Masafumi Azumi

Effect of Toroidal Field Ripple on Fast Ion Behavior in a Tokamak

Computational studies have been performed for fast ion behavior in a Tokamak with toroidal field ripple. Collisionless behavior of fast ions relating to rippletrapping, ripple-detrapping and banana drift is of essential importance in fast ion loss processes in case of quasi-perpendicular neutral beam injection. The collisionless ripple trapping and ripple-enhanced banana drift produce a large number of loss bands in velocity space and enhance the loss of fast ions. The fast ion loss associated with ripple can be categorized into two groups; ripple-trapped loss and banana-drift loss. The amount of loss particles due to the respective loss process is significantly influenced by the effect of finite banana size of fast ions. The ripple-trapped loss particles are localized in a specific region on the first wall surface.

Nondimensional transport study of JT-60U plasmas

Nondimensional local transport analysis of JT-60U plasmas has been carried out for NBI heated L-mode phase. Plasmas with the same poloidal cross section and almost the same profiles of nondimensional parameters such as ν * , β th , q , T e / T i except for ρ * are compared. The ρ * dependence of the one fluid thermal diffusivity, χ eff , the electron thermal diffusivity, χ e , and the ion thermal diffusivity, χ i , has been examined. The property of χ eff is between the weak gyroBohm type diffusion and the Bohm type diffusion in the region a /3≤ r ≤2 a /3, which is consistent with the results of the global confinement study done before. We have also found that χ e is weak gyroBohm type diffusion while χ i tends to be Bohm like diffusion.

Prandtl Number of Toroidal Plasmas

Theory of the L -mode confinement in toroidal plasmas is developed. The Prandtl number, the ratio between the ion viscosity and the thermal conductivity is obtained for the anomalous transport process which is caused by the self-sustained turbulence in the toroidal plasma. It is found that the Prandtl number is of order unity both for the ballooning mode turbulence in tokamaks and for the interchange mode turbulence in helical system. The influence on the anomalous transport and fluctuation level is evaluated. Hartmann number and magnetic Prandtl number are also discussed.

Passive burn control in a tokamak plasma using toroidal field ripple

This paper reports on the feasibility of passive burn control method using toroidal field ripple-degraded alpha-particle confinement with free expansion of the major radius that has been confirmed by a 1.5-dimensional transport code. In this transport code, a scaling of the ripple loss of alpha particles dived from the results of an orbit-following Monte Carlo code is used. For passive burn control, however, {gt}5% of the major radius margin is necessary and the resulting ripple-induced power loss of alpha particles exceeds 20%. Passive burn control in combination with feedback control of the field ripple, a hybrid burn temperature control. In hybrid burn control, the necessary major radius margin and the controlled field ripple are only 2 to 3% and {delta}{sub c} {approx gt} 1%, respectively. The resulting total power loss of alpha particles is {lt}15%.

Time Behavior of Heat Diffusivity during L-H-L Transitions in JT-60U

The L-H-L transitions have been analyzed mainly for high-field pulses (Bt ≈4 T) in JT-60U. The simultaneous response of electron temperature T e has been clearly observed during L-H-L transitions over a wide plasma region. This T e evolution is described as the result of the fast jump of electron heat diffusivity, δχ e, over the wide plasma region. Values of δχ e were obtained as 0.5 m2/s < | δχ e | < 1 m2/s, which usually increase with radius. The simultaneous response of ion temperature T i was also observed to be that of similar to the T e response. The jump of ion heat diffusivity, δχ i, is similar to that of δχ e and usually increases with radius, as does δχ e. In a low-field pulse (B t ≈2.5 T), | δχ i | is as large as 2 m2/s. Values of δχ are consistent with the change of the energy confinement time during transitions.

Effect of electron and ion viscosity on sawtooth crash in a tokamak

The effect of anomalous electron and ion viscosity induced by the stochastization of magnetic field line on sawtooth crash in a tokamak is investigated by using the reduced set of resistive MHD equations. As the perturbation grows beyond some amplitude, the most unstable mode is turned from the pure resistive mode into the one induced by the anomalous electron viscosity, and the growth rate suddenly increases, as observed in some experiments. After that, the growth of perturbation is decreased due to the anomalous ion viscosity. The sawtooth crash time is prolonged in spite of the explosive growth due to the electron viscosity. However, it is not completely suppressed by the effect of anomalous ion viscosity.

Comparison between theoretical analyses and experimental results of an ICRF loading in JT-60

A cavity resonance of ICRF fast wave was observed in early experiments on the second harmonic ICRF heating in JT-60. The purpose of these experiments was to confirm an antenna-plasma coupling. We discuss the loading impedance of ICRF waves in a linear theory, since the loading condition was in a low-power regime. The experimental results are compared with a numerical analysis of one-dimensional ICRF wave model. Good agreement between the experimental results and the theoretical analysis is obtained qualitatively.

Equilibrium Vacuum Magnetic Field of an Axisymmetric Torus with an Elliptical Plasma Cross-Section

A method obtaining a vacuum magnetic field around a prescribed axisymmetric toroidal plasma equilibrium is described. The equilibrium vacuum field which fits the boundary conditions on the plasma-vacuum interface is obtained assuming a large aspect ratio. The equilibrium solution is given explicitly for an elliptical plasma cross section and the singularities of the resultant magnetic field are examined.

Improvement of l=2 Stellarator and Torsatron

Magnetic field systems with l =2 helical windings of fairly short pitch have large rotational transform and high shear. This high shear promises the formation of average magnetic well, when a vertical field is externally applied. A hybrid field having mainly l =2 and l =3 helical fields can be produced by deforming l =2 helical windings. This field has a deep well and its current system is very simple. Numerical calculations using Biot-Savarts law show that these improvements are realized in practice.

Current-Diffusive Ballooning Mode in Low Shear and Negative Shear Regions of Tokamaks

The 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 upon the reduction of the geodesic curvature driving force. The reduction of thermal conductivity in the limit of very weak shear or negative shear in comparision with standard shear is also shown.