Changes In Plasma Density Research Articles

Reactor optimization strategies for remote plasma sources: Numerical insights into argon inductively coupled plasma at Torr pressures

The semiconductor industry increasingly relies on remote plasma sources (RPS) for advanced processing techniques. In this study, we numerically explored the performance optimization of inductively coupled plasma at pressures above 1 Torr, suitable for RPS applications. Using a two-dimensional fluid model, we examined how process parameters affect plasma density and analyzed the contributions of various chemical reactions to plasma density changes in an argon discharge. Our findings show that increasing radio frequency (RF) power, gas pressure, and flow rate elevates electron and ion densities in the downstream region of the RPS. The increase in RF power generates strong inductive heating, which leads to convective transport of thermal energy in the downstream region of the RPS. This transferred thermal energy is expected to efficiently transfer radicals downstream through dissociation reactions with low threshold energy. Increased flow rates boost ion flux and improve axial electron transport, while elevated pressures lower electron temperatures and reduce the ambipolar field. We also observed that ion distribution is influenced by multi-component diffusion downstream. Thus, optimizing power, flow rate, and pressure enhances radical transport efficiency to the lower stage of the RPS. These results were validated experimentally using a Langmuir probe in argon discharge, confirming our numerical predictions.

Influence of plasma density gradient on the tearing mode with the poloidal shear flow

The influence of the plasma density gradient on the m/n = 2/1 and m/n = 4/1 (m is the poloidal mode number and n is the toroidal mode number) tearing mode instability with poloidal flow and flow shear is investigated. Using the analytical solution that we obtained in a previous work and mainly focused on the factors of plasma density and poloidal shear flow, we found that the plasma density has a stabilizing effect on the classical tearing mode, and the poloidal equilibrium flow can intensify this beneficial effect. The density gradient was detrimental to the stability of the tearing mode. The effects of both density and density gradient are slight, but the effect of poloidal flow on the plasma density is significant. Considering that the plasma density changes with the poloidal flow, the values of the tearing mode stability index ∆′ clearly change. Our investigation also found that compared with the negative flow shear, the positive flow shear is beneficial to the stability of the tearing mode, and a larger poloidal flow shear has a better stabilizing effect on the classical tearing mode.

An angle-selective photonic crystal for multi-physical sensing applications.

This study reports a high-transmission photonic crystal (PC) that was optimized for TM waves at a frequency of 43.8 GHz and engineered using photonic band gap (PBG) principles to achieve angle selection. The structure demonstrated a remarkable transmission from -68° to 0°, consistently exceeding 80% efficiency. Assessing the fragility of the medium within the PC using the critical angle, a multi-physical sensor (MS) comprising both refractive index sensing (RIS) and plasma density sensing (PDS) functions was proposed. The PDS could detect concentrations from 0.4 × 1018 m-3 to 0.8 × 1018 m-3 with a sensitivity of 10.925° per m-3. For RIS, with the change in magnetic field intensity, it could detect refractive index in the ranges of 2.45-2.33 at 1.25 T, 2.33-2.21 at 1.15 T, 2.21-2.09 at 1.05 T, and 2.09-1.97 at 0.65 T, with respective sensitivities of -28° per RIU, -16° per RIU, -18.33° per RIU, and -9.38° per RIU, showcasing broad detection ranges and high sensitivities. Notably, the MS could maintain high transmission (greater than 0.8) in the RIS range from -60° to 0°, enabling dynamic angle selection for refractive index and plasma density. Therefore, it holds promising prospects in the real-time monitoring of refractive index and plasma density changes in healthcare- and environment-related applications, such as in early disease diagnosis, air quality monitoring and detecting metabolic activity or harmful substances.

The effect of scattering instability induced by high intensity seed on backward Raman amplification

Backward Raman amplification (BRA) in plasma has become one of the most promising techniques for further promoting light intensity. In this work, BRA in plasma is simulated using one-dimensional particle-in-cell code with different seed intensity, showing that higher seed intensity (in the range of 2×1011–2×1015 W/cm2) will increase energy conversion efficiency with pump pulse of fixed intensity (2×1014 W/cm2), which is pre-depleted by spontaneous Raman scattering. We find that the scattering instability of amplified seed pulse is enhanced and the plasma wave fluctuation level is promoted with seed intensity at 2×1016 W/cm2 because the local plasma density changes in the pump scattering depletion region. In the simulations, we also found that the enhanced Raman scattering and secondary Raman scattering of amplified seed pulse causes a rapid energy consumption and earlier saturation of the amplification, which significantly affects the temporal shape of the amplified seed pulse to form a double peak shape. In addition, we explore the evolution of the energy and the pulse duration of the double peaks in detail and reveal the law of pulse energy growth with different pulse durations in a Raman amplifier.

Conjugate observations of power line harmonic radiation onboard DEMETER

The effects of power line harmonic radiation (PLHR) on near-Earth space have been debated for many years. Since some ultrahigh voltage AC and DC power projects were added to the Chinese power grid, increasingly numerous radiations at 50 Hz and their harmonics have been caught by the microsatellite DEMETER in the geomagnetically conjugate topside ionosphere during the daytime of winter and spring. Distribution of SYM-H index has been studied, showing geomagnetic disturbances may be unnecessary for the occurrence of PLHR. The propagation of PLHR between hemispheres without much attenuation illustrates that PLHR can be amplified when crosses the equatorial zone. Eliminating seasonal and diurnal factors, dependence of PLHR propagation on the change in plasma density is disclosed. The abrupt variation of electron density gradient may create a favored condition for PLHR waves propagating between hemispheres.Graphical

First Analysis of In Situ Observation of Surface Alfvén Waves in an ICME Flux Rope

Alfvén waves (AWs) are ubiquitous in space and astrophysical plasma. Their crucial role in various physical processes has triggered intense research in solar–terrestrial physics. Simulation studies have proposed the generation of AWs along the surface of a cylindrical flux rope, referred to as surface AWs (SAWs); however, the observational verification of this distinct wave has been elusive to date. We report the first in situ observation of SAWs in a flux rope of an interplanetary coronal mass ejection. We apply the Walén test to identify them. We have used Elsässer variables to estimate the characteristics of SAWs. They may be excited by the movement of the flux rope’s footpoints or by instabilities along the boundaries of the plasma magnetic cloud. Here, the change in plasma density or field strength in the surface-aligned magnetic field may trigger SAWs.

On a Guiding of Whistler‐Mode Waves by Density Gradients

AbstractObservations from satellites demonstrate that in the magnetosphere, VLF whistler‐mode waves are frequently detected in the narrow transition regions, where the plasma density changes its magnitude over a short distance across the ambient magnetic field. These observations suggest that the small‐scale, isolated density gradients can guide the VLF whistler‐mode waves along the field. We investigate the guiding of the whistler‐mode waves by the transverse density gradients with a size much less than the characteristic perpendicular size of the wave. We found analytical solutions describing these waves in the plasma with a sharp density discontinuity between two homogeneous regions, and confirm with time‐dependent, two‐dimensional simulations that these waves are indeed guided by the discontinuity. Simulations also reveal that the parameters of the guided waves (the frequency and parallel wavelength) relate to the parameters of the plasma.

Self-focusing of a Bessel–Gaussian laser beam in plasma under density transition

The model given by Patil et al., for the self-focusing of lowest-order Bessel–Gaussian laser beam, is revisited by introducing the exponential plasma density ramp in this paper. Plasma density changes due to combined effect of the relativistic and ponderomotive nonlinearities, which arise due to the nonuniform spatial profile of the laser beam leading to stronger self-focusing. Under paraxial ray approximation, the expression for the evolution of laser-beam width parameter is obtained and solved numerically in order to achieve self-focusing. We have observed that as Bessel beam propagates through the plasma, it does not diffract and spread out, which is in contrast to the usual behavior of light that spreads out after being focused down to the small spot. Laser spot size decreases considerably under the presence of exponential density ramp. This study will be useful for better understanding the self-focusing mechanism of lasers as it becomes effective for the optimized parameters under the proposed situation. Bessel laser beams play an important role in enhancing the energy gain in laser-driven accelerators. In addition, for all laser-driven plasma-based accelerators, it appears possible to guide a laser beam over large distances using a plasma density channel, thus enhancing the acceleration length and the energy gain.

Electron cyclotron current drive under neutral beam injection on HL-2M

Based on OMFIT framework and HL-2M parameters, this paper comprehensively considers the changes in plasma density, temperature, and other transport quantities caused by the interaction of neutral beam injection (NBI) and electron cyclotron wave (ECW) with plasma. The changes in the Shafranov shift of the plasma magnetic surface center are also evaluated. Theoretically, the influence of NBI on the deposition location and current drive efficiency of the ECW is studied. According to the findings, NBI affected the position location and efficiency of the electron cyclotron current drive (ECCD) deposited on both high field side (HFS) and low field side (LFS). NBI can relocate the ECW power deposition location to the core and increase the current drive efficiency when the ECW power is deposited on the LFS. When the NBI power increases to 7 MW, the ECCD deposition location can shift to the core by roughly 0.15 normalized small radii, and the current drive efficiency can be improved by 1.3 times. Moreover, as NBI power increases, the radial region where the dimensionless current drive efficiency equals to zero gets closer to the plasma edge. When ECW power is deposited on the HFS paraxial, increasing NBI power causes the ECW deposition location to move toward the plasma edge, thus lowering current drive efficiency. This trend is caused by an increase in NBI power, which can increase the Shafranov shift of the plasma center, increase the electron density, and change the electron temperature. These studies hold great significance for achieving more effective current drive and controlling the plasma current profile and neoclassical tearing mode instability.

Studies of aluminum erosion by neutral particles using quartz crystal microbalance and low energy neutral particle analyzer on EAST

The neutral-induced material erosion is still an unsolved issue for ITER and future reactors, which influences the lifetime of the first wall material and tritium retention rate. A low energy neutral particle analyzer (LENPA) and a quartz crystal microbalance (QMB) were recently installed together on the equatorial port of sector H in EAST to study neutral-induced material erosion. The QMB can provide real-time and in-situ measurements on the erosion process of the wall material, while the neutral energy spectrum given by LENPA can help to calculate the material erosion rate for comparison with QMB results. In the 2021 EAST summer campaign, neutral-induced Al erosion was successfully studied with the two available diagnostics for the first time. In a series of long-pulse discharges, the average Al erosion rates measured by the QMB are consistent with the theoretical calculations using the LENPA data. Furthermore, the changes in plasma density, heating power and the real-time lithium powder injection result in different neutral flux and energy, which lead to changes in the neutral-induced material erosion rates.

An equivalent model of discharge instability in the discharge chamber of Kaufman ion thruster

The industrial application of the Kaufman ion thruster in its arc stage is limited owing to the instability of the discharge pulse. Presently, a complete prediction model that can predict the discharge pulse in the high-current stage does not exist. In this study, a complete prediction model for the pulse in the ion thruster is established using the zero-dimensional plasma discharge model and equivalent circuit model. The zero-dimensional plasma discharge model is used to obtain the corresponding plasma parameters by calculating the beam current, discharge current, voltage, and gas flow under actual working conditions. The input parameters of the equivalent circuit model are calculated using empirical formulae to acquire the estimated discharge waveforms. The pulse waveforms obtained using the model are found to be consistent with the experimental results. The model is used to evaluate the process of rapid changes in plasma density. Additionally, this model is employed to predict changes in the pulse waveforms when the volume of the discharge chamber and grid plate transmittance are changed.

A phase processor used in terahertz interferometer for tokamak

The commonly used phase processing techniques include coherence method, zero-crossing comparison method and FFT method. Due to the influence of noise, amplitude modulation and frequency modulation, these phase processing methods are easy to cause errors, thus affecting the measurement accuracy. In this paper, a phase processor based on all-phase fast Fourier transform is developed. The principle of the phase processing algorithm is analyzed, and the implementation process of the algorithm on FPGA is described. The effectiveness of the method is tested under different conditions. The experimental results show that the standard deviation of phase processing is 0.387° when the signal is sampled simultaneously under 90% AM modulation. When asynchronous sampling and 50% FM, the phase processing error is 0.371°. This phase processer has been used to the terahertz interferometer of the HL-2M device, the phase change of the output signal of the interferometer can be accurately processed when the plasma density changes dramatically, and the loss of phase processing information can be effectively avoided in the plasma diagnosis process. Based on the technical research proposed in this paper, a more accurate phase processing reference scheme is provided for plasma diagnosis of interferometer.

Daily Variations of Plasma Density in the Solar Streamer Belt

Abstract Improved space weather diagnostics depend critically on improving our understanding of the evolution of the slow solar wind in the streamer belts near the Sun. Recent innovations in tomography techniques are opening a new window on this complex environment. In this work, a new time-dependent technique is applied to COR2A/Solar Terrestrial Relations Observatory observations from a period near solar minimum (2018 November 11) for heliocentric distances of 4–8 R ⊙. For the first time, we find density variations of large amplitude throughout the quiescent streamer belt, ranging between 50% and 150% of the mean density, on timescales of tens of hours to days. Good agreement is found with Parker Solar Probe measurements at perihelion; thus, the variations revealed by tomography must form a major component of the slow solar wind variability, distinct from coronal mass ejections or smaller transients. A comparison of time series at different heights reveals a consistent time lag, so that changes at 4 R ⊙ occur later at increasing height, corresponding to an outward propagation speed of around 100 km s−1. This speed may correspond to either the plasma sound speed or the bulk outflow speed depending on an important question: are the density variations caused by the spatial movement of a narrow streamer belt (moving magnetic field, constant plasma density), or changes in plasma density within a nonmoving streamer belt (rigid magnetic field, variable density), or a combination of both?

Links of the Plasmapause With Other Boundary Layers of the Magnetosphere: Ionospheric Convection, Radiation Belt Boundaries, Auroral Oval

The plasmapause marks the limit of the plasmasphere and is characterized by a sudden change in plasma density. This can influence the other regions of the magnetosphere, including due to different waves circulating inside and outside the plasmasphere. In the present work, we first compare the positions of the plasmapause measured by the NASA Van Allen Probes in 2015 with those of the Space Weather Integrated Forecasting Framework plasmasphere model (SPM). Using the Van Allen Probes and other satellite observations like PROBA-V, we investigate the links that can exist with the radiation belt boundaries. The inward motion of the outer radiation belt associated with sudden flux enhancements of energetic electrons can indeed be directly related to the plasmapause erosion during geomagnetic storms, suggesting possible links. Moreover, the position of the plasmapause projected in the ionosphere is compared with the ionospheric convection boundary. The equatorward motion of the plasmapause projected in the ionosphere is related to the equatorward edge motion of the auroral oval that goes to lower latitudes during storms due to the geomagnetic perturbation, like the low altitude plasmapause and the outer radiation belt. The links between these different regions are investigated during quiet periods, for which the plasmasphere is widely extended, as well as during geomagnetic storms for which plumes are generated, and then afterwards rotates with the plasmasphere. The magnetic local time dependence of these boundaries is especially studied on March 14, 2014 after a sudden northward turning of the interplanetary magnetic field (IMF) and for the geomagnetic storm of August 26, 2018, showing the importance of the magnetic field topology and of the convection electric field in the interactions between these different regions eventually leading to the coupling between magnetosphere and ionosphere.

The effect of acetylene flow rate on the uniform deposition of thick DLC coatings on the inner surface of pipes with different draw ratios

Thick multilayered DLC coatings were deposited on the inner surface of pipes with different draw ratios by hollow cathode plasma immersion ion processing (PIIP). By tuning the acetylene (C2H2) flow rate, the change of plasma density caused by different draw ratios can be compensated to fulfill the uniform deposition of thick DLC coatings along the pipes. The influence of C2H2 flow rate on the mechanical and tribological properties of the DLC coatings were investigated. With the increase of draw ratio, the deposition temperature and gas pressure declined, and the plasma density became more asymmetrical in the axial direction. Increasing the flow rate can timely replenish the plasma inside the pipe and resolve the mass depletion problem, resulting in better thickness and structure uniformity of the DLC coatings. The greatest deposition rate of 28.6 μm/h was obtained at the 250 sccm C2H2 flow with a draw ratio of 5:1. The results of Raman spectrum and XPS indicated that the content of sp2-carbon bonds decreased with the increasing draw ratio for lower impact energies of ions. The higher sp3 carbon bonds content contributed to the formation of carbonaceous transfer layers, which resulted in the lower friction coefficient and wear rate.

The effect of facility background pressure on hollow cathode operation

The effect of facility background pressure on the operation of a hollow cathode in a Hall thruster-like axial magnetic field is experimentally characterized. Facility pressure was varied between 10 and 88 μTorr-Xe using a secondary flow of xenon into the test facility, and cathode operation was studied using a combination of telemetry and plasma diagnostic measurements. Increasing pressure resulted in decreased discharge voltage, cathode orifice plate temperature, and voltage and current oscillation magnitudes. The plasma diagnostics, which consisted of a radially mounted retarding potential analyzer and an ion saturation probe and emissive probe mounted to a fast reciprocating motion stage, showed that increasing pressure resulted in decreased radial high-energy ions, decreased centerline maximum plasma potential and electron temperature, and increased plasma density farther than 40 mm downstream of the cathode. The effects of background pressure on the cathode plume were largely constrained to between 50 and 100 mm downstream of the cathode, where the largest gradients of plasma potential and electron temperature occur. Experimental measurements were combined with neutral density simulations to identify that the confluence of changes in electron temperature, plasma density, and neutral density result in an increase of almost an order of magnitude in the calculated ionization rate in the same location. These results have implications for both standalone cathode tests and for improving the understanding of facility effects on cathode coupling in Hall thrusters.

Diagnosis of Post-Arc Residual Plasma in Vacuum Breaker

The residual plasma in the gap between the anode and cathode has an important influence on the recovery characteristics of the medium after the arc current zero in the vacuum arc extinguishing chamber. A diagnostic system for post-arc residual plasma is proposed and designed to diagnose the density of residual plasma during sheath expansion. The experimental platform is built on the basis of the synthetic circuit and detachable vacuum chamber. The diagnostic system uses Langmuir probe array to diagnose the post-arc plasma density change in vacuum interrupter, which reflects the dynamic process of sheath expansion in the gap. The effect of different current sizes and contact spacing on residual plasma is discussed in this paper. The plasma densities diagnosed by different position probes are used to analyze the post-arc characteristics of vacuum circuit breakers from a micro-level. The accuracy and reliability of the diagnostic system are verified by comparing with the previous studies, which provides some reference value for the future research on the post-arc micro-characteristics of vacuum breaker.

Theoretical Investigation of a Sensor Based on One-Dimensional Photonic Crystals to Measure Four Physical Quantities

In this study, a sensor based on the located defect mode resonance is proposed, which can be used to simultaneously measure changes in magnetic induction intensity, plasma density, refractive index, and incident light angle. Plasma is introduced as the defect into a one-dimensional periodic structure, exciting the located defect mode resonance. The sensitivity, linear range, and figure of merit of the sensor are investigated using the transfer matrix method. The increase in the number of cycles can be used to improve the quality factor and FOM. We also consider the influence of the loss tangent on the sensor to a certain extent. The one-dimensional layered structure is utilized, which has the merits of small volume and simple manufacture. In addition, compared with the traditional sensors design, which focuses on the improvement of performance parameters, our proposed sensor concentrates on the study of multiple physical quantities. Therefore, we hope that our work can have some application potential in the field of measurement.

Feedback Suppression in the Plasma Relativistic Microwave Noise Amplifier with Inverse Configuration

The noise amplification mode in a plasma maser with inverse geometry was obtained experimentally for the first time. It is shown that the introduction of a microwave absorber transfers the maser operating mode from the oscillator to the amplifier. In both modes, the tuning of the radiation frequency has been shown when the plasma density changes. This work is a step in the implementation of a plasma relativistic noise amplifier with a short REB pulse.

Challenges to Understanding the Earth's Ionosphere and Thermosphere

AbstractWe discuss, in a limited way, some of the challenges to advancing our understanding and description of the coupled plasma and neutral gas that make up the ionosphere and thermosphere (I‐T). The I‐T is strongly influenced by wave motions of the neutral atmosphere from the lower atmosphere and is coupled to the magnetosphere, which supplies energetic particle precipitation and field‐aligned currents at high latitudes. The resulting plasma dynamics are associated with currents generated by solar heating and upward propagating waves, by heating from energetic particles and electromagnetic energy from the magnetosphere and by the closure of the field‐aligned currents applied at high latitudes. These three contributors to the current are functions of position, magnetic activity, and other variables that must be unraveled to understand how the I‐T responds to coupling from the surrounding regions of geospace. We have captured the challenges to this understanding in four major themes associated with coupling to the lower atmosphere, the generation and flow of currents within the I‐T region, the coupling to the magnetosphere, and the response of the I‐T region reflected in the neutral and plasma density changes. Addressing these challenges requires advances in observing the neutral density, composition, and velocity and simultaneous observations of the plasma density and motions as well as the particles and field‐aligned current describing the magnetospheric energy inputs. Additionally, our modeling capability must advance to include better descriptions of the processes affecting the I‐T region and incorporate coupling to below and above at smaller spatial and temporal scales.