Kimitaka Itoh

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.

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.

Anomalous bootstrap current due to drift waves

Abstract An anomalous parallel current driven by radial flux in a tokamak is discussed. Drift waves, which cause an anomalous cross field diffusion, can generate a parallel current in a sheared magnetic field, if the fluctuation level has a radial dependence. The anomalous pinch effect is also discussed.

Electric rotation effect on ripple transport in heliotron/stellarator

The effects of the radial electric field on the ripple transport in the stellarator/heliotron are studied. The pitch angle dependence of the detrapping frequency is kept and the transport coefficient is calculated for the arbitrary values of Ω E /ν eff (Ω E : E r × B rotation frequency ν eff : effective collision frequency of ripple-trapped particles). A simple interpolation formula is derived and the electron energy confinement time is estimated.

Analysis of minority velocity distribution and power deposition profile in icrf heating in a tokamak

The power deposition profile in the two-ion hybrid resonance heating regime in tokamaks is studied by including the modification of the minority ion velocity distribution. The Fokker-Planck equation and the kinetic wave equation are solved numerically in order to describe the time evolution of the velocity distribution and the deposition profile. The power that is transferred coUisionally to the electrons and majority ions from the minority ions is also calculated. Application to the JET plasma demonstrates strong electron heating near the plasma centre. Control of the deposition profile by varying the minority ion concentration and the magnitude of the magnetic field is also discussed.

Analyses of ICRF waves on JET plasma

A theoretical and a computational study of ICRF waves were conducted for JET plasma. The present experimental parameters are analysed and various prediction studies for future experiments are considered using a combination of two kinetic codes. The wave field was solved for obtaining a power deposition profile of each plasma species and the antenna impedance. A two-ion hybrid case was simulated. Strong minority heating in the central core region was observed and a direct input to electrons was found at most 10% of the total absorption. However, for the higher-temperature target plasma, the deposition profile to the electrons became broader and the off-center and its input reached 30%. An efficient non-central deposition to ions by a combined use of NBI is also discussed. A global coupling study is also given for a cold plasma with dampings.

Effect of ion loss on ICRF heating

The effect of spatial loss on ICRF (ion cyclotron range of frequency) wave heating is studied. A simple model for the energy-dependent loss term is adopted in the Fokker-Planck equation. The case of the minority ion heating by fundamental cyclotron resonance is analysed. It is found that the high energy particle loss imposes a limit on the heating power density, above which an increase in the heating power does not raise the energy transfer to the bulk particles, i.e. the heating efficiency deteriorates. This power limit is serious for losses with strong energy dependence such as ripple diffusion. Small spatial loss can annihilate the increment of the fusion reaction by tail generation.

Impurity Injection using Multiple-Shell Pellet

A model of a three-layered multiple-shell pellet is studied in order to apply it to an impurity injection experiment. The mass dependence of the ablation is examined by employing the neutral-cloud shielding model. The localization width of the impurity is estimated.

ICRF wave catalyst in beam-heated plasma

A concept of RF catalyst by use of ion cyclotron wave is presented. The ICRF wave is excited by the external antenna, amplified by the beam particles and absorbed by the bulk plasma. The circulating power in the plasma is much larger than the wave energy which is radiated from the antenna: the injected wave works as catalyst to stimulate the energy flow from beam particles to the bulk plasma. Solving the kinetic wave equation in a model Tokamak geometry, the amplification factor is calculated, and the applicable plasma parameters in present experiments are discussed.

Multiple-shell pellet for plasma fueling

A pellet with shell structure is proposed to reduce the tritium circulation in magnetic fusion devices. As an example, the tritium core is covered by a deuterium layer in order to deposit tritium ions in the central core region of hot plasma.