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High-k turbulence (1 < kθρs < 5) spectrograms have been directly measured in density modulation experiments of the EAST tokamak using the collective Thomson scattering diagnostic system. Density-peaking modulation is achieved in Lower Hybrid Wave (LHW) modulation experiments, revealing that the broadband turbulence is sensitive to the power modulation of LHW and the concomitant variation of density-peaking. Additionally, the contributions of 2.45/4.6 GHz LHW to density peaking differ when Neutral Beam Injection (NBI) is on/off, displaying distinct responses of broadband turbulence in low/high frequency ranges. It is found by cross-coherence analysis that the phase angle is negative in cases without NBI, while in NBI cases it can be modulated from negative to positive, indicating a significant transition of radial turbulence propagation. These findings highlight the active control of density-peaking via auxiliary heating in tokamak plasmas and suggest the potential role of high-k turbulence in density-peaking modulation. [ABSTRACT FROM AUTHOR]

In high density, high temperature plasmas, the plasma sheath that develops can result in extremely high electric fields, on the order of tens to hundreds of V/nm. Under the right conditions, these electric fields can reach magnitudes that can increase the probability of electron tunneling ionization to occur, resulting in one or more electron-ion pairs. The presence of tunneling ionization can then modify the development of the plasma sheath, as well as properties such as the ion and electron densities and plasma potential. The tunnel ionization process for hydrogen atoms is demonstrated, in this work, as implemented in a Sandia National Laboratories, particle-in-cell code Aleph. Results are presented for the application of the tunnel ionization process to a one-dimensional, undriven plasma sheath. Additional results for cases that consider warm ions and neutrals, the inclusion of electron–neutral collisions, and the injection of neutral particles, as well as the application to various plasma devices, will be discussed. [ABSTRACT FROM AUTHOR]

A planar Langmuir probe, which can be used for both ion electric propulsion beam characteristics measurement and plasma diagnosis, was developed and applied to plume measurement and diagnosis experiments of an 8 cm Kaufman ion thruster to address the growing demand for miniaturized ion thruster plume measurement and diagnosis. The experimental results show that the plume area exhibits good symmetry along the central axis, and the beam dispersion angle of the 8 cm ion thruster is approximately 22.7°. The radial distributions of the floating potential, plasma space potential and ion density are similar, showing a high distribution in the middle and a relatively low distribution at the two ends. The miniaturized ion thruster plume measurement and diagnosis system is not only easy to manipulate, but also fast and efficient for plasma plume measurement and diagnosis, which provides more complete data support for analyzing the working state of the miniaturized ion thrusters and optimizing the design of the thruster structure. [ABSTRACT FROM AUTHOR]

The Faraday probe and cylindrical Langmuir probe were used to characterize the downstream ion and electron spatial evolution of a 300 W class low-power Hall thruster. The time-averaged ion current density, electron energy probability function, plasma potential, electron temperature, and electron density measurements were conducted at discharge voltages of 200–400 V and anode mass flow rates of 0.65 and 0.95 mg s−1 in the range of 100–500 mm axially and −100 to 100 mm radially downstream of the thruster. The results show that the ion and electron flows exhibit a bipolar diffusion characteristic along the radial direction. Meanwhile, the radial diffusion rate of ions in the plume is greater than the axial diffusion rate. The plasma potential decreases from 18 V at 100 mm axially from the thruster exit to 4 V at 500 mm axially and 100 mm radially. Correspondingly, the electron temperature decreases from 4.2 to 1.0 eV. The electron number density decreases from 2.6 × 1016 to 4 × 1014 m−3. A variable exponential relationship between electron temperature and electron density was observed from the measurements of electron energy probability distribution functions, with an adiabatic factor γ ranging between 1.3 and 1.4 (below the adiabatic value of 5/3). The adiabatic factor γ is considered to correlate with the anode mass flux and the spatial location of plasma, which suggests a possible dependence on the collision rate. These data are of great importance for plume model validation, improvement, plume effect evaluation, and protection mechanisms. [ABSTRACT FROM AUTHOR]

The cathode layer of surface dielectric barrier discharge plasma is investigated experimentally by using the surface potential measurement under different bias voltages. Positive charge accumulation is found near the edge of the exposed electrode (x = 0 mm) under negative DC bias voltage. In addition, the plasma electromotive force (EMF) distribution curve near the edge of the exposed electrode diverges with the negative DC bias voltage. These phenomena are in good agreement with the conclusion that the cathode layer shows positive potential in time average. Five actuators with different exposed electrode widths are designed to study the relationship between the cathode layer and the mechanical properties of the actuator. With the widening of the exposed electrode, the positive charges in the cathode layer increase, and the plasma EMF divergence is more pronounced. However, the thrust force decreases. Moreover, the variation of thrust force with DC bias voltage is consistent with the change of the plasma EMF. The experimental results show that the narrow-exposed electrode actuator for the cathode layer with a small size and low number of positive ions has better mechanical properties. By analysis, this is mainly due to the lower number of positive ions of the cathode layer, producing less shielding from the applied voltage. More electrons are emitted from the cathode layer. This paper provides a method for the detection and study of the cathode layer. In addition, it can provide insight into how the cathode layer affects the development of thrust force. [ABSTRACT FROM AUTHOR]

Fluctuations in plasma floating potential in an atmospheric pressure micro-plasma jet are investigated experimentally. The transport of charged particles toward the ambient air is often seen to be affected by plasma fluctuations. The dependence of fluctuations on operating parameters such as applied voltage, gas flow rate, and gas mixture ratio (He:Ar) is investigated so as to be able to control the fluctuations in the intended application. A single pin probe measures the fluctuations in the jet in a region where the plasma emerges out of the capillary. The fluctuations are characterized by fast Fourier transform and time-frequency analysis (TFA). It is found that with an increase in applied voltage at a fixed flow rate (1 l/min), the fluctuations increase and reach a peak value at ∼11 kV and thereafter decrease at higher voltages due to a high value of discharge current (∼4.77 mA) at 11 kV arising from the intense ionization taking place in the jet. When the gas flow rate is increased at a fixed voltage (14 kV), the fluctuations get enhanced for flow rates beyond 2 l/min due to transition to turbulent flow occurring at a buoyancy induced Reynolds number of ∼474. In the case of gas mixing, an increase in the concentration of Ar and a reduction in He at a fixed applied power (80.6 W) and flow rate (1 l/min) give rise to a higher level of fluctuations, which is considered to be due to lower thermal conductivity and ionization potential of Ar, leading to gas heating, and ponderomotive force, giving rise to filamentation. The TFA shows the time evolution of fluctuation frequencies, which can be tuned by varying the experimental parameters. Understanding the influence of experimental parameters is crucial in controlling the fluctuations in the micro-plasma jet. [ABSTRACT FROM AUTHOR]

Ability to control the electric potential inside the plasma is promising for various applications of E × B plasma because the trajectory of particles, which meets the desired purpose, is mainly determined by the drift motions in the external electric and magnetic fields. In the E × B source, it is well known that the equilibrium states and plasma density and temperature gradient can be sources of plasma instability, which, in turn, affects global plasma parameters. However, the effect of instability on the electric potential has not been verified so far. In this work, correlation of the plasma potential and instability is investigated through simplified-circuit modeling. The consideration of the transverse conductivity of electrons across the magnetic field reflecting the turbulence collision frequency well explains the plasma potential closer to the anode potential despite the presence of negatively biased cathodes. Eventually, this result indicates that the instability can restrict the variation of plasma potential in partially magnetized plasma. [ABSTRACT FROM AUTHOR]

Bombardment of the keeper electrode by high-energy ions limits the lifetime of the ion thruster. To investigate its sputtering mechanism, this paper established a two-dimensional simulation model and applied the Particle-In-Cell method to simulating the generation process of the high-energy ions and their sputtering process on the keeper electrode surface. The Monte Carlo Collision method was used to address the collisions between particles. The results showed that the plasma potential downstream of the keeper electrode oscillated; therefore, ions in this area were accelerated and their energy was increased from 0.085 to 35 eV, which was enough to erode the keeper electrode's surface. double charged xenon (Xe++) ions and charge exchange xenon (CEX) ions were the two kinds of high-energy ions. The most severe position was the center of the keeper electrode surface, and its erosion rate was about 2.0 × 10−4 m/1000 h, while the average erosion rate was about 10−5 m/1000 h when the frequency and the amplitude of the plasma potential were 50 Hz and 35 eV, respectively. Plasma potential oscillation greatly affected the sputtering mechanism of the keeper electrode. [ABSTRACT FROM AUTHOR]

The time-resolved measurements of the plasma parameters are carried out in a pulse-modulated low pressure capacitively coupled discharge operated in argon and argon–oxygen mixtures. Measurements are performed at a constant radio-frequency (RF) power of 130 W at 13.56 MHz, with the pulse repetition frequency of 1 kHz and 50% duty cycle. A single RF compensated Langmuir probe, synchronized with the modulated signal, is used to investigate the effects of filling gas pressure and O2 content in the discharge on the temporal evolution of electron density ne, electron temperature Te, plasma potential Vp, and electron energy probability function (EEPF). ne shows a rapid increase with time during the start of the pulse-on phase, followed by a gradual increase to attain a steady state value. During the pulse-off phase, ne decreases gradually. ne drops at a higher pressure and with increasing O2 content in the discharge. The temporal variations in Te and Vp, on the other hand, are more abrupt than the temporal variation in ne during the rise and the fall of the pulse. Both Te and Vp show an increase with rising pressure and with increasing O2 content in the discharge. The investigation of the temporal evolution of the EEPF reveals that the discharge is in a highly non-equilibrium state at the start of the pulse and evolves from bi-Maxwellian to Maxwellian distribution with the passage of time. The density of low energy electrons decreases, while the density of high energy electrons increases with increasing pressure and O2 content in the discharge. Moreover, several peaks and broadening in the EEPF profile are also noted with increasing O2 content. [ABSTRACT FROM AUTHOR]

Theoretical analysis and a large number of experiments have proved that plasma components do not satisfy Boltzmann–Gibbs statistics and can be well described by nonextensive statistical mechanics, while sheath potential coefficients in plasma with nonextensive distribution are not investigated deeply and comprehensively. Here, we investigate the ion sheath formed around a nonextensive single electric probe in plasma described by nonextensive statistical mechanics, and find that the sheath potential coefficient is related to the electron nonextensive parameter, besides the extensive limit the results return to the case of the Boltzmann–Gibbs statistical framework. The sheath potential coefficient presents different dependences on the electron nonextensive parameters in different regions. We also have calculated the corresponding method error and evaluated with a set of real experiment data, and found that the error is as high as 83.91% indicating that the effect of nonextensive parameters should be considered in the actual measurement. [ABSTRACT FROM AUTHOR]

The particle-in-cell-Monte Carlo collision method has been widely used in simulation studies of ion thrusters. Due to the huge computational demand of such simulations, numerical acceleration techniques are required. However, the effects of such numerical acceleration techniques on the simulation results are not clear. In this study, the effects of three numerical acceleration techniques are investigated using a series of simulations that implement different numerical acceleration factors. The resulting plasma potentials, plasma density distributions, and plasma currents in the discharge chamber are compared for simulations accelerated by increasing the vacuum permittivity, reducing the mass of heavy particles, and making use of self-similarity. The results demonstrate that increasing the permittivity thickens the sheath. When the sheath expands enough to extend to the cusps, the distributions of the potentials and the plasma densities are affected, influencing the current parameters. Reducing the masses of heavy particles greatly influences ion properties, especially the plasma density. Thus, it causes significant errors in the potential and current parameters. Errors in the beam current can be significantly decreased by correcting the beam current using an exponent relationship between the mass scaling factor and the plasma density. The use of self-similarity simultaneously thickens the sheath and decreases the particle number density and may slightly reduce the plasma current values. A number of suggestions for employing these techniques are also provided. [ABSTRACT FROM AUTHOR]

In this work, an experimental design for generation of the controllable electron density gradient is achieved through a large area oxide coated cathode and two stainless meshes. The large area oxide coated cathode is adopted to produce a large area, uniform and high density plasma, and these two metal meshes are specially designed to control the density and potential of the plasma. A boundary layer with independent electron density gradient and uniform plasma potential is subsequently generated. The controllable electron density gradient can be achieved, and the accompanied electric field can be compensated simultaneously using this design, which could be potentially extended to study the excitation of the plasma instabilities when the strong plasma density gradient exists. [ABSTRACT FROM AUTHOR]

We investigated the application of a less hydrogen-dilute and low gas pressure hollow-anode plasma to fast chemical-vapor deposition of photosensitive hydrogenated microcrystalline silicon (μc-Si:H). The hollow-anode plasma was remotely induced at a processing space by transferring a hollow-cathode plasma through a nozzle attached to a partition plate, which operated as an anode and separated the processing space from a hollow-cathode discharge space in an ultrahigh-vacuum hollow-electrode-enhanced glow-plasma transportation (HEEPT) system. The hollow-cathode plasma was excited by applying a very-high-frequency (VHF, 105 MHz) power to a cathode in the hollow-cathode discharge space. Through the use of this hollow-anode plasma under a gas flow rate ratio ([H2]/[SiH4]) of 1.25 (30 sccm/24 sccm), pressure of 80 Pa, and VHF power of 150 W (the highest power tested in this work), we fabricated a well-crystallized and photosensitive μc-Si:H thin film with a highly preferred crystal orientation along the <110> direction at a growth rate of 13 nm/s. Electrical analysis on the self-bias voltage of the cathode (Vdc) revealed that hollow-cathode discharges in the HEEPT system were approximately equivalent to symmetric discharge, i.e., Vdc ≒ 0 V. Optical analysis indicated that the hollow-anode plasma produced an enough amount of atomic hydrogen to grow well-crystallized μc-Si:H thin films, even at the lowest [H2]/[SiH4] ratio (1.25). Optical and electrical analyses and computational plasma simulation demonstrated that the hollow-anode plasma had a lower electron temperature and higher plasma space potential compared with those features of a glow discharge plasma enhanced by a conventional parallel-electrodes system. [ABSTRACT FROM AUTHOR]

Plasma potential measurements using the conventional Langmuir probe may cause an error due to the space charge effect. To solve the problem, a tube probe is proposed in this study which can minimize the space charge effect by collecting electrons with an orifice instead of the solid surface of the Langmuir probe. The I-V characteristic of the tube probe exhibits a clear turning point, accurately indicating the plasma potential. Comparing with the results of the conventional Langmuir probe, it suggests that the plasma potential measured by the Langmuir probe may be underestimated by about 0.1-0.2 Te/e, which may cause underestimation of the electron density by about 10%-20%. Combination use of the tube probe and the Langmuir probe is suggested for accurate measurement of the electron density. [ABSTRACT FROM AUTHOR]

The problem of generating a stationary electric field in a magnetized radio-frequency discharge (rf) plasma is studied experimentally. Helmholtz coils produce magnetic field in a cylindrical vacuum chamber with diameter of 85.6 cm and length of 220 cm. RF discharge is generated at a frequency of ~5 MHz. The rf power absorbed by plasma lies in the range 0.5-1.5 kW. Electrodes defining a negative potential are placed at the ends of the chamber. Two pairs of circular flat electrodes with diameter of 5.5 and 45 cm are investigated. The working gas is argon. Radial profiles of electron density and temperature are obtained. Radial profile of the plasma potential is investigated, as well as the dependence of plasma potential on the voltage applied to the end-electrodes. [ABSTRACT FROM AUTHOR]

In the work, the formation of an ionospheric depletion was simulated in a controlled laboratory plasma. The experiment was performed by releasing chemical substance sulfur hexafluoride (SF6) into the pure argon discharge plasma. Results indicate that the plasma parameters change significantly after release of chemicals. The electron density is nearly depleted due to the sulfur hexafluoride-electron attachment reaction; and the electron temperature and space potential experience an increase due to the decrease of the electron density. Compared to the traditional active release experiments, the laboratory scheme can be more efficient, high repetition rate and simpler measurement of the varying plasma parameter after chemical releasing. Therefore, it can effective building the bridge between the theoretical work and real space observation. [ABSTRACT FROM AUTHOR]

An electron emitting probe in saturated floating potential mode has been used to investigate the temporal evolution of plasma potential and the effect of substrate RF biasing on it for pulsed dual frequency (2 MHz/13.56 MHz) inductively coupled plasma (ICP) source. The low frequency power (P2MHz) has been pulsed at 1 KHz and a duty ratio of 50%, while high frequency power (P13.56MHz) has been used in continuous mode. The substrate has been biased with a separate bias power at (P12.56MHz) Argon has been used as a discharge gas. During the ICP power pulsing, three distinct regions in a typical plasma potential profile, have been identified as 'initial overshoot', pulse 'on-phase' and pulse 'off-phase'. It has been found out that the RF biasing of the substrate significantly modulates the temporal evolution of the plasma potential. During the initial overshoot, plasma potential decreases with increasing RF biasing of the substrate, however it increases with increasing substrate biasing for pulse 'on-phase' and 'off-phase'. An interesting structure in plasma potential profile has also been observed when the substrate bias is applied and its evolution depends upon the magnitude of bias power. The reason of the evolution of this structure may be the ambipolar diffusion of electron and its dependence on bias power. [ABSTRACT FROM AUTHOR]

This study shows that the propagation of plasma bullets along one dielectric tube is strongly affected by many discharge parameters, such as the waveform of applied voltage (AC or pulsed DC), peak voltage, He flow rate, and the frequency of AC voltage. Analysis indicates that the density and velocity of plasma bullets are mainly determined by the electric field at the front of plasma bullets. These discharge parameters may significantly influence the distribution of plasma potential along the tube, thus control the electric field at the front of plasma bullets and their propagation. An increase in the pulsed DC voltage with its rise time of <40-50 ns can lead to an obvious improvement in the electric field at the front of plasma bullets, resulting in generation of a plasma in the high density gas and a fast propagation of plasma bullets. He flowing through the tube can contribute to the surface diffusion of charged species, and greatly increase the electric field at the front of plasma bullets. During the propagation of plasma bullets, their density is decreased due to the surface recombination of charged species, such as electrons and ions. [ABSTRACT FROM AUTHOR]