Hiroki Kojima

Mass spectroscopic investigation of the CH3 radicals in a methane rf discharge

Neutral CH3 radicals in a capacitively coupled rf discharge in methane have been detected with a quadrupole mass spectrometer utilizing a threshold ionization technique. The absolute density of CH3 radicals was measured at pressures from 0.5 to 20 mTorr, together with the ionic composition of the methane plasma. The principal ionic species were CH+5 and C2H+5 , except in the low‐pressure region, suggesting the importance of ion‐molecule reactions in the plasma. The lifetime of CH3 radicals in the afterglow of pulsed rf discharges was measured and explained in terms of the recombination reaction CH3 +CH3 →C2 H6.

Observation of CH2 radical and comparison with CH3 radical in a rf methane discharge

Methylene radical (CH2) in a rf methane discharge was detected using threshold ionization mass spectrometry. The absolute density of CH2 radical (∼109 cm−3) for the discharge in 10 mTorr CH4 with 10 W rf power was two orders of magnitude less than the density of methyl radical (CH3). The CH2 density in the afterglow of a pulsed rf discharge turned out to decay on a time scale (≤10 ms) much shorter than the decay time of CH3 radical (∼100 ms). The observed rapid loss of CH2 radical was attributed to a large sticking coefficient and the resultant surface loss, in addition to gas phase losses due to the CH2 reactions with CH3 and CH4.

Spatial distribution of CH3 and CH2 radicals in a methane rf discharge

Spatial distributions of neutral radicals CH3 and CH2 in a capacitively coupled rf glow discharge of methane were measured by threshold ionization mass spectrometry. A strong asymmetry of the density profile was found for the CH2 radical in the high‐pressure (∼100 mTorr) discharge. In addition, comprehensive measurements of electron energy distribution, ionic composition, and radical sticking coefficient were made to use as inputs to theoretical modeling of radicals in the methane plasma. The model predictions agree substantially with the measured radical distributions.

Surface charge accumulation and partial discharge activity for small gaps of electrode/epoxy interface in sf 6 gas

The electrical insulation reliability of solid spacers in gas insulated switchgears (GISs) is an important issue to achieve a safe operation of such equipment. Among different phenomena, charge accumulation represents the most important matter that can degrade the overall performance of these insulation systems. For this respect, this paper discusses the contribution of partial discharge (PD) activity by ac voltage application to charge accumulation in the small gap at the electrode/epoxy interface as one of the weakest points in GIS solid spacers. The partial discharge inception voltages for non-accumulated charge case (PDIV0) and after exposing to PD activity (PDIVn) are measured among different gap lengths, simulating delamination at the electrode/epoxy interface. The PD activity is generated using applied voltage with 1.2×PDIV0 for all gap lengths examined in this study (50~500 ¿m). In these measurements, PDIV increased with increasing the number of PD pulses as a result of accumulated charges. The accumulated surface charge density is estimated using the boundary equations and is compared for the different gap lengths. The accumulated charge density was larger for the smaller gap lengths. Comparing PD parameters with accumulated charge density enabled us to identify that the number of negative and positive PD pulses is the main parameter that corresponds to charge accumulation process.

Evaluation of UHF method for partial discharge measurement by simultaneous observation of UHF signal and current pulse waveforms

The partial discharge - current pulse waveform analyzing (PD-CPWA) system can measure partial discharge (PD) current pulses continuously in high speed, and is being used for the study of PD mechanisms in SF6 gas in a laboratory. On the other hand, the ultra high frequency (UHF) method is becoming popular for the on-site monitoring of gas-insulated switchgears (GIS). The former can detect PD with high accuracy and make it possible to analyze and understand discharge physics in detail. The latter is suitable to detect PD in actual power equipment, even in noisy site condition. Clarification of coincidence of PD characteristics measured with both methods is very important for further application of the UHF method. In this paper, PD current pulses generated from artificial defects in SF6 gas were synchronously measured with both methods. Measured data were analyzed and compared in many PD characteristics. As a result, many characteristics coincided well between them. However, the scatter of PD and the limited equipment performance of the UHF method may cause some detection failures. An appropriate and advanced use of the UHF method was pointed out.

Progress in Development of Superconducting Fault Current Limiting Transformer (SFCLT)

We have been developing Superconducting Fault Current Limiting Transformer (SFCLT), which has multifunction of both a superconducting transformer in a steady state as well as a superconducting fault current limiter in a fault condition. This paper introduces the progress in our SFCLT project since 1998, from Step-1 to the latest Step-5, with the concept, design, fabrication and test results of SFCLT. In the latest Step-5, we developed a 2 MVA, 22/6.6 kV class SFCLT with YBCO coated conductors and verified the fundamental function as a transformer, effective current limiting function as a fault current limiter, and recovery characteristics after the fault clearance with its operational criterion.

High-Tc Superconducting Fault Current Limiting Transformer (HTc-SFCLT) With 2G Coated Conductors

We developed a 3-phase, 100 kVA, 6600 V/210 V high temperature superconducting fault current limiting transformer (HTc-SFCLT) with functions of both superconducting transformer and fault current limiter. The HTc-SFCLT is characterized by the application of 2G coated conductors with the higher current limiting performance and flexibility than that of the HTc-SFCLT with 1G Bi2212/CuNi composite bulk material. Fundamental tests of the HTc-SFCLT were carried out, and the design parameters as a superconducting transformer and as a superconducting fault current limiter were verified.

Partial discharge activity in electrical insulation for high temperature superconducting (HTS) cables

This paper discusses electrical insulation characteristics of liquid nitrogen (LN2) / polypropylene (PP) laminated paper composite insulation system for the practical electrical insulation design of high temperature superconducting (HTS) cables. Partial discharge (PD) inception, propagation and breakdown (BD) characteristics under ac voltage application were obtained and discussed at atmospheric and pressurized conditions. Experimental results were analyzed quantitatively and revealed that void- type discharge was categorized as an important form of discharge to characterize the transition of PD activity leading to BD.

Contribution of molecular activated recombination to hydrogen plasma detachment in the divertor plasma simulator NAGDIS-II

We have performed experiments on detached hydrogen plasmas associated with molecular activated recombination (MAR) in a linear divertor plasma simulator. Reductions of the ion particle flux and heat load to the target plate were clearly observed in hydrogen plasmas with the hydrogen gas puff. Detailed analysis of Balmer series spectra with the Collisional-Radiative Atomic-Molecular data (CRAMD) code shows that MAR mainly appears as a weak dependence of the Balmer series emission intensities on experimental conditions in detached hydrogen plasmas. The plasma conditions necessary to obtain plasma detachment through MAR or EIR (electron ion recombination including the radiative and three-body recombination) in tokamak divertors are also discussed.

Current Limiting and Recovery Characteristics of 2 MVA Class Superconducting Fault Current Limiting Transformer (SFCLT)

We have been developing Superconducting Fault Current Limiting Transformer (SFCLT) with the function of both superconducting transformer in steady state and superconducting fault current limiter in fault condition. As the Step-5 of SFCLT project, 2 MVA class HTS-SFCLT using YBCO coated conductors with the ratings of 22 kV/6.6 kV has been developed. One of the important concerns for the development of HTS-SFCLT is the current limiting and recovery characteristics as well as the transformer function. Thus, in this paper, current limitation and recovery tests of the HTS-SFCLT are carried out. The current limiting test have revealed that the HTS-SFCLT exhibits the excellent current limiting function as a superconducting fault current limiter, where the prospective short-circuit current IPRO=784 Apeak is reduced to 34% at the first peak and 18% at the 5th cycles. The HTS-SFCLT has been able to recover into superconducting state immediately after the fault clearance. According to the transition of generated resistance of the HTS-SFCLT, the criteria of recovery into superconducting state are quantified for different combinations of load current before the fault and prospective short-circuit current during the fault.