Increasing Repetition Frequency Research Articles

Efficient numerical simulation of atmospheric pulsed discharges by introducing deep learning

Plasma simulation is an important but sometimes time-consuming approach to study the discharge behaviors of atmospheric pulsed discharges. In this work, an efficient simulation method is proposed by introducing deep learning to investigate the discharge characteristics driven by very short pulsed voltages. A loss function is designed and optimized to minimize the discrepancy between the Deep Neural Network (DNN) and the verified fluid model. The prediction data obtained via well-trained DNN can accurately and efficiently reveal the key discharge characteristics, such as the waveforms of discharge current and gap voltage, spatial profiles of charged particles density and electric field. The spatial distributions of charged particles density and electric field obtained from DNN are also given to unveil the underlying mechanisms. Additionally, the predictions from deep learning and the formula analysis both highlight that the breakdown voltage and current density can be effectively reduced by increasing repetition frequency, which quantitatively agrees well with the experimental observations. This study provides a great potential promise for vastly improving the simulation efficiency by introducing deep learning in the field of atmospheric plasmas computation.

Modeling study on the effects of pulse rise rate in atmospheric pulsed discharges

In this paper, we present a modeling study on the discharge characteristics driven by short pulsed voltages, focusing on the effects of pulse rise rate based on the fluid description of atmospheric plasmas. The numerical results show that the breakdown voltage of short pulsed discharge is almost linearly dependent on the pulse rise rate, which is also confirmed by the derived equations from the fluid model. In other words, if the pulse rise rate is fixed as a constant, the simulation results clearly suggest that the breakdown voltage is almost unchanged, although the amplitude of pulsed voltage increases significantly. The spatial distribution of the electric field and electron density are given to reveal the underpinning physics. Additionally, the computational data and the analytical expression also indicate that an increased repetition frequency can effectively decrease the breakdown voltage and current density, which is consistent with the experimental observation.

Behaviors of field emitters under pulsed voltages

In this paper we described our study of the behaviors of field emitters driven by square-wave voltages. We observed phenomena under pulsed voltages that generally do not manifest themselves under direct-current voltages. We interpreted these phenomena with the cathode and anode combined treated as equivalent to a resistor and a condenser in series connection. First, because of the delay caused by the charging process of the condenser, the waveform of the voltage across the cathode-anode gap was remarkably distorted. Second, the resistor led to considerable attenuation in field emission, which was clearly observable within each pulse and became more dramatic with increasing repetition frequency of the pulses. Furthermore, the field emission currents under direct-current voltages were lower than those under pulsed voltages. This disparity is attributed to rising resistance in the circuit with rising temperature. We also discussed the restrictions that the waveform distortion and current attenuation could impose on potential field emitter applications.

Analysis of damage threshold of K9 glass irradiated by 248-nm KrF excimer laser

The theoretical model of K9 glass irradiated by a 248-nm KrF excimer laser was established, and a numerical simulation was performed to calculate temperature and thermal stress fields in the K9 glass sample using the finite element method. The laser-induced damage thresholds were defined and calculated, and the effect of repetition frequency and the number of pulses on the damage threshold were also studied. Furthermore, the experiment research was carried out to confirm the numerical simulation. The damage threshold and damage morphology were analyzed by means of a metallurgical microscope and scanning electron microscopy. The simulation and experimental results indicated that the damage mechanism of K9 glass irradiated by a KrF excimer laser was melting damage and stress damage, and the stress damage first appeared inside the K9 glass sample. The tensile stress damage threshold, the compressive stress damage threshold, and the melting damage threshold were 0.64, 0.76, and 1.05 J/cm2, respectively. The damage threshold decreased with increasing repetition frequency and number of laser pulses. The experimental results indicated that the damage threshold of K9 glass was 2.8 J/cm2.

The streamer-to-spark transition in a transient spark: a dc-driven nanosecond-pulsed discharge in atmospheric air

We present a study of the streamer-to-spark transition in a self-pulsing dc-driven discharge called a transient spark (TS). The TS is a streamer-to-spark transition discharge with short spark duration (∼10–100 ns), based on charging and discharging of the internal capacity of the electric circuit with repetition frequency 1–10 kHz. The TS can be maintained under relatively low energy conditions (0.1–1 mJ pulse−1). It generates a very reactive non-equilibrium air plasma applicable for flue gas cleaning or bio-decontamination.Thanks to the short spark current pulse duration, the steady-state gas temperature, measured at the beginning of the streamers initiating the TS, increases from an initial value of ∼300 K only up to ∼550 K at 10 kHz. The streamer-to-spark transition is governed by the subsequent increase in the gas temperature in the plasma channel up to ∼1000 K. This breakdown temperature does not change with increasing repetition frequency f. The heating after the streamer accelerates with increasing f, leading to a decrease in the average streamer-to-spark transition time from a few µs to less than 100 ns.

The Electronic and Optical Properties of IZO Thin Films Prepared by Pulsed DC Magnetron Sputtering

The electrical and the optical properties of In2O3-ZnO (IZO) films grown by using the pulsed dc magnetron sputtering method are investigated. We find that the average transmittance of the films in the visible range becomes over 85 % and that the optical band gap is about 3.3 eV, irrespective of the repetition frequency. The X-ray diffraction patterns for the film with a Zn content of 22 at.% show the crystalline structure to be amorphous. The growth rate and the sheet resistance of the film are shown to increase with increasing repetition frequency. Also, in a range of relatively thin thickness, the films grown with higher repetition frequency have higher refractive indices while, in the range of relatively thick thicknesses, the refractive index is nearly independent of the frequency.

The effect of pulse repetition frequency on the uptake into electropermeabilized cells in vitro with possible applications in electrochemotherapy

Electrochemotherapy is a technique where electric pulses in combination with chemotherapeutic agents are applied to tumor cells. In general, patients find electrochemotherapy tolerable, in spite of unpleasant sensations associated with contraction of muscles located beneath or in the vicinity of the electrodes. These contractions are due to the intensity of the electric pulses required for effective electropermeabilization of tumor cell membranes. Since a train of eight electric pulses with repetition frequency of 1 Hz is usually applied to the tumors, each pulse in the train excites underlying nerves and provokes muscle contractions. Therefore, for patients involved in electrochemotherapy, the use of pulses with repetition frequency higher than the frequency of tetanic contraction would represent reduced number of muscle contractions and associated unpleasant sensations. Our results of the uptake of Lucifer Yellow into electropermeabilized cells in vitro show that with increased repetition frequency the uptake stays at similar levels even at frequencies up to 8.3 kHz. On the basis of these results the possibilities for the clinical use of pulses with high repetition frequency in electrochemotherapy are considered.

Multiple hollow-cathode vacuum-ultraviolet plasma source excited by pulsed glow discharge

A multiple hollow-cathode vacuum-ultraviolet source excited by a pulsed glow discharge in a mixture of Xe and He has been developed. A MgF2 window with a diameter of 55 mm was used, and the repetition frequency was varied between 3 and 9 kHz. Stable output was maintained for about 3 h. Measurements of 147 nm radiant illumination were made at various distances from the window at various repetition frequencies. The peak radiant illumination decreased while the average radiant illumination increased with increasing repetition frequency. The output power density near the center of the window could be estimated from the relationship between measured radiant illumination and distance from the window. The peak and average output power densities at 6 kHz were 30 and 1.5 W/m2, respectively.

Phase and amplitude stability of an external cavity laser diode modelocked by optoelectronic feedback

The authors have investigated the stability of an external cavity laser diode modelocked by optoelectronic feedback. The measured picosecond pulses at repetition frequencies of up to 1.2 GHz exhibit an amplitude and phase jitter of the order of a few percent, increasing with increasing repetition frequency.