Yasushi Yamano

Charging characteristics and electric field distribution on alumina as affected by triple junctions in vacuum

This paper describes the dependence of the charging characteristics on the electric field distribution on the alumina (Al/sub 2/O/sub 3/) surface as affected by the triple junction in vacuum. For HV electrical insulation design of vacuum interrupter, surface flashover in vacuum is very important problem to be solved. Attention should be paid to the fact that the insulation characteristics on the dielectric surface are strongly influenced by field emission of electrons from the triple junction and the accumulated charges on the dielectric surface. In order to clarify the charging mechanism, we measured the charging characteristics for various types of triple junctions. In particular, we focused on the influence of the electric field distribution along the solid dielectrics and near the cathode triple junction (CTJ) on the charging characteristics. The results confirmed that the electric field distribution strongly affected the 2-dimensional (2D) distribution of the surface charge on the dielectric sample. Consequently, it was found that positive charging was generated on alumina, when the incident angle of the electric line of force on the alumina surface became >60/spl deg/.

Charging characteristics on dielectric surface by different charging processes in vacuum

This paper describes charging characteristics on a dielectric surface in vacuum by electron irradiation and field emission by a triple junction under negative and positive DC HV applications. The authors measured the 2-D distribution of electrostatic charging on a dielectric surface in situ. Experimental results revealed that the negative charge distribution caused by the electron beam had a conical shape over the whole surface. On the other hand, for a triple junction, it was an acute distribution around the triple junction. Moreover, they quantitatively investigated the difference of the 2-D charging distribution, using certain shape parameters, between the two charging processes mentioned above. In addition, they examined the time decay characteristics of the surface potential on the dielectric in vacuum.

Effect of mechanical finishes on secondary electron emission of alumina ceramics

The effect of surface roughness of insulator on the secondary electron emission (SEE) coefficients was investigated with changing the incident angle of primary electrons. The SEE coefficients were measured using a scanning electron microscope with a single-pulse electron beam (100 pA, 1 ms). As a result, the SEE coefficients increased with the incident angle for smooth surface, while those of rough surface almost did not change with the incident angle. The SEE coefficients of commercial alumina ceramics with three different surface finishes, i.e. as-sintered, as-ground and mirror-finished samples, were also measured before and after annealing treatments. This treatment was carried out in air at 1400 degC for 1 h. After annealing, the SEE coefficients were higher than those of unannealed samples. This increase is proposed to be due to the defect recoveries as well as neutralizations of charging, which significantly influence the SEE. For relative low purity (95% or less), the SEE coefficients of annealed alumina became lower after mechanical grinding operations and lower again after mirror-finished operations. The SEE coefficients of 99.7% purity alumina were almost unaffected by the mechanical finishes.

Recent R&D Activities of Negative-Ion-Based Ion Source for JT-60SA

The JT-60 Super Advanced (JT-60SA) tokamak aims to perform the ITER support and to demonstrate steady-state high-beta plasma project with the collaboration between Japan and EU. To attain these objectives, the negative-ion-based NBI (N-NBI) system is required to inject 10 MW for 100 s at the beam energy of 500 keV. On JT-60U, the present N-NBI ion source has injected 3.2 MW for 21 s at 320 keV; however, three key issues should be solved for the JT-60SA N-NBI ion source. One is to improve the voltage holding capability of the large negative ion source, where the available acceleration voltage has been limited to less than ~400 kV due to breakdowns. The accelerator of the JT-60U ion source is composed of large three-stage grids and three fiberglass reinforced plastic (FRP) insulators. Recent R&D tests suggested that the FRP insulators were not the main factor to trigger the breakdowns at the early conditioning stage. The accelerator with a large area of grids and their supporting structure may need a high margin in the design of electric field and a long time for conditioning. The second issue is to reduce the power loading of the acceleration grids. It was found that some beamlets were strongly deflected due to beamlet-beamlet interaction and strike on the grounded grid in the accelerator. Moreover, the electrons generated in the accelerator caused the grid loading and the overheating of the beamline components. The acceleration grids for JT-60SA are to be designed by taking account of the beamlet-beamlet interaction and the applied magnetic field in 3-D simulation. Third is to maintain the D production for 100 s. Although a constant D- beam power was confirmed on JT-60U for 21 s, an active cooling system is required to keep the temperature of the plasma grid (PG) under optimum condition during 100-s operation. A simple cooling structure is proposed for the active cooled PG, where a key is the temperature gradient on the PG for uniform D- production. In the present schedule, design work, reflecting the latest R&D progress, will continue until ~2011. The modified N-NBI ion source will start on JT-60SA in 2015.

Charging characteristics on dielectric surface by different charging process in vacuum

This paper describes the charging characteristics on a dielectric surface in a vacuum by different types of charging process: electron irradiation by electron beam; and field emission by triple junction under negative DC high voltage application. The authors measured the 2-dimensional distribution of electrostatic charging on the dielectric surface in-situ. Experimental results revealed that the negative charge distribution by the electron beam had a conical shape all over the surface. On the other hand, for the triple junction, it was an acute distribution around the triple junction. Moreover, they quantitatively investigated the difference of the 2-dimensional charging distribution using some shape parameters between above mentioned two charging processes. In addition, the authors examined the time decay characteristics of the surface potential on the dielectrics in vacuum.

Influence of electric field distribution on charging mechanism on alumina dielectrics by triple junction in vacuum

Surface flashover characteristics in vacuum is very important for HV insulation design of vacuum interrupter. It should be paid attention to the fact that the insulation characteristics on the dielectric surface are strongly influenced by field emission of electron at a triple junction and the accumulated charges on the dielectric surface. In order to clarify charging mechanism, the authors measured the charging characteristics for various types of triple junction configuration. In particular, they focused on the influence of electric field distribution along solid dielectrics and near the cathode triple junction (CTJ) on the charging characteristics. Through the discussion on experimental results, they confirmed that electric field distribution strongly affected 2-dimensional (2D) distribution of surface charge on dielectric sample. Consequently, it is found that positive charging is generated on Al/sub 2/O/sub 3/, when incident angle of electric line of force on the Al/sub 2/O/sub 3/ surface becomes larger than 60 degree.

Energy Spectra of Bremsstrahlung X-Rays Emitted From an FRP Insulator

Energy spectra of X-rays emitted from the surface of a fiberglass-reinforced plastic (FRP) insulator were measured at three different positions and compared with those of the vacuum gap between electrodes. Near the anode, the X-ray spectrum was dominated by monoenergetic electrons. Near the cathode, the spectrum peak shifted to low energy as compared with that near the anode. This result showed that a large amount of low-energy electrons was generated on the surface of the FRP insulator near the cathode.

Investigation of 2-dimensional charge distribution on dielectric surface in vacuum by real-time measurement technique

For the establishment of the vacuum insulation technique, the electrical insulation characteristics under the existence of the solid insulator need to be given attention. In particular, a study of a charging mechanism on the solid surface in vacuum is very important. For this reason, we measured a 2-dimensional surface charge distribution using a real-time measurement technique. Under electron beam irradiation to the dielectric surface, we measured the time-transition of surface charge accumulation by electrostatic probes. By varying the electron beam current and the beam energy, we investigated the dependence of such parameters on the charging process. From the measurement results, we clarified that such a measurement technique allowed us to obtain the time varying 2D charging characteristics, which resulted in the clarification of the charging mechanism caused by electron irradiation in vacuum.

Laser-induced plasma and laser-triggered discharge characteristics in low vacuum

This paper describes the characteristics of laser-induced plasma and laser-triggered discharge in low vacuum. Experimental results revealed that laser-induced plasma characteristics, such as plasma light intensity, probability of plasma production and plasma profile highly depended on residual gas pressure. Furthermore, the laser-induced plasma gives rise to discharge generation under lower voltage than the static breakdown. These experimental results will be fundamental data to clarify the relation among outgassing, charging and discharging in vacuum for space power technology.

Vacuum discharge phenomena under non-uniform field conditions

For the development of space power applications, the analysis of the discharge phenomena in low and medium vacuum, in particular under nonuniform electric field, is required. Although the data under such conditions are very important for the insulation design of space power apparatus, few studies have been reported. This paper describes the review results of the research activities on discharge phenomena under nonuniform fields in low and medium vacuum conditions. DC discharge inception voltage for various electrode configurations in He gas, Ar gas and air in vacuum range from 10/sup 5/ to 10/sup -1/ Pa were measured. In order to discuss quantitatively the change of the discharge characteristics with the vacuum pressure, an image processing method was introduced. Based on the analysis, the dependence on the discharge inception voltage and length of the discharge path on the residual gas pressure under nonuniform field conditions was clarified and agreed well with the dependence formerly obtained for the uniform field condition.