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In the alternative current field measurement (ACFM) technique, the nonzero value of liftoff distance for the magnetic sensor acts as a low-pass filter on surface crack signals, causing errors in crack detection and sizing. We present a blind deconvolution algorithm for liftoff evaluation and surface crack signal restoration. The algorithm employs the available closed-form expressions for the distribution of electromagnetic fields at the metal surface in the vicinity of a crack. To examine the accuracy of the algorithm, we use the original and the restored signals for crack sizing by a wavelet network inversion method. We present simulated and experimental results to demonstrate the role of the proposed algorithm in improving the inversion process. Index Terms--AC field measurement, crack sizing, deconvolution, liftoff, nondestructive evaluation.

The [V.sub.t] close-curve measurement shows that there exists a surface (X-Y) discharge contour in spite of a large sustain gap of 200/[micro]m greater than a barrier rib height of 125 [micro]m. This indicates that there are two different discharge modes, i.e., one mode is to produce the surface discharge, and the other is to produce the surface discharge including the plate gap discharge. The two discharge modes are examined and analyzed based on the [V.sub.t] close-curve movement on the cell voltage plane. The two discharge modes show the different discharge characteristics such as the sustain voltage level, [V.sub.t] close-curve behavior, luminance, and luminous efficiency. In addition, the spatial wall charge, electron density behaviors, and their current flows as a function of time are investigated in detail using the numerical analysis. Index Terms--Alternating current plasma display panel (ac-PDP), different discharge modes, large sustain gap, [V.sub.t] close curve.

Artificial pollution tests have been carried out to study ac flashover performances of various types of porcelain, glass, and polymeric insulators in this paper. The experimental results show that the polluted flashover voltage [U.sub.f] is affected by equivalent salt deposit density [[rho].sub.ESDD] and nonsoluble deposit density [[rho].sub.NSDD], the influences of which are independent of each other. Based on the analyses of the flashover voltages, the correction formulas of the flashover voltages [U.sub.f] of various insulators at various [[rho].sub.ESDD] and [[rho].sub.NSDD] levels are pointed out. The experimental results also show that there is a distinct difference among the flashover voltages of various types of polluted insulators. At the same pollution degree, the antipollution performance of the fog-type glass insulator is better than those of the outer-rib porcelain insulators. In terms of the effectiveness of creepage distance of insulators, the antipollution performance of polymeric insulators is superior to those of porcelain and glass insulators. Index Terms--[[rho.sub.ESDD], [[rho].sub.NSDD], ac, artificial pollution test, flashover performance, insulator, shape.

This paper deals with ac particle-triggered corona discharge as a follow-up to our previous research with dc voltage to clarify the particle-triggered corona discharge process in S[F.sub.6] gas. Corona current pulses, charges associated with a corona current pulse, and corona light pulses were observed with an aluminum ellipsoidal particle suspended in a parallel plane electrode system under a SF6 gas pressure range of 30 kPa [less than or equal to] P [less than or equal to] 50 kPa by changing the particle position. Corona mode, phase ([phi])-charge (q) characteristics as well as corona discharge processes were discussed and the following results were obtained. Ac corona mode depended on the instantaneous applied voltage, voltage gradient as well as the particle position and then, the [phi]-q characteristics were also affected by those parameters. Charges flow into the floating particle due to coronas on the both tips of particle and excite field fluctuations around the opposite side of particle in addition to the applied ac field. The field fluctuation in S[F.sub.6] gas by the corona charges was about 4% of the applied field and much lower than that in air gap which was about 70%. That is, the corona development was suppressed effectively by high electron affinity of S[F.sub.6] gas even in the case of floating particle. The less effective interference between coronas on the both side of particle in S[F.sub.6] gas results in an obscure local minimum in the breakdown voltage characteristics as the particle is in the vicinity of electrode as contrasted with a drastic fall in the breakdown voltage by the particle in air. Index Terms--GIS, S[F.sub.6], Ac corona mode, Ac particle-triggered corona process, [phi]-q characteristic