Role of Nonuniform Superthermal Ions for Internal Transport Barriers
Influence of inhomogeneity of plasma flow on the magnetic surface, which is driven by the anisotr... more Influence of inhomogeneity of plasma flow on the magnetic surface, which is driven by the 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 large nonlinear growth rate. This new mechanism of turbulence suppression provides the model of improved confinement associated with the reversed magnetic shear.
Based on the electric field bifurcation model, a possibility to realize the internal transport ba... more Based on the electric field bifurcation model, a possibility to realize the internal transport barrier in helical plasmas is discussed. Condition to relaize the electric domain interface and reduction of turbulent transport is analyzed. Critical heat flux for inducing the bifurcation and transport barrier is obtained for the Heliotron/torsatron plasmas. In the high temperature plasmas which are confined in present helical devices, the anomalous energy transport dominates over the neoclassical energy transport. The improved energy confinement, if it exists, could be realized when the anomalous transport is suppressed. Based on the turbulent transport model, the improved confinement has been analyzed in the geometry of the Heliotron configuration (1). The possible formation of internal transport barrier has been theoretically predicted (2); the electric field domain interface (3) could be established, and transport reduction at the interface takes place. The electric field domain inte...
A kind of large eddy simulation model is proposed to incorporate the effect of microturbulence in... more A kind of large eddy simulation model is proposed to incorporate the effect of microturbulence into MHD. This model is applied to the analysis of magnetic island dynamics in the presence of strong pressure gradient. It is found that the growth of magnetic island is accelerated by the anomalous resistivity and the propagation frequency of magnetic island is dictated by turbulent viscosity.
Based on the method of self-sustained turbulence, the anomalous transport coefficient is derived ... more Based on the method of self-sustained turbulence, the anomalous transport coefficient is derived for various toroidal plasmas. The L-mode confinements in the tokamaks, stellarators, H/T systems and RFP are explained by this formula, showing the influence of the magnetic geometry. The mechanism of the confinement improvement is discussed. Associated with the H-mode, the bifurcation physics is described. The catastrophic event at the β-limit is described. 30 refs., 5 figs.
Isotope effect on the energy confinement time is discussed for the DT plasma. The transport theor... more Isotope effect on the energy confinement time is discussed for the DT plasma. The transport theory which is based on the ballooning mode turbulence is applied. When the DT- plasma is produced under the condition of beta_ p > 1, the energy confinement time of DT- plasma (50% mixture) is expected to be about 1.2 times better than the D plasma with the same operation condition.
Recently, kinetic calculation of the current diffusivity (lambda) was made and it was commented t... more Recently, kinetic calculation of the current diffusivity (lambda) was made and it was commented that the fluid model of anomalous transport, in which the self-sustained turbulence and L-mode transport has been obtained [Itoh et al., Phys. Rev. Lett. 69 (1992) 1050], has overestimated lambda [Biglari, et al., Phys. Fluids B5]. This comment was misled by the improper evaluation of the wave number. The kinetic estimate of lambda is in the same order of the one in the fluid model. This would be one of the reasons that the transport theory, which was derived by using the fluid equations, explains well the present experimental results.
The effect of finite gyroradius on the current diffusive ballooning mode is examined. Starting fr... more The effect of finite gyroradius on the current diffusive ballooning mode is examined. Starting from the reduced MHD equations including turbulent transports, coupling with drift motion and finite gyroradius effect of ions, we derive a ballooning mode equation with complex transport coefficients. The eigenfrequency, saturation level and thermal diffusivity are evaluated numerically from the marginal stability condition. Preliminary results of their parameter dependence is presented.
Model of the L-mode confinement in tokamaks is developed based on the microscopic ballooning mode... more Model of the L-mode confinement in tokamaks is developed based on the microscopic ballooning mode instability. The anomalous transport coefficients determine the stability below the critical beta against the ideal MHD ballooning instability. The current-diffusivity has strong destabilizing effect, while the thermal transport chi and ion viscosity tend to stabilize the mode. The stability boundary for the least stable microscopic ballooning mode determines the anomalous transport coefficients. The obtained formula is compared with experimental observations on the L-mode confinement. The predictions on (1)the radial shape of chi, (2)the temperature-profile resilience, (3)the global confinement characteristics, such as the power degradation and the dependences on the plasma current, current profile and mass number, and (4)the ratio of the perturbative chi to the energy balance chi are consistent with experimental database.
Stability of the current diffusive ballooning mode in tokamaks with high toroidal mode number is ... more Stability of the current diffusive ballooning mode in tokamaks with high toroidal mode number is analyzed in the region of second-stability against the ideal magnetohydrodynamic mode. It is found that the growth rate of the current diffusive ballooning mode is decreased by the reduction of the geodesic curvature driving force. The reduction of thermal conductivity in the limit of very weak shear or negative shear in comparison with standard shear is also shown.
Anomalous transport coefficients in toroidal helical plasmas are studied, based on the innovative... more Anomalous transport coefficients in toroidal helical plasmas are studied, based on the innovative theoretical method. The self-sustained turbulence is analyzed by balancing the nonlinear growth due to the current diffusivity with the nonlinear damping by the ion viscosity and thermal conductivity. Interchange and ballooning mode turbulence is investigated, and the geometrical dependence of the anomalous transport coefficient is clarified. Variation of transport owing to the geometrical difference in toroidal helical plasmas is illustrated. The mechanism for confinement improvement is searched for. To verify the nonlinear destabilization and the self-sustained state, the nonlinear simulation of the interchange mode turbulence is performed in sheared slab. It is demonstrated that the nonlinear enhancement of the growth rate occurs when the fluctuation amplitude exceeds the critical level. In the saturation stage, the fluctuation level becomes higher associated with the enhanced nonlin...
Bench mark test of the code for nonlinear simulation of plasma turbulence is performed. A new cod... more Bench mark test of the code for nonlinear simulation of plasma turbulence is performed. A new code describing four fields (B) is compared to the existing code (A) which treats three fields. Examining the results from two codes under the physically identical conditions, characteristics of the deviations are analyzed. It is found that the infinitesimally small initial noise, due to the cancelling, grows in accordance with the nonlinear development of turbulence mode. Interaction with an intrinsic nonlinearity of the system makes the noise grow, whose contribution becomes similar magnitude to the fluctuation itself of the results. The instantaneous deviation shows the chaotic characteristics. The spectrum analysis is made. These show the intrinsic nonlinearity of the plasma turbulence.
Subcritical excitation and dynamics of plasma turbulence
Nonlinear dynamics of the interchange (or ballooning mode) turbulence is studied theoretically by... more Nonlinear dynamics of the interchange (or ballooning mode) turbulence is studied theoretically by taking into account the electron inertia effect. Method of dressed-test mode is developed, and Lagrange nonlinearity is renormalized in a form of turbulent transport coefficient. The effective mass of electrons becomes heavier due to fluctuations, causing the nonlinear growth above the threshold level. The amplitude of the stationary fluctuations is expressed in terms of the pressure gradient. It is shown that the backward bifurcation appears near the linear stability boundary. The subcritical nature of the turbulence is explicitly shown. The critical pressure gradient for the transition from collisional transport to the turbulent transport is obtained. The self-sustained turbulent state is found to maximize the entropy production rate. The result provides a prototype for the system far from the thermal equilibrium. The pressure gradient plays a role of the order parameter in the nonlin...
This article overviews some aspect of the recent theoretical activities in Japan on the problem o... more This article overviews some aspect of the recent theoretical activities in Japan on the problem of turbulent transport in confined plasmas. The method of self-sustained turbulence is discussed. The process of the renormalization is shown and the turbulent Prandtl number is introduced. Nonlinear destabilization by the electron momentum diffusion is explained. The nonlinear eigenmode equation is derived for the dressed-test-mode for the inhomogeneous plasma in the shear magnetic field. The eigenvalue equation is solved, and the least stable mode determines the anomalous transport coefficient. Formula of thermal conductivity is presented for the system of bad average magnetic curvature (current diffusive interchange mode (CDIM) turbulence) and that for the average good magnetic curvature (current diffusive ballooning mode (CDBM) turbulence). The transport coefficient, scale length of fluctuations and fluctuation level are shown to be the increasing function of the pressure gradient. Ve...
Spatial Structure of Compound Dither in L/H Transition
To study the plasma evolutin and spatial structure at the L/H transition, the double hysteresis i... more To study the plasma evolutin and spatial structure at the L/H transition, the double hysteresis is examined by use of the 1-dimensional transport model equations. Three mechanisms for the bipolar losses, i.e., the loss cone loss, collisional bulk viscosity loss of ions and the anomalous loss are simultaneously retained. five-fold multiple bifurcations are found to exist at the plasma edge, similar to the previous 0-dimensional study. Double hysteresis causes a self-generated oscillation, which is attributed to the compound dither, a kind of ELMs. Spatio-temporal evolution of the compound dither is analyzed.
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Papers by Masatoshi Yagi