RF Induction Research Articles

Langmuir probe measurements of a radio frequency induction plasma

In this work a planar, radio frequency induction plasma source is characterized in terms of ion density, electron temperature, and plasma potential using a single Langmuir probe in oxygen and noble gases. Probe measurements of density were also verified using microwave interferometry. Measured argon ion densities increase nearly linearly with power from 1×1011 cm−3 at 300 W rf power to 6×1011 cm−3 at 1.2 kW at 1×10−3 Torr. Krypton ion densities are also linear with power but saturate above 1 kW at a density of 2×1012 cm−3 at 1×10−3 Torr. Electron temperatures increase with decreasing pressure from 3 eV at 26×10−3 Torr to 7 eV at 0.3×10−3 Torr. Plasma potentials are typically 15–30 V and increase with decreasing pressure. Ion saturation current in oxygen at 5×10−3 Torr is 2.5% uniform over diagonals of 20 cm when a magnetic multipole bucket is used to confine the plasma. Ion generation energy cost in argon is 100–250 W/A.

Electromagnetic fields in a radio-frequency induction plasma

The electromagnetic fields which drive a radio-frequency induction plasma are both modeled and measured. The plasma source consists of a planar, square coil separated from a low pressure plasma chamber by a 2.54-cm-thick quartz window. A small loop antenna, which is sealed in a pyrex tube, is immersed in the discharge to determine the magnitude and direction of the rf magnetic field. The measured B field is primarily radial and axial. Typical rf field strengths vary from 2 to 7 G for rf powers of 0.1–1 kW. The radial B field decays exponentially in the axial direction. The skin depth of the electromagnetic field is 1.6–3.6 cm which is consistent with Langmuir probe measured ion densities (typically 3×1011 cm−3) in argon. Invoking Maxwell’s equations to deduce the rf electric field from the measured B field, we find the E field to be primarily azimuthal. Peak field strengths increase from 100 V/m at 100 W to 200 V/m at 600 W where they saturate for higher powers. Finally, we present a 3D finite element solution for the fields produced by this plasma source which employs a cold, collisionless plasma model to relate the relative plasma permittivity εr to the electron plasma frequency, ωpe, using εr=1−(ωpe/ω)2. The measured fields support this numerical solution.

Low-pressure microwave plasma nucleation and deposition of diamond films

Low-pressure microwave plasma nucleation and deposition of diamond films were investigated in the pressure range 10-mtorr to 10 torr, at substrate temperatures 400-750 C and with CH4 and O2 concentrations in H2 plasma of 2-15 percent and 2-10 percent, respectively. The experiments were performed in a microwave plasma system consisting of a microwave plasma chamber, a downstream deposition chamber, and an RF induction heated sample stage. Scanning electron microscopy of diamond films deposited at 600 C with 5 percent CH4 and 5 percent O2 in H2 plasmas showed high-quality well faceted crystallites of 1/2 micron size. Cathodoluminescence measurements of these films showed very few nitrogen impurities and no detectable silicon impurities.

RF induction plasma spraying: State-of-the-art review

Radio frequency (RF) induction plasma technology has gained wide acceptance for the preparation of plasma spray coatings and structural free-standing parts of metals, ceramics, and metal-matrix composites. Its principal advantages are the relatively large volume and low velocity of the discharge, which coupled with the ease of axial injection of the powder into the plasma allows for the melting of relatively large particles at high throughput. The absence of electrodes offers the added advantage of ease of operation under a wide range of conditions at atmospheric and low pressure, with an inert, reducing, or oxidizing atmosphere. Highlights of recent advances in induction plasma melting and deposition of materials are presented in this article. The first section deals with advances in induction plasma spraying technology including system and torch design. This is followed by a brief overview of mathematical modeling and diagnostic studies of the induction plasma deposition process. Examples of applications of induction plasma spraying for the preparation of protective coatings and fiber-reinforced metal matrix composites are presented in the final section.

Diamond film synthesis on Mo in thermal RF plasma

Deposition of diamond films was performed in a sub-atmospheric thermal RF induction plasma reactor. The pressure varied from 600 to 200 mbar (60 to 20 k Pa) and the axial distance of the substrate from the plasma centre varied from 6 to 15 cm. The deposits varied from individual diamond crystallites to very smooth and homogeneous diamond films. The grain size varied from about 10 μm to between 0.01 and 0.1 μm in the finest case depending on deposition conditions. The deposition rate of the films varied between 4 and 11 μm/h.

Formation of diamond films from low pressure radio frequency induction discharges

Diamond films have been deposited in a low pressure, radio frequency (r.f.) induction plasma-assisted chemical vapor deposition system. The r.f.-induction system confines the plasma at the low pressures of operation 0.010–10.00 Torr to permit efficient dissociation of the reactant gases. A variety of chemical systems have been used to deposit diamond, including traditional H 2-CH 4 discharges containing 0.5–2.0% CH 4; H 2-CF 4 discharges containing 4–16% CF 4, and water vapor discharges containing high concentrations of alcohol and/or acetic acid vapors. No molecular hydrogen is admitted to the growth chamber for the water-based processes. The water vapor becomes the functional equivalent of the molecular hydrogen used in more traditional H 2-CH 4 discharges. The success of the low pressure r.f.-induction plasma for diamond growth from the wide variety of chemical systems is predicated on the generation of a high electron density plasma. Parent gaseous molecules are converted into appropriate high temperature stable products such as H, H 2, CO, and C 2H 2 as they traverse the plasma. Quadrupole mass spectroscopy has been used to study the conversion of the water-alcohol vapors to H 2, CO, and C 2H 2 as they pass through the r.f. plasma, 99% of the CH 4O is converted into H 2, H 2O, and C 2H 2. These studies show plasma conversion of H 2O into molecular H 2. The excess oxygen is rapidly converted into CO through interactions of the O, presumably with solid carbon sources. Optical emission from both the water-based discharges and the molecular hydrogen-based discharges shows the propensity for atomic hydrogen generation from these low pressure r.f.-induction discharges.

A Mathematical Model for Chemical Vapor Infiltration with Volume Heating

A detailed mathematical model is presented to investigate the chemical vapor infiltration (CVI) of fiber‐reinforced ceramic composites with a volume‐heating source. Volume heating may be achieved by using microwave power or radio frequency (RF) induction in the case of conductive substrates. The analysis includes a set of constitutive equations describing the space and time dependence of species concentration, temperature, pressure, and porosity. The infiltration of carbon‐fiber preforms with carbon resulting from methane decomposition is selected as a model system for analysis. Particular emphasis is placed on the impact of absorbed power on deposit uniformity and processing time. CVI with volume heating may lead to complete densification with considerably lower processing times when compared to conventional CVI processes. It is shown that when a constant power is used, there exists a critical power value above which accessible porosity is trapped within the composite. Several power modulation schedules are suggested to achieve rapid and complete densification without residual accessible porosity.

An rf plasma source for D + and D − ions

The development of an rf induction source for D + and D − ions is described. The plasma generator is a magnetic multipole source of dimensions 55 × 31 × 20 cm 3. The rf system consists of a 50 kW, 2 MHz oscillator with a matching unit to couple the power efficiently to an internal antenna. The use of rf-driven plasmas offers the potential of higher proton yields and allows the investigation of the influence of wall materials on the source performance as opposed to filament-driven dc plasmas which coat the wall with the filament materials. The source and its derivatives have applications in fusion, particle accelerators and plasma processing.

Characterization of MOCVD fluid dynamics by laser velocimetry

The process of metalorganic chemical vapor deposition (MOCVD) is critically dependent on the fluid dynamics of the reacting and carrier gases. Quantitative understanding of the basic fluid dynamics associated with a reactor design is central to the application of engineering techniques to achieve design improvements in efficiency, bulk production and growth uniformity. A laser velocimetry (LV) system has been adapted specifically to acquire such quantitative information for the low Reynolds number mixed convective flows commonly encountered in MOCVD reactors. The instrument uses three color-separated wavelengths of an argon-ion laser to make three simultaneous independent orthogonal measurements of the flow field at specified interior locations. The technique is noninvasive, provides high spatial resolution, and is compatible with reactors operating at growth temperatures. The fluid dynamics of a horizontal MOCVD reactor used for GaAs growth were characterized by laser velocimetry measurements. The full three-component LV system was employed to determine a three-dimensional map of the flow field inside the quartz reactor vessel. Data have been obtained from the vessel both at room temperature and heated to growth temperature by RF induction. The two cases were compared to evaluate the effects of gravity-induced convection and buoyancy. Significant convective effects were found in the heated reactor, while fluidic switching effects were observed during room temperature operation. Instrumentation details and flow field measurements are presented.

A gap type RF induction heating system for ferromagnetic hyperthermia

A gap type RF induction heating system for ferromagnetic hyperthermia

Measurements of the radiation source strength in argon at temperatures between 5000 and 10 000 K

The volumetric radiative source strength has been measured for argon using a 50 kW rf induction plasma torch operated at atmospheric pressure with temperatures ranging between 5000 and 10 000 K. At temperatures below 7500 K the results differ by over an order of magnitude from the only previously published measurements [Phys. Fluids 10, 1125 (1967)]. Spatially resolved total radiation measurements were made with a calibrated pyroelectric detector system of flat response in the wavelength range 250 to 2500 nm. The corresponding temperature measurements were made through spectroscopic examination of relative and absolute line intensities, and absolute continuum intensities. The data were interpreted in terms of a partial equilibrium model in which the free and excited bound electrons are found to be mutually in equilibrium, irrespective of departures from equilibrium with the ground state. The differences from the previous radiation source strength measurements are interpreted in terms of unaccounted for nonequilibrium effects in that study. The present measurements are in satisfactory agreement with theoretical predictions. A simple yet complete set of calorimetric measurements taken independently supports these results.

Radio-frequency induction plasmas at atmospheric pressure: Mixtures of hydrogen, nitrogen, and oxygen with argon

Numerical calculations are reported which simulate atmospheric-pressure radiofrequency induction plasmas consisting of either pure argon or mixtures of argon with hydrogen, nitrogen, or oxygen. These calculations are compared to observations of laboratory plasmas generated with the same geometry and run conditions. The major features of the laboratory plasmas are predicted well by the calculations: the pure argon plasma is the largest, with the argon-oxygen plasma slightly smaller. The argon-nitrogen plasma is considerably smaller and the argon-hydrogen plasma is the shortest, although somewhat fatter than the argon-nitrogen case. The calculations are not entirely successful in predicting the exact location of the plasmas relative to the coils. A likely explanation is that there is significant uncertainty regarding the actual power coupled to the laboratory plasmas.

Phase equilibrium study of the YBaNiO system and structural characterization of the new quasi-one-dimensional oxide Y 2BaNiO 5

Phase equilibria in the Y 2O 3BaONiO system were studied in the temperature range 1000–1350°C in air. In addition to previously reported compositions, the new phase Y 2BaNiO 5 was identified. Conditions for synthesis and stability of Y 2BaNiO 5 are reported, as well as the dependence of oxygen nonstoichiometry and lattice parameters on conditions of preparation. Single crystals were obtained by rf induction melting and the structure was refined by four circle X-ray diffraction. Y 2BaNiO 5 is isomorphous with Nd 2BaNiO 5 and consists of isolated, highly compressed chains of corner-shared NiO 6 octahedra.

Free surface shape and AC electric current distribution for float zone silicon growth with a radio frequency induction coil

The float zone process involves a molten silicon zone between a melting polycrystalline feed rod above and a growing single crystal below. Frequently the feed rod is melted and the float zone is kept molten by the joulean heating from AC electric currents induced by a radio frequency induction coil around the float zone. The shape of the melt's free surface is determined by a balance of the hydrostatic pressure in the melt, the surface tension force and the inward electromagnetic body force due to the AC electric currents near the surface of the melt. For a given geometry of the induction coil, the distribution of AC electric currents depends on the free surface shape. Therefore the determination of the free surface shape and the determination of the distribution of the AC electric currents due to the induction coil are intrinsically coupled. This paper presents an iterative method to determine both the free surface shape and the AC electric current distribution. This treatment assumes that the zone's radial and axial dimensions are known. This method is illustrated with four cases combining two different induction coil shapes and two different AC voltages applied to the induction coil.

Measurements of nonequilibrium effects in thermal plasmas

Abstract

Analysis of an RF induction plasma torch with a permeable ceramic wall

AbstractAn analysis is made for an rf plasma torch with a permeable ceramic tube as the contaminant tube. The temperature, flow, and electromagnetic fields in the rf torch are obtained along with the argon flow, the gas and matrix temperatures. The results show that about 80% of the plasma radiation losses are trapped by the ceramic wall. For optimum cooling, a wall with variable thickness is proposed. The results also indicate that argon is not a suitable gas for cooling the permeable wall.

A turbulent flow model for the rf inductively coupled plasma

A mathematical representation is given for the turbulent fluid flow and energy transfer in an rf induction plasma. The flow and temperature fields are obtained through the solution of the two-dimensional rotationally symmetric turbulent Navier–Stokes equations along with the energy and the one-dimensional Maxwell’s equations for the electric and magnetic fields. The turbulent viscosity is determined using the standard k-ε model. Results are given for an argon plasma under atmospheric conditions. Different aspects of turbulent flows and their implications in rf plasmas are discussed. The results indicate the presence of both laminar and turbulent regimes in the same flow field. The effect of swirl in the plasma gas is to increase the overall turbulence level in the torch.

Heavy-ion inertial fusion: 1988 symposium summary

The 1988 International Symposium on Heavy-Ion Inertial Fusion was held at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, FRG, June 28–30, 1988. About 130 participants from 12 countries gave some 74 invited, oral and poster papers. Progress in developing high-current accelerators was noted for both rf linacs and induction linacs. In the area of beam-target interaction physics the German and French programs have moved to an experimental phase on topics such as energy deposition of heavy ions in hot, dense matter. These experiments will become a primary goal upon completion of a major expansion of the GSI accelerator complex. Unlike prior symposia, no new material was presented on system studies for heavy ion fusion. This review consists of highlights of the four major international programs together with summaries of selected topics.

Float zone shape and stability with the electromagnetic body force due to a radio-frequency induction coil

Previous studies of the shape and stability of the free surface in the float zone process have only included the surface tension and the hydrostatic pressure. If a radio-frequency induction coil is used to melt the feed rod, then it produces an inward electromagnetic body force at the free surface. The previous treatments of free surface shape and stability are extended to include this electromagnetic body force. This force pinches the float zone and produces a smaller minimum radius, relative to the feed rod and crystal radii. For a coil which is close to the free surface, a sufficiently strong electromagnetic force destabilizes the surface.

A review of free-electron lasers

Free-electron laser (FEL) theory and experiments are reviewed. The physical mechanism responsible for the generation of coherent radiation in the FEL is described and the fundamental role of the ponderomotive wave in bunching and trapping the beam is emphasized. The relationship of the FEL interaction to the beam–plasma interaction is pointed out. Various FEL operating regimes are discussed. These include the high-gain Compton and Raman regimes, both with and without an axial guiding magnetic field. The linear and nonlinear regimes are examined in detail, with particular emphasis on techniques for achieving efficiency enhancement. The quality of the electron beam used to drive FEL’s is a critical factor in determining their gain and efficiency. The subject of electron beam quality, for different accelerators, is discussed. Key proof-of-principle experiments for FELs in an axial guiding magnetic field, as well as those driven by induction linacs, rf linacs, electrostatic accelerators, and storage rings, are reviewed. Finally, the requirements on wigglers and resonators are discussed.