Suppressing an electrical breakdown of vacuum gap is one of principal factors to achieve higher performance and reliability of advanced facilities, such as particle accelerators, space crafts, vacuum interrupters, etc. Electrical breakdown mechanism of a vacuum gap is quite different from that of a gap in gases, since the electron-gas molecule collision process in vacuum is inconceivable. It is regarded that the electrical breakdown in vacuum is initiated by particles generated on the electrode surface. This article refers to breakdown mechanisms of vacuum gaps on the basis of particle generation on the electrode surface. Firstly, difference of the breakdown mechanism in gas and vacuum is explained. Secondly the outline of field electron emission based breakdown mechanism, namely anode initiated breakdown and cathode initiated breakdown, is given. Then the microparticle based mechanism, so called clump theory, is explained. In addition several experiments to improve hold-off voltage of vacuum gap are described.

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  This article focuses on the insulation along solid dielectrics exposed to high voltages or high electric fields in vacuum. Solid insulators in vacuum acquire charge due to electron impacts. Coincidently with the impacts, adsorbed gas molecules are released from the insulator surface. Discharge, or flashover, occurs in the desorbed gas layer. As the amount of gas molecules is closely related to the surface charge distribution, charging phenomena are firstly explained from experimental and theoretical viewpoints. Then, the theory for estimating the flashover voltage is explained and its applicability is discussed based on the experimental flashover data for various materials and surface conditions.

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  It is well known that spacecrafts on orbits are charged up due to the interaction with space plasma and high energy charged particles. Those charging phenomena are categorized into two. One is “surface charging” and the other is “internal charging”. The surface charging involves absolute charging of the conductive satellite body and differential charging occurring on different surface materials on the satellite. On the other hand, the internal charging occurs on the insulating materials and/or electrically floating conductors installed inside the spacecraft body. When the level of the differential charging and/or internal charging reaches some threshold, ESD's (Electrostatic Discharges) will possibly occur. The author reviewed these ESD phenomena based on the ground experiments simulating space environments.

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  Voltage holding capability of a large negative ion source for fusion application is experimentally examined, which is characterized by multiple-stage acceleration with multiple-apertures over 1000 on large-area grids of 2 m² for the multiple-beamlet accelerations. From the observation of the vacuum discharge between the grids, it was found that the aperture generated 10 times larger dark current than the flat region and initiated the vacuum discharge associated with the breakdown. As a result, it was found that the sustainable voltages were dominated by not only the surface area but also the number of the apertures. Because these effects were originated in the area effects by weak and strong electric field profiles, these results implied the surface integration of the electric field were the key parameter for the vacuum insulation.

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  We constructed two types of coincidence analyzers those can be used for Auger photoelectron coincidence spectroscopy and electron ion coincidence spectroscopy. The first one is designed for high electron energy resolution measurements, and consists of a double-pass coaxially symmetric mirror electron energy analyzer (DP-ASMA), a double-pass cylindrical mirror electron energy analyzer (DP-CMA), a time-of-flight ion mass spectrometer (TOF-MS), a magnetic shield, a conflat flange, an xyz stage, and a tilt adjustment mechanism. The second one is dedicated for high sensitivity measurements, and involves an ASMA with a large pinhole, DP-CMA with a large pinhole, and a TOF-MS.

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