K. Itoh

Scalings of energy confinement and density limit in stellarator/heliotron devices

The paper presents a study of empirical scaling of energy confinement observed experimentally in stellarator/heliotron devices (Heliotron E, Wendelstein VII-A, L2, Heliotron DR) for plasmas heated by electron cyclotron heating and/or neutral beam injection. The proposed scaling of the gross energy confinement time is: , where P is the absorbed power (MW), n is the line average electron density (1020 m?3), B is the magnetic field strength on the plasma axis (T), a is the average minor radius (m) and R is the major radius (m). The empirical scaling of the density limit obtainable under the optimum condition is proposed to be: . These scalings for helical systems are compared with those in tokamaks. The energy confinement scaling has a similar power dependence as the L-mode scaling of tokamaks. The density limit scaling for helical systems seems to indicate an upper limit of the achievable density similar to that in many tokamaks. From the energy confinement time and the density limit , a beta limit can be derived: , which can be lower than the stability/equilibrium beta limit. Thus, from the viewpoint of designing a machine, the values of B, a and R should be selected with care because the dependence of the confinement time (or n?ET) and of the above beta limit on these values is different.

Model of the H-mode in tokamaks

The paper presents a theoretical model of the H-mode in tokamaks which is based on the bifurcation of the radial electric field at the plasma edge. The electric field is determined by the balance of the non-ambipolar fluxes of ions and electrons at the edge. It is found that bifurcations of the radial electric field, the particle flux and the convective energy loss occur when the edge gradient reaches a critical value. This is attributed to L-H or H-L transition. The critical conditions are examined and the role of neutral particles as well as the effect of impurities are incorporated in the model. Combining the confinement scaling laws for both the core plasma and the scrape-off layer plasma, the threshold power for the transition is derived. The temporal evolution associated with the transition is studied.

Global energy confinement H-mode database for ITER

Describes the content of an H-mode confinement database that has been assembled for the ITER project. Data were collected from six machines of different sizes and shapes: ASDEX, DIII-D, JET, JFT-2M, PBX-M and PDX. A detailed description of the criteria used in the selection of the data and the definition of each of the variables is given. The authors also present an analysis of the conditions of the database, the scalings (power law and offset linear) of the data with both dimensional and dimensionless variables, and predictions of the expected confinement time for ITER

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.

Loss cone region in toroidal helical systems

An analytic formula is derived for the loss cone in toroidal helical systems. Particular emphasis is put on the loss region for the particles that are barely trapped in the helical ripples. Effects of the radial electric field and shifts of the magnetic axis are discussed. Comparison with the numerical calculation confirms that the simple analytic formula gives good approximations.

Numerical studies on divertor experiments

Abstract Numerical analysis on the divertor experiments in JFT-2M tokamak is made by the use of the two-dimensional time-dependent simulation code. The plasma in the scrape-off layer (SOL) and divertor region is solved for the given particle and heat sources from the main plasma, Λ p and Q t . The effects of the direction of the toroidal magnetic field is studied. It is found that the heat flux, which is proportional to b × ▽ T i , influences the divertor plasma conspicuously, but has only a small effect on the plasma parameters on the midplane in the framework of the fluid model. With inclusion of the b × ▽ T i , term for the radial heat flux, the behavior of the divertor plasma observed in the experiments is explained at least qualitatively.

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.

Effect of the radial electric field on alpha particle loss in tokamaks

The effect of the radial electric field near the plasma edge on the ripple trapped loss of fusion alpha particles is discussed. The order of magnitude of the potential difference, which substantially affects the localized heat load on the first wall, is also studied. If the potential difference is of the order of the plasma temperature, the peaking of the localized heat deposition on the first wall becomes weaker owing to the energy distribution of the ripple trapped loss particles.

Application of the intermediate frequency range fast wave to the JIPP TII-U plasma

A series of experiments has been conducted on the JIPP TII-U tokamak since 1989, using the newly constructed 130 MHz radiofrequency system. It has been predicted theoretically that the fast wave in this frequency range interacts weakly with particles. Two mechanisms of wave absorption have been identified in the experiment: electron Landau damping/transit time damping and 3rd harmonic ion cyclotron heating. The former mechanism is intimately connected with fast wave current drive and the latter can provide a new regime of plasma heating or a possible method of controlling the transport of alpha particles. It is found that the efficiency of the 3rd harmonic ion cyclotron heating is improved by using it in combination with neutral beam injection and ion cyclotron range of frequency heating. The heating efficiency obtained is as high as that of conventional heating. The experimental results are also analysed on the basis of a global wave theory which takes into account wave-particle interactions. These mechanisms of interaction are competing with each other; this will also be the case under more realistic reactor conditions.

Effect of Electric Field Inhomogeneities on Drift Wave Instabilities and Anomalous Transport

Effect of inhomogeneity of radial electric field on drift wave instabilities is investigated. Curvature of static potential gives rise to an additional Landau resonance. Critical conditions for the stability due to sheared E × B motion of ions are estimated for various modes. Effect on anomalous transport coefficient is also discussed.