Paper ID- JVST-07-05-2023-1623
Tunable diode laser absorption spectroscopy was used to record the space-and time-resolved number density of argon metastable atoms, Ar(1s3) (Paschen notation), in plane-to-plane dielectric barrier discharges operated in a Penning Ar-NH3 mixture at atmospheric pressure. In both lowfrequency (LF 650 V, 50 kHz) discharges and dual LF-radiofrequency (RF 190 V, 5 MHz) discharges operated in α-γ mode, the density of Ar(1s3) revealed a single peak per half-period of the LF voltage, with rise and decay times in the sub-microsecond time scale. These results were compared to the predictions of a 1D fluid model based on continuity and momentum equations for electrons, argon ions (Ar+ and Ar2 +) and excited argon 1s atoms as well electron energy balance equation. Using the scheme commonly reported for Ar-based dielectric barrier discharges in the homogeneous regime, the Ar metastable kinetics exhibited much slower rise and decay times than the ones seen in the experiments. The model was improved by considering the fast creation of Ar2* excimers through 3- body reactions involving Ar(1s) atoms and the rapid loss of Ar2* by vacuum ultraviolet light emission. In optically thin media for such photons, they can readily reach the dielectric barriers of the DBD electrodes and induce secondary electron emission. It is shown that Ar2* and photoemission play a significant role not only on the Ar metastable kinetics, but also on the dominant ionization pathways and possible α-γ transition in dual frequency RF-LF discharges
Paper ID- JVST-07-05-2023-1622
In this study, a cylindrical triode ultrahigh vacuum ionization gauge with a screen-printed carbon nanotube (CNT) electron source was developed, and its metrological performance in different gases was systematically investigated using an ultrahigh vacuum system. The resulting ionization gauge with a CNT cathode responded linearly to nitrogen, argon, and air pressures in the range from ~4.0 ± 1.0 × 10−7 to 6 × 10−4 Pa, which is the first reported CNT emitter-based ionization gauge whose lower limit of pressure measurement is lower than its hot cathode counterpart. In addition, the sensitivities of this novel gauge were ~0.05 Pa−1 for nitrogen, ~0.06 Pa−1 for argon, and ~0.04 Pa−1 for air, respectively. The trend of sensitivity with anode voltage, obtained by the experimental method, was roughly consistent with that gained through theoretical simulation. The advantages of the present sensor (including low power consumption for electron emissions, invisible to infrared light radiation and thermal radiation, high stability, etc.) mean that it has potential applications in space exploration.
Paper ID- JVST-07-05-2023-1620
The high amplitude and steep overvoltage generated by the breaking of the vacuum circuit breaker in the wind farm damages the inter-turn insulation of the transformer. There is a certain difference between the simulation results of the traditional reignition model and the measured overvoltage. It is necessary to improve the simulation model to simulate the overvoltage condition of the transformer more accurately and then select appropriate overvoltage protection measures. In this paper, based on the physical process of dielectric recovery during the opening process of the vacuum circuit breaker, a model of dielectric strength recovery is built to simulate the arc reignition of the vacuum circuit breaker. The model was applied to compare the overvoltage protection effects of RC snubbers, surge arresters, and choke coils. The simulation results show that the overvoltage amplitude and reignition times calculated by the model proposed in this paper are closer to the measured values. Compared with the traditional linear curve reignition model, the accuracy was increased by 24% and 51.2%, respectively. The parameter value of RC snubbers, the connection mode of surge arresters, and the combination mode of choke coil have an influence on overvoltage suppression. Finally, a suitable suppression scheme is proposed by installing a combined arrester on the high-voltage side of the transformer and connecting a choke coil in series, which can limit the phase-to-ground voltage and the phase-to-phase voltage to 2.43 p.u and 3.24 p.u, respectively, and reduce the steepness from 157.2 kV/μs to 22.3 kV/μs.
Paper ID- JVST-07-05-2023-1619
Simultaneous plasma nitriding and ageing were performed on 15-5 PH stainless steel, being the treatment time (2, 4 and 6 h) and temperature (390, 440 and 490◦ C) the control variables. The highest value for core hardness was obtained at the longest time (6 h) and the lowest temperature (390◦ C). A long treatment time (6 h) and high temperature (490◦ C) have favoured the surface layer formation. The formation of expanded martensite αN and expanded austenite γN was observed using low nitriding temperature, while raising the temperature favoured the formation of the γ and CrN phases. When the treatment was carried out at temperatures of 440 and 490◦ C, a drop in the corrosion resistance was observed. Treatment at 390◦ C led to an improvement in the corrosion potential (EI=0), however, the samples processed at 390◦ C did not show a clear passive region.
Paper ID- JVST-07-05-2023-1618
For successful operation of Wendelstein 7-X (W7-X) control of plasma impurity content and fuel recycling is required. This can be achieved by using wall conditioning methods. During the first divertor operation campaign (OP1.2a) of W7-X glow discharge conditioning (GDC), weekly in hydrogen and daily in helium for impurity and hydrogen removal respectively, was used in the absence of the magnetic field. He electron cyclotron resonance heating (ECRH) discharges were applied for density control in hydrogen plasmas during experimental days. The optimization of GDC and He ECRH wall conditioning on W7-X are presented. Solutions for glow discharge ignition problems are examined. The suitable He – GDC parameters, i.e. anode current and neutral gas pressure, are defined to keep the balance between maximum possible hydrogen removal rate and minimum plasma – facing component (PFC) erosion. Sequences of short He – ECRH pulses, so-called pulse trains, has been successfully implemented. The effect of pulse train main parameter variation such as gas prefill, input power, pulse length, duty cycle is described. The efficiency of single He recovery discharges and pulse trains are compared. The results of this work show significant improvement of wall cleaning efficiency.