Hitoshi Nihei

Initial results of RFP experiments in REPUTE-1

Experiments of a reversed field pinch device REPUTE-1 constructed in University of Tokyo started in 1984. The conductivity electron temperature and the line-averaged electron density increase approximately linearly with the increase of plasma current in the range up to 240 kA. The study of the effect of the bias toroidal field on the discharge characteristics shows that a higher bias field gives lower resistivity and a higher flat top plasma current. The toroidal flux at the current flat top tends to be a selective value independent of the initial bias field.

Performance characteristics of a microwave plasma source using an axial mirror and multipole magnetic fields

The performance characteristics of a microwave plasma source are presented. The plasma source consists of two chambers: chamber 1 generates a high‐density plasma over which a magnetic field is applied at electron cyclotron resonance condition or greater, and chamber 2 makes the plasma uniform in its lower region by surrounding the peripheral region with magnetic multipole line‐cusp fields. Plasma parameters were measured with a movable Langmuir probe located 1 cm above a plasma grid which is attached at the bottom of chamber 2. The dependence of plasma parameters on the magnetic field configuration, gas pressure and microwave power were examined, and radial distributions were measured. Uniform high‐density plasmas over a 9‐cm‐diam area were produced on the plasma grid in a suitable magnetic field configuration, and uniformity was maintained over a wide pressure range. The uniform density region was determined by the magnetic multipole fields existing in chamber 2’s peripheral region. The ion beam current ...

Microwave plasma source for ion sources with multipolar and axial magnetic fields

A microwave (2.45 GHz) plasma source for ion sources has been constructed and its chracteristics have been examined. It consists of the part of the source generating a plasma and that making the plasma uniform. The experiments have been conducted in four kinds of magnetic field configurations generated by solenoid coils and arrays of permanent magnets. The plasma parameters have been measured with a Lngmuir probe located at 2 cm above a plasma grid. The uniform plasma over an area of 8~10 cm in diameter and the density of 2.6×1011 cm-3 has been produced. The magnetic field intensity near the plasma grid is nearly zero.

Remarkes on Relaxation Phenomena in Toroidal Discharge

MHD relaxation phenomena have been studied for a toroidal plasma in a safety factor q <1 regime. Ultra low- q (ULQ) equilibria with ( n +1) -1 < q < n -1 ( n ≧1) are setup through MHD relaxation, and are sustained quasistatically. A ULQ equilibrium has a q profile with d q / d r <0 ( r : minor radius) for a major region in the plasma, so that it is stable against global MHD modes. In the time-scale of the classical field-diffusion, the ULQ equilibrium deforms to take a peaked-current profile and destabilizes a global mode. Then a catastrophic transition to a higher q (lower n ) state is observed.

Density Distribution in a Plasma Source with Cylindrical Configuration

The density distribution in a cylindrical plasma source is obtained by solving the diffusion equation based on a plasma model. In this model, the side wall of the cylindrical chamber is surrounded by magnetic multipole fields in order to obtain a uniform high-density plasma on a target wall. The form of the initial density distribution at the upper stream of the source is also discussed. Some calculated examples are shown. The results give the density distribution in the source and the plasma particle flux flowing into the target wall. The equations derived in this paper are useful for the design of a plasma source capable of generating a uniform high-density plasma over a wide area.

Magnetic Multipole Field Generated by Columns of Permanent Magnets Arranged on a Cylindrical Surface

The magnetic multipole field generated by columns of permanent magnets arranged on a cylindrical surface at equal intervals is calculated. Each magnet is replaced by two sheet currents flowing in opposite directions through each side of the magnet. The results have been compared with measured values and found to be in good agreement with them. These expressions are simple and very convenient for the design of a plasma source surrounded by magnetic multipole fields.

Magnetic field fluctuations in ripple reduction experiments on the REPUTE-1 RFP

The loop voltage of REPUTE-1 RFP is decreased by reducing the toroidal field ripple. The low frequency part of the toroidal field fluctuations, which is mainly composed of the m = 1 mode and has good coherence in space and time, is found to be reduced, especially outside the reversal surface, when a trimming field is applied. High frequency fluctuations, which have little coherence, are not influenced effectively.

3D-Computer Simulation of Ion-Beam Deflection Caused by the Displacement of Extraction Electrodes

A three-dimensional simulation code for ion-beam formation has been developed for the quantitative evaluation of the beamlet deflection arising from the displacement of the extraction electrode. The code gives a good simulation of the experimentally-observed relations between the deflection angle of the beamlet and the mutual displacement among the extraction electrodes.

Development of Proton Detection System with Thin Ruby Scintillator Insensitive to Neutrons and Gamma Rays for Neutron Lifetime Measurement

A proton detector for lifetime measurement of neutrons using a thin ruby scintillator which can be used under an environment of neutrons and γ-rays has been developed. The detector is almost insensitive to neutrons and γ-rays. This was confirmed by setting the detector adjacent to the ultra-cold neutron (UCN) facility installed in Kyoto University Reactor. When a single proton was injected into the scintillator, many scintillation signals were observed. The number of scintillation signals decreases with decreasing proton energy. Although the output signals generated by the scintillation and by the dark currents of a photomultiplier have approximately the same pulse heights, the proton can be detected by counting the signals within the time width corresponding to the decay time of scintillation. This neutron lifetime measurement system can be applied to the detection of particles other than protons.

Analyses of Plasma Flux and Density Distribution in Plasma Sources Surrounded by Magnetic Multipole Fields

Plasma confinement characteristics of plasma sources, the side walls of which are surrounded by magnetic multipole fields, are analyzed using theoretical equations. Experimental data used are those obtained with microwave plasma sources. The plasma flux, the pressure range over which a uniform plasma is produced, the plasma density distribution at a plasma grid and the cusp leak width are discussed. If the leak width is the geometric mean value of the ion and electron Larmor diameters, the results estimated using the theoretical equations are consistent with the experimental ones. It has also been observed from the traces due to the plasma irradiation that the leak width corresponds to the geometric mean value. The validity of the equations has been confirmed. These equations can be used as a basis for the design of a plasma source capable of producing a uniform high-density plasma over a wide area.