Increasing Radio Frequency Power Research Articles

Power transfer efficiency in an air-breathing radio frequency ion thruster

Abstract Due to a series of challenges such as low-orbit maintenance of satellites, the air-breathing electric propulsion has got widespread attention. Commonly, the radio frequency ion thruster is favored by low-orbit missions due to its high specific impulse and efficiency. In this paper, the power transfer efficiency of the radio frequency ion thruster with different gas composition is studied experimentally, which is obtained by measuring the radio frequency power and current of the antenna coil with and without discharge operation. The results show that increasing the turns of antenna coils can effectively improve the radio frequency power transfer efficiency, which is due to the improvement of Q factor. In pure N2 discharge, with the increase of radio frequency power, the radio frequency power transfer efficiency first rises rapidly and then exhibits a less steep increasing trend. The radio frequency power transfer efficiency increases with the increase of gas pressure at relatively high power, while declines rapidly at relatively low power. In N2/O2 discharge, increasing the N2 content at high power can improve the radio frequency power transfer efficiency, but the opposite was observed at low power. In order to give a better understanding of these trends, an analytic solution in limit cases is utilized, and a Langmuir probe was employed to measure the electron density. It is found that the evolution of radio frequency power transfer efficiency can be well explained by the variation of plasma resistance, which is related to the electron density and the effective electron collision frequency.

Ion flux measurements using a Mach-Langmuir probe in the ITER prototype neutral beam injection ion source

Neutral beam injection systems as foreseen for ITER use radio-frequency (RF) ion sources at low pressure, where negative hydrogen ions are mainly produced via surface conversion of neutral atoms and positive ions at a plasma facing grid (PG). Up to now there is only limited knowledge about how fluxes and directed velocities of the positive ions are affected by external parameters such as power, pressure and the horizontal magnetic filter field which causes plasma drifts and vertical asymmetries in the vicinity of the PG. For this reason a combined Mach-Langmuir-probe diagnostic is used at multiple positions in the expansion and close to the extraction system in the prototype RF ion source (1/8 of the full ITER ion source size) to measure the positive ions directed velocity and flux as well as the plasma parameters simultaneously. With increasing RF power the flux towards the PG is found to increase linearly, its magnitude being controlled by the plasma density. Towards ITER-relevant pressures the ion flux decreases, in contrast to the directed velocity, which increases non-linearly, reaching around 5 km s−1 at a pressure of 0.3 Pa. The magnetic filter field is discovered to strongly bent down the ion flow in front of the PG. As a result, the ions at the lower half of the PG flow almost exclusively parallel to it, wherefore the flux which impinges onto the lower PG half is reduced by around one order of magnitude.

The Effect of Transient Power Ramp‐Up on Structural and Optical Properties of CuO Thin Films Prepared by Radio Frequency Magnetron Sputtering

Transition metal oxides have found application as charge transport materials (CTMs) in perovskite solar cell devices due to their better environmental stability and superior electronic properties compared to organic CTMs. Many researchers have used radio frequency (RF) magnetron sputtering to deposit inorganic CTMs on inorganic layers. However, such efforts on perovskites have been limited by the distortion of high‐energy ions of the sputtering gas energized by the RF power and temperature. The ions may also distort the interface between transparent conducting oxides and the charge transport layers, thereby increasing optical scattering. Thus, the effect of ramp power during RF magnetron sputtering deposition on the structural and optical properties of copper oxide thin films is investigated. While the optical transmittance of the films decreases with increased RF power, it remains the same for all ramped power films; however, it has a similar profile as the highest RF power film. As the ramp power increases, the X‐ray diffraction shows that the films become more polycrystalline with a monoclinic crystal structure. The energy‐dispersive X‐ray spectroscopy results reveal that the atomic concentration of copper slightly increases with the RF power, whereas oxygen concentration slightly decreases.

Study of optical and passivation properties of hydrogenated silicon carbide thin films deposited by reactive magnetron sputtering for c-Si solar cell application

In this paper, optical and passivating properties of hydrogenated silicon carbide synthesized by reactive magnetron sputtering for c-Si solar cell application were studied. SiC:H films were synthesized by sputtering a SiC target in an argon-hydrogen ambient at a Radio Frequency (RF) power of 100–250 W. The thickness, density, and roughness of the films were measured by X-ray reflectometry. The optical properties of the films were studied by UV–visible spectroscopy. It is shown that with an increase in RF power, the refractive index increases from 1.53 to 2.32 (at λ = 633 nm), and the extinction coefficient does not exceed 0.009 in most of the visible and near-IR spectrum. Based on the measured optical constants using computer simulation the optimal thicknesses were calculated to maximize antireflection effect. IR spectroscopy showed correlation between the structure and physical properties of the films. It has been established that, at a lower RF power (100–150 W), particles are deposited instead of atoms. This leads to the formation of voids at the particle boundaries, which are subsequently filled with oxygen. Using the non-contact microwave photoconductance decay method, it was shown that the maximum effective lifetime (τeff) is achieved at RF power of 250 W. In addition, influence of preliminary surface cleaning procedure and post-annealing in vacuum on the τeff was considered.

Effects of RF magnetron sputtering power on the structure and nanohardness of high-entropy alloys (TiVCrNbSiTaBY)N hard coatings

High-entropy alloys (TiVCrNbSiTaBY)N hard coatings were deposited on silicon wafers and stainless steel plates using radio-frequency (RF) magnetron sputtering. The effects of RF power on the elemental composition, morphology, structure and nanohardness of coatings are evaluated. The coatings exhibit smooth surface with low roughness less than 1 nm. Higher RF power introduces more silicon into coatings and densifies the columnar structure. The coatings present nanocomposite structure of face-centered cubic phase and amorphous Si3N4. With increasing RF power, the content of amorphous phase rises and crystal grains are refined. The changes in structure lead to a tendency for nanohardness and elastic modulus of coatings to first increase and then decrease. The coating grown at 800 W has the highest nanohardness and highest elastic modulus of ∼38.1 GPa and ∼451.2 GPa, respectively. The strengthening mechanism is explained by the nanocrystal-TiN/amorphous-Si3N4 model. The annealing treatment in air demonstrates that the coating exhibits excellent oxidation resistance at 800 °C.

Deposition of amorphous carbon nitride thin films using pressure-gradient RF magnetron sputtering and their chemical bonding structures

Amorphous carbon nitride (a-CNx) thin films were deposited through radio frequency (RF) magnetron sputtering under increasing nitrogen gas pressure around a graphite target to provide nitrogen radicals at high concentrations. The basic properties of the films, mainly chemical bonding structures, were evaluated using X-ray photoelectron spectroscopy (XPS), soft X-ray emission spectroscopy (SXES), and near-edge X-ray absorption fine structure (NEXAFS), and compared with other films deposited through the common sputtering mode, i.e., at a uniform gas pressure in the chamber. To increase the gas pressure around the graphite target, the deposition rate was slightly increased, and the chemical bonding structure of the a-CNx films was changed. However, the nitrogen content in the a-CNx films obtained from XPS analysis did not increase when the nitrogen gas pressure around the graphite target was increased. The effect of nitrogen incorporation on the a-CNx films was comparatively more effective at higher RF powers. The intensity ratio of sp3-σ and sp2-σ in the SXES spectra of the films increased with increasing RF power, from 85 W to 130 W, then decreased with a further increase in the RF power. This trend was confirmed by the peaks in the C K-edge of the NEXAFS spectra.

The effect of plasma beta on error-field penetration in radio-frequency wave heated plasmas in EAST

The plasma-beta effect on the n = 1 resonant magnetic perturbation (RMP) field penetration in purely radio-frequency (RF) wave heated discharges has been investigated in EAST. The experimental results show that the dependence of the threshold RMP coil current for field penetration, , on the total absorbed power P tot scales as approximately , indicating that the error-field tolerance is improved with increasing RF power. This is benefited by the increased electron perpendicular flow dominated by a counter-current electron diamagnetic flow with increasing RF power. However, theoretical scaling in cylindrical geometry overestimates the power index. Assuming an additional term for the normalized beta in the scaling, it is shown that the fitted from the experimental observation is around −1, indicating a degradation effect of plasma beta. To clarify the underlying physics of the plasma-beta effect that was not included in the theoretical scaling in cylindrical geometry, the MARS-Q code with full toroidal geometry is employed for simulation of nonlinear field penetration (Liu et al 2013 Phys. Plasmas 20 042503). The MARS-Q simulation results reproduce the dependence well, and hence the P tot scaling of the threshold current in experimental observations. The main reason for this is that the net total torque, which is mainly contributed by the neoclassical toroidal viscosity (NTV), increases with increasing plasma . The results demonstrate that the nonlinear toroidal coupling effect via NTV torque plays an important role in determining field penetration, even in cases with relatively low , which is far less than the no-wall beta limit.

Broadband microwave absorption effects in 2D nitrogen capacitively coupled plasma under different operating conditions

In recent years, the interaction of electromagnetic waves (EM) with plasma sources under argon and helium discharges has been extensively studied due to its potential applications in plasma stealth. However, nitrogen, as a more economical discharge gas, has been ignored in terms of its absorption of EM waves and stealth effect. In this work, a numerical calculation model combining two-dimensional capacitively coupled plasma (CCP) fluid model and EM wave model was developed to investigate the plasma uniformity degree and broadband microwave absorption effects in helium and nitrogen CCP. It is concluded that the two-dimensional model in this paper has more accurate and reasonable through comparison with the one-dimensional and experimental results in helium CCP. Nitrogen CCP shows better broadband absorption effects than that of helium, and helium plasma has better uniformity than nitrogen under the same discharge parameters. But the uniformity degree of nitrogen plasma is not much different from that of helium under the same electron density, which means that nitrogen can significantly improve its broadband wave absorption properties to some extent without loss of uniformity degree. Based on the above conclusions, the absorption characteristics of nitrogen CCP under different radio frequency (RF) power and pressure are analyzed. The attenuation effect of nitrogen CCP increases with the increasing RF power, and it is interesting that the influence of pressure on the attenuation of EM waves is not monotonically increasing, and the related mechanism is discussed. Finally, discussion of skin depth under different RF power and pressure validates the above conclusions. The absorption band of nitrogen CCP under the best parameters in this work can reach the X-band, which shows great application potential in plasma stealth.

Effects of plasma power on ferroelectric properties of HfO2-ZrO2 nanolaminates produced by plasma enhanced atomic layer deposition

This study introduces the use of plasma-enhanced atomic layer deposition (PEALD) for growing ultrathin Zr-doped hafnium oxide (HfO2-ZrO2 or HZO) nanolaminates and the effect of radio frequency (RF) plasma power on the electrical properties of these fluorite-structure oxide films, which were employed in ferroelectric devices. The effect of the PEALD plasma power in a range of 50∼200 W on the degree of the orthorhombic phase and the number of oxygen vacancies in the crystallized films can be found more clearly after the additional heat treatment by rapid thermal annealing (RTA). Our measurement results reveal the notable effect of plasma power in using PEALD on the growth and properties of ferroelectric oxide films. Particularly, the increased plasma power can decrease the crystallized films' remnant polarization (Pr) from 12.68 to 4.29 μC/cm2. Besides, a relationship between the leakage current and the electronic structure causing the films’ polarization is revealed. The spectroscopically resolved band-edge electronic revealed the conversion of orthorhombic HfO2 in the laminar stacking with ZrO2 to the homogeneous HfZrxOy compounds as increasing RF power. In general, these findings show the opportunity to utilize our developed HZO nanolaminates grown by PEALD for many trending applications, particularly polarization-driven memory and ferroelectric-based advanced transistors.

Low-Temperature Deposition of High-Quality SiO2 Films with a Sloped Sidewall Profile for Vertical Step Coverage

SiO2 is one of the most widely used dielectric materials in optical and electronic devices. The Josephson voltage standard (JVS) chip fabrication process has rigorous requirements for the deposition temperature and step-coverage profiles of the SiO2 insulation layer. In this study, we deposited high-quality SiO2 insulation films at 60 °C using inductively coupled plasma-chemical vapor deposition (ICP-CVD) to fulfill these requirements and fabricate JVS chips simultaneously. SiO2 films have a high density, low compressive stress, and a sloped sidewall profile over the vertical junction steps. The sidewall profiles over the vertical junction steps can be adjusted by changing the radio frequency (RF) power, ICP power, and chamber pressure. The effects of sputtering etch and sloped step coverage were enhanced when the RF power was increased. The anisotropy ratio of the deposition rate between the sidewall and the bottom of the film was lower, and the sloped step coverage effect was enhanced when the ICP power was increased, or the deposition pressure was decreased. The effects of the RF power on the stress, density, roughness, and breakdown voltage of the SiO2 films were also investigated. Despite increased compressive stress with increasing RF power, the film stress was still low and within acceptable limits in the device. The films deposited under optimized conditions exhibited improved densities in the Fourier transform infrared spectra, buffered oxide etch rate, and breakdown voltage measurements compared with the films deposited without RF power. The roughness of the film also decreased. The step-coverage profile of the insulation layer prepared under optimized conditions was enhanced in the junction and bottom electrode regions; additionally, the performance of the device was optimized. This study holds immense significance for increasing the number of junctions in future devices.

Double-row dipole/loop combined array for human whole brain imaging at 7T.

Important issues in designing radiofrequency (RF) coils for human head imaging at ultra-high field (UHF; ≥7T) are the inhomogeneity and longitudinal coverage (along the magnet axis) of the transmit (Tx) RF field. Both the homogeneity and coverage produced by Tx volume coils can be improved by means of three-dimensional (3D) RF shimming, which requires the use of multirow Tx-arrays. In addition, according to recent findings of the ultimate intrinsic signal-to-noise ratio (UISNR) theory, the loop-only receive (Rx) arrays do not provide optimal SNR near the brain center at UHF. The latter can be obtained by combining complementary conductive structures carrying different current patterns (e.g., loops and dipole antennas). In this work, we developed, constructed, and evaluated a novel 32-element hybrid array design for human head imaging at 7T. The array consists of 16 transceiver loops placed in two rows circumscribing the head and 16 folded-end Rx-only dipoles positioned in the centers of loops. By placing all elements in a single layer, we increased RF power deposition into the tissue and, thus, preserved the Tx-efficiency. Using this hybrid design also simplifies the coil structure by minimizing the total number of array elements. The array demonstrated whole brain coverage, 3D RF shimming capability, and high SNR. It provided ~15% higher SNR near the brain center and, depending on the RF shim mode, from 20% to 40% higher Tx-efficiency than a common commercial head array coil.

Mathematical Model of Radio Frequency Assisted Heat Pump Drying of Ganoderma Lucidum (Ganoderma Boninense)

This study proposed a mathematical model which was used to predict the temperature and moisture content of Ganoderma lucidum (Ganoderma boninense) during the radio frequency (RF) assisted heat pump (HP) drying process. The simultaneous heat and mass transfer equations were established based on the physical and flat plate models for drying Ganoderma lucidum and the specific assumptions for the proposed problem. The finite difference method was used to solve the proposed heat transfer and moisture transfer problem, in which, pdepe function was adjusted and complemented with Matlab software. The temperature and moisture content of the material in the drying process was predicted. The effect of the output RF power on the temperature and moisture content of the material was evaluated. The drying process's input parameters were drying air temperature of 40oC, air velocity of 1.2 m/s, and an output RF power of 1.95 kW, 0.65 and 0 kW. The results showed that when the output RF power increases, the heating rate increases, and the drying time is shortened significantly. The drying material temperature reaches the drying air temperature in 20 minutes, 30 minutes, and 220 minutes and the drying time is 465, 555, and 700 minutes, corresponding to output RF power of 1.95 kW, 0.65 and 0 kW. The predicted temperature and moisture content of the material were compared with experimental drying data, and a considerably good agreement was found.

3 T: the good, the bad and the ugly.

It is around 20 years since the first commercial 3 T MRI systems became available. The theoretical promise of twice the signal-to-noise ratio of a 1.5 T system together with a greater sensitivity to magnetic susceptibility-related contrast mechanisms, such as the blood oxygen level dependent effect that is the basis for functional MRI, drove the initial market in neuroradiology. However, the limitations of the increased field strength soon became apparent, including the increased radiofrequency power deposition, tissue-dependent changes in relaxation times, increased artifacts, and greater safety concerns. Many of these issues are dependent upon MR physics and workarounds have had to be developed to try and mitigate their effects. This article reviews the underlying principles of the good, the bad and the ugly aspects of 3 T, discusses some of the methods used to improve image quality and explains the remaining challenges and concerns.

Experimental diagnosis of electron density and temperature in capacitively coupled argon plasmas: Triple-frequency discharges and two-dimensional spatial distributions

An approach combining optical emission spectroscopy with a collisional radiative model (OES-CRM) has been used to diagnose electron density and electron temperature in low pressure capacitively coupled argon plasmas. The electron density and electron temperature obtained by the OES-CRM show reasonable agreement with the results measured by a Langmuir probe. The method was first applied to diagnosis in triple-frequency (2, 13.56, and 27.12 MHz) plasmas. Compared with dual-frequency (2 and 27.12 MHz) discharges, it was found that in triple-frequency discharges, the high frequency source controls electron density more independently and with less influence on electron temperature as the intermediate frequency power increases. Next, the method was extended to a two-dimensional diagnosis based on the use of a charge coupled device camera and optical bandpass interference filters. The results showed that the axial and radial distributions of the electron density are more uniform at a lower radio frequency (RF) power. The axial uniformity of the electron density is better at a lower discharge frequency while the radial profiles of the electron temperature are flatter at a higher frequency. In all the cases, the electron temperature is highly uniform within the bulk plasma. Moreover, a mode transition from an α to a γ mode is observed at 13.56 MHz with the increasing RF power, and this is accompanied by a significant enhancement in electron density and a sharp reduction in electron temperature.

Magnetic and Pressure Effects on E–H Mode Transition Power and Electron Energy Distribution in Helical Antenna-Coupled RF Plasma

We used helical antenna-coupled radio frequency (RF) plasmas to study the evolution of the mode transition power and electron kinetics under different applied magnetic fields and argon pressures. Langmuir probe and emission spectrum data indicate that the discharge mode changes from the capacitive coupling E mode to the inductive coupling H mode through a strong mixed mode (E + H). One reason for this strongly mixed ionization is the antenna structure, which increases the capacitive coupling. Furthermore, the $\text{E}\to \text{H}$ transition threshold power decreases with increasing pressure regardless of whether there is a magnetic field, and the presence of the magnetic field significantly increases the transition power. The electron energy distribution function (EEDF) curve has a Druyvesteyn shape at low RF power (E mode). The presence of a magnetic field increases the electron energy range, and the binding of the electrons by the magnetic field causes the EEDF to deviate significantly from the Maxwellian shape. As RF power increases, the EEDF tends toward the Maxwellian shape during the transition from the E mode to the H mode. The magnetic field appears to act like a drag on this process, particularly at very low pressures.

Interface analysis of SrWO4:Er3+-Yb3+/Si thin films prepared by radio frequency magnetron sputtering for upconversion emission

Upconversion (UC) emission of Er–Yb doped strontium tungstate (SrWO4:Er3+-Yb3+) thin films on silicon substrates prepared by the radio frequency (RF) magnetron sputtering method using different RF powers has been investigated and reported. The X-ray diffraction patterns showed that the tetragonal phase of SrWO4 formed with an improvement in the crystallinity of the films at higher RF power. Atomic force microscopy images demonstrated the homogeneity of the grains on the surface and an increment of grain size with an increase in RF power during deposition. Auger electron spectroscopy was used for the analysis of the interface of the thin films, the Si interdiffusion as well as the role of the thickness with an increase in the sputtering power of the RF deposition. The UC emission of the films prepared at the different RF powers excited at 980 nm was recorded. An enhancement in the intensity with an increase in the RF powers was obtained. Three different bands of UC emission were recorded, which were assigned to the 2H11/2/4S3/2 → 4I15/2 (two green bands) and the one red band (4F9/2 → 4I15/2) transitions for the Er3+ ions. The maximum intensity of UC emission was recorded at 240 W RF power due to a better crystallinity and the bigger size of the grains in the film as well as a better defined interface between the thin film and the substrate.

Investigating optical, structural and morphological properties of polycrystalline CdTe thin-film deposited by RF magnetron sputtering

In this work, optimization of the physical properties of cadmium telluride (CdTe) thin films has been carried out to improve its performance by the influence of RF power. CdTe Nanocrystalline thin films were characterized by using various techniques such as atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD). XRD study revealed that CdTe films are polycrystalline and have preferential growth in (111) direction. FESEM images showed a continuous and dense morphology of CdTe films on glass substrates. The AFM result shows that the surface roughness of films increases with annealing. Optical properties were investigated with the help of the UV–visible absorption spectrum which showed that the bandgap decreases with an increase in Radio-frequency power. Further, density functional theory (DFT) calculations showed that the optimum bandgap i.e. 1.57 eV for CdTe films deposited at 100 W RF power with 2% N2 is in good agreement with the experimental result. This study showed that CdTe films deposited at 100 W RF power with 2% N2 can be used as an excellent absorbing material for the application of optoelectronic devices solar cells.

E-H mode transitions and high-energy electron characteristics of helical antenna coupled plasma

Metastable and high-energy electron characteristics obtained from optical emission spectroscopy are used to analyze the dependence of the H mode on the magnetic field strength and discharge pressure. The results show that the H-mode characteristics gradually appears as the magnetic field strength is increased, the reason being that electrons undergo multiple acceleration-collision cycles at high magnetic field strength, thereby the metastable ionization will be increased. This improves energy utilization and making the H mode appearing. The variation in the density of metastable states and the Langmuir probe data shows that the electron energy distribution function evolves from non-Maxwellian to Maxwellian. The radial constraint of the magnetic field to the electrons and thus reduces the electron heating efficiency. Moreover, the increase in electric field strength with magnetic field leads to an increase in energy obtained by the electrons per unit distance. The competition between the two makes the number of high-energy electrons decrease rapidly first, and then increase slowly with magnetic field strength increasing. The turning point increases with the increase of discharge pressure and radio-frequency (RF) power. And the higher the pressure the lower the high-energy electron. For fields between 105.5 G and 212.7 G. In the H-mode regime, and with increasing RF power, the number of high-energy electrons will be sudden rise after experiencing a steady increase. The sudden rise RF power increase with magnetic field and decrease with discharge pressure increase. However, at high magnetic fields (>265 G) and high power (>450 W), the high-energy electron density decreases with power increasing.

Experimental Investigation of Molybdenum Oxide Coatings Prepared by RF Magnetron Sputtering

Abstract Molybdenum oxide films were deposited by radio frequency (RF) reactive magnetron sputtering. The study of surface structure and wettability with variations in deposition power and deposition time was carried out. The RF power was varied in the range of 125 W to 200 W, which led to evolution of (540), (001), (015), and (100) textures of molybdenum oxide. The X-ray diffractometer results show an increment of preferred orientation along the (001) plane for deposited molybdenum oxide films. The average crystallite size is within the range 17–18 nm with an increase of RF power from 125 W to 200 W and 19–31 nm with an increment in deposition time from 15 to 60 minutes. The outcome demonstrated that RF power and the time of deposition are the capacities that influence the static and dynamic contact angle framed by water and formamide. The surface roughness or unpleasantness increases in increments from 9.12 to 27.77 nm with an expansion in RF power from 125 W to 200 W and from 39.56 to 238.77 nm with increases in time of deposition from 15 min to an hour.

The Polaris Linear Plasma Device

We report the construction of Polaris, a new linear plasma device with an axial magnetic field dedicated to research and education at the American University of Beirut in Lebanon. The goal is to initiate the investigation of topics related to basic magnetized plasma physics and nuclear fusion. We discuss the novel technical solutions found to set up various components of the device. We start with the magnetic coils where we use power cables to generate the required axial magnetic field. They are included in a stainless steel case for water-cooling. Then, we present the vacuum and the gas inlet systems with which we control the neutral pressure. The way we manufacture the water-cooled radio frequency (RF) antenna allows the production of a variety of designs. Polaris is thus built to operate in a steady state. Using a reciprocating Langmuir probe, we determine the plasma properties as a function of the various control parameters. As the plasma remains in the “blue mode,” we show that the increase in the main magnetic field leads to an increase in the density but a decrease in the plasma temperature. The increase in neutral pressure yields the same effects. However, the increase in RF power leads to an increase in both density and temperature. Finally, plans for future investigations are briefly described.