Effect of external circuit on the self-bias of capacitively coupled plasma driven by tailored voltage waveforms

Radio-frequency capacitively coupled plasmas that incorporate structured electrodes enable increases in the electron density within spatially localized regions through the hollow cathode effect (HCE). This enables enhanced control over the spatial profile of the plasma density, which is useful for several applications including materials processing, lighting and spacecraft propulsion. However, asymmetries in the powered and grounded electrode areas inherent to the hollow cathode geometry lead to the formation of a time averaged dc self-bias voltage at the powered electrode. This bias alters the energy and flux of secondary electrons leaving the surface of the cathode and consequentially can moderate the increased localized ionization afforded by the hollow cathode discharge. In this work, two-dimensional fluid-kinetic simulations are used to demonstrate control of the dc self-bias voltage in a dual-frequency driven (13.56, 27.12 MHz), hollow cathode enhanced, capacitively coupled argon plasma over the 66.6–200 Pa (0.5–1.5 Torr) pressure range. By varying the phase offset of the 27.12 MHz voltage waveform, the dc self-bias voltage varies by 10%–15% over an applied peak-to-peak voltage range of 600–1000 V, with lower voltages showing higher modulation. Resulting ionization rates due to secondary electrons within the hollow cathode cavity vary by a factor of 3 at constant voltage amplitude, demonstrating the ability to control plasma properties relevant for maintaining and enhancing the HCE.

In recent years the evolution of more sophisticated military avionic systems has required a corresponding reduction in component size and an increase in component life, precision, and reliability. To satisfy these more difficult design requirements, silicon solid-state devices are being used more extensively in power switching applications. Silicon controlled rectifiers (SCR's) can be used in place of relays or to drive relays with the proper ignition and extinction control circuits. The most general purpose external control circuits are the use of a transformer winding in series or in parallel with the SCR for quenching and a transformer winding connected between the trigger and the cathode for ignition. The use of separate transformers for exact timing and control of SCR's in dc circuits is an optimum control method since transformers have no moving parts and are usually more reliable than relays or pure transistor networks. In ac circuits, where waveform zeros are not used for switching, transformers can provide a convenient, precise method of SCR control. The purpose of this paper is to provide an optimum procedure for the designing of single primary and secondary winding ignition and extinction transformers (as shown in Fig. 1) for controlling SCR's. In addition, this paper provides a transformer core and winding specification synthesis that minimizes the over-all transformer size. A transformer secondary can provide a voltage zero in series with the SCR which allows the center p-n region to naturally recombine its excess charge carriers. It can also reset an SCR by providing both an external short across the SCR and the required SCR reverse current to deplete the excess charge carriers in the region of the center junction. This paper presents six circuits and their design parameters for setting and resetting SCR's. There are two ignition circuits and four quench circuits. For each of the six transformer circuits a relizability theorem, using the external circuit parameters, is provided so that a nonrealizable synthesis can be avoided. Ignition and extinction of SCR's require either a current impulse or a voltage impulse. Consequently the transformer design procedure is based upon certain essential elements of the full equivalent circuit, which is shown in Fig. 2. These transformers are intermittent in operation and do not require phase and amplitude linearity; hence the significant circuit design parameters are the turns ratio N, coil resistances R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ss</inf> and R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</inf> , and primary shunt inductance L <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</inf> . The procedure developed in this paper is based upon these fundamental transformer parameters with derived criteria for maximum values of leakage inductance L <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> and equialent shunt capacitance C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</inf> (both referred to the primary side). A simple synthesis procedure is also presented in this paper for the determination of actual transformer core and winding dimensions based upon the equivalent circuit parameters of Fig. 2 and the known external circuit environment.

Impedance matching is crucial for optimizing plasma generation and reducing power reflection in capacitively coupled plasmas (CCP). Designing these matchings is challenging due to the varying and typically unknown impedance of the plasma, especially in the presence of multiple driving frequencies. Here, a computational design method for impedance matching networks (IMNs) for CCPs is proposed and applied to discharges driven by tailored voltage waveforms (TVW). This method is based on a self-consistent combination of particle in cell/Monte Carlo collision simulations of the plasma with Kirchhoff’s equations to describe the external electrical circuit. Two Foster second-form networks with the same structure are used to constitute an L-type matching network, and the matching capability is optimized by iteratively updating the values of variable capacitors inside the IMN. The results show that the plasma density and the power absorbed by the plasma continuously increase in the frame of this iterative process of adjusting the matching parameters until an excellent impedance matching capability is finally achieved. Impedance matching is found to affect the DC self-bias voltage, whose absolute value is maximized when the best matching is achieved. Additionally, a change in the quality of the impedance matching is found to cause an electron heating mode transition. Poor impedance matching results in a heating mode where electron power absorption in the plasma bulk by drift electric fields plays an important role, while good matching results in the classical α-mode operation, where electron power absorption by ambipolar electric fields at the sheath edges dominates. The method proposed in this work is expected to be of great significance in promoting TVW plasma sources from theory to industrial application, since it allows designing the required complex multi-frequency IMNs.

Atmospheric pressure non-thermal plasma discharges are attractive for a wide range of applications due to their operational flexibility. Among the different atmospheric pressure nonthermal plasma sources, atmospheric pressure non-thermal micro plasma discharge is a very recent development. However, the micron scale size of these discharges makes it difficult to measure basic plasma characteristics (species density, gas and electron temperature, electric field etc) and also to analyze the different interplaying physico-chemical processes. In this work comprehensive multi-dimensional hybrid models were developed to simulate DC and RF atmospheric pressure non-thermal plasma discharges, and validated against experimental data. The basic plasma characteristics and different physico-chemical processes were explored by performing systematic parameter variations. The developed discharge models included detailed discharge physics, gas phase chemistry, surface chemistry and external circuit model. DC micro plasma discharges in argon, hydrogen and methane-hydrogen gas were simulated. The effect of the external circuit was found to be crucial in predicting the discharge characteristics accurately. The simulations indicated the discharge to be operating as a 'normal' glow discharge. Predictions from the model compared favorably to the experimental measurements. A detailed surface chemistry model was developed for a DC methane-hydrogen micro glow to study the possibility of using these micro plasma discharges in plasma enhanced chemical vapor deposition. RF plasma discharges in argon gas together with an extensive external circuit was simulated; special attention was devoted to the effect of external circuit parameters on the discharge characteristics. The circuit elements were found to trigger change in the mode of operation. An atmospheric pressure plasma jet in helium-oxygen feed gas was also simulated to study the possibility of surface decontamination applications. To the author's knowledge, this is the first attempt where detailed simulation of atmospheric pressure micro plasma discharge has been conducted together with an external circuit. The discharge models were used to investigate the plasma characteristics, physicochemical processes and study the effect of the external circuit and process parameters on the discharge. Better understanding of these processes will enable the tailoring and optimization of the operating conditions.

Hot cathode ion sources are widely used in large-scale Neutral Beam Injection (NBI) systems, but in practical engineering, there are still some problems that affect their stable operation, so further research is needed on the mechanism of arc initiation and the operation of hot cathode ion sources. In a complex electromagnetic field environment, it is challenging to measure the initial discharge process at the ns scale. This research focuses on this transient process in the Experimental Advanced Superconducting Tokamak (EAST) NBI hot cathode ion source, exploring its mechanism through a 3D multiphysics coupled model, and further highlights the function of designing a “floating plate”. The effects of the main parameters of the ion source are discussed, including floating plate size, external circuit parameters, and magnetic field. Results show that, in the initial arcing process, the floating plate controlled by the external circuit has a slightly stronger space charge limitation on the filaments compared to the discharge chamber structure with pure anode side wall. But the entire arcing process can reach the steady state faster, and the wall loss of the emitted electrons in the subsequent steady state is less. Compared with the conventional assumption that the floating plate is fixed at 50V, in the case of considering the external circuit, the space charge limitation is smaller, and the filament can also quickly transfer to the required thermionic emission limited mode. When the floating plate area accounts for 15% of the side wall area of the discharge chamber, the above characteristic is more obvious. The arc voltage almost determines the energy of electrons at different positions and has an important influence on ionization efficiency. The divider resistor of the external circuit can affect the stability of the discharge process. The magnetic field generated by filament current seriously affects the electron trajectory and restricts the emission of primary electrons. The multiphysics coupled model in this research offers analysis methods and optimization ideas for similar ion sources, providing references for the design of hot cathode ion source.

Simultaneous Potential and Circuit Solution for Two-Dimensional Bounded Plasma Simulation Codes

Summary form only given. The paper reports results of an investigation and optimization of single-turn coilgun performance aimed at attaining the maximum velocity at a specified accelerated mass and specified constraints on the maximum permissible mechanical and thermal loads. Current distributions in conductors were calculated using the hybrid finite element boundary method modified to take into account the effect of the external circuit with constant inductance, resistance, and capacitance. Results are presented on the acceleration of metal solids with masses of a few grams to several hundred grams to velocities of 0.2 km/sec - 3 km/sec with an efficiency of up to 40% for realistic parameters of the external circuit.

Evolutions of distortions in voltage and current waveforms in atmospheric pressure radio frequency (RF) symmetrical alpha-mode discharges were obtained by varying the driving frequency and the length of connection coaxial cable in the external circuit. Based on the fast Fourier transform analysis on waveforms, the pronounced current and voltage waveforms are, respectively, attributed to series and parallel resonances occurring between the atmospheric pressure RF plasma and the external circuit. Despite the weak nonlinearity due to the geometrical symmetry and high collision-frequency at atmospheric pressure, variations of the external circuit such varying the length of the connecting cable can still influence discharges at an equal power.

Roles of external circuit and rhizosphere location in CH4 emission control in sequencing batch flow constructed wetland-microbial fuel cell

This paper deals with the analysis of mechatronic systems including piezoelectric materials used as sensors or actuators for stabilization and damping of mechanical vibration. This work presents the analysis of the flexural vibrating one-dimensional mechatronic system – a cantilever beam and the piezoelectric transducer bonded on the beam surface by means of a glue layer. The external RC circuit is connected to the transducer clamps. Dynamic equations of motion of the considered mechatronic system were written down using the discrete-continuous mathematical model, taking into consideration the influence of the connection layer and the external electric circuit. Dynamic flexibility of the mechatronic system was assigned on the basis of the approximate Galerkin method.

A generalized external circuit model for electrostatic particle-in-cell simulations

In this study, a one-dimensional particle-in-cell/Monte Carlo collision model is developed to investigate the effects of harmonic number and magnetic field variation on electron beam modulation discharges. The results show that increasing the harmonic number leads to greater voltage waveform asymmetry, which enhances sheath expansion on the powered electrode side. This enables more effective electron acceleration, resulting in higher plasma density, elevated electron temperature near the powered sheath, and the formation of a self-bias voltage. Additionally, under a fixed harmonic number, increasing the magnetic field strength initially increases the plasma density due to extended electron paths that enhance ionization. However, at stronger fields, the electron Larmor radius decreases, leading to more localized ionization near the injection side and reduced overall density. The ion energy distribution function at the powered electrode becomes narrower with decreasing peak energy, while that at the grounded electrode shows an increasing and broadened energy spectrum. These findings provide guidance for optimizing plasma density and ion energy control in plasma processing applications.

This paper discusses the feasibility of a terahertz-signal source made of an AlGaN/GaN superlattice. The negative differential conductivity, electrical domain formation, current oscillations, and power efficiency of a perspective source are described. The superlattice geometry and conduction band profile, which are distorted by polarization fields, are related to the oscillation frequency and power efficiency of the device. The optimal Al content, superlattice period, and the parameters of the external circuit that favor submillimeter wave generation are determined.

An analytical theory of high voltage reverse-biased p+-n-n+-structures picosecond switching into conducting state by pulsed lighting has been developed and a numerical simulation of this process has been performed. Combining the results of theory and simulation allowed us to obtain a simple relation between the parameters of structure, light pulse, external circuit and main characteristics of the process - the load current pulse amplitude and duration of switching process.

Effects of external circuit on MHD channel flow of a rarefied gas