Sanae-Inoue Itoh

Change of Transport at L- and H-Mode Transition

A new refined model of the L-mode and H-mode transition in tokamaks is presented based on the bifurcation of the radial electric field, E r , near the edge. The radial gradient of E r is newly introduced to explain the sudden change of fluctuations as well as plasma fluxes at the onset of transition. This nodel predicts that the L- to H-mode transition is associated with the decrease of d E r /d r causing reduction of particle and energy fluxes at the critical gradient.

Kinetic description of propagation and absorption structures of ICRF waves

Propagation and absorption of waves in the ion cyclotron range of frequencies (ICRF) are investigated using the kinetic theory in a high-temperature plasma. The wave equation which includes kinetic effects (such as finite-gyroradius effect and wave/particle interactions) is solved as a boundary-value problem for a two-ion-component plasma (majority deuterium and minority hydrogen) with one-dimensional inhomogeneities. The global structure of the wave field and the absorption mechanism are explained. The power deposition profile for each plasma species and the coupling to the antenna are obtained. The mode conversion of the fast wave to the ion Bernstein wave is associated with the two-ion hybrid resonance, and the latter is absorbed by electrons and deuterons via Landau damping and collisional damping, respectively. Protons absorb the wave which tunnels through the cut-off layer (for strong-field-side excitations) by cyclotron damping. The existence of cavity resonances is also confirmed; these considerably influence the energy absorption.

Global waves in hot plasmas

Abstract This article reviews theoretical and numerical study on the global structure of Alfven and ICRF waves in the high temperature plasmas confined in tokamaks. The kinetic theory of the wave equation in an inhomogeneous and dispersive medium is developed. The wave equation is solved as a boundary value problem to provide a consistent description of the excitation by an antenna, propagation and absorption of the rf waves. The wave forms, power deposition profile, power partition by plasma species, and the antenna impedance are obtained. The numerical procedure and accuracy of the solution as well as the computational performance are also discussed.

Averaged Force and Flow Induced by RF Waves in Inhomogeneous and Dispersive Plasmas

A general formula of the macroscopic averaged force induced by the rf waves in a dispersive nonuniform plasma is derived. The force is presented in terms of the spectra of waves and the mobility tensor of the plasma, where the kinetic processes can be also included. One mechanism to generate the net force is the absorption of the wave through wave-particle interactions. The other is the space-time inhomogeneities of the wave envelope and the background plasma. The nonmagnetohydrodynamic contribution to the force in the Vlasov plasma is calculated and compared to the MHD ponderomotive force. The macroscopic plasma flow induced by the rf force is also studied.

Two-dimensional analysis of limiter/divertor transition in scrape-off layer plasmas

The structures of scrape-off layer and divertor plasmas have been studied numerically with a neutral code and a two-dimensional fluid code. Doublet-III is taken as an example for an open divertor configuration. A decisive parameter is the distance between the plasma surface (determined by the magnetic separatrix) and the limiter, which is varied in order to assess the interaction of the plasma with the limiter as well as the effect of neutrals on the main plasma. The minimum value of the limiter clearance needed to prevent plasma-limiter interaction is determined. The scaling of the edge temperature and the dependence of the e-folding length of the scrape-off layer plasma on the heating power are obtained.

Detached and Attached Plasma in Stellarators

Detached and attached plasmas in stellarators are studied. The energy balance equation in the peripheral plasma is solved by use of a simple model of the radiation loss. Criterion for the detached/attached plasma is obtained. The relation with the density limit is also discussed.

A Model of Peaked Density Profile and Inward Pinch in Tokamaks

A model theory of inward pinch and peaked density profile of ohmic discharges in the tokamak is presented. Ion anomalous viscosity in the presence of sheared rotation causes the drift across the magnetic field. Radial electric field, E r , can cause an inward pinch of electrons as well. The ratio of viscosities and the diffusion coefficient, the Prandtl number, determines the structures of E r and the density profile n ( r ) in a stationary state. In viscous plasmas, peaked profiles of both density and rotation velocity are expected. Reduction of edge neutrals changes the boundary condition and can induce further density peaking. Change of d E r /d r propagates into the center, causing ion viscous heating associated with the damping of velocity shear.

Steady-state operation regime of tokamak reactor plasma; Consistency analysis

In this paper the consistency of physics constraints imposed on a core plasma in a tokamak reactor is investigated. Conditions for the steady-state operation of the International Thermonuclear Experimental Reactor (ITER)-grade plasma are listed, i.e., the density limit, the critical beta, feasibility of full current-drive and diverter functions, etc. The parameter regime, in which these guidelines are simultaneously satisfied, is investigated. Based on the available data base, the consistency of the conditions is examined. The L-mode scaling of the energy confinement time is employed for extrapolation to the ITER-grade plasma.

Density clamping and power deposition profile in rf heated plasma in toroidal helical systems

Radial transport driven by rf heating in toroidal helical systems with the large helical ripple is studied. The cyclotron resonance heating changes the pitch angle of resonating particles, so that the transition from the transit orbit to the trapped one occurs. The deviation of the average minor radius of the particle orbit from the magnetic surface strongly depends on the pitch angle of the particle velocity. Owing to this dependence, the slight change in the pitch angle by heating can generate a large step in the radial location. The approximate forms of the driven radial flow, which is rf-power-dependent, and the deformation of the heating profile are obtained. It is found that the deposition profile in this configuration is hardly made sharper than a critical gradient length. The injection of the heating power from the inside of the torus is preferable to improve the heating efficiency and to overcome the density clamping.

Microinstability, Entropy Production and Plasma Confinement

A variational method to obtain the plasma confinement properties in magnetic fusion devices is proposed. The most probable steady state of an inhomogeneous plasma open to external sources is seeked out by using a non-equilibrium thermodynamical and statistical mechanical approach. As a thermodynamical potential to cause anomalous loss fluxes, an irreversible entropy production rate is used, which is associated with the dissipation due to low-frequency electromagnetic fluctuations in the presence of the pressure inhomogeneity. Its conditional minimum (extremum) state, with respect to the finite inhomogeneities of the plasma parameters and the distribution of the fluctuation spectrum, corresponds to the required steady state of a dissipative structure. An external source condition is included as a constraint which makes an effective change of the potential.