Hiroshige Watanabe

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.

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.

On the Minimization of Displacement of Plasma Column in a Toroidal Magnetic Field with a Non-Planar Axis

The system with a toroidal helical magnetic axis is re-examined numerically to obtain the minimum displacement of the plasma column in an infinitely conducting chamber wall. The results of the numerical calculation show that in order to minimize the displacement of the plasma column in the toroidal helical system, we should have a device with a large number of pitch n , for example n =32, and in such a device the β value of the equilibrium state is permitted up to 100 percent. A rule which determines the optimum condition for the minimization of the displacement is shown by the figure.

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.

Investigation of magnetic field properties in the helical axis stellarator: Asperator NP-4

Experimental measurements of the magnetic field properties of a helical axis stellarator were carried out. The results, together with the numerical calculation, are described herein. The toroidal device, Asperator NP-4, has a helical magnetic axis with a pitch angle of 45? and a multi-field period of n=8. The magnetic field of NP-4 is produced by the currents of helical solenoids and L=+1, -1 helical windings. By an electron beam mapping of field lines, the existence of a high rotational transform angle and magnetic surfaces are confirmed and the position if the helical magnetic axis is obtained. The other fundamental magnetic properties such as the shear and the specific volume of the configuration are also deduced from the measurements. Numerical calculations lead to a good agreement with the results of the experiment.

Toroidal Effect on Magnetic Line of Force in Non-Planar Helical Magnetic Axis System (Asperator NP-3)

The behavior of magnetic line of force in a stellarator-type toroidal device with three dimensional helical magnetic axis (Asperator NP-3) is investigated both numerically and analytically, especially taking into account of the toroidal perturbation. To avoid or decrease the influence of the crucial 2/3 resonance, the absolute value of ellipticity of magnetic surface e should be over 1/3, or both e and triangularity Q should be near zero simultaneously. The swing δ employed in the helical windings is revealed to be very effective to the structure of magnetic surface.

Collisional Damping of the Whistler Mode in a Dense Plasma

The effect of Coulumb collisions on the transverse wave along an external magnetic field in a fully ionized dense plasma is investigated by means of the Fokker-Planck equation. Expressions for the dispersion relation and the spatial damping constants of the whistler mode are obtained. The spatial damping constants, to first order, are given by κ_i=\frac{2}{3\sqrt{\pi}}(\frac{k\nu_{c}}{\omega_{c}-\omega})[1+\frac{7}{5}(\frac{kv_{0}}{\omega-\omega_{c}})^2], κ_e=\frac{14}{15\sqrt{2\pi}}(\frac{k\nu_{c}}{\omega_{c}-\omega})(\frac{kv_{0}}{\omega-\omega_{c}})^2, where νc is the electron collision frequency, v0 is the electron thermal velocity, k is the wave number and ωc is the electron cyclotron frequency. Finally, their applications to the experimental results are briefly discussed.

Current Equilibrium in a Toroidal Device with a Non-Planar Magnetic Axis (Asperator NP-3)

The results of the experiment on plasma equilibrium in Asperator NP-3, a toroidal device with a three-dimensional magnetic axis, are described. It is shown that the equilibrium of a current column in this system is mainly determined by the vacuum masnetic surfaces produced by solenoids with a spatial axis. By the application of an l =1 transverse field, a method for controlling the current position by the external field is confirmed experimentally.

Design Study of Modular Coil System in a Toroidal Helical Axis Stellarator

Analytical and numerical studies on the design of vacuum magnetic field configuration produced only by planar non-circular coils with elliptical and triangular deformations are made systematically in a closed system with a three dimensional magnetic axis. The aim is to find out favorable configurations with a magnetic well, large rotational transform and small aspect ratio (∼10). The present study shows that it is difficult to realize the configuration which satisfies these requirements simultaneously, because of the deterioration and the destruction of magnetic surfaces in the outer region due mainly to the discreteness of coils. Supplementary measures for increasing the non-circularity of magnetic surfaces, which is appropriate from the engineering point of view, should be taken.