceeds a critical value, the difference between the Lorenz model and the five components model is observed, i.e., the former has only the periodic solutions, while the latter contains the chaotic solutions. Some examples are shown. The limitation to a model with a few degrees of freedom is also discussed.

On the chaotic nature of turbulence observed in benchmark analysis of nonlinear plasma simulation

Simulational results of two dissipative interchange turbulence (Rayleigh-Taylor-type instability with dissipation) models with the same physics are compared. The convective nonlinearity is the nonlinear mechanism in the models. They are shown to have different time evolutions in the nonlinear phase due to the different initial value which is attributed to the initial noise. In the first model (A), a single pressure representing the sum of the ion and electron components is used (one-fluid model). In the second model (B) the ion and electron components of the pressure fields are independently solved (two-fluid model). Both models become physically identical if we set ion and electron pressure fields to be equal in the model (B). The initial conditions only differ by the infinitesimally small initial noise due to the roundoff errors which comes from the finite difference but not the differentiation. This noise grows in accordance with the nonlinear development of the turbulence mode. Interaction with an intrinsic nonlinearity of the system makes the noise grow, whose contribution becomes almost the same magnitude of the fluctuation itself in the results. The instantaneous deviation shows the chaotic characteristics of the turbulence. (c) 1997 American Institute of Physics.

Probabilistic Excitation of Plasma Transitions

Statistical property of an excitation of plasma transition is analyzed, which takes place in a strongly turbulent state. Transition characteristics which have been studied in a deterministic picture are re-examined. Model equations of transition, with a hysteresis nature in flux-gradient relation in the presence of strong self-noise, are solved and the dynamics of plasma gradient and turbulent-driven flux is studied. The excitation probability is shown to be fairly large well below the conventional critical condition. Probability distribution function, as a function of the gradient, for an onset of the transition is obtained both for power law noise.

SUPPRESSION OF PLASMA TURBULENCE BY ASYMMETRIC HOT IONS

The influence of plasma flow inhomogeneity on the magnetic surface, which is driven by anisotropic hot ions, on the micro-turbulence of the low/negative magnetic shear tokamak is investigated. It is found that the poloidal inhomogeneity is effective in suppressing the current-diffusive ballooning mode turbulence which has a large nonlinear growth rate. This new mechanism of turbulence suppression provides the model of improved confinement associated with the reversed magnetic shear.

Nonlinear simulation of electromagnetic current diffusive interchange mode turbulence

The model equation for the electrostatic current diffusive interchange mode turbulence is extended for both electrostatic and electromagnetic turbulence. Not only convective nonlinearity but also the electromagnetic nonlinearity which is related to the parallel flow are incorporated into the model equation. The two-dimensional nonlinear simulation of the electromagnetic current diffusive interchange mode is performed based on the extended fluid model. Two cases are investigated. In the first case, we neglect the terms and in Ohms law, but retain the electromagnetic nonlinearity. This case corresponds to a simple extension of previous work. It is found that the electromagnetic nonlinearity gives rise to the normal cascade and higher saturation of the fluctuating energy. In the second case, we retain and terms as well. In this case, the turbulent level develops higher than that in the electrostatic limit, and the strong oscillatory behaviour of the fluctuating energy is observed. The strong inverse cascade is observed, which overcomes the normal cascade due to the electromagnetic nonlinearity. The quasi-linear effect and the system size effect on the saturation level are also discussed in the context of the second model.