Makoto Katsurai

An FDTD formulation for dispersive media using a current density

A novel finite-difference time-domain (FDTD) formulation for dispersive media called the JE convolution (JEC) method is derived using the convolution relationship between the current density J and the electric field E. The high accuracy of the JEC method is confirmed by computing the reflection and transmission coefficients for a nonmagnetized plasma slab in one dimension. It is found that the new method has an accuracy comparable to the auxiliary differential equation (ADE) while having the same computational efficiency as the less accurate recursive convolution (RC) method. Numerical simulations also show that the JEC method exhibits significantly higher accuracy than the RC method in modeling wave attenuation inside the plasma.

Experimental investigation of three-dimensional magnetic reconnection by use of two colliding spheromaks

Experimental investigation of three‐dimensional (3‐D) effects of magnetic reconnection dynamics has been extended by use of axially colliding spheromaks [M. Yamada et al., Phys. Fluids B 3, 2379 (1991)]. The two toroidal shape spheromak plasmas with major radii of 15–20 cm and with parallel toroidal currents of up to 30 kA collide to merge in an external equilibrium field. It is important to note that the present experimental setup allows one to investigate magnetic reconnection comprehensively from both local and global points of view. Reconnection angle θ between the merging field lines is varied by changing the polarity of the internal toroidal field and the magnitude of an external toroidal field. It is observed that the speed of counterhelicity merging with θ∼180° is about three times faster than that of cohelicity merging with θ∼90°. This suggests the significance of a 3‐D effect on the reconnection process. This difference is attributed to the property of the neutral current sheets with and without the magnetic field component parallel to the reconnection (X) line. In the counterhelicity merging, the neutral current sheet is compressed in much shorter time than in the cohelicity merging, resulting in much higher current density and subsequent faster decay of the current sheet. This induces a faster magnetic reconnection. The reconnection speed increases proportionally with the initial approaching speed of the spheromaks, suggesting that a compressible driven reconnection model is consistent with the present reconnection experimental results.

High-beta characteristics of first and second-stable spherical tokamaks in reconnection heating experiments of TS-3

Since 1986, the centre solenoid (CS)-less formations of ultra-high-beta CT/ST plasmas have been developed in the TS-3 merging experiment using high power heating of magnetic reconnection. In the cohelicity (Type-A) merging, two STs were merged together to build up the plasma beta to βT≈0.5. In the counterhelicity (Type-B) merging, an oblate FRC formed by two merging spheromaks with opposing toroidal field Bt, was transformed into an ultra-high-beta (βT≈0.8) ST by applying external toroidal field Bt. We made the BALLOO code stability analyses of the produced STs and concluded that formations of the first-stable/marginally second-stable STs were obtained by Type-A merging and the second-stable STs by Type-B merging and also unstable STs by both mergings. The ballooning-stable regime calculated from the experimental data was almost consistent with the measured high-n instabilities. The stable regime became larger significantly by increasing the hollowness of current profile and broadness of pressure profile. This paper also addresses normalized betas βN of thus produced STs as large as 6–17 for comparison with the Troyon scaling and a promising B2 scaling of the reconnection heating. These facts indicate that the axial merging is one of the most efficient startup method for high-beta ST without powerful CS.

Objectives and design of the JT-60 superconducting tokamak

A fully superconducting tokamak named JT-60SC is designed for the modification programme of JT-60 to enhance economical and environmental attractiveness in tokamak fusion reactors. JT-60SC aims at realizing high-β steady-state operation in the use of low radio-activation ferritic steel in a low ν* and ρ* regime relevant to the reactor plasmas. Objectives, research issues, plasma control schemes and a conceptual design for JT-60SC are presented.

Initial results from investigation of three‐dimensional magnetic reconnection in a laboratory plasma

A comprehensive laboratory experiment has been proposed to investigate the fundamental 3‐D physics of magnetic reconnection regions and their associated hydromagnetic flows. Two toroidal plasma rings, with equal or opposite magnetic helicity, are formed and then brought together, contacting along a toroidally symmetric line. This research addresses three important questions: (1) How does magnetic helicity affect reconnection? (2) Will three‐dimensional processes arise spontaneously and modify the usual Sweet–Parker or Petschek picture of two‐dimensional reconnection where the global configuration is that of an axisymmetric X‐point line? (3) How does the reconnection rate respond to global forcing? In a preliminary experiment carried out at the University of Tokyo [Phys. Rev. Lett. 65, 721 (1990)], the direction of the toroidal field plays an important role in the merging process. It is found that plasmas of opposite helicity merge appreciably faster than those of similar helicity. It is also found that th...

Fabrication and testing of a micro superconducting actuator using the Meissner effect

The fabrication and testing of a micro superconducting actuator is reported. The levitation and driving were achieved by the Meissner effect and the Lorentz force. For the stator, a YBaCuO superconducting film and etched-copper film in polyimide was used. For the slider, an Nd-Fe-B permanent magnet was used. Noncontact and remote measurements were made of the levitation heights by using a laser displacement meter. Measurements were made on the sliders movement by using image-processing techniques. The results show that the 8 mg slider stably levitated 0.99 mm above the stator at 280 G. The levitation height was decreased by the increase of the weight of the slider and the attractive driving force. The levitation height exhibits a hysteresis characteristic as a function of the applied magnetic field.<<ETX>>

A proposal for a superconducting actuator using Meissner effect

A superconducting actuator with magnetic levitation using Meissner effect in mu m order is proposed to remove the friction between moving elements and fixed elements in micromachines. The actuator, called Meissnac, is analyzed by the discrete surface current method. A driving method for the actuator is proposed. The driving force can be obtained by using the difference of the pitch of the stator and that of the slider, and the control of superconducting and normal states of superconductors. The control of the state can be achieved either by applying current over J/sub c/ to the superconductor or by heating it above T/sub c/. Maximum values of the levitating force and the driving force are 1.7 and 0.26 (N/m/sup 2/A/sup 2/). The lateral movement in a particular direction of a slider is obtained by choosing appropriate superconductors and by transforming them into the normal state. A scale model is fabricated using a YBaCuO high-T/sub c/ superconductor. The levitating force and the driving force of a scale model are 84 and 31 (mgf) when the slider is composed of a permanent magnet.<<ETX>>

Numerical analysis of surface wave excitation in a planar-type nonmagnetized plasma processing device

Two-dimensional numerical simulations based on the finite-difference time-domain approximation to Maxwells equations coupled with nonmagnetized cold electron plasma equations are used to study surface wave propagation in a four-layer planar type surface wave plasma processing structure (Komachi et al., 1994) under the assumption that the plasma is overdense. Simulations are used to verify the existence of surface waves along the plasma-dielectric window interface as well as to investigate the excitation mechanism of surface waves. In addition, simulations are used to study the effect of the air gap on the electric field distribution at the plasma-dielectric window interface. It is found that metal edges located along the processing chamber wall sufficiently close to the interface are needed to excite surface waves along the interface. It is also found that the air gap layer helps achieve a more uniform electric field distribution over the processing chamber.

A levitation-type linear synchronous microactuator using the Meissner effect of high-Tc superconductors

Abstract A linear synchronous microactuator with a superconducting stator, whose slider is levitated by the Meissner effect and driven by the Lorentz force, has

Three‐dimensional numerical simulations of the relaxation process in spheromak plasmas

Nonlinear evolution of three‐dimensional magnetohydrodynamic (MHD) instabilities of a toroidal spheromak in a cylindrical flux conserver has been studied by numerical simulations for various initial equilibrium states with different q profiles. In spheromaks with qa>1, where qa is the safety factor on the magnetic axis, nonlinear evolution of the resonant internal kink mode dominates with a poloidal mode number m=1 and a toroidal mode number n=1 that causes the poloidal flux amplification. This process corresponds to that of the internal disruption model for tokamaks by Kadomtsev [Sov. J. Plasma Phys. 1, 389 (1975)]. In spheromaks with a very high qa, namely qa≳3, the gross n=1 kink mode grows extensively in the region including the major axis of the torus, which causes the flux conversion from the toroidal to poloidal directions. For spheromaks with a low qa, namely qa≲0.5, the internal kink mode with a toroidal mode number n∼1/qa is first destabilized, and the excitation of the modes with lower n number...