On plasma-neutral gas interaction

The paper emphasises the importance of plasma-neutral boundary layer in a wide variety of physical phenomena occurring in laboratory and cosmic plasmas. The interaction of a magnetised plasma stream penetrating a neutral gas cloud is discussed in the light of Alfvén’s critical velocity and Varma’s threshold velocity on the ionising interaction. Interaction of a moving magnetised plasma with a stationary neutral gas has been studied and described. The device comprises of a plasma gun and an interaction region where neutral gas cloud is injected. The interaction region is provided with a transverse magnetic field of upto 1000 G. Several diagnostics deployed at the interaction region to make measurements on the macroscopic parameters of plasma and neutral gas are described. The parameters of discharge circuits are measured with high current and voltage probes. An interaction between a magnetised plasma stream and a neutral gas cloud is demonstrated. It is shown that this interaction does not have Varma’s threshold on their relative velocity. The Alfvén’s critical velocity phenomenon is shown to depend on the integrated column neutral gas density that a plasma stream encounters while penetrating through it and not on the neutral gas density in the range of 10¹⁷–10²¹ m^?2.     

Acoustic perturbations in a pulsedRF-plasma

The response of a stationary weakly ionized plasma to a density perturbation in the neutral gas component was studied in a neon plasma with the following typical properties: electron density ¯N _e≈8×10¹² cm^?3, electron temperature on the axis of the vesselT _e0≈3.0 eV; neutral gas densityN _n≈1×10¹⁷cm^?3 and neutral gas temperatureT _n0≈600 °K. A neutral density perturbation, generated 50 cm apart from the plasma, produces a fluctuation in the ion density and a sharp spike in the differential voltage of a floating double probe. The experimental observations demonstrate the propagation of an ion sheath and of an electric field perturbation together with the neutral density perturbation. An interpretation of the plasma response to acoustic wave pulses has been proposed by Ingard and Schulz in a theory on acoustic wave modes in a weakly ionized gas. The experimental results are in good agreement with the theoretical expectations.     

A measurement of deuterium neutrals by the balmer-series in the STP-2 high beta screw pinch tokamak

The neutral particle density of a deuterium plasma in the STP-2 screw pinch tokamak has been determined spectroscopically. The particle density for the ground state of deuterium is calculated from the rate equations for population density of the excited levels with principal quantum numbers 3 and 4; both of the population densities were obtained from intensities of the Balmer alpha and beta lines, and the rate coefficients for electronic collision were estimated from the electron density (10¹⁴/cm³) and temperature (10 eV) measured by laser scattering. We find that the neutral density is approx. 2 × 10¹²/cm³ at the center of the plasma and 2 × 10¹⁴/cm³ on the periphery. The time history of the neutral deuterium density is consistent with the observed increase in plasma density.     

Electrical Conductivity of Noble Gas Plasmas in the Warm Dense Matter Regime

The electrical conductivity of noble gas plasmas is investigated by using the relaxation‐time approximation in the density range 10^–5–10 g cm^–3 and temperature range 10⁴–10⁵ K. The electrical conductivity calculated shows reasonable consistency with shock wave experiments and theoretical simulations at above 2 × 10⁴ K where the Coulomb interaction dominates. A nonmetal to metal transition in helium plasma is predicted at 2.4 g cm^–3 and shown reasonable agreement with the most recent shock wave experiment (above 1.9 g cm^–3). Furthermore, the insulator‐metal transition densities of all the noble gas plasmas are predicted and compared with available results (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Plasma–wall transition in the quasi‐neutral region of collisional and stationary plasmas in a magnetic field enclosed by totally absorbing walls

The structure of stationary electron–positive ion plasmas in spatially limited vessels is analysed with special emphasis on the plasma–wall transition using different physical models. Basic investigations are carried out in a two‐fluid model, which is supplemented by ionization and oblique magnetic fields. Collisions between the two particle species were taken into account, as well as the dependence of the collision frequency on the particle density. For the case of non‐vanishing magnetic fields, electrons are not assumed to be in Boltzmann equilibrium. The investigated one‐dimensional domain is limited by totally absorbing walls on each side. Stationary states are considered, in which ionization sources balance the wall losses. To also take into account kinetic effects, simulations in a quasi‐neutral hybrid model are performed. The hybrid model assumes the electrons as a fluid and treats the ions using a particle‐in‐cell (PIC) method. A new way of ensuring the Bohm criterion is used by removing those superparticles impeding the wall. When comparing the results, both models reveal differences, especially when ionization from a resting neutral gas or weakly magnetized plasmas is considered, causing a broadening of the ion distribution or anisotropy effects, respectively.     

Effect of neutral pressure on the He plasma flow measurement in a linear device

Axial and azimuthal flow velocities have been measured in a linear plasma device called NAGDIS-II (NAGoya DIvertor Simulator-II), along with plasma density and electron temperature, using a vector Mach probe composed of two Mach probes, one of which is for the axial flow, and the other is for the azimuthal flow. To study the effect of neutral pressure on the deduction of the Mach numbers, the ratio of upstream to downstream currents are measured by changing the neutral pressure for the deduction of flow velocities. Helium plasma was generated with pressure of 2–35 mTorr. Since the ion gyro-radius at the magnetic flux of 300 G is larger than the probe size, an unmagnetized collisionless Mach probe theory was used for the deduction of Mach numbers and their variations. In order to check the range of collisionality, plasma density (n_e = 10¹⁰–10¹¹ cm^?3) and electron temperature (T_e = 2–9 eV) are measured by a single electric probe using a conventional collisionless probe theory. Variations of Mach number, electron temperature and plasma density with collisionless models are to be compared with those using collisional models for different pressures where ionization and ion-neutral collision are included. Mach numbers by the collisionless model are found to be overestimated by 120% for the maximum difference even in weakly collisional plasmas. A clear flow reversal exists in the axial direction with higher pressure plasma, even in the linear machine. Azimuthal flows are also measured simultaneously along with axial flows, yet they seem to be very small in the present cold ion plasma (T_i/T_e << 1).     

Kinetic analysis of the ionization ratio effects on a three‐species plasma

In partially ionized plasmas, the energy transferred to electrically charged species by the electromagnetic field can be partly channelized to the population of neutrals, due to interspecies collisional processes. Depending on the relative density of neutrals, these effects may govern the collective plasma dynamics by drastically modifying particle dynamics and energy‐transport processes with respect to the fully ionized plasma‐approximation models. In this work, the influence of the ionization ratio r_i on a partially ionized plasma is analysed by means of a three‐species one‐dimensional kinetic model to compute transient and steady state velocity‐dependent distribution functions. The conservative collision operators accounting for charge–charge and charge–neutral interactions allow studying several plasma scenarios with the same entire number of particles per unit of volume but for an increasing r_i parameter, in the presence of a modulated signal‐like electric field. For a sequence of plasma scenarios of fixed r_i, ranging from typical weakly ionized to highly ionized plasma values r_i ～ 10^?7–10^?4, the mass species flows are examined. These flows behave linearly with respect to r_i up to a value r_i ? 10^?5 from which the quasi‐linear dependence is critically altered. The convection–diffusion equations are solved with the semianalytical Propagator Integral Method, which behaves well to deal with conservative operators, density, and field discontinuities, allowing for the use of collision terms of disparate time and spatial characteristic scales. The results can be relevant to a wide class of plasma systems and to analyse the ionization ratio effects on transport coefficients.     

OH number densities and plasma jet behavior in atmospheric microwave plasma jets operating with different plasma gases (Ar,Ar/N<Subscript>2</Subscript>, and Ar/O<Subscript>2</Subscript>)

OH radical number density in multiple atmospheric pressure microwave plasma jets is measured using UV cavity ringdown spectroscopy of the OH (A–X) (0–0) band at 308 nm. The plasma cavity was excited by a 2.45 GHz microwave plasma source and plasma jets of 2–12 mm long were generated by using three different plasma gases, argon (Ar), Ar/N₂, and Ar/O₂. Comparative characterization of the plasma jets in terms of plasma shape, stability, gas temperature, emission intensities of OH, NO, and N₂, and absolute number density of the OH radical was carried out under different plasma gas flow rates and powers at various locations along the plasma jet axis. With three different operating gases, the presence of OH radicals in all of the plasma jets extended to the far downstream. As compared to the argon plasma jets, the plasma jets formed with Ar/N₂ and Ar/O₂ are more diffuse and less stable. Plasma gas temperatures along the jet axis were measured to be in the range of 470–800 K for all of the jets formed in the different gas mixtures. In each plasma jet, OH number density decreases along the jet axis from the highest OH density in the vicinity of the jet tip to the lowest in the far downstream. OH density ranges from 1.3 × 10¹² to 1.1 × 10¹⁶, 4.1 × 10¹³ to 3.9 × 10¹⁵, and 7.0 × 10¹² to 4.6 × 10¹⁶ molecule/cm³ in the Ar, Ar/N₂, and Ar/O₂ plasma jets, respectively. The OH density dependence on plasma power and gas flow rate in the three plasma jets is also investigated.     

Rotating relativistic electron beam-plasma interaction and formation of a field-reversed configuration

Experimental results on interaction of a rotating relativistic electron beam with plasma and neutral gas are presented. The rotating relativistic electron beam has been propagated up to a distance of 150 cm in a plasma. The response of the plasma to the rotating electron beam is found to be of magnetic diffusion type over a plasma density range 10¹¹–10¹³ cm⁻³. Excitation of the axial and azimuthal return currents by the rotating beam and subsequent trapping of the azimuthal return current layer by the magnetic mirror field are observed. A field-reversed configuration has been formed by the rotating relativistic electron beam when injected into neutral hydrogen gas. We have observed field reversal up to three times the initial field in an axial length of 100 cm.     

大气压下绝缘毛细管内等离子体放电及其特性研究

在石英毛细管内利用两个边缘锋利的中空针型电极间的放电形成了63 cm长的大气压弧光等离子体.通过记录放电图片和测量电流-电压特征波形及伏安特性曲线的方法对管内等离子体从反常辉光状态过渡至超长弧光状态的过程做了细致的研究,发现管内等离子体在弧光状态下的电子密度不低于10¹⁴ cm^-3.另外,还进一步考察了两电极的间距和电源工作频率对放电伏安特性的影响以及通过发射光谱法测得的等离子体气体温度随外加电压的变化规律.当活性气体(氧气)按一定比例混合到氩等离子体中时,通过 关键词： 大气压等离子体 反常辉光放电 弧光放电 发射光谱     

Target plasma formation by UHF power

The properties of plasma injected into an open magnetic trap of uniform field from an independent UHF source have been investigated. Plasma is created in the UHF source at the frequency of 2400 MHz (power input 150 W) in the electron cyclotron resonance (ECR) regime at the pressure of neutral argon (10⁻⁵−10⁻²) torr. It is established that a rather quiescent target plasma with controlled density within the range of (2 × 10⁸−2 × 10¹²) cm⁻³ and temperature 2–3eV is accumulated in the trap. It turned out that plasma lifetime in the trap is determined by a classical mechanism of particle escape at the expense of collisions, at fixed value of magnetic field in the trap it practically is not changed with the variation of neutral gas pressure and reaches the value ≈ 4×10⁻³ s at the magnetic field strength in the trap equal 1600 Oe.     

The impact of the Ramsauer effect on the frequency of elastic collisions in inductive RF discharges in inert gases

This paper presents the results of investigating the power absorption mechanism of an inductive RF discharge plasma. Dependences of the frequency of elastic electron collisions with inert gas atoms (helium, neon, argon, and krypton) on the pressure are given. In the frequency range of 3 × 10⁶–3 × 10⁷ s^?1, an equivalent plasma resistance and the power input into the plasma are determined by the values of collision frequency and electron density within a skin layer and do not depend on the type of gas within the limits of experimental error. Upon reaching the electron temperature of ～1 eV, the energy of the main part of electrons lies in the range of Ramsauer’s minimum for elastic cross section. This leads to a decreasing elastic-collision frequency in heavy inert gases as compared to helium.     

超短超强脉冲激光在空气中产生的电离通道的寿命研究

对超短超强激光脉冲在大气中传播时形成的电离通道的寿命进行了理论研究.综合考虑了通道中自由电子，正离子，负离子的复合，自由电子和中性分子的吸附以及在后续 激光作用下的退吸附过程.推导出了退吸附激光强度恒定时通道中带电离子密度的速率方程 的解析解.计算结果表明，通过引入退吸附激光抑制电子和中性分子的吸附作用能够在微秒 的时间尺度上将电子密度维持在10¹²—10¹³cm^-3的水平，在相同的波长 和平均功率下，短脉冲序列的退吸附效果要略好于连续激光 关键词： 等离子体通道 复合 吸附 退吸附 寿命     

Measurement of neutral gas temperature in inductively coupled plasmas

Dependence of the neutral gas temperature on the gas pressure and discharge power in an inductively coupled plasma source has been investigated using optical emission spectroscopy. Both nitrogen and argon plasmas have been studied separately. In the case of argon plasma, about 5% nitrogen was added to the total gas flow as an actinometer. The maximum temperature was found to be as high as 1850 K at 1 Torr working pressure and 600 W radiofrequency power. The temperature increases almost linearly with the logarithm of the gas pressure, but changes only slightly with the discharge power in the range of 100–600 W. In a nitrogen plasma, a sudden increase in the neutral gas temperature occurs when the gas pressure is increased at a given discharge power. This suggests a discharge-mode transition from the H-mode (high plasma density) to the E-mode (low plasma density). In the H-mode, the gas temperature is proportional to the logarithm of the gas pressure, as in the argon plasma. However, the gas temperature increases almost linearly with the discharge power, which is in contrast to the case of argon plasma. The electron density in the nitrogen plasma is about 10% of that in the argon plasma. This may explain the observation that the nitrogen neutral temperature is always lower than the argon neutral temperature under the same discharge power and gas pressure.     

Influence of atom-atom collisions on the collisional-radiative ionization and recombination coefficients of helium plasmas

Coefficients for volume recombination and ionization have been calculated for a dense helium plasma of low degree of ionization. The calculations are based on a collisional-radiative model in which electron-atom, electron-electron-ion, atom-atom, and electron-atom-ion collisions intervene. Molecular species such as He ₂ ^* and He ₂ ⁺ have not been taken into account. The essential results are: At low temperatures and high neutral gas densities the recombination coefficient is proportional to the number density of neutral helium atoms. At high temperatures the presence of neutral particles practically does not influence the recombination process compared to pure ion-electron-electron recombination. At high neutral particle densities, high atom temperatures and low electron densities the ionization process is mainly due to atom-atom collisions. In this point our calculations are in relatively good agreement with recent shock tube measurements of Kalra and Measures (Phys. Fluids14, 2544 (1971)). It is emphasized that the simple two-step model for ionization by shock waves in the noble gases should be replaced by a more general collisional-radiative model in which the atomic level structure intervenes in more detail.     

A One‐Dimensional Particle‐in‐Cell Model of Plasma Build‐Up in Vacuum Arcs

Understanding the mechanism of plasma build‐up in vacuum arcs is essential in many fields of physics. A one‐dimensional particle‐in‐cell computer simulation model is presented, which models the plasma developing from a field emitter tip under electrical breakdown conditions, taking into account the relevant physical phenomena. As a starting point, only an external electric field and an initial enhancement factor of the tip are assumed. General requirements for plasma formation have been identified and formulated in terms of the initial local field and a critical neutral density. The dependence of plasma build‐up on tip melting current, the evaporation rate of neutrals and external circuit time constant has been investigated for copper and simulations imply that arcing involves melting currents around 0.5–1 A/μm², evaporation of neutrals to electron field emission ratios in the regime 0.01 – 0.05, plasma build‐up timescales in the order of ～ 1 – 10 ns and two different regimes depending on initial conditions, one producing an arc plasma, the other one not. Also the influence of the initial field enhancement factor and the external electric field required for ignition has been explored, and results are consistent with the experimentally measured local field value of ～ 10 GV/m for copper (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reduced Brillouin backscatter in CO2 laser-target interaction

Substantially reduced Brillouin reflection has been found for CO₂ laser irradiated high density gas targets. In contrast to high reflectivity (60%) previously observed for underdense hydrogen plasma, total backscatter (stimulated plus specular) is found to peak at 30% for incident intensity 5 × 10¹² W/cm² and decrease thereafter to 18% at 10¹³ W/cm². Ponderomative effects are postulated to account for these observations.     

Plasma in open magnetic trap injected from independent UHF source

The properties of plasma injected into the open magnetic trap of uniform field from an independent UHF source have been investigated. It is established that a rather quiescent plasma with control density within the range of 2×10⁸–2×10¹² cm^–3 and temperature 2–3 eV is accumulated in the trap. It turned out that plasma lifetime in the trap is determined by a classical mechanism of particle escape at the expense of collisions, at fixed value of magnetic field in the trap it is not practically changed with the variation of neutral gas pressure and reaches the maximum value 4×10^–3 s at magnetic field strength in the trap equal 1600 Oe. Besides, the experimental data are analyzed on the basis of balance equations.     

K-line X-ray fluorescence from highly charged iron ions under dense astrophysical plasma conditions

In the present work, we report an investigation of plasma environment effects on the atomic parameters associated with the K-vacancy states in highly charged iron ions within the astrophysical context of accretion disks around black holes. More particularly, the sensitivity of K-line X-ray fluorescence parameters (wavelengths, radiative transition probabilities, and Auger rates) in Fe XVII–Fe XXV ions has been estimated for plasma conditions characterized by an electron temperature ranging from 10⁵ to 10⁷ K and an electron density ranging from 10¹⁸ to 10²² cm⁻³. In order to do this, relativistic multiconfiguration Dirac-Fock atomic structure calculations have been carried out by considering a time averaged Debye-Hückel potential for both the electron–nucleus and electron–electron interactions.     

Generation of uniform atmospheric pressure argon glow plasma by dielectric barrier discharge

In this paper, atmospheric pressure glow discharges (APGD) in argon generated in parallel plate dielectric barrier discharge system is investigated by means of electrical and optical measurements. Using a high voltage (0–20 kV) power supply operating at 10–30 kHz, homogeneous and steady APGD has been observed between the electrodes with gap spacing from 0.5 mm to 2 mm and with a dielectric barrier of thickness 2 mm while argon gas is fed at a controlled flow rate of 1 l/min. The electron temperature and electron density of the plasma are determined by means of optical emission spectroscopy. Our results show that the electron density of the discharge obtained is of the order of 10¹⁶ cm^???3 while the electron temperature is estimated to be 0.65 eV. The important result is that electron density determined from the line intensity ratio method and stark broadening method are in very good agreement. The Lissajous figure is used to estimate the energy deposited to the glow discharge. It is found that the energy deposited to the discharge is in the range of 20 to 25 μJ with a discharge voltage of 1.85 kV. The energy deposited to the discharge is observed to be higher at smaller gas spacing. The glow discharge plasma is tested to be effective in reducing the hydrophobicity of polyethylene film significantly.