Kenro Miyamoto

Recent stellarator research

This paper intends to review recent stellarator research in various laboratories. Efforts are continuously and actively being made to improve the confinement of plasmas in stellarator fields. Advances in confinement theory are summarized. Experimental results concerning Ohmically heated plasmas as well as currentless plasmas are reviewed. Finally, stellarator reactor studies are briefly introduced.

Magnetohydrodynamic instabilities in a current-carrying stellarator

Numerical studies on the stability of kink and resistive tearing modes in a current-carrying linear stellarator are presented for various current profiles and helical fields. In the case of an l = 2 helical field, the magnetic shear vanishes and the stability diagram is given by the straight lines with ισ + ιδ = const., where ισ is the rotational transform due to the plasma current and ιδ is that due to the helical field. For the l = 2 stellarator with , the magnetohydrodynamic stability against kink and tearing modes is improved compared to that in tokamaks. When an l = 3 helical component exists, magnetic shear plays an important role in the stability properties. The stability diagrams become fairly complex; they are, however, understandable from properties of the Euler equation. It should be noted that the internal kink modes are made more unstable than in a tokamak, because of the l = 3 helical field.

Image Evaluation by Spot Diagram Using A Computer

The development of electronic computers has brought greater possibilities in evaluation methods for lens systems, and the theory of transfer functions is now being applied to practical lens design. In this paper, the evaluation by spot diagrams is discussed. A method of plotting spot diagrams is proposed, utilizing the line printer of a computer as the X-Y plotter. The geometrical optical intensity distribution Ig (x,y) given by the spot diagram is expressed in the form Ig(x,y)=N-1∑i=1Nδ(x-xi,y-yi).The convolution t (x,y) with the turbidity r (x,y) of the image receiver, the total impulse response, is given by t(x,y)=N-1∑i=1Nr(x-xi,y-yi).Intensity distributions calculated by the computer are compared with the measured ones for an actual lens, and the agreement is satisfactory. This procedure can also be applied to the case of the transfer function. The Strehl definition t (0,0) is calculated for several cases of primary and secondary spherical aberrations, and it is confirmed that there are two extremum positions of the spot diagram in some cases.

Expansion and thermalization processes of plasma flow on the BSG-II device

The expansion process of a collision-dominated theta-pinch plasma in a homogeneous magnetic channel and the thermalization process of the kinetic energy of this plasma flow by a magnetic mirror field are studied on the BSG-II device. Previously it was reported that shock phenomena due to a mirror field were observed by several methods of measurement. In this paper four experimental results are presented: (1) the expansion of plasma along the homogeneous magnetic channel is adiabatic; it is well described by a model for one-dimensional isentropic flow of collision-dominated plasma; (2) the flow is supersonic and the occurrence of a shock in front of the mirror field is in accordance with a criterion concerning the supersonic-sonic transition of plasma flow; (3) the rate of plasma loss through the mirror field is reduced by the occurrence of the shock; (4) the values of the plasma parameters measured before and behind the shock front satisfy the Rankine-Hugoniot relations, though plasma pressure, flow velocity and Mach number are time-dependent.

Conceptional Design of the Vertical Control System in JIPP T-II

A conceptional design of a system for controling the plasma position in a toroidal discharge is described. It is expected that a system consisting of magnetic probes, a digital computer and phase-controlled thyristors enables feedforward-feedback control which suppresses the plasma displacement sufficiently with a help of resistive shell.

THERMALIZATION OF THE KINETIC ENERGY OF A PLASMA FLOW BY A MAGNETIC MIRROR FIELD IN BSG-I EXPERIMENTS.

The purpose of the experiment using the BSG-I device described here is to study the expansion of a plasma, produced in a strong magnetic field by a theta pinch, into a weak magnetic channel and the thermalization of the kinetic energy of this plasma flow by a magnetic mirror field. The plasma, to which the fluid model is applicable, treated here is collision dominated and low-β. Shock phenomena due to the mirror field at the far end of the device are observed by several kinds of measurements and it was concluded that some part of the kinetic energy of the plasma flow was changed to thermal energy by the shock phenomenon. Plasma parameters measured in front of and behind the shock front were consistent with the Rankine-Hugoniot relations. Thus the ion temperature rises after the shock and approaches that of the initial plasma as produced in the theta pinch region before the expansion.

Propagation of Laser Light

Light propagation in connection with communication by laser light is discussed. As a model of the field, with extension α and radius of curvature of wave surface l, uA = α−1 exp[−(ρ2/2α2)+jk(ρ2/2l)] is chosen. The field up after propagation to distance Z is up = β−1je−iδ exp[−(r2/2β2)−jk(Z+γr2)].The position of minimum extension β is not the ordinary focus because of the significant effect of diffraction by the aperture. By use of these results, the condition of α and l is analyzed in order to transfer the light with minimum loss over the distance D through a pipe with the linear dimension ϕ, and the proper relay optical system is discussed.

Project MACETIE (Multistage adiabatic compression and equi-temperature irreversible expansion)

The principle of Project MACETIE and the preliminary experiment are described. The basis of our plan is the thought that the goal of controlled thermonuclear fusion may be accessible by the multistage alternating processes of adiabatic compression and irreversible expansion of plasma. In the latter process, the plasma compressed by a strong magnetic field is expanded into a region of weaker magnetic field. A static consideration of this expansion suggests that plasma temperature will not change appreciably in this process, and a second adiabatic compression of plasma in the weak field may further increase its temperature. Preliminary experiments are being carried out to examine the foregoing principle of irreversible expansion.

Rotational Transform Angle of the Stellarator Field with Twisted Coils

The stellarator field produced by a system of twisted coils in a linear configuration is studied analytically. The current of this system can be represented by the superposition of the azimuthal and helical components. The rotational transform angle of this system is analyzed. It is found that the fundamental harmonic helical components of the current distribution gives the major contribution to the rotational transform angle. The numerical results in the case of the toroidal configuration are compared with analytical ones.

Electron Temperature Measurement in JIPP-Ib Stellarator by the Method of Time History

The time variation of the line emission of the impurities in the vacuum-ultraviolet spectral range is measured. With the use of experimental data, the electron temperature of the low β plasma confined in JIPP-Ib stellarator has been determined by the method of time history. The helium plasma produced by the ohmically heating current is used. The determined electron temperature is around 120 ev at 0.8 ms after the start or discharge. The results of the simulation based on the model introducing the effect of inhomogeneity show that the electron temperature determined by this method corresponds to the temperature at the plasma center.