Validity conditions for complete and partial local thermodynamicequilibrium of nonhydrogenic level systems and their application tocopper vapor arcs in vacuum

Validity conditions for complete and partial local thermodynamic equilibrium (CLTE and PLTE) of homogeneous, time-dependent, and optically thin plasmas are derived. For Cu I, electron densities of n_e⩾(5×10²²-5×10²³ ) m^-3 are required for the establishment of CLTE. For Cu I and Cu II, n_e⩾(5×10²¹-5×10²¹ -5×10²²) m^-3 is necessary for PLTE (for electron temperatures of 1-2 eV). Application to low-current copper vapor arcs in vacuum shows that CLTE can be expected for r<200-600 μm (r=distance from the cathode spot). A further limitation follows for temperatures of 2 eV or higher if diffusion effects are taken into consideration. Consequently, the use of the LTE formulas in plasma spectroscopy of low-current vacuum arcs is very limited     

The type and extent of non-LTE in argon arcs at 0.1-10 bar

A 30-A, 3-mm-diameter, wall-stabilized argon arc with 1% hydrogen is examined spectrometrically at pressures of 0.1-10 bar. Values of T_g, and T_exa diverge as pressure decreases below 5 bar (n_e⩽1×10¹⁷ cm^-3) at r=0. T_exβ is 20-40% larger than the other temperatures. The results are dependent on the transition probability scale used     

Analytic expression for the electric potential in the plasma sheath

An expression for the spatial dependence of the electric potential in a collisionless and source-free planar plasma sheath is presented. This expression is derived by analogy with Child's law and approaches Child's law asymptotically as the potential drop φ_W across the sheath becomes large, |eφ_W/kT_e|>10⁴. Here k is Boltzmann's constant, T_e is the electron temperature, and e is the electronic charge. Comparison with numerical solutions of the model equations indicates that the sheath thickness and potential variation predicted by this improved Child's law are accurate for |eφ_W/kT_e|>10. In contrast, the authors find that Child's law is accurate only when |e φ_W/kT_e|>10⁴     

Analytical solution for capacitive RF sheath

A self-consistent solution for the dynamics of a high voltage, capacitive radio frequency (RF) sheath driven by a sinusoidal current source is obtained under the assumptions of time-independent, collisionless ion motion and inertialess electrons. Expressions are obtained for the time-average ion and electron densities, electric field and potential within the sheath. The nonlinear oscillation motion of the electron sheath boundary and the nonlinear oscillating sheath voltage are also obtained. The effective sheath capacitance and conductance are also determined. It was found that: (1) the ion-sheath thickness S _m is √50/27 larger than a Child's law sheath for the DC voltage and ion current density; (2) the sheath capacitance per unit area for the fundamental voltage harmonic is 2.452 ϵ₀ /S_m, where ϵ₀ is the free space permittivity; (3) the ratio of the DC to peak value of the oscillating voltage is 54/125; (4) the second and third voltage harmonics are, respectively, 12.3 and 4.2% of the fundamental; and (5) the conductance per unit area for stochastic heating by the oscillating sheath is 2.98 (λ_D/S_m)^2/3 (e ²n₀/mu_e), where n ₀ is the ion density, λ_D is the Debye length at the plasma-sheath edge, and u_e is the mean electron speed     

Effect of a high-frequency field on the reflection of ion acousticwaves from the sheath at a grid

Experiments are described which show that the reflection coefficient for ion acoustic waves (IAW) from the sheath at a grid is affected by an HF electric field with a frequency f_HF≲5f_pi(f_pi =ion plasma frequency). For peak-to-peak amplitudes of the HF voltage drop across the sheath Φ₀≲k_B T_e/e and f_HF>f _pi, the energy distribution of the ions passing through the grid develops a hot tail and the reflected wave suffers enhanced Landau damping. If Φ₀≳k_BT_e/e and f_HF<f_pi, a large-amplitude IAW is excited at the grid; a well-defined ion beam is formed; and local growth of the reflected wave is observed. Test waves launched from the grid show the same propagation characteristics as the reflected waves     

Radiation cooling rates of a selenium plasma

The development of a scalable hydrogenic ionization model is described. The model utilizes correct energy and level structure data for each ionization stage and can be coupled self-consistently to a radiation transport calculation of the full nonhydrogenic X-ray spectrum. Thus it can be used to calculate accurately the effects of opacity on radiation emission rates that are of relevance to plasmas designed to produce recombination pumped population inversions due to rapid plasma cooling. The model is applied to a selenium plasma. It is found that, at ion densities where X-ray lasing has been observed, line emission from Δn≠0 transitions is a more important contributor to selenium's cooling rate than line emission from the Δn=0 transitions. Plasma opacity can also play an important role     

Competition of modes resonant with arbitrary cyclotron harmonics ina gyrotron with nonfixed axial structure of the high-frequencyfield

A general theory is developed for self-consistent calculations of mode competition in a gyrotron with nonfixed axial structure of the RF field for arbitrary cyclotron harmonics. The theory is applied to the gyrotron operating at the Kernforschungszentrum Karlsruhe with a frequency of 150 GHz. The formalism presented allows a self-consistent calculation of mode competition for the operating and parasitic modes at the cyclotron resonance at arbitrary harmonics. Specific calculations are carried out for the cases n₀=1, n₁ =2 and n₀=2, n₁=1. It is emphasized that the formalism considered applies only to the case in which the azimuthal orthogonality condition is satisfied: n₀m₁≠n₁ m₀. There are circumstances when this condition is not satisfied     

The role of dust particles with large gyroradii in the `2/3'fall-down process

The authors consider the Alfven-Arrhenius fall-down process and propose a mechanism whereby the Rosseland electric field (the field needed to maintain quasineutrality) may be responsible for the capture and confinement of large-gyroradius dust particles within a plasma shell stratified along the direction of the magnetic-field lines. For these particles, the effect of the magnetic force is rather weak, and they move with a constant z component of the angular momentum in a one-dimensional equivalent potential (gravitational plus centrifugal). This has a maximum at the equator and a minimum at the `2/3' points, i.e. the points where the field-aligned components of the gravitational and centrifugal forces balance. It is shown that under suitable initial conditions these are points of maximum dust density and minimum plasma density. The plasma-planetisemal transition is therefore expected to take place at the `2/3' points in accordance with the Alfven-Arrhenius mechanism. It is also shown that the fraction of infalling dust particles that can accrete onto the equatorial plane by the Alfven-Arrhenius and Rosseland mechanisms is rather small (~(L/ R_e)≪1), L being the thickness of the plasma shell, and R_e, a characteristic length scale of the field line     

Simulation of pulsed electropositive and electronegative plasmas

A particle-in-cell (PIC) code with nonperiodic boundary conditions, including ionization and ion motion, is used to simulate the approach to equilibrium and the decay in the postdischarge of model electropositive and electronegative plasmas in a symmetric RF diode. In the electropositive plasma the density decreases to 1/e in ~10 μs. The electronegative ion plasma density is about four times higher and decreases to 1/e in ~50 μs, with the electron temperature and density decreasing to zero in a few microseconds. During this latter time scale, a weak field is set up to drive the negative ions to the boundaries at the same rate as the ion diffusion velocity. In the postdischarge this is close to thermal, hence some hundreds of microseconds are required to remove most of the negative ions from the system. As negative ions are thought to be the precursors for particulate formation, plasmas pulsed at around 1 kHz will severely decrease the lifetime of the negative ions and thereby reduce the possibility of particulate growth     

A slow wave free-electron laser in a longitudinal wiggler field

Linearized Vlasov-Maxwell equations are solved to obtain the growth rate of free electron laser instability from a tenuous relativistic electron beam propagating in a partially dielectric loaded waveguide immersed in combined axial and longitudinal wiggler magnetic fields. The instability appears via cyclotron resonance interactions for wave perturbations very close to w-kV_z-w_c=nk ₀V_Z where n is the general harmonic number. For n=0, the gain is similar to a slow wave cyclotron amplifier. For n⩾1, the growth rate is substantially larger than the standard slow wave free electron laser scheme utilizing a transverse wiggler field     

Plasma deposition of thin films utilizing the anodic vacuum arc

Anodic vacuum arcs operating with cold cathodes in the spot mode and hot evaporating anodes are investigated to explore their technical potential as a plasma deposition technique. This discharge provides a unique source of a highly ionized, metal vapor plasma by autogeneration of the working gas to evaporation of the anode. This gas-free and droplet-free metal vapor plasma expands into the ambient vacuum (10^-4 mbar) and produces thin metallic films at the surface of substrates. An analysis of Al and Cu plasmas at the position of a possible substrate for arc currents between 20 and 200 A leads to the following results: electron densities, 10¹⁵-10¹⁸/ m³; degree of ionization, 0.5-25%; directed ion energy, 5 eV; and electron temperatures, 0.2-1 eV. Metallic coatings generated with deposition rates between 0.1 and 100 nm/s show the following properties: purity, 99.9%; polycrystalline structure with grain sizes between a few and a few hundred nm, same mass density as the respective bulk material, electrical conductivity rather close to that of the bulk material, and excellent optical properties. The coatings show good adhesion, which can be enhanced by a plasma-supported pretreatment of the substrate surface and by an acceleration of the ions towards the substrate     

Superheat turbulence in gas discharged plasma

The influence of the electron concentration and temperature fluctuations on local thermodynamic equilibrium (LTE) in a gas-discharge plasma due to superheat turbulence development is analyzed. Data for the noble-gas atmospheric plasmas Ar and He (T=6-18 kK) and air ( T=4-9 kK) are given. It is shown that superheat turbulence causes deviation from LTE when parameter-space gradients are absent. As a result, the influence of superheat turbulence for weakly radiative gases (He, H₂) is considerably greater than for strong radiative gases (Ar, Xe, etc). The artificial excitement of superheat turbulence in plasma without any current by means of external electric field fluctuations is demonstrated     

Relativistic harmonic content of nonlinear electromagnetic waves inunderdense plasmas

The relativistic harmonic content of large-amplitude electromagnetic waves propagating in underdense plasmas is investigated. The steady-state harmonic content of nonlinear linearly polarized waves is calculated for both the very underdense (w_p/w ₀)≪1 and critical density (w_p/w₀)≃1 limits. For weak nonlinearities, eE₀/mcω₀<1, the nonlinear source term for the third harmonic is derived for arbitrary w_p/w₀. Arguments are given for extending these results for arbitrary wave amplitudes. It is also shown that the use of the variable x-ct and the quasi-static approximation leads to errors in both magnitude and sign when calculating the third harmonic. In the absence of damping or density gradients the third harmonic's amplitude is found to oscillate between zero and twice the steady-state value. Preliminary PIC simulation results are presented. The simulation results are in basic agreement with the uniform plasma predictions for the third-harmonic amplitude. However, the higher harmonics are orders of magnitude larger than expected and the presence of density ramps significantly modifies the results     

Electron collision cross-section data for plasma modeling

An overview of the current status in electron collision cross section databases for plasma modeling is presented. Emphasis is on dissociation, ionization, and dissociative ionization. The effect of the operating conditions on the relative importance of different collision mechanisms is discussed. For high density plasmas with low gas pressure, a second collision with electrons becomes likely and metastable states may play a role in the dissociation and ionization processes. A number of outstanding theoretical and experimental issues in the cross-section data are discussed. For the total ionization cross section, a four-parameter fitting formula is found to represent the numerical data well and the parameters for some molecules used in plasma modeling are presented     

Evaporative ion plating: process mechanisms and optimization

The basic discharge mechanisms pertinent to ion plating process are reviewed and some recent findings are highlighted, relating in particular to thermionically enhanced ion plating with an evaporative vapor source. By enhancing the discharge, the cathode sheath can approach collisionless conditions, which allows most of the energy to be transported to the substrate by ions. The energy distribution is also improved, this being significant if a critical energy E* is thought to be required in order to achieve high-quality coatings. It is suggested that E* is about 40 to 70 eV, and that higher bombarding energies may not necessarily improve coating quality, this being alternatively achievable by increasing the ionization efficiency. The role of metal vapors is outlined with reference to evidence from enhanced discharges that clusters of metal atoms exist in the vapor phase and that most of the ion current is transported to the substrate by the metal species. Further practical aspects are discussed such as the spatial distribution of bombardment intensity, which is shown to decrease exponentially with distance from the thermionic emitter used for discharge enhancement. It is suggested that positioning the thermionic emitter in close proximity to the vapor source will provide a more consistent ratio between energetic bombardment effects and vapor arrival rates throughout the deposition volume     

Propagation in a warm collisional magnetoplasma enclosed in aconducting cylinder

A study is presented of the propagation of electron plasma waves in a warm collisional plasma filling a conducting cylinder and magnetized strongly in the axial direction, the plasma parameters being taken to be such that the electron-ion collision is the dominant damping process. The attenuation and phase constants are derived in suitably normalized form by using the electrodynamic method. The dispersion and attenuation characteristics for propagation in the lowest order mode in a hydrogen plasma are obtained for various values of the normalized plasma radius αω_pe/c, the electron temperature and electron density being held constant, and the characteristics are compared with those given by the usual quasi-static approximation, which is found to be valid only if αω_pe /c≲1, where α is the plasma radius, ω _pe is the plasma frequency, and c is the velocity of light in free space. The effect of the plasma frequency on the characteristics is investigated     

A two-electron group model theory for radio-frequency ionization ofnoble gases with turbulent flow

Equations are derived for predicting the current-voltage characteristic curves of axial RF discharges in noble gases, with turbulent flow. The electrons are considered to be made up of two Maxwellian groups: bulk and tail electrons. The bulk electrons are described by a temperature T_b, and have kinetic energies (1/2 mv²=eV) from 0 to eV _l (eV_l=the threshold energy of the first dominant inelastic collision process). The electrons of the depressed tail of the distribution function are described by another temperature, T_t<T_b, and have (eV>eV_l). The terms in these equations correspond to the prevailing processes occurring inside the noble gas discharge. The rate coefficients given are derived, based on the two-electron group model. The effect of the high velocity flow is accounted for by the terms giving the divergence of the flux of particles in the redirection of flow in each of the continuity equations for the primary species and by adding a diffusion coefficient due to turbulence to the static discharge diffusion coefficients of the ions and metastables     

Pair-ion plasma generation using fullerenes

We have developed a novel method for generating pure pair plasma which consists of positive- and negative-charged particles with an equal mass. The pair-ion plasma without electrons is generated using fullerene as an ion source through the processes of hollow-electron-beam impact ionization, electron attachment, preferential radial diffusion of ions, and resultant electron separation in an axial magnetic field. Basic characteristics of this plasma are discussed in terms of the differences from ordinary electron-ion plasmas, such as a phenomenon in the absence of sheath and potential structure formation.     

铯47D精细能级超冷里德堡原子自由演化的动力学研究

用连续窄线宽激光器将超冷铯里德堡原子分别激发到47D_3/2, 47D_5/2精细态, 观察了处于里德堡精细态的铯原子向超冷铯等离子体自由演化的过程, 详细对比了不同精细态的铯里德堡原子预电离时间、电离速率以及等离子体的转化效率. 将里德堡原子快速转化为等离子体的过程解释为局域势阱内由预电离产生的电子与里德堡原子的快速碰撞导致的雪崩电离.     

Plasma motion in a filtered cathodic vacuum arc

A model is proposed for the flow of a plasma originating from a cathodic vacuum arc into a curvilinear magnetic field. The model gives good agreement with measurements obtained from a filtered cathodic-arc thin film deposition system. The important parameters involved in the motion of a vacuum arc plasma beam through a magnetic filter are examined. The analysis is based on the use of the guiding center approximation to describe the motion of the charged particles produced in the plasma where the thermal energy is negligible compared to the mass flow energy. Electron-ion collision effects are included within the framework of the drift model. It is shown that under the limiting condition of a collision frequency which is much higher than the cyclotron frequency of the electron, the motion of the plasma ions around the bend becomes independent of the magnetic field, with the number of ions traversing the filter significantly reduced. However, in the collisionless plasma case (cyclotron frequency higher than the collision frequency), the model predicts a square-law relationship between ion-saturation current and magnetic field , I_p ∞B²