Norio Satomi

Compact Toroidal Plasma with Toroidal and Poloidal Magnetic Fields

Compact toroidal plasma has been experimentally obtained in a drum-type copper vessel on injecting an annular plasma stream from a magnetized plasma gun. Toroidal and poloidal magnetic fields, B t and B p , of the plasma toroid show a typical exponential decay with 110 µs and no MHD instability is observed. Experimental data of B t and B p configuration for long confinement are described fairly well by the model, which is the fundamental-mode state of the minimum energy force-free equilibrium estimated from the low- β limit Grad-Shafranov equation. The configuration of the stable plasma is hence fully-determined by the metal vessel geometry.

Temporal evolution of the decaying spheromak in the CTCC-I experiment

In the CTCC-I spheromak experiment, after low-Z impurities have been suppressed by titanium coating on the inner surface of the flux conserver, MHD activity increases to the point that instabilities are present in every discharge at some time during the decay process. Most of the instabilities occur intermittently, as evidenced by the O V line radiation intensity which rises gradually and then drops abruptly. In some experiments, a conducting pole is inserted along the symmetry axis to increase the magnetic shear of the configuration. This conducting pole effectively suppresses a destructive instability, but other MHD instabilities are still present. Magnetic field measurements indicate that the occurrence of the instability is correlated with a relaxation process during which the peaked current profile at the magnetic axis is flattened by the MHD instability. An estimate of the energy loss during the subsequent relaxation process is given.

Internal stress induced in the process of boron coating

Internal stresses induced in boron films formed by vacuum deposition were studied to find the generation mechanism of compressive stresses by an optically levered laser method. It was found that the internal stresses of boron thin films changed from tensile to compressive with increasing substrate temperatures and decreasing deposition rates. Deliberate addition of oxygen gas in an atmosphere with a pressure of 0.02 mTorr did not clearly change internal stresses, though oxygen content was increased. According to the observation by a transmission electron microscope (TEM), the film structure was in an amorphous state based on icosahedral subunits. An interatomic distance for the high deposition rate (0.5 nm/s) was slightly larger by 1.7% than that for the low deposition rate (0.06 nm/s). This result seems consistent with a film formation model based on adatom migration.

Effect of irradiation ion species on internal stress in boron thin films

Abstract Internal stress has been studied under concurrent ion bombardments during the deposition of boron thin films. Boron thin films prepared by the vacuum evaporation exhibited fairly large compressive stress of about 0.5 GPa even without ion irradiation. Hydrogen ion irradiation with 100–400 eV energy resulted in the large compressive stress of 0.8–1.2 GPa due to the ion peening effect. On the other hand, under the irradiation with 100–400 eV helium ion the stress relief was observed and the compressive stress decreased by a factor of about 2. This may be caused by the large sputtering (5–15%) that will modify film structure in the bulk and surface morphology, resulting in porous, rather dense film.

Mechanical properties of boron coatings

Abstract Internal stress of coatings will cause reliability problems, such as adhesion failure and peeling. We measured the internal stress in boron coatings, which was prepared by the ion plating method, with an apparatus based on the optically levered laser technique. The boron coatings exhibited large compressive stress in the range from −0.5 GPa to −2.6 GPa. It was found that these compressive stresses were decreasing functions of the deposition rate and were increasing functions of the ion bombardment energy.

Internal stress control of boron thin film

The occurrence of stress in thin films has led to serious stability problems in practical use. We have investigated the stress in the boron films to find the deposition condition of the boron films with less stress. It was found that the stress in the boron film varies sufficiently from compressive to tensile stress, that is from - 1.0 to 1.4 GPa, depending on the evaporation conditions, such as deposition rate and the substrate temperature. Hydrogen ion bombardment resulted in the enhancement of the compressive stress, possibly due to ion peening effect, while under helium ion bombardment, stress relief was observed. The boron film with nearly zero stress was obtained by the evaporation at a deposition rate of 0.5 nm s -1 and substrate temperature of 300°C.

Electron temperature measurement using the line-intensity ratio on the CTCC spheromak

Line-intensity ratio measurements of the electron temperature have been successfully made on the CTCC-II spheromak. The electron temperatures obtained in the core plasma are 20-80 eV, which are consistent with the values obtained by Thomson scattering. This measurement has the advantage of giving the time evolution of the electron temperature within a single plasma discharge. In the CTCC spheromak, intermittent instability (stepwise instability) and its relaxation are frequently observed. During the stepwise instability and its relaxation cycles, increase and decrease in the temperature are observed in the current-profile-peaking phase and the nonlinear saturation phase of the n=2 mode. It is demonstrated that the temperature profile in the peripheral plasma can be controlled by an additional magnetic field (choking field), which can reduce the error field in the entrance hole of the flux conserver.

A Two-Stage Ion Heating Obtained in a Rapid Theta-Compression Experiment. : II

The process of a two-staged ion heating of counter streaming plasma flows under a rapidly increasing magnetic field is studied by measuring plasma parameters and changes of fields. In the early stage of the compression, ions of the initial plasma are heated up to 300∼400 eV till 240 ns by the reflected ions from the magnetic piston existing near the wall. By a stable and strong azimuthal current sheet around the plasma column existed already in the encounter process, a large magnetic well is formed after 240 ns. By the interaction with this moving magnetic barrier, ions are accelerated effectively and the ion temperature reaches 3∼4 keV at last. The density and the temperature of electrons are (2∼3)×10 15 cm -3 and 300 eV, respectively.

The Instability Observed in the CTCC-I Spheromak Plasma

In the CTCC-I experiment, the instability appears in almost all the discharges and occurs several times in the every discharge. The dominant toroidal mode number for this instability is the n =2, which is measured using a toroidally arrayed magnetic probe. This mode shows different behavior in the instabilities; in the earlier and intermediate phase of the discharge the saturation phase come to appear, but near the termination this saturation phase does not appear. These phenomena probably depend on the magnetic Reynolds number. The evolution of the magnetic field configuration is also investigated. From these results, the instability observed in the CTCC-I spheromak is found to be the n =2 ideal kink mode.

Impurity Behavior and Energy Balance on CTCC-Spheromak

In CTCC spheromak experiment, a coaxial gun produces initial plasma and ejects it into a confinement region surrounded by metal wall where magnetic flux is conserved, i.e. Flux Conserver (FC). A time of production is only about 0.1 ms and a extinction time of the plasma is about 1.5 ms. There is no supply of energy to spheromak during this duration, and there is scarcely any occurrence of ruinously global instabilities, except for a intermittent instability (stepwise instability). Then spheromak is gradually decaying due to its own resistivity. The present plasma have been studied experimentally from view point of impurity behavior and the physical process about the temporal behavior of the spheromak have been also clarified by the use of zero dimensional energy balance model.