Keiichiro Sugita

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.

Analysis on influence of temporal and spatial profiles of disturbance on stability of pool-cooled superconductor [in LHD]

An influence of temporal and spatial profiles of thermal disturbance on the transient stability of superconductors is analyzed. We consider two types of pool-cooled superconductors: one conductor has the cross-sectional pattern where the stabilizer is located outside the superconducting cable; the other conductor has the stabilizer inside the superconducting cable. The computational result shows the former type of conductor is more affected by variations of the transport current and the temporal and spatial profiles of the disturbance than the latter. As a result, we recommend that the superconducting cable be located near the coolant in high current monolithic superconductor.<<ETX>>

Investigation of the Transient Glow Discharge in Nitrogen

With the development of the pseudospark device, glow discharge with high power has recently become of interest. In this study, the voltage across a gap, the discharge current and the glow duration have been measured during transient glow discharge in the pressure range of 40-130 Torr in nitrogen. The results show that there exists a large difference between a previous investigation by Chalmers (1971) and the present study in the quantity of energy dissipated in the gap during the transient glow. However, in regard to the volume density of energy dissipated in the cathode fall, the values obtained by Chalmers and the present authors almost agree. From this result it is considered that the energy density is more essential than the energy quantity with respect to the transition of discharge from a transient glow to an arc.

Numerical analysis of stability and quench characteristics of superconductors for the LHD coils

A numerical analysis has been performed on the stability and quench characteristics of superconductors for helical coils of the Large Helical Device. Two types of conductors are dealt with, both of which consist of a NbTi/Cu superconducting cable stabilized by pure aluminium. The analysis is based on a two-dimensional model and the heat conduction equation was solved numerically. The distributions of temperature and current density are clarified for the cases of recovery and quench. The minimum input energies required to initiate a quench are calculated and compared with the experimental values.<<ETX>>

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.

Primary electron avalanche under an induced toroidal electric field in H2, He and N2

Reports a study on the development of primary electron avalanche under a toroidal electric field. The applied toroidal field is an induced electric field of E0=86.8 V cm-1 and the gases used are H2, He and N2. The avalanches are generated and make steady progress in the torus at 0.8-2.4 Torr in H2, at 1.6-6.3 Torr in He and at 0.4-1.2 Torr in N2 under the given field. It is suggested that the considerably high pressure range of He is closely related to the large value of alpha /p at low E/p. The development velocities of avalanche in N2 and He are nearly equal to the electron drift velocity at the E/p used. The development velocity in H2 is about twice as large as that of the others. This difference is presumably due to the multiplied effect of the large ionisation coefficient and the small absorption coefficient.

Two-dimensional analysis of the stability in superconductors proposed for the LHD coil

Abstract The thermal stability of the superconductor KISO-3B, which has been one of conductors developed for LHD coil, is analyzed numerically on a two-dimensional model. The computational results show that the threshold of thermal disturbance from a recovery to a quench is clear and the change from a recovery to a quench is produced by a slight difference of several percent in the magnitude of thermal disturbance. In the case of a recovery the superconductor state area spreads over the whole conductor instantaneously after it appears locally in the cross section of the conductor by the decrease in temperature according to the relation between the rates of Joule heat generation and heat transfer.

Diamagnetic Signal in a Plasma Column with an Inflection Point

This letter describes the open ended device with a non-planar magnetic axis of which the torsional angle can be changed easily. It is shown experimentally that the value of the diamagnetic signal obtained at a non-planar configuration is more than that at a planar one under the same condition.

Optical measurement of a primary electron avalanche in an induced toroidal electric field

The authors describe a primary electron avalanche which is accelerated by the induced electric field and develops in an electrodeless torus. Waveforms obtained by an optical method have two peaks, one corresponding to the first passage of the avalanche round the torus and the other to the stage in which the ionisation proceeds rapidly, resulting in the formation of a nearly uniform plasma.

EXPERIMENTS ON CYCLOTRON RESONANCE AND NONADIABATIC TRAPPING OF HYDROGEN IONS IN A MIRROR MACHINE.

Ion cyclotron resonance and nonadiabatic trapping of the hydrogen ions in a mirror machine investigated. A hydrogen plasma (about 10 8 ions/cm 3 ) diffuses into a magnetic mirror field along the magnetic lines of force and an r.f. electric field of 4.5 Mc/s is applied to the plasma through a pair of plane electrodes at the midplane of the magnetic mirrors. It is observed that the magnetic moments of the ions are increased non-adiabatically at cyclotron resonance field for the molecular ions (5.8 kgauss) and the atomic ions (2.9 kgauss). The latter is attributed to the harmonic resonance of the molecular ions. Nonadiabatic trapping of the molecular ions in a mirror field is also observed at 5.8 kgauss and 2.9 kgauss. About 15% of the injected ions is reflected from the far mirror field. The most of the ions escape through the far mirror field because the applied r.f. electric field is shielded by the plasma.