Three-dimensional modelling of inductively coupled plasma torches

A three-dimensional model has been developed for simulating the behaviour of inductively coupled plasma torches (ICPTs), using customized CFD commercial code FLUENT ^?. The helicoidal coil is taken into account in its actual 3-D shape, showing the effects of its non-axisymmetry on the plasma discharge. Steady state, continuity, momentum and energy equations are solved for argon optically thin plasmas under the assumptions of LTE and laminar flow. The electromagnetic field is obtained by solving the 3-D vector potential equation on a grid extending outside the torch region. In order to evaluate the importance of various 3-D effects on calculated plasma temperature and flow fields, comparisons of our new results with the ones obtainable from conventional 2-D models and from an improved 2-D model that includes 3-D coil effects are presented. The presence of wall temperature hot spots due to plasma discharge displacement from the torch axis is evidenced, while the use of the new 3-D code for optimization of induction coil geometry and plasma gas inlet features is foreseen. Received 5 September 2002 Published online 13 December 2002 RID="a" ID="a"e-mail: colombo@ciram.ing.unibo.it     

Numerical studies on velocity, temperature history and heat transfer to the particles injected into the argon plasma

A new approach to study the particle velocity in a thermal plasma in relation to input parameters (power, gas flow rate, injection velocity of the particle and particle size) and nozzle dimensions (nozzle length and diameter) has been made. Injected particle's temperature and thermal history were calculated for particles of three different materials (alumina, tungsten and graphite) in argon plasma. Allowable powder feed rate was calculated for the particles. Heat transfer per particle injected in to the plasma is reported. Liquid fraction of the particle after it reached the melting point is also reported. Particle velocity is found to increase with increase in power, gas flow rate and injection velocity and decrease with increase in particle size, nozzle length and nozzle diameter. Thermal histories of the particles in relation to the plasma temperature and particle diameter are presented. Particle's residence time is found to increase with increase in diameter of the particle. Allowable powder feed rate for complete melting of the particle is higher at higher percentage utilisation of the plasma power. Powder feed rate is seen to decrease with increase in particle size and it is higher for tungsten and lower for graphite particle. Heat transfer rate from plasma to particle is seen to decrease with increase in time and the same is higher for plasmas of higher temperature and smaller sized particle. Received 4 May 2000 and Received in final form 15 March 2001     

Experimental observation and numerical analysis of arc plasmas diffused by magnetism

At atmospheric pressure, an intensified charge-coupled device (ICCD) camera with a narrow-band filter is used to capture the unsaturated images of a magnetically rotating arc. Comparison of its configurations with different arc current and external axial magnetic field (AMF) strength shows that the strong electromagnetic force may impel the arc to diffuse. Under the fully diffuse mode, the plasma is distributed throughout the electrode gap and no anode attachment can be seen in the cross-section of the torch. The fully diffuse plasma runs more steadily and its intensity distribution is more uniform, while its voltage fluctuation is reduced significantly. Using a commercial CFD (computational fluid dynamics) code FLUENT, the fluid flow and heat transfer of the fully diffuse plasma in an assumed magnetron torch have been simulated for qualitatively discussing the AMF effects. Numerical results show that the AMF significantly impels the plasma to retract axially and expand radially. As a result, the plasma intensity distribution on the cross section of the torch gets to be more uniform.     

Electron temperature measurement in a surface-wave-produced argon plasma at intermediate pressures

The main objective of this work is to obtain the electron temperature in an argon surface-wave-produced plasma column at intermediate gas pressures. After proving that argon upper excited states remain in Excitation Saturation Balance, the value of electron temperature along the plasma column has been obtained using a modified Saha equation and a corrected Boltzmann-plot. Moreover, the electron energy distribution function has been verified to be nearly Maxwellian in a 0.8-2.8 torr intermediate pressure range. Received 24 July 2000 and Received in final form 19 January 2001     

Application of a ferroelectric plasma cathode as a high-current switch

In this paper the parameters of two types of high-current switches based on ferroelectric BaTiO₃ ignition are presented. Both types of switches showed a reliable and controllable operation with a repetition rate of several Hz. The first type is a vacuum two-electrode switch ignited by the plasma which is generated by a BaTiO₃ cathode. This type of switch was tested in the voltage range of 3-25 kV and switched current amplitude of 2-15 kA with either negative or positive polarity of the high-voltage electrode. The second type is a BaTiO₃ surface flashover strip-like switch ignited by a driving pulse which has an amplitude of several kV. It was shown that the application of the driving pulse (>10 kV) leads to the appearance of many non-complete surface discharges which transform further to a multi-channel discharge. This type of switch was tested in the voltage range of 1-25 kV and current amplitude of 0.5-15 kA. The design of the switches, their lifetime, the time jitter and the parameters of the switched current for different discharge conditions are presented. Received 5 November 2001     

Excitation of hydrogen atom by positron using hylleraas time-dependent approach

The time-dependent treatment of positron-hydrogen scattering for a zero total angular momentum has been presented. The initial wavefunction of the positron-hydrogen scattering system has been expanded in terms of three dimensional dynamical wave functions to include all higher angular momenta by solving a set of three coupled differential equations. This wavefunction is then time evolved using Taylor series expansion of the evolution operator. The excitation probabilities are monitored as the wavefunction propagates until there is no more change in the probabilities. The positron impact excitation cross-sections extracted from the final wavefunction are compared with the available results of converged close coupling approach. Received 23 July 2001 and Received in final form 25 November 2001     

Plasma Modelling for the PSI Linear Plasma Device

Deuterium discharges in the PSI device have been modelled using the coupled package of the B2 hydrodynamic plasma code and the Eirene Monte‐Carlo neutral code. Radial and axial plasma profiles have been calculated for different magnetic field configurations, various radial diffusion laws and for different values of the flux limiter in the parallel electron heat conduction law used in the B2 code. The results are compared with experimental findings. The axial variation of the magnetic field strength is found to have an important influence on the plasma state via the axial plasma flow which closely resembles the neutral gas streaming through a series of laval nozzles. For particular magnetic field configurations, an appropriate ansatz for the parallel electron heat conduction turns out to be a crucial point for the applicability of hydrodynamic models to linear devices like the plasma generator PSI. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

直流非转移弧热等离子体喷枪加热效率分析

利用计算机模拟技术,采用有限容积方法,针对自制的低能耗、高效率内送粉等离子喷涂设备,对喷枪内部影响喷涂热效率的因素进行了分析。研究结果表明通过适当提高进气流量、提高混合气体中的氮气或氢气含量和减少喷枪通道长度,均能提高等离子喷枪热效率。而电流的大小以及喷枪的通道直径对热效率无显著影响。     

An analytic colllslonai-radlatlve model Incorporating non-LTE and optical depth effects

In this paper we analyze the variations in line intensities ratios due to a non-equilibrium situation and to optical depth effects. A four level model is proposed and the two particular situations for the possible transitions are considered. Electron density and temperature as well as the source thickness are used as independent parameters to find out in which way and extent they modify the ratios of levels populations compared with the ideal case of an equilibrium state and optically thin source. Accordingly with the ion of interest, electron temperatures ranging from I/20 to I/7 eV (I being the ionization energy), whereas electron densities in the interval from 10¹⁴ to 10²⁰ cm^-3 will be considered. These ranges are of special interest for diverse applications such as LIBS and measurement of transition probabilities. Some results are presented for real ions and a new expression for the escape factor is also proposed for general plasma conditions. Received 12 June 2001 and Received in final form 24 October 2001     

Numerical simulation for the characterization of operating conditions of RF-RF hybrid plasma torches

A two-dimensional (r,z) numerical code has been developed to investigate the prominent features of RF-RF hybrid plasma torches with two stages of different diameter. Flow and temperature fields have been calculated within the assumptions of laminar flow and local thermodynamic equilibrium for the optically thin argon plasma operated at atmospheric pressure, taking into full account the electromagnetic interaction between the primary and the secondary stage of the hybrid device. Results from a detailed parametric study for various geometric, gas flow and electric configurations aim at putting into evidence the wide range of operating conditions that can characterize the use of RF-RF hybrid plasma torches for industrial applications, showing also their possibility to give high enthalpy plasma jets with high torch efficiencies. The magnetofluidynamic modelling described in this work can be an effective tool for providing the theoretical framework for a deep understanding of RF-RF hybrid plasma torches and for designing them as suitable sources for chemical processing of materials, when utilized within an integrated approach that would match the induction plasma torch simulation with the RF generator operating conditions to evaluate the total source efficiency for each particular hybrid configuration.Received: 23 December 2003, Published online: 10 February 2004PACS: 52.75.Hn Plasma torches - 52.65.-y Plasma simulation - 52.80.Pi High-frequency - RF dischargesA colour version of the figures is available in electronic form at http: //ww.edpsciences.org     

Aging and energy landscapes: application to liquids and glasses

The equation of state for a liquid in equilibrium, written in the potential energy landscape formalism, is generalized to describe out-of-equilibrium conditions. The hypothesis that during aging the system explores basins associated to equilibrium configurations is the key ingredient in the derivation. Theoretical predictions are successfully compared with data from molecular dynamics simulations of different aging processes, such as temperature and pressure jumps. Received 7 August 2002 / Received in final form 8 October 2002 Published online 19 December 2002 RID="a" ID="a"Present address: Laboratoire de Physique Théorique des Liquides, Université Pierre et Marie Curie, 4 place Jussieu, Paris 75005, France e-mail: mossa@lptl.jussieu.fr     

Ionization of iridium ions in the Dresden EBIT studied by X-ray spectroscopy of direct excitation and radiative recombination processes

Ir^q+ ( 41≤q≤64) ions with open-shell configurations have been produced in the electron beam of the room-temperature Dresden Electron Beam Ion Trap (Dresden EBIT) at electron excitation energies from 2 keV to 13 keV. X-ray emission from direct excitation processes and radiative capture in krypton-like to aluminium-like iridium ions is measured with an energy dispersive Si(Li) detector. The detected X-ray lines are analyzed and compared with results from multiconfigurational Dirac-Fock (MCDF) atomic structure calculations. This allows to determine dominant produced ion charge states at different electron energies. The analysis shows that at the realized working gas pressure of 5×10^-9mbar for higher charged ions the maximum ion charge state is not preferently determined by the chosen electron beam energy needed for ionization of certain atomic substates, but by the balance between ionization and charge state reducing processes as charge exchange and radiative recombination. This behaviour is also discussed on the basis of model calculations for the resulting ion charge state distribution. Received 12 July 2001 and Received in final form 10 September 2001     

Electrothermal efficiency, temperature and thermal conductivity of plasma jet in a DC plasma spray torch

A study was made to evaluate the electrothermal efficiency of a DC arc plasma torch and temperature and thermal conductivity of plasma jet in the torch. The torch was operated at power levels from 4 to 20 kW in non-transferred arc mode. The effect of nitrogen in combination with argon as plasma gas on the above properties was investigated. Calculations were made from experimental data. The electrothermal efficiency increased significantly with increase in nitrogen content. The plasma jet temperature and thermal conductivity exhibited a decrease with increase in nitrogen content. The experiment was done at different total gas flow rates. The results are explained on the basis of dissociation energy of nitrogen molecules and plasma jet energy loss to the cathode, anode and the walls of the torch     

Metastable states of the Ising chain with Kawasaki dynamics

We consider a ferromagnetic Ising chain evolving under Kawasaki dynamics at zero temperature. We investigate the statistics of the blocking time, as well as various characteristics of the metastable configurations reached by the system, including the statistics of the final energy, the spin correlations, and the distribution of domain sizes. Results of extensive numerical simulations are compared with analytical predictions made for the a priori ensemble of all blocked configurations with equal weights. Qualitative differences are found, e.g. in the domain sizes, which are found to be neither statistically independent nor exponentially distributed. Received 24 October 2002 / Received in final form 13 January 2003 Published online 1st April 2003 RID="a" ID="a"e-mail: luck@spht.saclay.cea.fr RID="b" ID="b"URA 2306 of CNRS     

Exact propagator of the Fokker-Planck equation with a space-dependent diffusion coefficient and a time-dependent mean-reverting force

We have investigated the algebraic structure of the Fokker-Planck equation with a variable diffusion coefficient and a time-dependent mean-reverting force. Such a model could be useful to study the general problem of a Brownian walker with a space-dependent diffusion coefficient. We also show that this model is related to the Fokker-Planck equation with a constant diffusion coefficient and a time-dependent anharmonic potential of the form V(x, t) = ?a(t)x ² + b ln x, which has been widely applied to model different physical and biological phenomena, e.g. the study of neuron models and stochastic resonance in monostable nonlinear oscillators. Using the Lie algebraic approach we have derived the exact diffusion propagators for the Fokker-Planck equations associated with different boundary conditions, namely (i) the case of a single absorbing barrier, and (ii) the case of two absorbing barriers. These exact diffusion propagators enable us to study the time evolution of the corresponding stochastic systems. Received 23 October 2001 and Received in final form 24 December 2001     

Non-iterative optimal design of quantum controls

A non-iterative means for quantum control design is introduced with the aim of offering practical designs that can later be fine-tuned with laboratory closed-loop techniques. The procedure recognizes that Hamiltonians for realistic system control applications are rarely known accurately. The algorithm takes advantage of this fact by allowing for managed deviations in the equations of motion, thus removing the standard Lagrange multiplier. Suitable time-dependent cost functional weights are introduced that eliminate the traditional final time matching condition, thereby producing non-iterative design equations as an initial value problem. Removal of the final time condition also eliminates the demand that the target state be reached at any artificially imposed time. Tests on a simple molecular system indicate that the algorithm leads to well-behaved designs and that the weight functions are adequately estimated by order of magnitude analysis. Received 30 June 2000 and Received in final form 22 November 2000     

NMR study of the dissolution of laser-polarized xenon

NMR of laser-polarized xenon is used to probe the dissolution behaviour of the noble gas in different liquids. The dissolution and self-relaxation rates are extracted via a macroscopic model, and comparison of the decay rate of the xenon magnetization in deuterated and non-deuterated solvent pairs allows the determination of the pure dipole-dipole contribution to relaxation. A transient convective effect, tentatively assigned to the xenon concentration gradient, is observed and characterized by diffusion encoding MRI experiments. The flow of xenon penetrates inside the solvent near the walls of the NMR tube, the longitudinal images showing a “” shape, the transverse ones a “O” shape. This convection effect has implications for delivery conditions of laser-polarized xenon in continuous flow experiments and magnetic resonance imaging. Received 29 April 2002 / Received in final form 26 July 2002 Published online 22 October 2002 RID="a" ID="a"e-mail: hdesvaux@cea.fr RID="b" ID="b"URA CNRS/CEA 331     

AC Stark effect in toroidal carbon nanotubes threaded with an ac magnetic flux

The ac Stark effect is investigated in the toroidal carbon nanotube system threaded with an ac magnetic flux. The Floquet theory is employed to deal with the time-dependent quantum problems. The time-averaged energy of the system is derived and is found to exhibit a strong relationship with an external field, and the modified energy gap has been presented. The ac flux enhances energy gaps to cause metal-semiconductor transition. The steady current has been obtained by employing the free energy approach, and the persistent current is a special case as the magnitude of the ac flux approaches zero. The photon-assisted current is quite different from the persistent current due to the absorption and emission of photons. The local density of states is obtained by calculating the Green's function in the Floquet state, and photon-resonant structures are observed. All of the novel features are associated with the ac Stark effect, which is caused by the modification of energy levels. Received 20 November 2002 / Received in final form 7 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: zhaohonk@yahoo.com     

Electronic correlation effects on the properties of an half-filled octahedron cluster in a magnetic field

The present study focuses on electronic correlation effects on magnetic energy, the spin-spin correlation function of an octahedron cluster in the (3↑, 3 ↓) electronic configuration threaded by a magnetic field. Some other spin configurations are also discussed and various field directions are considered. An accurate diagonalisation technique has been used to solve the Hubbard Hamiltonian. A result is analysed on a linear energy stabilisation at low magnetic flux. Moreover, two types of antiferromagnetic transition versus the flux occurring for a correlation term larger than a critical one have been observed, i.e. the likelihood of a charge excitation before the antiferromagnetic transition. Finally, a comparison between the results obtained from the exact diagonalisation and the Gutzwiller method has been carried out, leading to a suggested modification of the Gutzwiller approach in order to improve it. Received 23 June 1999 and Received in final form 28 July 2000     

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介绍了自行研制的直流非转移弧等离子体炬的结构和工作原理。运用相似理论,研究了该等离子体炬的电热特性与运行参数,比如气体流量、弧功率的关系,并将计算值与实验结果进行了比较。结果表明,计算值与实验结果吻合较好,可以利用获得的公式来对炬进行结构改进和参数优化。