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The discharge mode characteristics of cathodes may strongly influence the discharge stability and performance of electrostatic thrusters. In this article, discharge characteristics and mode transition phenomenon of the ring-cusp magnetically confined plasma bridge neutralizer (RCM-PBN) were experimentally studied using argon as the working gas. The dependences of anode current and oscillation amplitude on anode voltage, argon flow rate, heater power, and cathode-to-anode distance were investigated. Plasma properties were measured and plasma plume images were taken under different discharge modes. Two distinct discharge modes were observed during the experiments: high oscillation mode and low oscillation mode. In the high oscillation mode, the plasma plume appears dim, the anode current is low, and the oscillation level is more than 2%. While in the low oscillation mode, a spot-like structure close to the orifice is observed. The plume becomes brighter, the anode current increases, and the oscillation level decreases below 2%. The RCM-PBN was found to transition into the low oscillation mode by increasing anode voltage, flow rate, heater power and by decreasing the cathode-to-anode distance. [ABSTRACT FROM AUTHOR]

The plasma bridge neutralizer (PBN) based on a tungsten filament is a promising technique of a thermionic DC electron source where a hot filament is immersed in an inert gas flow and electrons are "bridged" from a small orifice to the ion beam. PBNs have been widely used in space propulsion and industrial applications due to their relatively simple structure and low power consumption. However, they have well-known disadvantages, namely, low emission current density and short lifetime. In this article, we propose a novel ring-cusp magnetically confined PBN (RCM-PBN) to address these issues. In the RCM-PBN, electrons are confined by a ring-cusp magnetic field, which improves the ionization efficiency and reduces the discharge chamber wall losses. Electrical insulation of the orifice plate from the chamber wall prevents a large number of electrons from being collected by the orifice plate, which greatly improves the extracted electron current. The effects of different operating parameters on the extracted electron current were studied through experiments. It was found that the increase in the extracted electron current with the extraction voltage was related to the anode spot formation. Analysis of the gas utilization factor and electron extraction cost shows that the optimal operating condition was obtained at an argon mass flow rate of 1.2 SCCM and a heater power of 45 W. At its optimum, a stable electron current of 1.1 A was extracted from the RCM-PBN with a gas utilization factor of 12.8 and an electron extraction cost of 143 W/A. [ABSTRACT FROM AUTHOR]

In this article, we show the feasibility of improving the performance of a high voltage and high repetition avalanche transistor pulse generator by applying a two-phase immersion cooling technique. The forced air cooling, single-phase immersion cooling, and two-phase immersion cooling techniques were applied to a 60-stage subnanosecond pulse generator. The experimental results indicate that the two-phase immersion cooling technique can effectively control the temperature rising of the avalanche transistor and, thus, reduce the time base jitter and voltage amplitude jitter. Five types of dielectric fluid with different boiling temperatures were comparatively studied. The FC-72 was finally adopted for the balanced performance on voltage amplitude and repetition rate. The two-phase immersion cooling technique could reduce the thermal resistance between the case and ambient and increase the maximum power from 330 to 876 mW. With this cooling method, the surface temperature of the transistor can be effectively controlled below 62 °C. The pulse generator could achieve outstanding performance with a voltage of 2350 V and a rise time of 180 ps. It can work stably at 200 kHz for more than 30 min, and the burst repetition rate could be 260 kHz within 1 min. This article offers new perspectives in the design of high repetitive avalanche transistorized subnanosecond generators. [ABSTRACT FROM AUTHOR]

The wall-less Hall thruster is expected to overcome the issue of wall erosion in miniaturized Hall thrusters. A unique structure of negative gradient magnetic field is used to constrain the plasma behavior. In this paper, permanent magnet rings with different thicknesses were used to adjust the magnetic field strength, and its effect on discharge characteristics and thrust performance was experimentally investigated. The experiment results show that increasing the magnetic field strength can reduce the current oscillation and increase the maximum operating voltage. Within a limited range, increasing the magnetic field strength can significantly improve thrust performance. However, excessive magnetic field strength leads to a decrease in anode efficiency. The optimal magnetic field strength appears at 121 mT, with a thrust of 5.5 mN, a specific impulse of 935 s, a thrust power ratio of 51 mN/kW, and an anode efficiency of 23.1 %. Based on the analysis of different efficiency parameters and plume images, under excessively high magnetic field strength, the shortening of the ionization region may lead to intensified leakage of neutral atoms, which may be the primary reason for the reduction in propellant utilization and anode efficiency. • The effect of magnetic field strength on discharge characteristics and thrust performance was experimentally studied. • Increasing the magnetic field strength can suppress the anode current oscillation and enhance the maximum operating voltage. • There is an optimal magnetic field strength to maximize the thrust performance. • The reduced propellant utilization under high magnetic field strength is the main reason for the decrease in performance. [ABSTRACT FROM AUTHOR]

This paper studied the plasma plume evolution process of a capillary discharge based pulsed plasma thruster. Time-resolved imaging and optical emission spectroscopy were applied to investigate the plume morphology and plasma species characteristics. It showed that ionized particles (mainly C II and F II) were accelerated early in the pulse and neutral particles (mainly C I and F I) later. An optical time-of-flight (OTOF) method was developed using a photodiode array combined with narrow bandpass filters. The equivalent streaming velocity of the plasma plume was evaluated with the OTOF technique. Measurements of individual species showed that both the ionized and the neutral species could be effectively accelerated by gasdynamic forces, and the ionized particles could reach a higher velocity. A Doppler shift measurement of the plasma plume was also performed to compare it with the findings from the OTOF method. The plasma plume streaming velocity of a thruster with a discharge energy of 5 J was measured and found to be (25.34 ± 0.17) km/s (OTOF) and (22.36 ± 4.02) km/s (Doppler shift). In addition, differences between the operation processes of the capillary discharge based pulsed plasma thruster and the electromagnetic pulsed plasma thruster were analyzed. [ABSTRACT FROM AUTHOR]

In this paper, a 60-stage Marx circuit with the two-phase immersion cooling technique is presented. The performance parameters of the pulse generator with phase change immersion cooling and forced air cooling are compared. The results show that the maximum repetition rate is increased from 80 to 260 kHz, the pulse drift is significantly reduced from 900 to 200 ps, and the temperature rise of transistors is controlled effectively to enhance the stability of the amplitude. We also studied the influence of temperature on the turn-on and turn-off processes of the transistors to show the importance of heat management under high repetition conditions. Finally, a pulse generator is developed with the repetition rate of 200 kHz, the rise time of 180 ps, and the amplitude of 2350 V on a matched 50 Ω resistive load, which can work for more than 30 min. [ABSTRACT FROM AUTHOR]

The external discharge plasma thruster removes the annular channel of the traditional Hall thruster, allowing the ionization region to appear outside the thruster, which is expected to reduce the plasma-wall interaction and extend the thruster lifetime. Due to its simpler structure and lighter weight, the external discharge plasma thruster is expected to be scaled down to lower power levels. In this work, the effect of radial scaling down on the performance of low-power external discharge plasma thrusters was experimentally investigated by comparing the discharge characteristics, thrust performance, beam characteristics, and spectral characteristics of thrusters with two different anode diameters of 15 and 30 mm. The 30 mm thruster has thrust of 78–235 mN, specific impulse of 566–1071 s, and anode efficiency of 11.8–23.4% at 78–235 W anode power. Compared with the 30 mm thruster, the anode power and thrust of the 15 mm thruster were decreased by approximately half. Plume diagnostic results show that the 15 mm thruster had a higher current utilization, higher ion peak energy and more active ionization due to better electron confinement. However, the propellant utilization of the 15 mm thruster was lower because of the shortened ionization region length. • The manuscript's highlights are as follows: • The performance of external discharge plasma thrusters with anode diameters of 15 and 30 mm were experimentally compared. • The 15 mm thruster had a higher current utilization, higher ion peak energy and more active ionization. • The shortened ionization region length leads to a lower propellant utilization. • The external discharge plasma thruster has an advantage in thrust density. [ABSTRACT FROM AUTHOR]

The Blumlein pulse forming network (PFN) has widely been used in pulsed power technology to generate square waves with short pulse widths. In this paper, we developed a repetitive frequency square wave generator based on Blumlein PFN and pseudospark switch (PSS). A Blumlein PFN with unequal capacitances has been proposed, and the PFN parameters have been optimized for better output waveforms. A single-gap PSS with a withstand voltage of 40 kV and a repetitive frequency of 100 Hz has been designed to switch the Blumlein PFN. The experiment results show that the square wave generator can output pulses with a voltage of 26 kV, a rise time of 25 ns, and a pulse width of 90 ns on a matched load of 11 Ω. It has operated steadily for 10 h with a repetitive frequency of 100 Hz, and the jitter remains at around 1 ns after 1.05 × 106 shots. [ABSTRACT FROM AUTHOR]

With the development of technology, low-temperature plasma plays an increasingly important role in industrial applications. The industrial application of low-temperature plasma has the following requirements for plasma, high electron energy, low macroscopic temperature, and uniformity. Low-temperature plasma driven by nanosecond pulses reflects more significant advantages in these aspects compared to direct current plasma and alternating current plasma. In this paper, a simple topology is proposed, which is based on the pseudospark switch and the diode opening switch. A pulse generator is developed, which can eventually output pulses with an amplitude of 106 kV, a rise time of 15.5 ns, a pulse width of 46 ns, and a maximum repetition rate of 1 kHz on a 260 Ω resistive load. The pulse generator can successfully drive needle-plate discharge plasma in ambient air. It has excellent parameters, stability, compactness, and a long lifetime. The proposed topology may be helpful for nanosecond pulse generators with amplitude ranging from tens to hundreds of kilovolts, which could be widely used in industry. [ABSTRACT FROM AUTHOR]

In recent years, several novel avalanche transistor-based power synthesis topologies have been proposed to improve the output performance of pulse generators based on avalanche transistors. The most promising is the topology based on avalanche transistors Marx Bank Circuits (MBCs) and linear transformer driver (LTD). However, it suffers from the same problems as other semiconductor switch-based LTD generators. The greater the number of LTD modules, the higher the requirements for synchronization and drive capability of the trigger system. This paper proposes a new self-triggering topology for pulse generators based on avalanche transistors MBCs and LTD, which significantly simplifies the entire generator's requirement for trigger system synchronization and driving capability. First, the circuit topology and its operation principle are introduced. Then, three prototypes with one trigger LTD module and three self-triggering LTD modules are developed. The output characteristics are experimentally investigated. The results verify the feasibility of the proposed topology. Finally, the output amplitude and the rise time are 3.35 kV/3.7 ns, 4.12 kV/3.7 ns, and 4.88 kV/4.0 ns on a 25 Ω resistive load, respectively. All generators can operate at 1 kHz. The topology proposed in the article maximally simplifies the requirements for synchronization and drive capability of the trigger system for generators based on avalanche transistor MBCs and LTD. [ABSTRACT FROM AUTHOR]

Low-temperature plasma technology is widely used in various industrial fields, which require the plasma to be of large volume, diffuse, and stable. Furthermore, previous studies have shown that better plasma performance has been obtained by using generators with a high voltage, a high repetition rate, a fast rise time, and a short pulse duration. In this paper, a novel topology is proposed for such generators, which is based on magnetic switches and diode opening switches. A prototype is developed, and its output characteristics are investigated by varying essential parameters, such as the load resistance and the power supply voltage. The experimental results show that it can generate pulses with a voltage of 30.6 kV, a rise time of 7.1 ns, a pulse duration of 8.2 ns, and a maximum repetition rate of 12 kHz on a 300 Ω resistive load. The prototype has been successfully used to drive uniform plasma in ambient air. In the proposed topology, a diode is added to make the magnetic cores independent of each other, significantly simplifying the design calculation. It may help develop nanosecond solid-state generators. [ABSTRACT FROM AUTHOR]

Three series of oligomers containing functionalized imide and thiophene rings, namely the acceptor type bisthiazole imide oligomer with different length (PBTzIn , n = 1–8), the donor-acceptor type derivatives by introducing three electron donating thiophene rings into BTzI (PBTzI3Tm , m = 1–4), and derivatives by substituting the two hydrogen of center thiophene in PBTzI3Tm with fluorine (PBTzI3T-2Fm , m = 1–4), were designed based on some experimentally synthesized oligomers to clarify the roles of oligomer length, thiophene donor, and fluorine-substitution on the charge transfer properties. The charge transfer properties of these imide-functionalized thiazole-based oligomers were systematically studied using density functional theory and traditional Marcus theory in term of geometric structures, frontier molecular orbitals, ionization potentials (IP), electron affinities (EA), reorganization energies, charge transfer integrals, crystal packing models, and charge transfer mobilities. The IP/EA decreased/increased with increasing oligomer length in PBTzIn , rendering long PBTzIn oligomers showing both p- and n-type characteristic of organic semiconductors. The orbital energy level and mobility show saturation characteristics for n = 6. PBTzI8 shows high and balanced hole/electron mobility of 1.089/0.249 cm2 V−1s−1 and thus is good ambipolar semiconductor. The introduced three thiophene rings in PBTzI3Tm (m = 1–4) enhances the backbone conjugate property and reduces the hole injection barrier. Substituting thiophene H atoms with F further enhances the interaction between the non-covalent bond N...S and S...F and thus increases the molecular planarity, rendering PBTzI3T-2Fm (m = 1–4) being better semiconductor. PBTzI3T3 is ambipolar organic semiconductor with hole/electron mobility of 11.760/1.396 cm2V−1s−1, while PBTzI3T-2F3 and PBTzI3T-2F4 show better ambipolar organic semiconductor performance with much larger hole/electron mobility of 16.325/3.674 cm2V−1s−1 and 60.019/5.086 cm2V−1s−1. • Three series of oligomers containing functionalized imide and thiophen rings were designed. • Roles of oligomer length, thiophen donor, and fluorine-substitution on the charge transfer properties were clarified. • Geometric structures, frontier molecular orbitals, ionizationpotentials (IP), electron affinities (EA) were studied. • Reorganization energies, charge transfer integrals, crystal packing models, and charge transfer mobilities were predicted. • New types of n- and ambipolar semiconductors with very high mobilities were designed. [ABSTRACT FROM AUTHOR]

• Different electronic donating/accepting groups (Na, MP, MB, BN) were introduced onto the C8 position of 5-phenyl-5H-benzo[ b ]phosphindole 5-oxide(PhBPO). • Substitution effect on the structure and spectral properties was revealed according to density functional theory (DFT) calculation. • ground state geometries, orbital energy gaps, infrared (IR) spectra, UV–vis spectra, and fluorescence spectra of these PhBPO derivatives were described systematically. • These compounds possess easy-to-modify sites and are good candidates as potential functional building block. • These compounds are good fluorescence materials and substitution helps increase the fluorescence strength. The properties of phosphole-based π-conjugated compounds could be well tuned via simple chemical modification. Four π-conjugated groups with different electronic nature including naphthalen-2-yl, 4-methoxyphenyl, 4-Methyl benzoate, and 4-benzonitrile were introduced to the C8 position of 5-phenylbenzo[ b ]phosphindole 5-oxide (PhBPO) to reveal the relationship between structures and properties. Density functional theory (DFT) calculations were carried out to comparatively study the ground state geometries, orbital energy gaps, infrared (IR) spectra, UV–vis spectra, and fluorescence (FL) spectra of the five PhBPO derivatives. Good correspondences between the simulated and experimental IR and UV–vis spectral properties were found, proving the feasibility of the DFT method. The IR and Uv-vis spectra were assigned and explained in detail on the basis of the calculation results and the substituional effect of different substituents on the structure and spectral properties was revealed. The large FL oscillator strengths indicate these PhBPO derivatives are good candidates for fluorescent materials. PhBPO derivatives possess easy-to-modify sites and thus can be further utilized as functional materials. Image, graphical abstract [ABSTRACT FROM AUTHOR]