Electrical characteristics of pulsed glow discharges

The i-v characteristics, energy partitioning, and time evolution of the discharge current and reduced field (E/N) for a nitrogen discharge are simulated using a self-consistent calculation of the electron energy distribution function and the vibrational level populations. The model includes diffusion losses and takes account of the external circuit parameters. The results discussed are for pressures of 1-100 torr, discharge currents in the range of 10^-3-5.0 A, and a reduced field (E/N) in the range of 150-250 Td. For a typical discharge in a tube of 2-cm diam. and a current of a few amperes, the results show that the energy stored in the vibrational manifold saturates a few milliseconds after the initiation of the discharge     

Stimulated acceleration and confinement of deuterons in focuseddischarges. II

For pt.I see ibid., vol.16, no.3, p.368-73, June 1988. Methods of increasing, by a factor greater than five, the neutron yield/short Y _n from D-D fusion reactions in a plasma focus (PF) enhance both the D⁺-ion acceleration to energy values E _d>1-8 MeV and the ion confinement in the pinch region. Nuclear activation of C and N in the (doped) filling gas of the discharge chamber and of solid targets of C and BN bombarded by the ion beam in the direction of the electrode axis (0°) confirms earlier determination of the energy spectrum of the trapped ions (dφ_t/dE∝φ_0tE ^-m) and of the ejected beam (dφ_b/dE∝φ_0bE _d^-m, m=2.5±0.5 for 0.1 MeV≲E≲3 MeV). A Thomson (parabola) spectrometer with nanosecond time resolution determines the time of emission t( E) of the beam at 0°. Ion acceleration and trapping occur within the small (filamentary) elements of the magnetic fine structure of the pinch, which can be dispersed on a relatively large confinement volume after the pinch disintegration. It has been found that φ_t/φ_b≳10-10³ for E_d≳1 MeV, depending on Y_n     

Monte Carlo simulation of electron swarms in nitrogen in uniformE×B fields

The motion of electrons in nitrogen in uniform E× B fields is simulated using the Monte Carlo technique for 240⩽E/N⩽600 Td (1 Td=1×10^-17 V cm²) and 0⩽B/N⩽0.45×10^-17 T cm³ . The electron-molecule collision cross sections adopted are the same cross sections as those used previously for the numerical solution of the Boltzmann equation. The swarm parameters obtained from the Monte Carlo simulation are compared with the Boltzmann solution and with the experimental data available in the literature. In relation to E×B fields, it is concluded that the Monte Carlo approach provides an independent method of substantiating the validity of the equivalent electric-field approach     

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     

Pressure limits for the vacuum arc deposition technique

Observations of the cathodic copper plasma expansion at low pressures of He, Ar, and SF₆ showed that, for background gas mass densities of ρ_g=1 to 4×10^-4 kg/m ³ and higher, the plasma and gas are separated into two volumes. A shock wave acts as a boundary between the two volumes. The boundary attains a stationary position once its expansion velocity decreases to the velocity of sound in the background gas. This position corresponds to a distance R_c to the cathode that agrees with a snowplow expansion model, giving R_c ^βf=E_r, where f is a function of the arc current and background gas characteristics, E _r is the erosion rate of the cathode, and β varies between 2.1 and 2.5. The interaction model is based on kinetic energy exchanges between two gas-like volumes without other energy losses. A maximum pressure limit for vacuum arc deposition is set for ρ_g /I=2 to 9×10^-6 kg/m³ A     

Disproof of Cowling's theorem

T.G. Cowling's theorem (1934) is a general statement that axisymmetric dynamos cannot be constructed in astrophysical bodies. The theorem is based on the assumption that the electrical conductivity is a scalar, σE=J. In a counterargument, the author demonstrates that this statement is not valid for a diffuse plasma. He suggests that, in an astrophysical plasma, collisional processes can violate Cowling's theorem by scattering current into a current-free channel     

氢离子轰击对TiC膜结构及界面影响的研究

本文研究荷能氢离子轰击对TiC薄膜的结构及其界面的影响。氢离子源为5A的脉冲离子注入器,能量25keV,脉冲宽度20ms。样品经轰击后发现。与受相同辐照的45~#钢样品相比,TiC薄膜表面无明显的损伤;TiC薄膜内的氢含量较高;薄膜的织构消除;TiC薄膜与基体间的结合力有明显提高。     

A study of the cathode emission and the discharge parameters of luminescent tubes

A method of continuously monitoring the emission of thermionic cathodes working under mercury-vapor discharge conditions, as well as a basis from which to estimate the parameters of the ion current which falls on the cathode of a discharge tube is described. The method proposed was tested on models of luminescent tubes working on direct current. It is shown that it is possible for a mercury-vapor type discharge to change to a glow discharge due to poisoning of the cathode by ion bombardment.     

The use of bubbles for plasma liners in ICF experiments

The production of thin, axially symmetric bubbles between electrodes in a plasma-focus discharge chamber at pressures below 1 torr is described. A theory of their use as plasma lines (PLs) is given. A mechanism of imploding such liners using a plasma-focus snowplow (SP) for the accumulation of magnetic energy is described. The transfer of the discharge current I from the SP to the PL should result in a substantial amplification of the dI/dt as seen by the PL, resulting in very high density of the latter. Possible applications of such dense plasma liners in ICF are mentioned     

Internal dynamics of a plasma propelled across a magnetic field

When a plasma is pushed across a magnetic field by some nonelectromagnetic force, ions and electrons get turned in opposite directions by the magnetic field. This creates an internal current as well as sheaths at the plasma surfaces and results in an electric field which allows the plasma to maintain some, or even most of its initial momentum in the form of E&oarr;×B&oarr; drift. An exact analysis of that process is presented for the internal region of the plasma. The energy provided by the initial push is used, in part, to create some gyrations inside the plasma. When the rest energy density of the plasma exceeds twice the magnetic energy density (or when the Alfven speed is less than c), there will be enough energy to spare for the plasma to continue across the magnetic field at half its initial momentum. Two cases are considered: an impulsive start and a gentle push such as provided by gravity. The amplitude of the resulting internal gyrations becomes small in the second case. The frequencies of the gyrations are those of extraordinary modes of very long spatial wavelength     

Electrical discharge initiation and a macroscopic model forformative time lags

Formative times in electrical discharges in overvoltaged gaps are analyzed with a model having no spatial dependence and with simple assumptions about discharge channel temperature T and discharge voltage. The model treats the early temporal evolution of the discharge. Specifically, the dissipative voltage drop, V*, across the discharge is taken to be a step function of T. Thermal quasi-equilibrium is assumed in the discharge medium, and it is shown that d(In t_d)/_d(In &thetas;)=-1, i.e., &thetas;t_d=constant, where &thetas; is the fractional overvoltage and t_d is the formative time lag, in agreement with measured values of t_d for much of the experimentally explored range of &thetas;. Highly-time-resolved (~92 ps) experimental data are presented for the first 10 ns of electrical discharge initiation; these data suggest that the authors' model should provide a reasonable representation of t _d when t_d>10-100 ns     

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     

Development and investigation of a metal ion source for accelerators

The design and operation of a new spray-type metal ion source are described. The source is built around a plasma emitter containing metal ions and using a two-step glow discharge with double plasma compression. The necessary temperature of electrons in a discharge chamber is reached by distributing the discharge power without heating. The design of the discharge chamber is optimized. The first set of experimental data for the mass-and-charge composition of a beam generated by this source is presented.     

Computer simulation of a carbon-deposition plasma in CH4

A transport and reaction model of a low-pressure, high-frequency (13.56 MHz) CH₄ plasma used for diamondlike carbon (a-C:H) deposition was developed. The model includes reactions among four molecular species (CH₄, C₂ H₆, C₂H₄, and H₂), five radicals and atom (CH₃, CH₂, CH, C₂H₅, and H), and four ions (CH₄⁺ , CH₃⁺, CH₅⁺, and C ₂H₅⁺). It also accounts for the influence of the sticking coefficient of species at the walls. Calculated values of the dissociation degree for several flow rates are in good agreement with experimental measurements made by quadrupole mass spectroscopy. A simple surface-model based on the hydrogen coverage of surface and ion flux and energy at the substrate surface was established. This model permitted the calculation of the deposition rate on the powered electrode as a function of the power applied to this electrode. Good agreement between experimental and calculated growth rates was obtained when CH₃, C₂H₅, and CH₂ were assumed to participate in film formation, and when hydrogen removal by ion bombardment with variable energy as a function of the power was included in the model     

ECR-PECVD制备SiO2薄膜中衬底射频偏压的作用

采用微波电子回旋共振等离子体增强化学气相沉积（ECR－PECVD）技术在单晶衬底上制备了SiO2薄膜,研究了射频偏压对薄膜特性的影响。通过X射线光电子能谱（XPS）、傅里叶变换红外线光谱（FTIR）、原子力显镜（AFM）和扫描隧道显微镜（STM）三维形貌图测量等手段,对成膜特性进行了分析。实验结果表明,通过改变射频偏压的参数来控制离子轰击能量,对ECR－PECVD成膜的内应力、溅射现象、微观结构和化学计量均有明显的影响。     

高压耦合高功率脉冲磁控溅射的增强放电效应

等离子体源离子注入与沉积技术作为一种可生产高结合力、高致密度涂层的真空镀膜技术,具有广阔的应用前景,尤其适用于高载荷工况下服役的功能涂层制备.该技术中金属等离子体源是关键,而现有的脉冲阴极弧源结构复杂,且由于伴随"金属液滴"而需要增加过滤装置.本文研究了另一种简单结构的金属等离子体源备选一高功率脉冲磁控溅射源(HPPMS)的放电特性,采用等离子体发射光谱仪探索了不同的耦合高压对HPPMS放电靶电流特性和等离子体特性的作用.发现耦合高压对HPPMS放电有明显的促进作用,相同靶电压下的放电强度大幅增加,相对于金属放电,耦合高压对气体放电的促进作用更加明显,但在自溅射为主的高压放电阶段对金属放电的促进作用明显增强.讨论了耦合高压对HPPMS放电的增强机制,发现耦合高压自辉光放电、耦合高压和HPPMS电压构成双向负压形成的空心阴极效应,以及耦合高压鞘层改善的双极扩散效应都对HPPMS放电的增强有明显作用.     

Some interruption criteria for short high-frequency vacuum arcs

If the contacts of a vacuum interrupter open shortly before a current zero, the transient recovery voltage (TRV) can cause a reignition and reestablish the arc. When the current in a diffuse vacuum arc passes through zero, there is a distinct pause before the TRV builds up (approximately 40 ns for copper). During this pause the gap carries conduction current only with an ion component which depends on dI /dt, varying between 3 A for dI/dt=60 A/μs and 60 A for dI/dt=1235 A/μs. The ion current subsequently decays in tens or hundreds of nanoseconds. It can be distinguished from the displacement current at this time by varying dV/dt, keeping the other parameters constant. Among the interruption criteria for short high-frequency vacuum arcs, dI /dt prior to current zero and initial dV/dt are the most important. High values of dI/dt are more likely to precipitate reignitions, but breakdowns can occur after lower dI/dt's if the gap has been subjected to a high current for a relatively long time (>100 μs)     

Effect of Ar in the source gas on the microstructure and optoelectronic properties of microcrystalline silicon films deposited by plasma-enhanced CVD

Hydrogenated microcrystalline silicon films have been prepared by plasma-enhanced chemical vapor deposition technique using silane diluted in H₂ or H₂ + Ar. The microstructures for silicon films have been evaluated by Raman scattering spectroscopy, X-ray diffraction and Fourier-transform infrared spectroscopy. Optical characterization has been done by UV-vis spectroscopy. It is found that the addition of Ar in diluent gases efficiently improves the deposition rate and crystallinity due to an enhanced dissociation of the source gas and the energy of deexcitation of Ar* released within the growth zone. Meanwhile, the enhanced crystallinity and the reducing of hydrogen ion bombardment with increasing Ar dilution lead to the polymerization and also a bad passivation of the hydrogen which cause the widening of the optical gap and increase of defect states in the μc-Si films. The absorption coefficient and dark conductivity are found to decrease basically with increasing Ar dilution corresponding to the widening optical gap and more defects. That the activation energy increases with increasing Ar dilution or decreasing hydrogen dilution is due to the fact that more defect states lead to a pulling down of the Fermi level.     

A Raman regime free-electron laser in a partially dielectric-loadedwaveguide

A partially dielectric-loaded waveguide supports modes with phase velocity ν_ph>c/∈^1/2, where ∈ is the dielectric constant of the lining. The coupling of a fast mode (with ν_ph>c) with a slow mode (with c /∈^1/2<ν_ph<c) via a negative-energy beam mode gives rise to the explosive growth of all the three waves at the expense of the energy of the beam. The slow mode of lower frequency can be launched from an external source and can become an efficient wiggler for the generation of the fast free-electron laser mode