Investigation on the shockwave induced by surface arc plasma in quiescent air

The shockwave induced by surface direct-current （DC） arc discharge is investigated both experimentally and numer- ically. In the experiment, the shockwave generated by rapid gas heating is clearly observed from Schlieren images. The peak velocity of the shockwave is measured to be over 410 m/s; during its upright movement, it gradually falls to about 340 m/s; no remarkable difference is seen after changing the discharge voltage and the pulse frequency. In the modeling of the arc plasma, the arc domain is not simulated as a boundary condition with fixed temperature or pressure, but a source term with a time-varying input power density, which could better reflect the influence of the heating process. It is found that with a reference power density of 2.8× 1012 W/m2, the calculated peak velocity is higher than the measured one, but they quickly （in 30 Its） become agreed with each other. The peak velocity also rises while increasing the power density, the maximum velocity acquired in the simulation is over 468 m/s, which is expected to be effective for high speed flow control.     

Electrical and thermal characterization of near-surface electrical discharge plasma actuation driven by radio frequency voltage at low pressure

The electrical and thermal characterization of near-surface electrical discharge plasma driven by radio frequency voltage are investigated experimentally in this paper. The influences of operating pressure, electrode distance, and duty cycle on the discharge are studied. When pressure reaches 60 Torr(1 Torr = 1.33322 × 10~2 Pa) the transition from diffuse glow mode to constricted mode occurs. With the operating pressure varying from 10 Torr to 60 Torr, the discharge energy calculated from the charge–voltage(Q–V) Lissajous figure decreases rapidly, while it remains unchanged between 60 Torr and 460 Torr. Under certain experimental conditions, there exists an optimized electrode distance(8 mm). As the duty cycle of applied voltage increases, the voltage–current waveforms and Q–V Lissajous figures show no distinct changes.     

Influence of air pressure on the performance of plasma synthetic jet actuator

Plasma synthetic jet actuator(PSJA) has a wide application prospect in the high-speed flow control field for its high jet velocity.In this paper,the influence of the air pressure on the performance of a two-electrode PSJA is investigated by the schlieren method in a large range from 7 k Pa to 100 k Pa.The energy consumed by the PSJA is roughly the same for all the pressure levels.Traces of the precursor shock wave velocity and the jet front velocity vary a lot for different pressures.The precursor shock wave velocity first decreases gradually and then remains at 345 m/s as the air pressure increases.The peak jet front velocity always appears at the first appearance of a jet,and it decreases gradually with the increase of the air pressure.A maximum precursor shock wave velocity of 520 m/s and a maximum jet front velocity of 440 m/s are observed at the pressure of 7 k Pa.The averaged jet velocity in one period ranges from 44 m/s to 54 m/s for all air pressures,and it drops with the rising of the air pressure.High velocities of the precursor shock wave and the jet front indicate that this type of PSJA can still be used to influence the high-speed flow field at 7 k Pa.     

Electric and plasma characteristics of RF discharge plasma actuation under varying pressures

The electric and plasma characteristics of RF discharge plasma actuation under varying pressure have been investigated experimentally. As the pressure increases, the shapes of charge–voltage Lissajous curves vary, and the discharge energy increases. The emission spectra show significant difference as the pressure varies. When the pressure is 1000 Pa,the electron temperature is estimated to be 4.139 e V, the electron density and the vibrational temperature of plasma are peak4.71×10~(11)cm~(-3) and 1.27 e V, respectively. The ratio of spectral lines I391.4/peak I380.5which describes the electron temperature hardly changes when the pressure varies between 5000–30000 Pa, while it increases remarkably with the pressure below 5000 Pa, indicating a transition from filamentary discharge to glow discharge. The characteristics of emission spectrum are obviously influenced by the loading power. With more loading power, both of the illumination and emission spectrum intensity increase at 10000 Pa. The pin–pin electrode RF discharge is arc-like at power higher than 33 W, which results in a macroscopic air temperature increase.     

Electrical and optical characteristics of the radio frequency surface dielectric barrier discharge plasma actuation

Electrical characteristics and optical emission spectrum of the radio frequency(RF) surface dielectric barrier discharge(SDBD) plasma actuation are investigated experimentally in this paper. Influences of operating pressure, duty cycle and load power on the discharge are analyzed. When the operating pressure reaches 30 kPa, the discharge energy calculated from the Charge–Voltage(Q–V) Lissajous figure increases significantly, while the effective capacitance decreases remarkably. As the duty cycle of the applied voltage increases, the voltage–current waveforms, the area of Q–V loop and the capacity show no distinct changes. Below 40 W, effective capacitance increases with the increase of load power, but it almost remains unchanged when load power is between 40 W and 95 W. The relative intensity I_(391.4)~peak/I_(380.5)~peak changes little as the operating pressure varies from 4 kPa to 100 kPa, while it rises evidently with the pressure below 4 kPa, which indicates that the RF discharge mode shifts from filamentary discharge to glow discharge at around 4 kPa. With the increase of load power, the relative intensity I_(391.4)~peak/I_(380.5)~peak rises evidently. Additionally, the relative intensity I_(371.1)~peak/I_(380.5)~peak is insensitive to the pressure,the duty cycle, and the load power.     

Modeling and optimization of the multichannel spark discharge

This paper reports a novel analytic model of this multichannel spark discharge, considering the delay time in the breakdown process, the electric transforming of the discharge channel from a capacitor to a resistor induced by the air breakdown, and the varying plasma resistance in the discharge process. The good agreement between the experimental and the simulated results validated the accuracy of this model. Based on this model, the influence of the circuit parameters on the maximum discharge channel number(MDCN) is investigated. Both the input voltage amplitude and the breakdown voltage threshold of each discharge channel play a critical role. With the increase of the input voltage and the decrease of the breakdown voltage, the MCDN increases almost linearly. With the increase of the discharge capacitance, the MDCN first rises and then remains almost constant. With the increase of the circuit inductance, the MDCN increases slowly but decreases quickly when the inductance increases over a certain value. There is an optimal value of the capacitor connected to the discharge channel corresponding to the MDCN. Finally, based on these results, to shorten the discharge time, a modified multichannel discharge circuit is developed and validated by the experiment. With only 6-kV input voltage, 31-channels discharge is achieved. The breakdown voltage of each electrode gap is larger than 3 kV. The modified discharge circuit is certain to be widely used in the PSJA flow control field.     

Experimental investigation on electrical characteristics and ignition performance of multichannel plasma igniter

Relighting of jet engines at high altitudes is very difficult because of the high velocity, low pressure, and low temperature of the inlet airflow. Successful ignition needs sufficient ignition energy to generate a spark kernel to induce a so-called critical flame initiation radius. However, at high altitudes with high-speed inlet airflow, the critical flame initiation radius becomes larger; therefore, traditional ignition technologies such as a semiconductor igniter(SI) become infeasible for use in high-altitude relighting of jet engines. In this study, to generate a large spark kernel to achieve successful ignition with high-speed inlet airflow, a new type of multichannel plasma igniter(MCPI) is proposed. Experiments on the electrical characteristics of the MCPI and SI were conducted under normal and sub-atmospheric pressures(P = 10–100 kPa). Ignition experiments for the MCPI and SI with a kerosene/air mixture in a triple-swirler combustor under different velocities of inlet airflow(60–110 m/s), with a temperature of 473 K at standard atmospheric pressure, were investigated. Results show that the MCPI generates much more arc discharge energy than the SI under a constant pressure; for example, the MCPI generated 6.93% and 16.05% more arc discharge energy than that of the SI at 30 kPa and 50 kPa, respectively. Compared to the SI, the MCPI generates a larger area and height of plasma heating zone, and induces a much larger initial spark kernel.Furthermore, the lean ignition limit of the MCPI and SI decreases with an increase in the velocity of the inlet airflow, and the maximum velocity of inlet airflow where the SI and MCPI can achieve successful and reliable ignition is 88.7 m/s and 102.2 m/s, respectively. Therefore, the MCPI has the advantage of achieving successful ignition with high-speed inlet airflow and extends the average ignition speed boundary of the kerosene/air mixture by 15.2%.     

中子辐照6H-SiC晶体中的钉扎效应

分别对两种不同的6H-SiC晶体进行了总通量为5.74×10~(18) n· cm~(-2)、8.2×10~(18) n· cm~(-2)和1.72×10~(19) n· cm~(-2)的不同剂量的中子辐照,利用X射线衍射等手段观测了中子辐照引起的晶体内部损伤.结果表明:两种样品在不同的中子通量辐照下,消除部分原始缺陷的、纯度较高的样品辐照后的内部损伤明显小于纯度较低、内部原始缺陷较多的样品.样品的辐照损伤不仅取决于中子辐照剂量,还取决于其辐照时的成分和结构.晶体的结构缺陷和杂质,作为一种"钉扎",在辐照过程中作用极大,它们导致了晶体的长程缺陷(如位错)的大幅度增加.     

Thermal and induced flow characteristics of radio frequency surface dielectric barrier discharge plasma actuation at atmospheric pressure

Thermal and induced flow velocity characteristics of radio frequency(RF) surface dielectric barrier discharge(SDBD)plasma actuation are experimentally investigated in this paper. The spatial and temporal distributions of the dielectric surface temperature are measured with the infrared thermography at atmospheric pressure. In the spanwise direction, the highest dielectric surface temperature is acquired at the center of the high voltage electrode, while it reduces gradually along the chordwise direction. The maximum temperature of the dielectric surface raises rapidly once discharge begins.After several seconds(typically 100 s), the temperature reaches equilibrium among the actuator's surface, plasma, and surrounding air. The maximum dielectric surface temperature is higher than that powered by an AC power supply in dozens of k Hz. Influences of the duty cycle and the input frequency on the thermal characteristics are analyzed. When the duty cycle increases, the maximum dielectric surface temperature increases linearly. However, the maximum dielectric surface temperature increases nonlinearly when the input frequency varies from 0.47 MHz to 1.61 MHz. The induced flow velocity of the RF SDBD actuator is 0.25 m/s.     

Calculation of prompt fission neutron spectrum for 233U（n,f）reaction by the semi-empirical method~

The prompt fission neutron spectra for the neutron-induced fission of ²³³U for low energy neutrons (below 6 MeV) are calculated using nuclear evaporation theory with a semi-empirical method, in which the partition of the total excitation energy between the fission fragments for the n_th+²³³U fission reactions is determined by the available experimental and evaluation data. The calculated prompt fission neutron spectra agree well with the experimental data. The proportions of high-energy neutrons of prompt fission neutron spectrum versus incident neutron energies are investigated with the theoretical spectra, and the results are consistent with the systematics. The semi-empirical method could be a useful tool for the prompt evaluation of fission neutron spectra.