Preionization Process Research Articles

Numerical investigation of discharge evolution and breakdown characteristics of ArF excimer lasers

The corona bar induced pre–ionization is a crucial preliminary process in the operation of ArF excimer lasers, directly impacting the uniformity and stability of output laser. The ultraviolet corona pre–ionization, as the mainstream method, is tightly coupled with the main discharge process, which complicates analysis. Here, we establish a numerical model of a single pulse discharge incorporating an external circuit to analyze the pre–ionization process and its influence on the breakdown characteristics. (1) By adopting detailed input parameters of photoionization model, we observe uniform and dispersed plasma propagation from the corona bar to the main gap. (2) An artificial boundary condition is proposed to investigate the phenomenological effect of high–energy electrons emission, emphasizing the influence of surface discharge along the cathode. (3) The propagation and breakdown characteristics of the two pre–ionization setup methods, photoionization and background electron density, are compared numerically. This study enhances the understanding of the pre–ionization process in ArF excimer lasers and provides theoretical insights for their optimization and design.

Micro-discharge in tortuous pores： “splitting-quenching” of primary ionization wave and the role of pre-ionization

Abstract Porous discharge has been widely studied recent years, however the discharge development inside the pores with complicated structure was little involved. In this paper, the discharge developments in vertical and tortuous pore are studied using a 2D fluid model established by COMSOL Multiphysics. The discharge develops symmetrically and bilaterally in vertical pore, and along the higher potential sidewall in tortuous pore, respectively. For tortuous pore, there are two pre-ionization process with negative and positive polarity successively, the latter one splits off from the former one and is in the form of multiple ionizing heads propagating simultaneously in string. The negative one can induce the primary discharge to deflect at the channel elbow joint in a “splitting-quenching” way or directly deflect it by bridging with it. For the postive one, the multiple ionizing heads can distort the electric field and short cut the potential and thus accelerate the primary discharge. Furthermore, the discharge can be enhanced deep inside the tortuous pore by the large number of seed electrons provided by pre-ionization. The influence of the dielectric constant is also studied, with the dielectric constant increases, the discharge can maintain bilateral morphology in tortuous pores for a deeper development distance.

Analysis of preionization effect of excimer laser

Excimer laser plays an important role in the national economic production because of its short wavelength, high repetition frequency and large energy. In the output characteristics of excimer lasers, high beam quality and high energy stability are the characters of high-end excimer light source, and whether the excimer laser system can sufficiently develop the preionization process is the key to output laser pulses with high beam quality and stable energy. This paper is mainly to study the preionization process of ArF excimer laser, and establish one-dimensional simulation calculation model and two-dimensional simulation calculation model. Based on the fluid calculation method of Comsol Multiphysics software, the numerical simulation analysis and calculation are carried out, with the study focusing on the ionization wave propagation process in the preionization process and the relationship between preionization and main discharge. The results show that the spatial structure of corona rod and main electrode should be considered comprehensively when an excimer laser system is designed. Appropriate preionization structure and voltage can make the main discharge triggered during the maintenance of free electrons, which can effectively reduce the discharge voltage, avoid streamer and arc discharge, and improve the output beam quality and system energy stability.

Experimental study of plasma gun-assisted field-reversed configuration formation

The field-reversed configuration (FRC) is a compact toroidal plasma system related to the research of alternative fusion reactors. Improving the FRC formation, which is critical to the improvement of its overall quality, is reported in this paper by using a plasma gun to assist the pre-ionization process. For the single-translated FRC, the poloidal magnetic flux increases around 30%, accompanied by an increase in the separatrix radius and plasma density, while its translation speed is found to decrease slightly. The estimated increase in particle inventory is more than 60%. The improvement of these parameters also extends the lifetime of the FRC.

A new rapid synthesis of hexagonal prism Zn-MOF as a precursor at room temperature for energy storage through pre-ionization strategy

In this paper, a new hexagonal prismatic Zn-MOF is rapidly synthesized at room temperature through a one-step precipitation method as precursor for the preparation of porous carbon. The SEM and GCD tests indicate that the pre-ionization process of BTC greatly accelerates the reaction speed between BTC and Zn ions, and only 0.5 h is required for the preparation of Zn-MOF with orderly morphology at room temperature, far less than 3–24 h of the existing hydrothermal synthesis. The derived porous carbon (BTCC) is provided with a considerable specific surface area of 1,464 m2 g−1 and suitable pores of 3.9 nm in size. Its richly porous structure offers a superior supercapacitor performance. The BTCC electrode offered a high specific capacitance and an excellent cycle stability. Furthermore, the assembled two symmetrical supercapacitors, C|1 M Na2SO4|C and C|6 M KOH|C, provide high energy density of 22.4 Wh kg−1 and 13.7 Wh kg−1, respectively. Their energy retention rates were 80.0% and 89.4%, respectively after 10,000 cycles at 20 A g−1. The proposed pre-ionization strategy is a facile, convenient and easy-to-industrial method for the preparation of new MOFs, thereby significantly reducing the manufacturing cost of porous carbon for energy storage.

Numerical study on the discharge pattern evolution in an atmospheric pressure helium dielectric barrier discharge under the variation of nitrogen admixture content

We report a systematic numerical study concerning the effect of traces of nitrogen admixtures on the discharge pattern dynamics in an atmospheric helium dielectric barrier discharge. By gradually increasing the N2 concentration from 1 to 1000 ppm, the discharge mode transformed from the stationary pattern, via quasi-uniform structure, stationary pattern, into the complementary pattern. Analyses revealed that Penning ionization exerted a significant influence on the electron distribution, surface charge deposition and electric field profile over the gap and thus induced the transitions in pattern structure. With the increase of N2 content, the average rate of Penning ionization within the afterglow stage presented a non-monotonic variation trend, which conformed to previous results, and seed electron level of the main discharge varied in a similar way. When the discharge was ignited with a large amount of seed electrons, the electron distribution prior to the breakdown phase tended to homogenize and the level of radial electric field during the pre-ionization process was restrained, contributing to the homogenization of the discharge. Furthermore, the formation of complementary discharge pattern was believed to be resulted from the excessive reduction of average electron density between two adjacent main discharges at above 500 ppm N2 level. The reduced electron concentration made the volume ionization more susceptible to the surface charge distribution and led to the disruption of discharge channel succession between positive and negative half cycles.

Pre-ionization by AC Ohmic coil operation in the TST-2 spherical tokamak

AC Ohmic coil operation experiments with frequencies up to 10 kHz were performed on the TST-2 spherical tokamak device, and the pre-ionization process was studied in detail. The minimum loop voltage for pre-ionization was 0.4 V, which corresponds to 0.5 V at the inboard limiter. Dependences of growth rate and saturation level of the process on various parameters were obtained, and they are compared with a time-dependent 0-dimensional model based on Townsend avalanche and loss along field lines. Most of the dependences are reproduced qualitatively by the model, and quantitative differences are within a factor of several. However, the external vertical field dependence of the appearance time, which is defined as the time to observe a plasma, and the isotope effect cannot be reproduced by the model. An ambipolar diffusion state which is predicted theoretically but mitigated experimentally is discussed. It was found that secondary electron emission at the limiter surfaces is a candidate mechanism to mitigate the state.

通道等离子体环境对稳态等离子体推进器壁面腐蚀的影响

稳态等离子体推进器工作过程中通道的等离子体环境参数会影响高能粒子束的分布，进而影响离子对壁面溅射的情况。本文就加入预电离腔及考虑电荷交换碰撞对壁面腐蚀的影响进行了研究。通过调整模型中预电离率参数，研究了预电离率大小对通道物理特性分布及对通道壁面常规腐蚀的影响。结果表明预电离不会影响通道内主要物理特性，但由预电离引入的额外等离子体会加剧壁面的常规腐蚀。通过加入蒙特卡洛碰撞模块得到了电荷交换碰撞对通道各项物理参数分布和壁面常规腐蚀的影响。结果表明当考虑电荷交换碰撞后，通道内高能离子密度减少，壁面常规腐蚀情况有所减缓。 The tunnel plasma environment can affect the distribution of high energy ion beam, and then the sputter-induced wall erosion. In this paper, the effects of preionization on the physical characteristic distributions and the wall erosion are investigated. The results indicate that the preionization do not affect the main physical characteristic in the discharge channel. While the wall erosion will be aggravated by the extra sputter plasma which is introduced by the preionization process. The effects of charge-exchange collision on the physical characteristic distributions and the wall erosion are also investigated. The results show that the ion density decreases and the wall erosion alleviates after taking the charge-exchange collision into consideration.

Exploration of strong-field double ionization of CS2 molecule in bichromatic counterrotating circularly polarized laser fields

By using classical ensemble method, we investigate the double ionization of CS2 molecule in linearly, the bichromatic counterrotating circularly polarized laser fields and the combination of bichromatic counterrotating circularly polarized laser fields and static field, respectively. The numerical results show that the ionization probability in the bichromatic counterrotating circularly polarized laser fields is about 2 order magnitude higher than that in linearly polarized laser field. When a static field is added, the ionization probability is the largest. Besides, the “knee” structure occurs at about 0.05PW/cm2 in linearly polarized laser field; whereas “knee” structure is disappeared in the bichromatic counterrotating circularly polarized laser fields and combined laser field. The corresponding momentum distribution of CS2 molecule presents a “finger-like” structure at about 0.05PW/cm2 in linearly polarized field. By analysing the energy distributions of double-ionized electrons versus time and corresponding trajectories, we find that, for linearly polarized case non-sequential double ionization (NSDI) is predominant at about 0.05PW/cm2, for bichromatic counterrotating circularly polarized laser fields, one electron ionizes after another which indicate sequential ionization process (SDI). When the static field is added, the two electrons undergoes a long pre-ionization process first and then ionizes one after another, and the pre-ionization process lasts longer than other two cases.

Implicit electrostatic particle-in-cell/Monte Carlo simulation for the magnetized plasma: Algorithms and application in gas-inductive breakdown**Project supported by the National Natural Science Foundation of China (Grant Nos. 11275007, 11105057, 11175023, and 11275039). One of the author (Wang H Y) is supported by Program for Liaoning Excellent Talents in University (Grant No. LJQ2012098).

An implicit electrostatic particle-in-cell/Monte Carlo (PIC/MC) algorithm is developed for the magnetized discharging device simulation. The inductive driving force can be considered. The direct implicit PIC algorithm (DIPIC) and energy conservation scheme are applied together and the grid heating can be eliminated in most cases. A tensor-susceptibility Poisson equation is constructed. Its discrete form is made up by a hybrid scheme in one-dimensional (1D) and two-dimensional (2D) cylindrical systems. A semi-coarsening multigrid method is used to solve the discrete system. The algorithm is applied to simulate the cylindrical magnetized target fusion (MTF) pre-ionization process and get qualitatively correct results. The potential application of the algorithm is discussed briefly.

Simulation and design of pre-ionization systems for Alborz tokamak

Alborz tokamak is an educational system for studying plasma phenomena in many physics and engineering experiments. A hot filament and a reverse discharge loop are used in the tokamak as the pre-ionization system. The hot cathode prepares a local initial electron density, then the reverse discharge loop trigger the ionization avalanche by means of inducing a toroidal electric field. The parameters of the hot filament are determined in order to produce the desired electron source. Filament temperature is simulated by using three-dimensional finite element method. The average values of filament temperature and electron density at the plasma core (at the end of pre-ionization process) were calculated and are about 2750 K and 1019m−3, respectively. The resultant electron density and equivalent plasma resistivity due to reverse discharge loop are also calculated. In this paper, the simulation results, optimum structural style, the obtained parameters, the temperature of different parts of the filament and produced electron density are presented and discussed.

The role of photo electrical effect in sustaining the preionization process in Plasma Focus device

In the present work an estimation shows that the photo effect on the cathode near the insulator has significant role in the breakdown of the Plasma Focus device. An analysis is made to establish the number of the emitted photons from the exited molecules and respectively the number of ejected electrons from the cathode due to the photo effect.

Performance Improvement of a Discharge-Pumped ArF Excimer Laser by Xenon Gas Addition

The effect of xenon (Xe) gas addition to a discharge-pumped ArF excimer laser was examined. When the partial pressure of the Xe gas was changed over a wide range, the output energy of the ArF excimer laser was found to become maximum at the Xe partial pressure of 20 mTorr and to be about three times higher than that without Xe addition. It was shown that Xe gas addition was effective in increasing the initial electron density which was produced in the preionization process, and that the increase of the initial electron density then produced a tendency to make the main discharge in the laser uniform in the direction of the optical axis.

Synchronization of ultraviolet-preionization in a Marx-driven TEA CO 2 laser

In a TEA CO 2 laser a two-stage spark gap was used to control the predischarge, which produced the ultraviolet-preionization, and to generate a trigger pulse for the Marx bank, which excited the main discharge. The time-sequence of the predischarge and the trigger pulse could be reversed so that the main discharge could commence before the preionization process. This set-up has enabled temporal synchronization on the laser performance, or basically the time delay between the pre- and the main discharges, to be investigated from about − 150 ns to 600 ns.

Reduction of Spectral Interferences from Na in Laser-Enhanced Ionization Spectrometry by Laser Pre-Ionization

Pre-ionization of Na atoms prior to the analysis of other analyte atoms using the Laser-Enhanced Ionization (LEI) technique in flames has been investigated in order to reduce the influence of spectral interferences (in this case of Na) to the measured LEI signal. A large fraction of the Na atoms present in the flame was ionized with the use of one laser system. A short time later (400 µs) another laser system was aimed at a position 4 mm above the first laser in the flame, thus probing the part of the flame in which the concentration of (neutral) Na atoms had been significantly reduced. Different excitation schemes were used for the preionization process. It was found that the degree of depletion of Na was 13% when the atoms were first excited to the 3 p state by 589-nm light and then photoionized by focused pump laser light (308 nm). When the atoms were excited in two resonant steps (3 s-3 p-7 d), 80% of all the Na atoms could be depleted. With all three lasers illuminating the atoms, finally, 83% of the Na atoms were depleted. (If laser-enhanced chemical processes can be neglected within the duration of the laser pulse, which is most probable in the case of ns pulse duration, the degree of depletion is equivalent to the degree of ionization.) The second laser system was then used for studies of the LEI signal from other elements in the same manner as above. It was then found that this Na pre-ionization did not interfere in any way with the detection of other elements which were detected 400 μs later (4 mm higher in the flame). This was illustrated by a measurement of Mg in a Na matrix in which the Mg signal recovery was 100% while the Na signal was decreased by a factor of 5 (resulting in a corresponding increase in selectivity).

New method for the excitation of a homogeneous discharge in a high-pressure CO2laser

A new method is proposed for the excitation of a homogeneous discharge in a high-pressure gas laser. It is shown that, under the experimental conditions employed, the preionization process is dominated by electrons which are formed in a preliminary discharge and penetrate into the cathode region of the main discharge. An important role is played by the photoionization processes in a double-discharge scheme utilizing a preliminary corona discharge.

Electron-impact ionization and appearance potentials

A method 1 for the computerized acquisition of ionization efficiency data followed by mathematical analysis using the energy-distribution difference technique,2 which is a considerable improvement on conventional mass spectrometric techniques, has been applied to the determination of the ionization and appearance potentials of a variety of compounds and processes. Comparisons of the electronimpact ionization potential data with photoelectron spectroscopic measurements show that it is possible to measure adiabatic ionization potentials by electron impact in favourable cases and indicates a reason for the consistently high values found by other methods. The accuracy of measurement of appearance potentials approaches that of photoionization methods and in one case evidence for a preionization process is found. A high activation energy process is shown to occur without the expected large kinetic shift in appearance potential, suggesting that this computerized method is very sensitive. The measurements also yield a heat of formation and ionization potential for the “phenyl” fragment ion from monosubstituted benzenes.

Photoionization and Absorption Coefficients of OCS

Absolute absorption and photoionization coefficients of OCS gas have been measured in the 600–960-Å region using a 1-m Seya–Namioka-type scanning vacuum ultraviolet monochromator. The Hopfield helium continuum was used as a background source with photoelectric detection of the dispersed light. A number of peaks were observed in both the absorption and photoionization spectra at wavelengths that agree well with those of a high-resolution photographic spectrum. A series of the “apparent emission type” which converged at 17.931 eV had the Fano shape for a preionization process in both the absorption and ionization spectra. The total continuum underlying the band structure had been estimated, as well as that fraction due to ionization and to dissociation. Fluorescence emission was observed for all wavelengths of excitation in the Hopfield continuum. Peaks in the spectrum of the total fluorescence yield coincided in wavelength with absorption bands of Rydberg-type series converging to a limit at 16.04 eV. Maximum of the continuum underlying the fluorescence was at about 810 Å.

Production of theE-Layer in the Oxygen Dissociation Region in the Upper Atmosphere

Starting with the generally accepted theory that the $E$-layer is formed by ionization of ${\mathrm{O}}_{2}$ in the region of its dissociation, the probable value of the absorption cross section of ${\mathrm{O}}_{2}$ for this ionization is calculated by utilizing the height distribution of ${\mathrm{O}}_{2}$ (in the transition region) as recently obtained by Moses and Wu. It is found that, depending upon the temperature gradient and the boundary temperature chosen for the region of dissociation, the necessary absorption cross section varies from 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ to 9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$. Hence, it follows that for the two current hypotheses of $E$-layer ionization, namely, pre-ionization by solar rays in the wavelength range 900A-1000A (Nicolet) and ionization by high energy photons emitted from the solar corona (Hoyle and Bates), the ionization cross section of ${\mathrm{O}}_{2}$ should also lie within this range. For the former, the rate of ion production (as obtained by application of simple Chapman formula and assuming the sun to be radiating like a blackbody) appears to be one hundred times more than the observed rate. The discrepancy is removed if it is assumed that of the molecules excited to the pre-ionization levels by absorption, only a small fraction (one in a hundred) undergoes ionization. For the high energy photons it is found that in order to have the necessary absorption cross section, the energy should be 181 ev rather than 325 ev as obtained by Hoyle and Bates. It is suggested that both the pre-ionization process and the ionization by high energy photons are operative in producing $E$-layer ionization. The former produces the normal $E$-layer and the latter (of different frequencies) intensifies the ionization at different levels producing the fine structure of the $E$-layer as reported recently.