Inductively-coupled Radio Frequency Research Articles

Omnidirectional Monolithic Marker for Intra-Operative MR-Based Positional Sensing in Closed MRI.

We present a design of an inductively coupled radio frequency (ICRF) marker for magnetic resonance (MR)-based positional tracking, enabling the robust increase of tracking signal at all scanning orientations in quadrature-excited closed MR imaging (MRI). The marker employs three curved resonant circuits fully covering a cylindrical surface that encloses the signal source. Each resonant circuit is a planar spiral inductor with parallel plate capacitors fabricated monolithically on flexible printed circuit board (FPC) and bent to achieve the curved structure. Size of the constructed marker is Ø3-mm ×5 -mm with quality factor > 22, and its tracking performance was validated with 1.5 T MRI scanner. As result, the marker remains as a high positive contrast spot under 360° rotations in 3 axes. The marker can be accurately localized with a maximum error of 0.56 mm under a displacement of 56 mm from the isocenter, along with an inherent standard deviation of 0.1-mm. Accrediting to the high image contrast, the presented marker enables automatic and real-time tracking in 3D without dependency on its orientation with respect to the MRI scanner receive coil. In combination with its small form-factor, the presented marker would facilitate robust and wireless MR-based tracking for intervention and clinical diagnosis. This method targets applications that can involve rotational changes in all axes (X-Y-Z).

Suspension plasma sprayed copper-graphene coatings for improved antibacterial properties

Copper has a long history of use as a biocidal agent. Recent studies have demonstrated that incorporating graphene into copper coatings improves their antibacterial properties. However, the implementation of copper-graphene composite coatings is currently limited by the cost and scarcity of copper, as well as the difficulty of achieving a homogeneous distribution of graphene within the copper matrix. In this study, a new approach was developed to utilize an inductively-coupled radio frequency plasma torch to spray a uniform composite coating of copper nanoparticles and graphene nanoflakes (GNFs) onto a metal substrate. The resulting composite coatings exhibit microstructural uniformity, coating splats cohesion, and GNFs retention and dispersion within the Cu matrix. Furthermore, the Cu-GNFs coatings demonstrated strong antibacterial properties, with a 99% reduction of Escherichia coli bacteria within 1 h.

Improvement of magnetic resonance imaging using a wireless radiofrequency resonator\xa0array

In recent years, new human magnetic resonance imaging systems operating at static magnetic fields strengths of 7 Tesla or higher have become available, providing better signal sensitivity compared with lower field strengths. However, imaging human-sized objects at such high field strength and associated precession frequencies is limited due to the technical challenges associated with the wavelength effect, which substantially disturb the transmit field uniformity over the human body when conventional coils are used. Here we report a novel passive inductively-coupled radiofrequency resonator array design with a simple structure that works in conjunction with conventional coils and requires only to be tuned to the scanner’s operating frequency. We show that inductive-coupling between the resonator array and the coil improves the transmit efficiency and signal sensitivity in the targeted region. The simple structure, flexibility, and cost-efficiency make the proposed array design an attractive approach for altering the transmit field distribution specially at high field systems, where the wavelength is comparable with the tissue size.

An optimized and flexible configuration for the magnetic filter in the SPIDER experiment

Magnetic fields in negative ion driven Neutral Beam Injectors (NBIs) are essential to reduce the electron temperature and density in the extraction region, along with filtering out the otherwise unavoidably co-extracted electrons. In SPIDER, the full scale ion source and extractor of the ITER NBI, a horizontal magnetic field up to 8 mT inside the plasma source is produced by a current of up to 5 kA flowing through the plasma-facing electrode and additional bus-bars. First SPIDER experimental campaigns showed an influence on the plasma generation when the magnetic field is increased beyond a threshold. This effect has been ascribed to the particular topology of the field in the plasma volume inside the 8 inductively-coupled radio-frequency drivers that generate the plasma.The paper describes the experiments aimed at identifying the problem and the design process for a new layout of bus-bars capable of creating an optimized magnetic field topology within the RF driver, while keeping it almost unchanged in the other regions where the required performances were already achieved. Particular attention was given for a design leaving the possibility of easily modifying the new topology, so as to significantly expand the operating margin of the experimental activity.

In situ XPS analysis of the electronic structure of silicon and titanium thin films exposed to low-pressure inductively-coupled RF plasma

Carbon contamination of synchrotron and free-electron lasers beamline optics continues to be a major nuisance due to the interaction of the intense photon beams with the surfaces of the optical elements in the presence of residual gases even in ultrahigh vacuum (UHV) conditions. Among the available in situ cleaning strategies, low-pressure radio frequency (RF) plasma treatment has emerged as a useful and relatively simple approach to remove such carbon contamination. However, the irreversible damage that the plasma may induce in such critical surfaces has to be carefully characterized before its general application. In this study, we focus on reducing the amount of carbon from UHV chamber inside surfaces via silicon and titanium coatings using a low-pressure inductively-coupled downstream plasma source and we characterize the surface alterations by in situ X-ray photoemission spectroscopy (XPS). The in situ mirror cleaning is simulated by means of silicon wafers. We observe upward band bending, which translates into lower binding energies of the photoemission lines, that we attribute to the generation of vacancies and trapped charges in the oxide layers.

Influence of surface materials on the volume production of negative ions in a radio-frequency driven hydrogen plasma

Negative atomic hydrogen ion (H−) densities were measured in a pulsed low-pressure E-mode inductively-coupled radio-frequency (rf) driven plasma in hydrogen by means of laser photodetachment and a Langmuir probe. This investigation focuses on the influence of different metallic surface materials on the volume production of H− ions. The H− density was measured above a thin disc of either tungsten, stainless steel, copper, aluminium, or molybdenum placed onto the lower grounded electrode of the plasma device as a function of gas pressure and applied rf power. For copper, aluminium, and molybdenum the H− density was found to be quite insensitive to pressure and rf power, with values ranging between 3.6 × 1014 to 5.8 × 1014 m−3. For stainless steel and tungsten, the H− dependency was found to be complex, apart from the case of a similar linear increase from 2.9 × 1014 to 1.1 × 1015 m−3 with rf power at a pressure of 25 Pa. Two-photon absorption laser induced fluorescence was used to measure the atomic hydrogen densities and phase resolved optical emission spectroscopy was used to investigate whether the plasma dynamics were surface dependent. An explanation for the observed differences between the two sets of investigated materials is given in terms of surface reaction mechanisms for the creation of vibrationally excited hydrogen molecules.

Effect of VUV radiation and reactive hydrogen atoms on depletion of fluorine from polytetrafluoroethylene surface

Modification of polytetrafluoroethylene (PTFE) surface upon treatment with reactive species from hydrogen plasma (ions, neutral atoms, and UV/VUV) as well as synergistic effects are presented and explained. PTFE samples were either directly exposed to hydrogen plasma or separately to UV/VUV radiation or atomic hydrogen, and the resulting evolution of the surface composition was determined by X-ray photoelectron spectroscopy. High-density plasma was created by an inductively-coupled radiofrequency discharge in the H-mode. Samples placed in specially designed holders were covered with various optical windows (magnesium fluoride, quartz glass, or borosilicate glass), to allow for separate treatments with either VUV, UV or visible radiation. Rapid fluorine depletion was observed either at a direct exposure to plasma or to VUV; however, the best results were observed at a combined treatment with VUV/UV and atomic hydrogen. For direct treatment, a minimum in the F/C ratio was observed at the shortest time of 1 s. The F/C dropped from 2.2 to 0.4, whereas at longer times it increased to 1. For treatment with UV/VUV and for the combined treatment with UV/VUV and atoms, the F/C decreased with treatment time and stabilized at 0.6 and 0.4, respectively. Exposure to the afterglow did not result in significant modification.

An Inductively-Coupled Plasma Electrothermal Radiofrequency Thruster

The “cubesat” form factor has been adopted as the defacto standard for a cost effective and modular, nano-satellite platform. Many commercial options exist for nearly all components re- quired to build such satellite; however, there is a limited range of thruster options that suit the power and size restrictions of a cubesat. Based on the prior work on the “Pocket Rocket” electro- thermal capacitively-coupled radiofrequency (RF) plasma thruster operating at 13.56 MHz, a new design is proposed which is based on an inductively-coupled radiofrequency plasma system operat- ing at 40.68 MHz. The new thruster design, which includes a compact and efficient radiofrequency matching network adjacent to the plasma cavity is presented, together with the first direct thrust measurements using argon as the propellant with the thruster immersed in vacuum and attached to a calibrated thrust balance. These initial results of the unoptimised first prototype indicate an up to 40% instantaneous thrust gain from the plasma compared to the cold gas thrust: typical total thrust at 100 SCCM of argon and 50 W RF power is ∼ 1.1 mN.

Evaluation of power transfer efficiency for a high power inductively coupled radio-frequency hydrogen ion source

Neutral beam injection (NBI) for plasma heating and current drive is necessary for International Thermonuclear Experimental reactor (ITER) tokamak. Due to its various advantages, a radio frequency (RF) driven plasma source type was selected as a reference ion source for the ITER heating NBI. The ITER relevant RF negative ion sources are inductively coupled (IC) devices whose operational working frequency has been chosen to be 1 MHz and are characterized by high RF power density (∼9.4 W cm−3) and low operational pressure (around 0.3 Pa). The RF field is produced by a coil in a cylindrical chamber leading to a plasma generation followed by its expansion inside the chamber. This paper recalls different concepts based on which a methodology is developed to evaluate the efficiency of the RF power transfer to hydrogen plasma. This efficiency is then analyzed as a function of the working frequency and in dependence of other operating source and plasma parameters. The study is applied to a high power IC RF hydrogen ion source which is similar to one simplified driver of the ELISE source (half the size of the ITER NBI source).

Third type boundary conditions for steady state ambipolar diffusion equation

A third type boundary condition for boundary value problem of electron balance in inductively coupled radio-frequency (ICRF) discharge is described. It is assumed that directions of diffusion and drift flows on boundary through the positive charge layer are coincided. Herein we study the effect of different boundary conditions on the parameters of ICRF discharge by computational solution of eigenvalue problems. It is showed that electronic temperature at boundary conditions derived in this study differ from the electronic temperature at boundary condition which mean a complete recombination of charged particles on the wall of gas discharge chamber (DC). The derived boundary conditions can be used for modelling ICRF discharges to determine optimal parameters of plasmatrons.

Low temperature deposition of a-SiC:H thin films applying a dual plasma source process

The utilization of silicon carbide (SiC) thin films is of great interest whenever a high robustness is requested for MEMS/NEMS-based devices, which are operated in harsh environments beyond the applicability of standard silicon technology. Hydrogenated amorphous SiC (a-SiC:H) is especially attractive for these scenarios, as it can be deposited at temperatures lower than 400°C while still being beneficial for enhanced device performance. Furthermore, the thin film properties can be tailored to a great extent during synthesis or via post-deposition annealing. This study reports on the deposition of a-SiC:H thin films at a substrate temperature of 250°C applying a dual plasma process by superimposing a radio frequency (RF) plasma which excites the substrate table to an inductively-coupled RF plasma. This auxiliary plasma source has a huge impact on the resulting thin film properties due to the interaction of various opposing processes during film growth. To understand these processes, the layer composition was determined using time-of-flight elastic recoil detection analyses, Fourier transform infrared spectroscopy and by mass effusion measurements up to 1000°C, thus revealing changes in chemical composition, but also the power dependent incorporation of gaseous species from the plasma. This, for example, allows modification of the residual stress ranging from nearly stress free to highly compressively stressed layers of more than −2GPa, Young's modulus values ranging from about 168GPa down to 48GPa, but also greatly affects the surface topography which can be adjusted to a very low root mean squared roughness of less than 0.1nm.

Surface properties of plasma-functionalized graphite-encapsulated gold nanoparticles prepared by a direct current arc discharge method

The graphite-encapsulated gold nanoparticles (Au@C NPs) fabricated by a direct current arc discharge method were surface-functionalized by an inductively-coupled radio frequency ammonia plasma with a particle explosion technique for enhancing surface modification efficiency. To investigate the structural and surface properties of Au@C NPs, characterizations using x-ray diffraction, high resolution transmission electron microscopy and x-ray photoelectron spectroscopy have been conducted on the untreated and plasma treated Au@C NPs. Based on the experimental results, we give insight into the possible formation of Au ions in the interface between the graphite layers and gold core particles of the Au@C NPs. Finally, the role of the plasma treatment on the surface functionalization of Au@C NPs with amino groups is discussed.

Tracking the Position and Rotational Orientation of a Catheter Using a Transmit Array System

A new method for detecting the rotational orientation and tracking the position of an inductively coupled radio frequency (ICRF) coil using a transmit array system is proposed. The method employs a conventional body birdcage coil, but the quadrature hybrid is eliminated so that the two excitation channels can be used separately. The transmit array system provides RF excitations such that the body birdcage coil creates linearly polarized and changing RF pulses instead of a conventional rotational forward-polarized excitation. The receive coils and their operations are not modified. Inductively coupled RF coils are constructed on catheters for detecting rotational orientation and for tracking purposes. Signals from the anatomy and from tissue close to the ICRF coil are different due to the new RF excitation scheme: the ICRF coil can be separated from the anatomy in real time, and after doing so, a color-coded image is reconstructed. More importantly, this novel method enables a real-time calculation of the absolute rotational orientation of an ICRF coil constructed on a catheter. Modified FLASH and TrueFISP sequences are used for the experiments. The acquired images from this technique show the feasibility of different applications, such as catheter tracking. Furthermore, applications where knowledge of the rotational orientation of the catheter is important, such as magnetic resonance-guided endoluminal-focused ultrasound, RF ablation, side-looking optical imaging, and catheters with side ports for needles, become feasible with this method.

Plasma immersion ion implantation into cylindrical bore using internal inductively-coupled radio-frequency discharge

Plasma immersion ion implantation (PIII) is a potentially excellent interior surface treatment technique due to no line-of-sight restriction. However, some problems have been encountered due to the low ion energy and ion fluence non-uniformity especially for treatment of the interior wall of a thin tube. In this paper, a new method for inner surface PIII using internal inductively-coupled radio-frequency (RF) discharge is described. A cylindrical inductive coil inserted inside the tube serves as both the plasma source and grounded electrode to avoid overlapping of the plasma sheath fronts propagating from opposite sides. The effects of the gas species, gas pressure, RF power, and number of coil turns are investigated. Our results demonstrate the feasibility of this novel inner surface treatment method and the number of turns in the coil has a critical influence on the discharge behavior. If the number of turns is little, the plasma density is low and non-uniform inside the tube due to the relatively intense capacitively-coupled RF discharge at the two ends. In contrast, the plasma density and uniformity are evidently improved by using more turns in the coil.

Plasma immersion ion implantation into cylindrical bore using internal inductively-coupled radio-frequency discharge

Plasma immersion ion implantation (PIII) is a potentially excellent interior surface treatment technique due to no line-of-sight restriction. However, some problems have been encountered due to the low ion energy and ion fluence non-uniformity especially for treatment of the interior wall of a thin tube. In this paper, a new method for inner surface PIII using internal inductively-coupled radio-frequency (RF) discharge is described. A cylindrical inductive coil inserted inside the tube serves as both the plasma source and grounded electrode to avoid overlapping of the two plasma sheath fronts propagating from opposite sides. The effects of the gas species, gas pressure, RF power, and number of coil turns are investigated. Our results demonstrate the feasibility of this novel inner surface treatment method and the number of turns in the coil has a critical influence on the discharge behavior. If the number of turns is little, the plasma density is low and non-uniform inside the tube due to the relatively intense capacitively-coupled RF discharge at the two ends. In contrast, the plasma density and uniformity are evidently improved by using more turns in the coil.

A catheter tracking method using reverse polarization for MR‐guided interventions

To conduct interventional procedures in MRI, reliable visualization of interventional devices such as catheters is necessary. For this purpose, the use of inductively-coupled radio frequency (ICRF) coils has been proposed. Without a wired connection, the signal around the ICRF coil is amplified, enabling catheters to be visualized. The wireless connection allows easy handling of catheters, in some pulse sequences, however, it might be difficult to differentiate the catheters from anatomical background information. In this work, a novel ICRF coil visualization method, which allows separation of the catheter and the anatomical information by using the reverse and forward polarization modes of a coil, is proposed. This method allows images of the anatomy and the catheter to be combined into a color-coded image. First, an ICRF coil with decoupling diodes was constructed; we call this a receive-coupled RF (RCRF) coil. The RF safety profile of the RCRF coil is shown to be better than the ICRF coil. Second, to demonstrate the feasibility of this method, a receive-only birdcage coil without a hybrid coupler was constructed and then connected to a scanner as a two-channel phased-array coil. MR signals acquired from two channels were added after phase adjustments to create the reverse and forward polarization mode images. The reverse polarization mode image contained signal only from the RCRF coil, but the forward polarization mode displayed both anatomical information and the RCRF coil. The performance of this novel tracking method was tested in phantom and animal experiments. Color-coded images demonstrate the feasibility of the method to track catheters using RCRF coils.

Effect of annealing temperature on properties of H 2/N 2 rf plasma-treated stainless steel

Nitriding of AISI 304 austenitic stainless steel was prepared using inductively coupled radio frequency (ICRF) plasma with different H 2/N 2 ratios. Thickness of nitriding layer, microhardness, and structural phases of the compound layer, surface microstructure and cross-section morphology were studied before and after annealing treatment. Little change is found in the microhardness values and thickness of the compound layer when the treated samples are annealed up to 400 °C. With further increase in the annealing temperature, the microhardness values decrease while the thicknesses increase. Due to the low temperature plasma processing for the 50% N 2 + 50% H 2 sample, post-annealing over 500 °C has enormous influence on the structural phases of the sample. However, the 25% N 2 + 75% H 2 sample shows little variation on the structural phases due to annealing process.

Electron release in the afterglow of a pulsed inductively-coupled radiofrequency oxygen plasma

A pulsed inductively-coupled radiofrequency plasma in oxygen is investigated by means of time-resolved microwave interferometry in a wide pressure range from 0.5 to 200 Pa. In the afterglow a peak of the electron density is observed. The effect is maximum for pressures around 50 Pa. The time-resolved measurements of the electron density are interpreted in the framework of a fluid model. This model points out the significance of negative ions. The overall electron density is comparatively small. Attachment and detachment processes nearly balance during the power-on phase. But when the power is switched off, the electron temperature drops very quickly. This means that the production of new negative ions is inhibited so that the negative ions are destroyed by collisions. These reactions quickly set free electrons in the afterglow and are the reason for the observed peak in the electron density after switching off the power.

Magnetic and transport properties of nanocomposite Fe/Fe3−δO4 and Fe3−δO4 films prepared by plasma-enhanced chemical vapour deposition

Reflective, pinhole-free Fe/Fe3O4 and Fe3−δO4/Fe2O3 nanocomposite films were obtained by reacting iron pentacarbonyl, Fe(CO)5, in an inductively-coupled radio frequency glow discharge reactor. The conductivity of the Fex/(Fe3O4)1−x (x > 7%) composite films exhibits metallic characteristics and the conductivity decreases as the α-Fe content decreases. The metal-to-insulator transition temperature of the Fex/(Fe3O4)1−x (x ∼0.07) films shifts to a higher temperature as compared with Fe3−δO4 due to the increased conductivity from α-Fe. The magnetization versus temperature (M–T) curves show the transition temperature ranging from 95 to 136 K, corresponding to the Verwey temperature, which is dependent on the film composition. The Fe3−δO4 film exhibits a negative magnetoresistance of about 4% and 8% at room temperature and 80 K, respectively.

Etching of Silicon and Silicon Dioxide in Dense Low-Pressure Inductively Coupled Radiofrequency Discharge Fluorocarbon Plasmas

The results of a study on the selective etching of SiO2 and Si in dense low-pressure (p < 1 Pa) inductively coupled radio-frequency discharge fluorocarbon (СHF3 and СHF3 + H2) plasmas in a nonuniform magnetic field are reported. It was found that the selectivity of etching SiO2/Si increased from 9 to 24 with the addition of hydrogen. In the etching of SiO2 in a СHF3 + H2 plasma under certain process conditions, groove ridges were formed at the bottom of the etched groove. The mechanisms of ridge formation and the role of ion bombardment in this process are discussed.