Masakazu Takayama

Frequency Spectrum of Fluctuations Near a Rational Surface in a Toroidal Heliac

Density and space potential fluctuations have been studied in electron cyclotron resonance heating (ECRH) plasma of a helical axis stellarator TU Heliac using Langmuir probe techniques. These fluctuations are coherent and global, which can be explained by a drift instability model in cylindrical geometry. A particular fluctuation mode vanishes inside a rational surface. The ratio of this mode, n/m, corresponds to the rotational transform ι/ of this rational surface, (m, n: poloidal, toroidal fluctuation modes, respectively). This phenomenon near the rational surface can also be explained by a drift instability theory.

LH transition by a biased hot cathode in the Tohoku University Heliac

In the Tohoku University Heliac (TU-Heliac), a helical axis stellarator, an electron injection electrode using a hot cathode made of LaB6 was developed and the transition mechanism to an improved mode has been intensively studied. In the electrode current-sweep mode of biasing experiments, a bifurcation phenomenon, i.e. a negative resistance feature in the electrode characteristics was observed accompanied with transition to an improved mode (H-mode) or transition from H-mode to L-mode, in helium plasma discharges of a wide range of collisionality. The ion viscous damping force was estimated from the J ? B driving force for poloidal rotation. The local maxima in viscosity were found at the poloidal Mach number around ?Mp ~ 1?3, as predicted by neoclassical theory. It was also found that the negative resistance (a bifurcation phenomenon) was observed when the poloidal viscosity showed a local maximum.

A New Capacitive / Resistive Probe Method for Studying Magnetic Surfaces

A new capacitive/resistive probe method for mapping the magnetic surfaces from resistance or capacitance between a magnetic surface and a vacuum vessel was developed and tested. Those resistances and capacitances can be regarded as components of a simple electrical bridge circuit. This method exploits electrical transient response of the bridge circuit for a square pulse. From equiresistance or equicapacitance points, the magnetic surface structure can be deduced. Measurements on the Tohoku University Heliac, which is a small-size standard heliac, show good agreement with numerical calculations. This method is particularly useful for pulse-operated machines.

Influence of biased electrode on plasma confinement in the Tohoku University Heliac

Observations of an increase in electron density triggered by electrode biasing are reported in the Tohoku University Heliac (TU-Heliac). The biased electrode, which is located deep inside the plasma, induces a strong positive radial electric field, after which the line density increases by a factor of 2, the electron density profile steepens and the fluctuation level drops. We estimate the damping force opposing poloidal rotation experimentally and compare it with various versions of the neoclassical theory. The experimental results agree in that the measured damping force has a local maxima at |M p|=1–2 and tends to be proportional to M p in the high speed region (|M p|>3–6), where M p is the poloidal Mach number. For |M p|<1–2, however, the dependence of the measured damping force on M p differs a little from theoretical predictions.

Transition of poloidal viscosity by electrode biasing in the Large Helical Device

Electrode biasing experiments were carried out in various magnetic configurations on the Large Helical Device (LHD). The transitions of poloidal viscosity, which were accompanied with bifurcation phenomena characterized by a negative resistance in an electrode characteristic, were clearly observed on LHD by the electrode biasing. The critical external driving force required for transition was compared with the local maximum in ion viscosity, and the radial resistivity before the transition also compared with the expected value from a neoclassical theory. The critical driving force increased and the radial resistivity decreased with the major radius of the magnetic axis Rax going outwards. The configuration dependence of the transition condition and the radial resistivity qualitatively agreed with neoclassical theories. The radial electric field and the viscosity were also evaluated by the neoclassical transport code for a non-axisymmetric system, and estimated electrode voltage required for the transition, which was consistent with the experimental results.

Electrode biasing experiment in the Large Helical Device

The transition to the improved confinement mode by electrode biasing was observed for the first time in the Large Helical Device (LHD). A negative resistance was observed in the confinement mode sustained by the cold electrode biasing. The electrode current showed a clear decrease against an increase in the electrode voltage and was characterized by a hysteresis in the transition phenomena. The decrease in the electrode current suggested an improvement in the radial particle transport. An increase in the energy confinement time and remarkable suppression of density fluctuations, which correspond to the transition, were also observed. These results indicated that the electrode biased plasma in LHD showed similar improvements in confinement to those in the H-mode plasmas in tokamaks and stellarators.

Electrostatic Field Variation in the Atmosphere Induced by Earth Potential Difference Variation During Seismic Wave Propagation

To clarify the relationship between earthquakes and electrical phenomena, signals of three parameters were observed continuously at Sennan in Akita Prefecture, Japan: ground acceleration (GAC), earth potential difference (EPD), and electrostatic field in the atmosphere (EFA). A large metal plate was used as an electrostatic antenna to observe EFA variation. For this paper, we used the digital natural observation (D-NOB) method as a signal processing technique and found clear EPD and EFA waveforms during seismic wave propagation. Our observation and examination results show that GAC causes EPD variation. Subsequently, the EPD variation induces EFA variation

Particle transport study with an electron beam in the Tohoku University Heliac

The particle transport of low density ( ~1012 m-3) and low energy ( <20 eV) electrons is studied with a directed electron beam in the Tohoku University Heliac. The particle diffusion coefficient is estimated experimentally using the emissive filament technique. We investigate the coefficients dependence on magnetic field strength, collision frequency and electron beam energy. The measured dependence is similar to the dependence calculated using the neoclassical transport theory.

Output characteristics of a surface permanent magnet-type vernier motor - Comparison of test results and calculation

This paper describes a surface permanent magnet (SPM)-type vernier motor that can generate high torque at low speeds, and a simple, high-speed method for analyzing its output characteristics is proposed. In this method, if the induced electromotive force, the synchronous reactance, and the iron loss are provided by finite element analysis (FEA), the output characteristics can be calculated immediately using the voltage and torque equations on the γ and d axes. The proposed method is simpler and faster than the general method that uses FEA only. In this paper, the voltage and torque equations of the SPM-type vernier motor are derived, and the proposed analysis method using these equations is presented. Moreover, the agreement of the results calculated using this method with the measured values confirms a high degree of accuracy.

Design of surface permanent magnet-type vernier motor using Halbach array magnet

A surface permanent magnet (SPM)-type vernier motor whose rotor uses Halbach array magnet is designed in this study. Voltage and torque equations are derived, and an output calculation method and design process based on these equations are proposed. A high-output power motor and a high-power factor, high-efficiency motor are designed using the proposed design process, and the output characteristics are clarified using the proposed calculation method and finite element analysis. A high-output power motor with a maximum output of 11.9 kW is designed. A high-power factor, high-efficiency motor with a maximum output exceeding 6 kW is also designed; the power factor and efficiency of this motor at 4 kW are more than 90% and more than 85%, respectively. The proposed calculation method and the design process are shown to be very effective.