Rf Cycle Research Articles

Ionization instability induced striations in low frequency and pulsed He/H2O atmospheric pressure plasmas

In previous work [Kawamura et al., Plasma Sources Sci. Technol. 25, 054009 (2016)] and [Kawamura et al., J. Phys. D: Appl. Phys. 50, 145204 (2017)], 1D kinetic particle-in-cell (PIC) simulations of narrow gap (1 to 4 mm), high frequency (27 MHz) or dc-driven, He/2%H2O atmospheric pressure plasmas (APPs) showed an ionization instability resulting in standing striations (spatial oscillations) in the bulk plasma. We developed a steady-state striation theory which showed that the striations are due to non-local electron kinetics. In both the high frequency and dc-driven cases, the equilibrium electron density n0 in the plasma bulk was stationary. In this work, we first conduct 1D PIC simulations of a 1 mm gap He/2%H2O APP, driven by a sinusoidal current at a low frequency of f = 50 kHz such that ω = 2πf is well below the ionization frequency νiz. In this case, n0 varies with time, and we observe a time-varying instability which quasistatically depends on n0(t). At each phase of the rf cycle, the discharge resembles a dc discharge at the same n0. At higher frequencies (200 kHz–1 MHz), ω approaches νiz, and quasistatic equilibrium at each phase breaks down. The discharge is also driven with a 200 kHz, 50% duty cycle square wave pulse with a short rise and fall time of 0.1 μs in an attempt to directly measure the striation growth rate s during the on-cycle before it saturated. However, the spike in current during the rise time leads to a spike in electron temperature Te and hence νiz and s at the beginning of the rise which saturated during the beginning of the on-cycle. To predict the instability growth rate and saturation during and after the current spike, we extend our striation theory to include time-varying n0, Te, νiz, as well as terms for the nonlinear saturation and noise floor of the striation amplitude. The time-varying global model predictions are compared to the PIC simulations, showing reasonable agreement.

Fasting/Refeeding Cycles Prevent Myocardial Dysfunction and Morphology Damage in the Spontaneously Hypertensive Rats.

BackgroundCaloric restriction is known to impair the cardiac function and morphology in hypertrophied hearts of spontaneously hypertensive rats (SHR); however, the influence of fasting/refeeding (RF) is unknown.ObjectiveTo investigate the fasting/refeeding approach on myocardial remodeling and function. In addition, the current study was designed to bring information regarding the mechanisms underlying the participation of Ca2+ handling and β-adrenergic system.MethodsSixty-day-old male SHR rats were submitted to food ad libitum (C), 50% food restriction (R50) or RF cycles for 90 days. Cardiac remodeling was assessed by ultrastructure analysis and isolated papillary muscle function. The level of significance considered was 5% (α = 0.05).ResultsThe RF rats presented lower cardiac atrophy than R50 in relation to C rats. The C rats increased weight gain, R50 maintained their initial body weight and RF rats increased and decreased weight during RF. The RF did not cause functional impairment because the isotonic and isometric parameters showed similar behavior to those of C. The isotonic and isometric cardiac parameters were significantly elevated in RF rats compared to R50 rats. In addition, the R50 rats had cardiac damage in relation to C for isotonic and isometric variables. While the R50 rats showed focal changes in many muscle fibers, the RF rats displayed mild alterations, such as loss or disorganization of myofibrils.ConclusionFasting/refeeding promotes cardiac beneficial effects and attenuates myocardial injury caused by caloric restriction in SHR rats, contributing to reduce the cardiovascular risk profile and morphological injuries. Furthermore, RF promotes mild improvement in Ca2+ handling and β-adrenergic system.

Possibility of operating quadrupole mass filter at high resolution

A new possibility of improving the resolving power of quadrupole mass filters has been studied theoretically in this work. The results show that with the use of two AC excitations, in addition to the main RF supply, it is possible to modify the first stability diagram for mass filtering by creating a narrow and long band of stability along the X boundary near the tip of first stability region. These newly developed stability regions (the X-band) are similar to higher stability regions, and offer high mass resolution and fast mass separation features. This approach overcomes the many limitations of the normal operation of quadrupole analyzers, while retaining the advantages of using the first stability region. The new operation mode could achieve up to 10,000 mass resolving power with the ion residence time of only 100 RF cycles. In addition, the ion transmission efficiency with the use of the X-band is not only compromised, but is greater than in the normal operation mode. Furthermore, the new mode features one-dimensional mass filtering (in the X direction only) that is not sensitive to nonlinear field distortions, which are particularly problematic for quadrupole mass filters which built with circular rods. Faster mass separation has been confirmed in simulations and theoretical computations of the exponential increment of the trajectory instability. Due to the location of the X-band near the tip of the first stability region, the new operation mode can still have the benefits of traditional techniques (delayed DC ramp) for overcoming the negative effects of fringe fields and improving the ion transmission efficiency. The theoretical simulations show that the method of improving the performance of quadrupole mass filters does not require any modifications of mechanical structures, and only needs different and a little more sophisticated method of electric applications.

The hollow cathode effect in a radio-frequency driven microhollow cathode discharge in nitrogen

A two-dimensional particle-in-cell Monte-Carlo code has been developed to study the physical mechanism of the hollow cathode effect (HCE) in an rf microhollow cathode discharge (rf-MHCD). Under the simulated conditions, the HCE in the rf-MHCD is the result of sheath-superposition, and both α ionization and γ ionization play a role. However, α ionization mode is predominant. Electrons undergo a pendular motion during the negative portion of the rf cycle. When the rf hollow electrode has a positive voltage, the majority of the electrons move toward the rf electrode, and the mean electron energy near the rf electrode is higher than that in a dc hollow cathode discharge, resulting in a large number of energetic electrons bombarding the hollow cathode wall, an important characteristic of the rf-MHCD. When the hollow cathode aperture is sufficiently small, many electrons strike the hollow electrode and are removed from the discharge space, so that the plasma density decreases. The average energy of the ions in the quasi-neutral plasma region near the axis is greater than the thermal energy in a molecular gas. Therefore, high density, high energy, and high chemical activity are characteristic advantages of rf-MHCD plasma sources.

PIC/MCC Simulation of Radio Frequency Hollow Cathode Discharge in Nitrogen**supported by Natural Science Foundation of Hebei Province, China (No. A2012205072)

A two-dimensional PIC/MCC model is developed to simulate the nitrogen radio frequency hollow cathode discharge (rf-HCD). It is found that both the sheath oscillation heating and the secondary electron heating together play a role to maintain the rf-HCD under the simulated conditions. The mean energy of ions (N2+, N+) in the negative glow region is greater than the thermal kinetic energy of the molecular gas (N2), which is an important characteristic of rf-HCD. During the negative portion of the hollow electrode voltage cycle, electrons mainly follow pendulum movement and produce a large number of ionization collisions in the plasma region. During the positive voltage of the rf cycle, the axial electric field becomes stronger and its direction is pointing to the anode (substrate), therefore the ions move toward the anode (substrate) via the axial electric field acceleration. Compared with dc-HCD, rf-HCD is more suitable for serving as a plasma jet nozzle at low pressure.

Universal nonmonotonic structure in the saturation curves of magneto-optical-trap-loadedNa+ions stored in an ion-neutral hybrid trap: Prediction and observation

We predict that the steady-state ion number $N_s$ for radio-frequency (rf) traps, loaded at a rate of $\lambda$ particles per unit time, shows universal non-monotonic behavior as a function of loading rate $\lambda$. The shape of $N_s(\lambda)$, characterized by four dynamical regions, is universal in the sense that it is predicted to manifest itself in all rf traps independently of the details of their construction. For $\lambda\ll$ 1 particles / rf cycle (Region I), as expected, $N_s(\lambda)$ increases monotonically with $\lambda$. However, contrary to intuition, at intermediate $\lambda \sim 1$ particles / rf cycle (Region II), $N_s(\lambda)$ reaches a maximum, followed by a minimum of $N_s(\lambda)$ (Region III). For $\lambda\gg 1$ particles / rf cycle (Region IV), $N_s(\lambda)$ again rises monotonically. In Region IV numerical simulations, analytical calculations, and experiments show $N_s(\lambda)\sim \lambda^{2/3}$. We confirm this prediction experimentally with MOT-loaded Na$^+$ ions stored in a hybrid ion-neutral trap.

Fundamentals of Switching RF Power Amplifiers

How to determine if an amplifier is actually switching, or is just operating in compression, is developed here. While compressed amplifier operation is any deviation from linear operation toward lower output at the highest amplifier output powers, switching has additional requirements that the transistor achieve two steady states within each RF cycle: OFF and ON. Conditions necessary for transistor switching operation to occur at RF signal frequencies are derived, and related to the transistor transition frequency metric $f_{T}$ . The degradation of available amplifier efficiency as the operating frequency approaches $f_{T}$ is evaluated. Testing methods to verify that transistor switching operation is achieved are proposed.

Structure of the velocity distribution of sheath-accelerated secondary electrons in an asymmetric RF-dc discharge

Low-pressure capacitively-coupled discharges with additional dc bias applied to a separate electrode are utilized in plasma-assisted etching for semiconductor device manufacturing. Measurements of the electron velocity distribution function (EVDF) of the flux impinging on the wafer, as well as in the plasma bulk, show a thermal population and additional peaks within a broad range of energies. That range extends from the thermal level up to the value for the ‘ballistic’ peak, corresponding to the bias potential. The non-thermal electron flux has been correlated to alleviating the electron shading effect and providing etch-resistance properties to masking photoresist layers. ‘Middle-energy peak electrons’ at energies of several hundred eV may provide an additional sustaining mechanism for the discharge. These features in the electron velocity (or energy) distribution functions are possibly caused by secondary electrons emitted from the electrodes and interacting with two high-voltage sheaths: a stationary sheath at the dc electrode and an oscillating self-biased sheath at the powered electrode. Since at those energies the mean free path for large-angle scattering (momentum relaxation length) is comparable to, or exceeds the size of the discharge gap, these ‘ballistic’ electrons will not be fully scattered by the background gas as they traverse the inter-electrode space. We have performed test-particle simulations in which the features in the EVDF of electrons impacting the RF electrode are fully resolved at all energies. An analytical model has been developed to predict existence of peaked and step-like structures in the EVDF. Those features can be explained by analyzing the kinematics of electron trajectories in the discharge gap. Step-like structures in the EVDF near the powered electrode appear due to accumulation of electrons emitted from the dc electrode within a portion of the RF cycle, and their subsequent release. Trapping occurs when the RF sheath voltage exceeds the applied bias, and is decreasing. The secondary electrons originating from the dc-biased surface also form a peak near the energy equal to the bias potential. Additional peaks, at lower energies, are formed by the electrons emitted from the RF electrode and eventually escaping to it. The latter can be grouped according to the number of bounces between the sheaths during their residence time in the discharge. Each of such groups may give rise to an individual peak in the distribution. The trap-and-release theory developed in this paper provides a convincing explanation for the observations of the ballistic and ‘middle energy peak’ electrons detected in experiments.

Electron heating during E-H transition in inductively coupled RF plasmas

A planar inductively coupled RF discharge (13.56 MHz) in argon and oxygen was exemplarily studied using space and phase resolved optical emission spectroscopy. The characteristic excitation rate pattern due to the electron heating during the sheath expansion was found for both gases in the E-mode. Furthermore, an intensive pattern in oxygen appears during the sheath collapse. This is associated with the electron heating caused by electric field reversal due to the strong electronegativity. The transition from the E- to the H-mode may be stepwise or continuous, depending on the gas type and total gas pressure. In the H-mode, significant differences in the excitation rate patterns exist. A broad and weakly modulated pattern is found over the RF cycle in argon, whereas in oxygen two separated patterns appear representing the electron heating for each half cycle. The reason may be the different excitation processes of the investigated resonant states and the influence of metastable argon atoms as well as attachment/detachment processes and dissociative recombination in oxygen. The E-H transition in oxygen at 5 Pa develops continuously and was studied in detail through the excitation rate. During the transition, the E- and H-mode are present and a hybrid mode was observed.

Spatio-temporally resolved electron temperature in argon radio-frequency capacitive discharge at atmospheric pressure

Due to the lack of convincing experimental evidence for electron information, there are still unclearly understood discharge phenomena in atmospheric pressure radio-frequency (rf) capacitive discharge, e.g. the electron heating, discharge structures, and the alpha–gamma mode transition. Thus, to perceive basic and meaningful principles with an unambiguous interpretation, simple and reliable electron diagnostics are required. Since bremsstrahlung emitted through electron-neutral atom interaction depends on electron density (ne) and temperature (Te), their diagnostic is possible. In particular, Te is easily estimated from the ratio of bremsstrahlung emissivities at two different wavelengths or more. In this paper, 2D Te distribution in an argon atmospheric pressure capacitive discharge measured by using a digital camera and optical band pass filters is described. Time-averaged Te in the bulk region obtained by a digital camera is consistent with that measured by an absolutely calibrated spectrometer. In addition, time-resolved emission spectra and the corresponding ne and Te during one rf cycle of the argon capacitive discharge are discussed. The result shows that Te varied from 2.3 to 3.0 eV, while ne did not change significantly.

A double-frequency rf gun for field emission

Cold cathodes have attracted a lot of attention in the field of accelerators in recent years. While the development of suitable cold cathodes is in progress, attempts have been made to combine the cold cathode with a rf structure. Due to the strong dependence on the electric field, field emissions peak at the wave crest, which is not the best injection phase, during a rf cycle. To make the injection phase adjustable, a flexible double-frequency rf gun is designed. The addition of a 3rd-harmonic field to the fundamental one in the half cell will move the wave crest toward a better injection phase and make the initial bunch length shorter. The full cell is resonant at the fundamental frequency. Since only the half cell is resonant at two frequencies, the gun can be easily tuned. Simulations show that the time-dependent rf effects on the transverse and longitudinal phase spaces of the electron bunch can be reduced by choosing proper rf parameters and the space charge effects can be compensated for by using an external solenoid field. Therefore, the gun is able to provide low emittance, low energy spread and short electron bunches with high average current.

Phase Resolved Imaging of a Repetitive Extrusion of Hydrogen Plasma From a Hollow Cathode Source

Hydrogen plasmas are useful in many areas of materials processing. In this paper, a hydrogen plasma is produced in a hollow cathode source and its expansion into a chamber over the course of an RF cycle (80 ns) is observed with an intensified CCD camera. The source is observed to launch plasma down into the expansion region once per RF cycle. The shape of the extrusion suggests a surface wave carries charge down the inside of the glass tube, before the plasma collapses back to the center of the tube downstream from the source. At the end of the RF cycle, the plasma in the tube is seen to completely detach from the source plasma.

Two-Dimensional Numerical Study on Characterization of Low-Pressure Capacitively Coupled Argon Discharges

This paper presents an investigation of argon capacitively coupled plasma at low pressure. A two-dimensional, time-dependent fluid model is used to describe the production, transport, and destruction of electrons and positive ions. The model is solved for a GEC(gaseous electronics conference) Cell reactor type (with 4cm diameter and 2.5cm interelectrode distance) operating at frequency 13.56MHz, pressure 1Torr and applied voltage 1000V, in pure argon. Two-dimensional distributions are presented in the stationary state, including electron temperature and species density i.e. electron, ion and metastable atom. The electric field and electric potential at different phases in one RF cycle is also studied.

Quadrupole mass filter operation under the influence of magnetic field

This work demonstrates resolution enhancement of a quadrupole mass filter (QMF) under the influence of a static magnetic field. Generally, QMF resolution can be improved by increasing the number of rf cycles an ion experiences when passing through the mass filter. In order to improve the resolution, the dimensions of the QMF or the operating parameters need to be changed. However, geometric modifications to improve performance increase the manufacturing cost and usually the size of the instrument. By applying a magnetic field, a low-cost, small footprint instrument with reduced power requirements can be realized. Significant improvement in QMF resolution was observed experimentally for certain magnetic field conditions, and these have been explained in terms of our theoretical model developed at the University of Liverpool. This model is capable of accurate simulation of spectra allowing the user to specify different values of mass spectrometer dimensions and applied input signals. The model predicts enhanced instrument resolution R>26,000 for a CO2 and N2 mixture with a 200-mm long mass filter operating in stability zone 3 via application of an axial magnetic field.

Space and phase resolved ion energy and angular distributions in single- and dual-frequency capacitively coupled plasmas

The control of ion energy and angular distributions (IEADs) is critically important for anisotropic etching or deposition in microelectronic fabrication processes. With single frequency capacitively coupled plasmas (CCPs), the narrowing in angle and spread in energy of ions as they cross the sheath are definable functions of frequency, sheath width, and mean free path. With increases in wafer size, single frequency CCPs are finding difficulty in meeting the requirement of simultaneously controlling plasma densities, ion fluxes, and ion energies. Dual-frequency CCPs are being investigated to provide this flexible control. The high frequency (HF) is intended to control the plasma density and ion fluxes, while the ion energies are intended to be controlled by the low frequency (LF). However, recent research has shown that the LF can also influence the magnitude of ion fluxes and that IEADs are determined by both frequencies. Hence, separate control of fluxes and IEADs is complex. In this paper, results from a two-dimensional computational investigation of Ar/O2 plasma properties in an industrial reactor are discussed. The IEADs are tracked as a function of height above the substrate and phase within the rf cycles from the bulk plasma to the presheath and through the sheath with the goal of providing insights to this complexity. Comparison is made to laser-induced fluorescence experiments. The authors found that the ratios of HF/LF voltage and driving frequency are critical parameters in determining the shape of the IEADs, both during the transit of the ion through the sheath and when ions are incident onto the substrate. To the degree that contributions from the HF can modify plasma density, sheath potential, and sheath thickness, this may provide additional control for the IEADs.

Nanosecond optical imaging spectroscopy of an electrothermal radiofrequency plasma thruster plume

Nanosecond optical imaging spectroscopy is employed to investigate the spatio-temporal dynamics of the plasma plume expanding from a 4.2 mm-diameter, 20 mm-long cylindrical capacitively coupled electrothermal radiofrequency (rf) driven thruster using 10 W of power at 12.50 MHz and an argon pressure of 1.5 Torr. On-axis, the plume exhibits four distinct peaks of optical emission intensity within the rf period. The plume has a spherical shape with a transient radial extension (during half of the rf cycle) at the thruster exit plane due to an rf current to ground when the grounded electrode acts as an anode.

Quadrupole Mass Filter: Design and Performance for Operation in Stability Zone 3

The predicted performance of a quadrupole mass filter (QMF) operating in Mathieu stability zone 3 is described in detail using computer simulations. The investigation considers the factors that limit the ultimate maximum resolution (Rmax) and percentage transmission (%Tx), which can be obtained for a given QMF for a particular scan line of operation. The performance curve (i.e., the resolution (R) versus number (N) of radio frequency (rf) cycles experienced by the ions in the mass filter) has been modeled for the upper and lower tip of stability zone 3. The saturation behavior of the performance curve observed in practice for zone 3 is explained. Furthermore, new design equations are presented by examining the intersection of the scan line with stability zone 3. Resolution versus transmission characteristics of stability zones 1 and 3 are compared and the dependence of performance for zones 1 and 3 is related to particular instrument operating parameters.

Formation of spatially periodic fronts of high-energy electrons in a radio-frequency driven surface microdischarge

The generation of spatially periodic fronts of high-energy electrons (>13.48 eV) has been investigated in a radio-frequency surface microdischarge in atmospheric-pressure argon. Optical emission spectroscopy is used to study the Ar I 2p1−1s2 transition surrounding a filamentary microdischarge, both spatially and with respect to the phase of the applied voltage. The formation of excitation fronts, which remain at a constant propagation distance throughout the RF cycle and for the duration of the pulse, may be explained by a localized increase in the electric field at the tip of surface-charge layers that are deposited during the extension phase.

Electron current extraction from radio frequency excited micro-dielectric barrier discharges

Micro dielectric barrier discharges (mDBDs) consist of micro-plasma devices (10–100 μm diameter) in which the electrodes are fully or partially covered by dielectrics, and often operate at atmospheric pressure driven with radio frequency (rf) waveforms. In certain applications, it may be desirable to extract electron current out of the mDBD plasma, which necessitates a third electrode. As a result, the physical structure of the m-DBD and the electron emitting properties of its materials are important to its operation. In this paper, results from a two-dimensional computer simulation of current extraction from mDBDs sustained in atmospheric pressure N2 will be discussed. The mDBDs are sandwich structures with an opening of tens-of-microns excited with rf voltage waveforms of up to 25 MHz. Following avalanche by electron impact ionization in the mDBD cavity, the plasma can be expelled from the cavity towards the extraction electrode during the part of the rf cycle when the extraction electrode appears anodic. The electron current extraction can be enhanced by biasing this electrode. The charge collection can be controlled by choice of rf frequency, rf driving voltage, and permittivity of the dielectric barrier.

Particle-in-Cell/Monte Carlo Collision Simulations of Striations in Inductively Coupled Plasmas

Striations in inductively coupled plasmas (ICPs) appear as a series of cocoons beneath a coil through which a source current is driven. In this study, we have simulated striations in ICPs using the particle-in-cell/Monte Carlo collision method. The simulated effects of pressure on striation characteristics are in good agreement with experimental observations. It is found that the discharge structure and electron heating mechanism of striated ICPs are markedly different from those of normal ICPs. In particular, electron heating occurs once in one rf cycle in striated ICPs but twice in normal ICPs. Electron heating in striated ICPs may be ascribed to a strong self-excited electric field induced by the oscillating motion of the electron density at approximately the ion density, resulting in a high electron temperature. Furthermore, the relationship between the spatial period of the striation and the discharge space is put forward as a generation criterion for striations in rf discharges. Striations tend to occur when the region of plasma generation due to ionization is adjacent to walls where plasma is lost.