Coaxial Source Research Articles

A low-power, linear-geometry Hall plasma source with an open electron-drift

This paper presents a discussion of the physics of modern Hall plasma thrusters and its impact on the design of new plasma thrusters of varying geometry and power. A particular emphasis is placed on the design and development of a linear-geometry (non- coaxial) source with an open electron-drift current. The operating characteristics of a linear-geometry Hall discharge scaled to operate in the 50 to 100 W power range are presented. Two thruster acceleration channels were fabricated—one of alumina and one of boron nitride. Differences in operation with the two channel materials are attributable to differences in the secondary electron emission properties. In either case, however, operation is achieved despite the lack of a closed electron current drift in the Hall direction, suggesting that there is an anomalous axial electron mobility, due to either plasma fluctuations or collisions with the channel wall. Strong low-frequency oscillations in the discharge current, associated with the depletion of propellant within the discharge, are seen to appear and vary with changes in the applied magnetic field strength. The frequency of this oscillatory mode is higher than that seen in larger (and higher power) discharges, due to the decreased residence time of the propellant within the channel.

Analysis of plasma generation and acceleration in pulsed coaxial hybrid source

The purpose of the research undertaken by the authors was to explain, on a theoretical basis, the phenomenon of pulsed reactive plasma generation in the new, coaxial hybrid source. The authors assumed that the separation of the stages of plasma generation and acceleration, and the occurrence of an external magnetic field, would enable them to apply lower acceleration energies and, as a result, improve the plasma homogeneity. This assumption was the basis for constructing and elaborating a new sputtering–evaporation plasma source called the pulsed coaxial hybrid source. It should enable an independent execution of the process of plasma generation and acceleration, and give the possibility of controlling its parameters and composition in each of the mentioned processes. The main problems solved by the authors, until now, and presented in this paper concern calculating the main characteristics of plasma generated in the stationary magnetron discharge and estimating their influence on the pulse plasma acceleration.

Space–time spectral investigation of the distribution of pulsed plasma generated by a new coaxial hybrid source

The purpose of the research described in the paper was to investigate the space–time distribution of individual elements of the plasma generated and accelerated in the new coaxial hybrid source worked out by the authors. Temporal courses of spectral signals of selected plasma components were recorded. The space–time distribution of plasma components during its transport in the deposition chamber was investigated utilising the Abel reconstruction method. Apart from spectral signals, the current–voltage characteristics were recorded. On the basis of the investigation results the influence of the source performance characteristics on the space–time plasma distribution inside the deposition chamber was defined.

Co-axial multicusp source for low axial energy spread ion beam production

A co-axial multicusp ion source has been designed and tested. This source uses a new magnetic filter configuration. This magnetic filter is efficient in modifying the plasma potential distribution which can reduce the axial energy spread of the extracted ion beam . Energy spreads as low as 0.6 eV have been obtained. The electron temperature in this source has also been found to be about 0.1 eV. Furthermore, the new source configuration is capable of adjusting the radial plasma potential distribution which can improve the transverse ion energy, which results in a low beam emittance . The co-axial source can be used for a number of different applications such as ion projection lithography and radioactive ion beam projets.

A novel on-wafer resistive noise source

We describe a novel on-wafer resistive noise source (ORNS) suitable for noise parameter characterization of microwave devices using the cold noise power measurement technique. The noise source can enhance measurement accuracy by providing a calibrated noise temperature directly at the device reference plane. A procedure to determine the excess noise ratio of the noise source is presented. Noise figure measurements performed up to 40 GHz with the on-wafer noise source and with a commercial coaxial noise source are in good agreement, thereby validating the technique. The novel source is effective as a noise standard up to millimeter-wave frequencies.

Finite difference calculations of electro-magnetic fields in arbitrarycurvilinear coordinates for coaxial microwave plasma sources

We present a finite difference frequency domain (FDFD) algorithm for the vector wave equation. Employing covariant formulations, arbitrary curvilinear, non-orthogonal computational grids are mapped onto equidistant Cartesian coordinates. This provides an intrinsic method for a local increase of computational accuracy and application of standard finite difference expressions. As an example this technique is applied to the microwave plasma torch (MPT). It is shown that there is a high field plasma generating region on top of the inner conductor. The interior of this region is of comparably low electrical field strength and supposed to be responsible for the hollow structure of the plasma observed in this type of plasma source.

Analytical calculation of the efficiencies of gamma scintillators. Part I: Total efficiency for coaxial disk sources

Total efficiencies of clad right cylindrical NaI(Tl) scintillation detectors exposed to photons emitted by circular disk sources have been calculated by using rigid mathematical expressions. Results are tabulated for various gamma energies.

Plasma source for ion and electron beam lithography

A new plasma source configuration, coaxial source, has been developed at the Lawrence Berkeley National Laboratory suitable for ion and electron beam lithography applications. The axial ion energy spread and electron temperature of the multicusp ion source have been reduced considerably from 2 and 0.3 eV to a record low of 0.6 eV by employing a coaxial source arrangement. Results of ion projection lithographic exposure at the Fraunhofer Institute demonstrate that feature size less than 65 nm can be achieved by using a filter-equipped multicusp ion source. Langmuir probe measurements also show that very low energy spread electron beams can be obtained with the multicusp plasma generator.

Growth of GaN by molecular-beam epitaxy with activation of the nitrogen by a capacitive rf magnetron discharge

It is shown that GaN films can be grown by molecular-beam epitaxy with plasma activation of the nitrogen by a magnetron rf discharge in a specially constructed coaxial source with capacitive coupling. A growth rate of ∼0.1 μm/h is obtained on GaAs and sapphire substrates, and ways are found for optimizing the design of the plasma source in order to increase the growth rate. The electrophysical and luminescence properties of undoped epitaxial films are investigated at temperatures ranging all the way to room temperature.

Electron cyclotron resonance applicator of the Wilkinson microwave power divider type for large diameter ion sources

We have designed and tested a new model of coaxial 2.45 GHz electron cyclotron resonance (ECR) ion source based on the splitting of the microwave power traveling along a coaxial line into a set of periodically distributed antennas. Our motivation was the study of the scaling-up feasibility of a coaxially fed ion source in which it is desirable to circumvent the transverse electric and magnetic (TEM) mode conversion into azimuthally unsymmetrical higher-order modes. Another important characteristic of this source is the possible incorporation of permanent magnets into the individual antennas in order to create the ECR zones around the latter. The validity of these concepts was tested on a 5 cm in external diameter coaxial ECR ion source in which the single thick coaxial antenna was replaced by a set of quarter wavelength magnetized rod antennas in order to enhance the magnetic field created by the external splitted ring magnets. For an incident power of 100 W and an argon pressure of 1.2×10−4 mbar, a 500 eV singly charged ion beam with a current density of 1 mA/cm2 was delivered through an accel-decel set of grids. Implementation of a scaled-up 20 cm in diameter ion source is in progress.

An illuminating retractor for intracranial microneurosurgery.

During certain microneurosurgical procedures, the illumination provided by current coaxial microscope light sources is suboptimal. One potential solution is the surgical light blade (SLB), a malleable retractor incorporating a flat fiberoptic bundle along its surface. The SLB provides intense illumination within the surgical field without decreasing the amount of limited operating space. A prototype of the SLB was tested in six patients. Results suggest that with further development the SLB could become useful for intracranial microneurosurgery.

High sensitivity, inductively coupled miniature magnetic probe array for detailed measurement of time varying magnetic field profiles in plasma flows

A modified Ḃ circuit design has been implemented as part of a miniature magnetic probe array for the Coaxial Plasma Source experiment [R. M. Mayo et al., Plasma Sources Sci. Technol. 4, 47 (1995)] at the North Carolina State University. This facility is currently being used for the generation of energetic plasma flows to allow laboratory study of magnetogasdynamics with particular emphasis on the importance of the Hall effect [D. C. Black et al., Phys. Plasma 1, 3115 (1994)] and plasma microinstabilities [R. M. Mayo et al., Phys. Plasma 2, 337 (1995)] to plasma transport in coaxial plasma sources. The miniature magnetic probe array consists of ten spatially separated coils wound on an Acetal form of dimensions 2.38 cm by 0.32 cm by 0.32 cm. At five positions, with roughly 0.32 cm separation, two mutually perpendicular coils are wound to measure the magnetic field in the θ̂ and ẑ directions. The modification to the signal processing circuitry consists of the use of a step-up transformer to boost the probe signal prior to filtering and acquiring the signal at the data acquisition system. This additional means of amplifying the signal allows for reduction in the size of the probe, and thus helps minimize the perturbing effect of the magnetic probe on the plasma. An additional advantage of using a signal transformer is that it provides electrical isolation between the experiment and the data acquisition system.

A compact electron cyclotron resonance proton ion source

The demand has increased for a high-current, long lifetime ion source in various fields of application. The present work concerning the production of atomic hydrogen ions H+ in competition with molecular ions H+2 has been essentially motivated by recent trends in the field of passivation of silicon [V. Le Thanh, M. Eddrief, and C. A. Sébenne, Appl. Phys. Lett. 64, 3308 (1994)]. For a given low neutral energy, passivation with a neutral atomic beam H instead of H2 is preferred. The most straightforward method to produce the neutral beams is by the extraction of low energy atomic ions, and neutralization by electron capture. We have thus developed an electron cyclotron resonance coaxial hydrogen ion source that produces a 30-mm-diam ion beam, is operated with a microwave power in the range of 50–130 W, and has a pressure of neutrals in the range of 6×10−4–4×10−3 mbar. We have compared the proportion of extracted H+ and H+2 ions with the two principal parameters of the plasma: the pressure of neutrals and microwave injected power. These parameters have been optimized for the production of an 83% atomic ion beam.

Coaxial excimer lamps pumped by barrier and longitudinal discharges

An experimental investigation was made of the radiation emitted by pulsed coaxial excimer light sources ('excilamps') pumped by barrier and longitudinal discharges. Stable operation of excilamps was demonstrated at pulse repetition frequencies up to 12 kHz. An average output radiation power was of the order of 0.2 W at λ ~ 308 nm. Excitation by a longitudinal discharge generated, for the same pump pulse energies, pulsed output radiation powers several times higher than excitation by a barrier discharge.

Stabilization and saturation of the ideal tilt mode in a driven annular field-reversed configuration

The coaxial slow source (CSS) is a device that was designed as a means of forming annular field-reversed configurations (AFRCs) for magnetic fusion on relatively slow (50–100 μs) time scales. Such configurations are predicted to be ideally unstable to magnetohydrodynamic tilting modes in which one-half of the torus shifts in the positive axial direction and the other half in the negative direction. These instabilities have been observed to grow and saturate at finite amplitude in the CSS as formation progresses. Stable operating regimes of fill pressure/loop voltage space have also been found. The relatively cold temperature (∼2 eV) of the plasma gives rise to a high classical resistivity, placing the Lundquist number (ratio of resistive to Alfvén time scales) between 0.5 and 10.0. It is proposed that resistivity, along with other dissipation mechanisms such as viscosity and ion–neutral friction, are responsible for the observed stability. Experiments were performed to explore these stability issues. A nonlinear, dynamical systems stability model that accounts for the effects of inductive current drive and dissipation has also been developed, and shows some agreement with the experimental results. The findings of this study are significant for two reasons: (1) It is one of the few clear observations of tilting modes in a FRC-like geometry; and (2) it provides an experimental study of ideally unstable modes in a configuration that does not escape the low Lundquist number regime, finding that the configuration appears stable below a Lundquist number of 0.5.

A magnetized coaxial source facility for the generation of energetic plasma flows

The design features and characterization of a new magnetized coaxial plasma source facility for the generation of energetic plasma stream flows are presented. Careful, yet simple design of a low-inductance (<or approximately=200 nH) external power delivery system provides high-current (>or approximately=130 kA) pulses to the plasma source with fast rise ( approximately 26 mu s). This allows for the production and acceleration of dense ( approximately 2*1015 cm-3), high-quality (10-40 eV), self-field plasmas to approximately 105 m s-1. We are then afforded the laboratory study of plasma streams with a wide variety of applications including, but not limited to, advanced thrusters for electric propulsion, astrophysical jets, critical ionization velocity, magnetic fusion, large-scale plasma etching and deposition, etc. Comparison to an ideal MHD model is made indicating reasonably good agreement in these self-field discharges, while elucidating the advantages of strong applied magnetization to provide nozzling in a magnetically constricted flow.

Fate of event hydrothermal plumes on the Juan de Fuca Ridge

Current meter moorings were deployed in August 1993 at the megaplume site of the June 1993 seismic event on the CoAxial segment of the Juan de Fuca Ridge and were recovered in April 1994. Record‐averaged flow was about 2 cm/s across axis, which is sufficiently large to advect plumes off axis in a few days. During July two megaplumes (1800 m level) disappeared between repeated CTD tows. A deeper plume (2150 m) had a relatively strong signature during August and extended southeastward in agreement with the observed flow. A CTD tow along the east flank of Axial Volcano did not locate remnants of the megaplumes but did find deeper plumes which could have come from off‐axis advection from the CoAxial sources followed by southward advection along the east side of ridge.

Initial Results on High Enthalpy Plasma Generation in a Magnetized Coaxial Source

Initial investigation on high enthalpy plasma stream generation in the North Carolina State University Coaxial Plasma Source[1] (CPS) facility is presented. Tenuous, yet high enthalpy, flows are produced from this Magnetized Coaxial plasma Gun (MCG) which allow laboratory study of plasma streams with a wide variety of applications. The applicability includes, but is not limited to, advanced thrusters for electric space propulsion, astrophysical jets and critical ionization phenomena, magnetic fusion compact toroid devices and tokamak fueling, large scale plasma etching and deposition, etc.

Tokamak formation and sustainment by coaxial helicity injection current drive

Experiments characterizing the coaxial helicity source and helicity injection current drive on the Prototype Helicity Injected Tokamak (Proto-HIT) have been performed. Toroidal currents over 30 kA can be sustained for times much greater than the plasma resistive time. The coaxial source operates with a large toroidal field that greatly enhances the source impedance, allowing a high rate of helicity injection (high source voltage) at a low source current (high efficiency), with results showing good agreement with a simple analytical model

Evaluation of Uncertainties of the Null-Balanced Total-Power Radiometer System NCS1.

Standard uncertainties are evaluated for the null-balanced, total-power, heterodyned radiometer system with a switched input that was recently developed at NIST to calibrate thermal noise sources. Eight significant sources of uncertainty due to systematic effects are identified, two attributable to the two noise standards, and one each to connectors, the input mismatch, the input switch asymmetry, the isolator, the broadband mismatch and the attenuator. The combined standard uncertainty of a typical coaxial noise source calibration at a representative frequency of 2 GHz is about 1%, A strategy for reducing uncertainties is discussed.