The surface of a solid dielectric insulator becomes electrically charged when subjected to a high-voltage stress in vacuum. A method for calculating the surface flashover voltage based on the assumption that the discharge occurs in a layer of desorbed gases from the insulator surface is proposed. The electric field strength required to cause surface flashover is calculated by taking into account the secondary electron emission characteristics of the dielectric material. The dependence of the surface flashover field on the insulating material is deduced. A dependence of the flashover voltage on the insulator length to a power law of 0.5 is theoretically predicted. The calculated surface flashover voltage is compared with the previously reported measurements and good agreement is obtained.

The surface flashover of Teflon, plexiglass, quartz, Pyrex glass, Macor glass-ceramic, and sapphire solid insulators has been measured in vacuum (∼10−8 Torr, ∼10−6 Pa) and in atmospheric air using dc, ac (60 Hz), and 1.2/50-μsec lightning impulse voltages. The dependence of the flashover voltage on the following parameters is investigated: (1) spacer material, (2) diameter of the spacer, (3) spacer length, (4) number of spacers stacked in series, (5) air pressure in the range 10−6–105 Pa, (6) electrode material, (7) spark conditioning, and (8) the external resistance in series with the gap. At a fixed insulator length the flashover voltage decreases with increasing spacer diameter. The withstand voltage of spacers stacked in series increases with increasing the number of spacers. The dc flashover voltage of different insulating materials is theoretically calculated in vacuum as a function of the length of the insulator and compared with the experimentally obtained results. Good agreement is obtained.

The surface flashover of a Macor glass-ceramic conical insulator in ultrahigh vacuum is investigated using direct current (dc), alternating current (ac at 60 Hz), standard lightning impulse (1.2/50 μs), and combinations of dc+1.2/50 μs and dc+ac. The dc and the 1.2/50-μs flashover voltages are found to be at about the same level when the cone angle θ, which the surface makes with the applied field, is varied from −55° to +55°. At a fixed positive value of θ and a fixed thickness of the insulator, the lightning impulse, and the dc flashover voltages are higher than the ac. The level and the polarity of the dc prestress influence the combined dc+impulse flashover voltage in conical insulators. The ac+dc flashover depends on the dc prestress level and increases with increasing the prestress until a saturation is reached which is at a slightly higher voltage than the dc flashover. The electric field and the potential distributions at the interface of the cone insulator are calculated for different cone angles in the range 5° to 55°.

in the full system development, which included subsystem level technologies involving Aeronautics, Mechanical and Chemical Engineering, Laser and Optics and Diagnostics and Instrumentation. He has published 14 papers in national and international journals and reports. He is a member of Indian Laser Association, ILA and many awards to his credit.

Articles you may be interested inSelf-consistent simulation of the initiation of the flashover discharge on vacuum insulator surface Phys. Plasmas 19, 073516 (2012); 10.1063/1.4737195 Surface flashover of solid insulators in atmospheric air and in vacuumThe influence of the metal-dielectric junction on the electric field distribution along the solid insulator-vacuum interface is analyzed in detail in two widely used electrode-insulator geometries: (1) a cylindrical solid insulator with metal inserts at both ends and (2) a cylindrical solid insulator placed in recessed electrodes at both ends. The effects of the length of the metal inserts and the angle t/J between the solid insulator and the recessed electrode on the electric field and the flashover voltage have been theoretically investigated and experimentally validated. The permittivity of the solid insulator materials used is varied from 2.1 to 100, The surface flashover voltages of Macor glass ceramic and Teflon are measured in vacuum ( < 10-8 Torr) in different electrode-insulator contact geometries using dc, ac (60 Hz), and 1.2/50-,us lightning impulse. The measurements show good qualitative agreement with the behavior predicted from the computation.

The surface of a solid insulator in vacuum (and in a high gas pressure) becomes electrically charged when subjected to a high voltage stress. The surface charge density is proportional to the applied voltage. The magnitude of the surface charge density depends on the secondary electron emission characteristic, the geometrical shape, and the material of the insulator. The field enchancement, contributing to a lower withstand voltage of solid insulators in vacuum (and in high pressure gases), due to the presence of surface charge is computed along the surface of the solid insulator and at the triple electrode-solid insulator-vacuum junction near the anode and cathode. Different patterns of surface charge density distribution has been considered in order to evaluate their effects on the field enhancement. The polarity, the magnitude and the shape of the distribution of the surface charge density has been found to have a considerable effect on the field enhancement. The observed influence of the charge distribution patterns on the electric field suggests that in order to obtain the experimentally observed field enhancements, the insulator surface must possess a positive surface charge except in the region close to the anode, where it is negatively charged. The electric field and the potential distributions of the interfacial boundary between the solid insulator and vacuum is computed for different realistic surface charge distributions. The computed electric field at the triple junction for varying values of the dc applied field is then compared with the measured values reported in the literature. Good agreement is obtained. The effects of the magnitude of surface charge density, the applied field and the length of the solid insulator on the field enhancement at the triple junctions are also investigated. Nine different solid insulator materials having a relative permittivity in the range 2.1–13 and electron impact energies in the range 20–60 eV are examined. The effect of the surface charge on the withstand voltage of the insulating vacuum gap bridged by a solid insulator is discussed.

Cobalt doped Titanium Dioxide (TiO 2 ) were synthesized by a standard solid-state reaction method. The dielectric constant and loss have been measured as a function of frequency (100 Hz-1 KHz) and temperature (50°C to 300°C). Similar behavior is observed between dielectric constant and dielectric loss. Both dielectric constant and dielectric loss are found to decrease with increasing frequency. Photoluminescence (PL) spectra were taken at different excitation wavelength. PL emission spectra shows a broad spectra in the range 400 nm to 500 nm.

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