Sheath Dynamics Research Articles

Dynamic Optic Nerve Sheath Diameter (ONSD) guided management of raised intracranial pressure in pediatric acute liver failure

The objectives were to evaluate the role of optic nerve sheath diameter (ONSD) to detect raised intracranial pressure (ICP) in pediatric acute liver failure (PALF), study the variations in ONSD with ICP-lowering measures and to evaluate its prognostic role. PALF with clinical evidence of raised ICP were enrolled as cases, while those without raised ICP were control group A. ONSD was measured at admission and repeated regularly. It was also measured at time of each new episode of raised ICP and 2h after the management of such episode. 31 PALF with raised ICP were included as cases and 15 without as control group A. ONSD was significantly higher in cases: 5mm (IQR: 4.7-5.4) as compared to control group A: 3.8mm (IQR: 3.3-4). ONSD greater than 4.55mm at baseline diagnosed clinically raised ICP with 87.5% sensitivity and 100% specificity. The mean ONSD was 5.44 ± 0.49mm during a total of 90 events of acute raised ICP. Clinical responders had a decrease in ONSD by 0.59 ± 0.24mm by 2h, whereas non-responders showed a decrease of 0.18 ± 0.23mm, p < 0.0005. ONSD persisting more than 4.6mm by 24h of management predicted poor outcome with sensitivity and specificity of 83.3% and 72.7%. ONSD is a simple, bedside, inexpensive, reproducible and repeatable modality to assess ongoing change in ICP in PALF. ONSD more than 4.55mm suggests raised ICP. The goal should be to bring ONSD down to less than 4.6mm within 24h by aggressive anti-ICP therapy to achieve favourable outcome.

A Global Model of the Collisional Plasma Boundary Sheath Using Step Model

Capacitively coupled plasmas (CCPs) are widely used for material processing, for example, for the manufacturing of semiconductors textile and modified surface. The ever-increasing requirements that companies face with respect to the quality, efficiency, and environmental friendliness of their manufacturing processes demand continuous progress. One simple and significant path to gain insight is the global model. Global models, which are, in engineering, called the lumped element model, allow for a complete and transparent mathematical analysis. Therefore, this work will study three models, the example of the exact model, and its verification using the step model to find the analytical relation between the sheath charge and the overall voltage drop across it. This formula is the missing factor in the global model that establishes the aim of the research. In addition, an effective lumped element model was studied, which allows describing the sheath dynamics in simple, though nonlinear engineering terms, such as resistance, capacitance, and inductance. The study achieves more detailed dynamics, such as the time-varying charge, voltage distribution, charge–voltage distribution $V_{\text {sh}}(Q)$ that controls the nonlinear sheath dynamics, current distributions, and the equation of the sheath distance for dc and ac sheaths. The results obtained at low computational complexity provide satisfactory calculations for nonlinear dynamics of collision sheath behavior.

Electron heating dynamics and plasma parameters control in capacitively coupled plasma

Capacitively coupled plasma (CCP) has gain wide attention due to its important applications in industry. The researches of CCP mainly focus on the discharge characteristics and plasma parameters under different discharge conditions to obtain a good understanding of the discharge, find good methods of controlling the charged particle properties, and improve the process performance and efficiency. The controlling of plasma parameters is based on the following three aspects: gas, chamber, and power source. Changing these discharge conditions can directly influence the sheath dynamics and the charged particle heating process, which can further influence the electron and ion distribution functions, the plasma uniformity, and the production of neutral particles, etc. Based on a review of the recent years’ researches of CCP, the electron heating dynamics and several common methods of controlling the plasma parameters, i.e. voltage waveform tailoring, realistic secondary electron emission, and magnetized capacitively coupled plasma are introduced and discussed in detail in this work.

Decoupling ion energy and flux in intermediate pressure capacitively coupled plasmas via tailored voltage waveforms

The discrete control of ion energy and flux is of increasing importance to industrially relevant plasma sources. The ion energy distribution functions (IEDFs) and net ion flux incident upon material surfaces in intermediate pressure (≈133 Pa, 1 Torr) radio-frequency capacitively coupled plasmas (rf CCPs) are coupled to the spatio-temporal sheath dynamics and resulting phase-averaged sheath potential. For single frequency driven discharges this co-dependence of ion energy and flux on the sheath potential limits the range of accessible operating regimes. In this work, experimentally benchmarked 2D fluid/Monte-Carlo simulations are employed to demonstrate quasi-independent control of the ion flux and IEDF incident upon plasma facing surfaces in a collisional (≈200 Pa, 1.5 Torr argon) rf hollow cathode discharge driven by multi-harmonic (n ⩾ 2) tailored voltage waveforms. The application of variable phase offset n = 5 tailored voltage waveforms affords a significant degree of control over the ion flux and mean ion energy , modulating each by factors of 2.9 and 1.6, respectively as compared to 1.8 and 1.6, achieved via n = 2 dual-frequency voltage waveforms. The disparate modulations achieved employing n = 5 tailored voltage waveforms demonstrate a significant degree of independent control over the mean ion energy and ion flux for collisional conditions, enabling access to a wider range of operational regimes. Maximising the extent to which ion energy and flux may be independently controlled enables improvements to plasma sources for technological applications such as plasma assisted material manufacture and spacecraft propulsion.

MHD Simulation of Physical Processes in Spherical Plasma-Focus Chambers with Allowance for Neutron Generation

The results of the development of a two-dimensional MHD code for carrying out computational studies of the dynamics of plasma current sheath in spherical chambers with a plasma focus are presented. Equations of magnetohydrodynamics with allowance for magnetic field diffusion, thermal conductivity and plasma radiation are used in this work. An implicit scheme is used in the calculation of the magnetic field, which makes it possible to describe the motion of plasma in a low-density region behind the plasma sheath. The formulas that take into account the possible appearance of anomalous resistance in the plasma are used to calculate the plasma conductivity. The neutron yield is calculated with allowance for thermonuclear and beam–target neutron generation mechanisms. The effect of the minimum residual gas density behind the plasma sheath on the cumulation of the plasma sheath is studied. The effects of magnetic field diffusion, thermal conductivity and anomalous plasma resistance on the plasma sheath dynamics are considered. The calculations are performed for two spherical plasma-focus chambers operating with currents up to 1 and 2 MA and neutron yields to 1012 and 1.5 × 1013 DT neutrons, respectively. The comparison of the calculated dependences with experimental data on the current, voltage and neutron yield made it possible to refine the parameters used in the calculations and achieve a satisfactory agreement between the simulation and experiment.

Experimental investigation of a low-energy linear plasma propulsion device

This paper presents preliminary results for a new design of a pulsed linear plasma device that generates electromagnetic plasma propulsion. The current sheath dynamics and the plasma propulsion along an extension tube have been studied. Breakdown of helium gas occurs between coaxial electrodes when applying a high-voltage pulse. This paper presents measurements with a number of probes, as well as camera images of the plasma propagation. A plasma is ejected and fills a 40 cm length extension tube. Additionally, the measurements show that the length of the propelled plasma column and its intensity inside the expansion tube is increased by increasing the charging voltage, while the plasma propulsion current is decreased along the extension tube. It is noted that a lower critical time; 4.7 μs, is needed to ionize the gas and form the plasma current sheath.

Effects of the dynamic cathode sheath on electron transport at the initial period of HiPIMS pulse studied by Langmuir probe measurements and 2D PIC-MCC simulation

In high power impulse magnetron sputtering (HiPIMS) discharge, the cathode sheath is a particularly vital section which determines the spatial distribution of electric field and the energy and transport of charged particles. In this work, a single Langmuir probe is employed to explore the effects of dynamic cathode sheath on electron transport at the initial period of HiPIMS pulse. Measurements show that the electron temperature is as high as ~20 eV and the plasma density is much lower than ~1015–1016 m−3 at the beginning of HiPIMS pulse t < 3 μs; correspondingly, a feature of small low-energy amplitude and extended high-energy tail is found in the electron energy distribution function (EEDF). And the high-energy electrons can escape the magnetic trap and gradually diffuse to further axial positions, even to z = 140 mm with only several microseconds delay. The diffusion coefficient of electrons D⊥ is larger than the typical Bohm diffusion at z > 32 mm in our discharge case. The two-dimension Particle-in-cell Monte Carlo collision (2D PIC-MCC) simulation results are well in agreement with the experimental results. The net positive charge density ∆n near the target is lower than ~+9.2×1014 m−3 at t = 0.5 μs, and the charge separation is up to ~0.4. In this case, the cathode sheath has a large thickness and the axial electric field EZ outside the sheath is as strong as dozens of 10 kVm−1. The simulation results confirm, for the case of the expanding sheath, (i) the electrons can be accelerated and escape the magnetic trap with weak constraint; (ii) the electron group can maintain their kinetic energy when they diffuse from the ionization region (IR) to bulk plasma (BP). When the net charge density increases to ~+3×1016 m−3 at t = 2.5 μs, the sheath thickness is condensed to ~1 mm with ~600 V voltage drop.

BASINÇ VE VOLTAJIN KÜRESEL PLAZMA ODAK CİHAZI DİNAMİ ÜZERİNE ETKİSİ

A spherical plasma focus device with two concentric electrodes is simulated using equal amount of deuterium and tritium mixture as a working gas. The developed model is used to investigate the effect of gas pressure and charging voltage on dynamics of current sheath in spherical plasma focus. While charging voltage is varied from 15 kV to 30 kV to see the effect of voltage, gas pressure is increased from 1 Torr to 20 Torr for gas pressure study. In this work, it is found that there is a strong correlation between current sheath dynamics and investigated parameters which are gas pressure and charging voltage.

Effects of Dynamic Plasma Sheath on Electromagnetic Wave Propagation and Bit Error Rate Under External Magnetic Field

The “magnetic window” is considered to be an effective technique for alleviating a vehicle’s atmospheric reentry blackout, whereas the dynamic variation characteristic of the plasma sheath is critical for the vehicle’s communication. In this article, the effects of dynamic plasma sheath on electromagnetic wave propagation and the bit error rate (BER) of two typical modulation signals under an external magnetic field are investigated. First, the hybrid matrix method is used to calculate the transmission coefficients of right-handed circularly polarized (RHCP) waves with different magnetic fields and plasma electron density fluctuation. Then, the average BER of two typical modulations signals, namely M-ary phase-shift keying (MPSK) and noncoherent M-ary frequency shift keying modulation (NC-MFSK) is numerically simulated based on the magnetized plasma sheath channel response. The results show that the magnitude in dB scale and phase of the RHCP transmission coefficient follow Gaussian distribution when the probability density function of the electron density is a Gaussian distribution. The BER performance of binary phase-shift keying (BPSK) and NC-MFSK is relevant to the incident wave frequency, the plasma stopband, and the intensity of electron density variation. Owing to the insensitivity of phase jitter, the NC-8FSK has a better performance than BPSK, followed by NC-4FSK and NC-2FSK.

Dynamic sheath formation and sub-THz radiation from laser–metal interactions

The generation of low-frequency radiation from a short pulse (∼100 fs) laser with mJ energy incident on a metal surface is investigated. The electrons within the metal surface absorb energy from the laser pulse, increasing in temperature to a few electron volts and resulting in some at the high-energy tail of the distribution to overcome the work function barrier. Emission of these electrons from the surface contributes to a surface current which sources secondary radiation emission. The Richardson–Dushman emission picture breaks down due to Coulombic effects and a modified emission model is presented. Previous precedence exists for modified thermionic emission models for laser–metal interactions of a similar nature, such as the one presented by Riffe et al. [J. Opt. Soc. Am. B 10, 1424–1435 (1993)] upon which we elaborate. Surface currents generated by such a mechanism are modeled with a particle-in-cell (PIC) simulation together with a Monte Carlo treatment of electron–neutral collisions with air molecules. The modified emission model, together with the PIC model, provides a numerical basis from which the radiated spectra are estimated due to the time-varying currents emitted from the surface of the metal. Experiments and numerical simulations for air pressure between 0.1 and 1 atm show qualitative agreement as to the pressure dependence of the measured signal energy from the secondary radiation, with Emeasured∝Pair−α for some α near but less than 1. Quantitative agreement between experiments and simulations is improved by including the effect of an additional contribution to secondary emission energy arising from inverse-bremsstrahlung collisions within a nm-scale vapor layer.

The PSK Channel Capacity Estimation under Dynamic Plasma Sheath Channel

During the reentry process, the plasma sheath covering the surface of the hypersonic aircraft will cause the amplitude attenuation and phase jitter of the communication electromagnetic waves. Channel parameters such as the electron density and collision frequency of the plasma sheath reflect the changing trend of the plasma sheath, and these parameters can be measured by physical means. However, these parameters cannot directly reflect the change of the channel communication ability and cannot directly serve the design of communication methods in the plasma sheath. Due to the particularity of the plasma sheath, the traditional channel estimation method for Additive White Gaussian Noise channels will no longer be applicable. This paper presents a channel capacity estimation method for dynamic plasma sheath. First, the plasma sheath is equivalent to a discrete input continuous output memoryless channel, and then the channel capacity expression is derived according to Shannon formula. Finally, the channel capacity of the dynamic plasma sheath is estimated by calculating the transition probability density function. The simulation results show that the channel capacity of the dynamic plasma sheath is affected by both the signal-to-noise ratio (SNR) and the dynamic parameters of the plasma sheath. When the electron density is small, the channel capacity is mainly affected by the SNR. As the electron density increases, the dynamic parameters of the plasma sheath gradually become the main factor affecting the channel capacity. This method is a theoretical analysis of the channel capacity when the channel parameters of the plasma channel are known, and it is meaningful for conducting the work of communication methods design.

A study of SCL sheath in presence of two dust species in a photoemissive environment

We have presented a basic model for a one dimensional collisionless space charge limited (SCL) plasma sheath consisting of electrons, ions, and two kinds of dust populations—a nano-dust population and a micrometer sized dust population. We have assumed that the nano-dust particles, plasma electrons, photoelectrons, and ions are the constituents of the plasma fluid and the micrometer sized dust particles behave as suspended impurities in the plasma without affecting the plasma dynamics. We have assumed that the charges of the micrometer-dusts are determined from the current balance on them and the charges on the nano-dusts are fixed. Due to photoemission, like the lunar surface charges to a positive potential, so the nano-dust and micrometer-dust particles charge to a positive potential. The electron densities have been derived by assuming both the electrons and photoelectrons to be Maxwellian, whereas the ion and nano-dust densities are derived from energy conservation of the ions and nano-dust particles and the continuity equations. Being a constituent of plasma, the nano-dusts do affect the sheath dynamics through the Poisson equation, which determines the plasma potential in the sheath region. The typical conditions for the levitation of dust particles in an SCL sheath including the maximum height and maximum size of the particles are calculated.

The effects of electron energy distribution function on the plasma sheath structure in the presence of charged nanoparticles

The effects of the electron energy distribution function (EEDF) on the structure of a dusty plasma sheath are investigated. Here, it is assumed that the electrons obey a Druyvesteyn-type distribution with a parameter $x$ controlling the shape of the EEDF. The Druyvesteyn-like distribution tends to a Maxwellian distribution as $x$ varies from 2 to 1. Using the orbital motion limited theory, the incident electron current on the dust is evaluated for a given $x$ . The results of numerical simulations are compared with those of a Maxwellian distribution. It was found that the sheath dynamics depends strongly on the magnitude of $x$ . The sheath thickness increases monotonically with increasing $x$ . However, the absolute dust charge decreases and, as a result, the accelerating ion drag force is weakened and thus the dust number density is enhanced. For a plasma with a Druyvesteyn-like distribution, the Bohm speed is a function of $x$ and increases with increasing $x$ .

Numerical investigation of dynamic properties of plasma sheath with pitching motion

Research on the dynamic properties of a plasma sheath coupled with pitching motion of the vehicle has great significance in solving the problem of communication interruption in the process of vehicle reentry. This paper investigates the dynamic properties of the plasma sheath by using the simplified conventional Burnett (SCB) equations and the Navier-Stokes (NS) equations with the thermochemical non-equilibrium effect. The eleven-species chemical kinetic models are applied to the comparison and there is verification of a dynamic plasma sheath simulation for the first time. After the introduction of vehicle pitching motion, the dynamic results are more consistent with the experimental data than the simulated results when treating it as static state. The plasma sheath characteristic parameters show periodic properties, whose changing period is the same as the pitching motion period. However, because of different velocities of the pitching motion, phase shifts exist in different positions of the vehicle. The enhancement of the rarefied effect weakens the disturbance to the plasma sheath. This research reveals the distribution and regularities of the dynamic plasma sheath. It is significant in solving the ionization blackout problem and the design of the reentry vehicle, and provides reliable data for further research on the dynamic plasma sheath.

Dynamic optic nerve sheath diameter changes upon moderate hyperventilation in patients with traumatic brain injury

BackgroundSonographic assessment of optical nerve sheath diameter (ONSD) has the potential for non-invasive monitoring of intracranial pressure (ICP). Hyperventilation (HV) -induced hypocapnia is used in the management of patients with traumatic brain injury (TBI) to reduce ICP. This study investigates, whether sonography is a reliable tool to detect dynamic changes in ONSD. MethodsThis prospective single center trial included patients with TBI and neuromonitoring within 36 h after injury. Data collection and ONSD measurements were performed at baseline and during moderate HV for 50 min. Patients not suffering from TBI were recruited as control group. ResultsTen patients with TBI (70% males, mean age 35 ± 14 years) with a median of first GCS of 5.9 and ten control patients (40% males, mean age 45 ± 16 years) without presumed intracranial hypertension were included. During HV, ICP decreased significantly (p < .0001) in the TBI group. An ONSD response was found for HV (p = .05). ConclusionWe observed a dynamic decrease of ONSD during moderate HV. This suggests a potential use of serial ONSD measurements when applying HV in cases of suspected intracranial hypertension.

Plasma Current Sheath Dynamics and Energy Dissipation in a Low-Energy Plasma Focus Device

This paper reports the measurements of plasma current sheath (PCS) dynamics, the energy dissipation processes, and the plasma focus (PF) electrical characteristics, particularly during the axial phase discharge in a Mather-type PF device (EAEA-PF1) energized with a 30 µF capacitor bank charged with 8, 10 and 12 kV. All these works are carried out under discharge conditions where the optimal PF action is achieved. At each charging voltage (Vch), 8 kV, 10 kV and 12 kV, the optimal PF action is studied for different Argon gas pressures (P) ranging from 0.4 to 1.2 Torr. The results show that the best PF is formed at Vch = 8 kV and P = 0.6 Torr, Vch = 10 kV and P = 0.8 Torr, and Vch = 12 kV and P = 0.8 Torr. The implosion velocity (Vz) results of PCS show that the maximum value of Vz (4.48 cm/µs) occurs at the end of the axial phase (i.e., at the coaxial electrode muzzle), which is detected at Vch = 12 kV and P = 0.8 Torr. Moreover, a less inefficient snowplow action is observed under these discharge conditions. The energy dissipation process data indicate that at Vch = 12 kV and P = 0.8 Torr, the ratio between the total energy dissipation and the input energy has a maximum value of ≅ 90%, and the minimum residual energy left on the condenser bank (175.39 J) is also achieved under these discharge conditions.

Estimation of surface flashover threshold in a vacuum II: flashover phase transition

Surface discharge in a vacuum occurs under an intense external field, also called a flashover. The flashover threshold over which the surface flashover occurs is of vital importance in the vacuum–solid interface. It is widely assumed that the flashover is initiated by a single surface multipactor and develops as a plasma discharge along a dielectric surface, namely the secondary electron emission avalanche (SEEA). Yet the SEEA model is not versatile for all practical vacuum–solid interfaces and there remain experiment results contradicting the theory predictions without a resolute answer. Here we further expand on previous theories and consider the conventional SEEA model under arbitrary field distributions. The example of a conical insulator is then given to test the validity of the SEEA model under an arbitrary field. It is found that the obtained flashover threshold is only consistent with the experiment at low cone angles where the field near the cathode triple junction is strong, while remarkable discrepancies appear when the anode field is significantly enhanced. The discrepancies can be understood if a different discharge model, i.e. an anode-initiated flashover (AIF) is included. Both particle-in-cell simulation and optical diagnostics are employed to visualize this unique discharge, which are then formalized on theoretical grounds to elucidate the early stage of the AIF. The transition between two flashover phases is then clarified. The plasma–surface interaction and sheath dynamics during the discharge are analyzed theoretically, showing distinct sheath evolutions in two flashover models. Additionally, a new factor reflecting field distortion is introduced to estimate the discharge threshold under an SEEA framework.

Bit Error Rate and Channel Capacity Performance of Telemetry Modulation Methods Under Typical Reentry Plasma Sheath Channel

Due to the severe impact of plasma turbulence on electromagnetic waves, the dynamic plasma sheath can degrade communication quality, even resulting in communication blackouts. This article studies the effect of different frequency bands and two typical telemetry modulation methods on communication signals under a time-varying plasma sheath channel. First, considering plasma dynamics on different scales, a typical reentry plasma sheath channel response is simulated and analyzed. Then, the average bit error rate (BER) and channel capacity of $M$ -ary phase-shift keying (MPSK) and $M$ -ary frequency-shift keying (MFSK) evaluating the telemetry communication performance are derived theoretically and simulated numerically based on the typical reentry plasma sheath channel response. The simulation results show that BPSK and noncoherent-MFSK (NC-MFSK) in the Ka -band and above bands have a preferable communication performance; when bit-to-noise ratio > 10 dB, BER $ and channel capacity loss $ for both BPSK and NC-MFSK ( $M\ge 8$ ) modulations in the especial reentry plasma condition (ERPC). Comparing BPSK and NC-MFSK in the Ka -band and above bands, the performances of BER and channel capacity follow the order: NC-MFSK ( $M \ge 8$ ) > BPSK > NC-4FSK > NC-2FSK. In particular, results indicate that under the dynamic plasma channel, terahertz communication can effectively improve communication performance and alleviate communication blackout.

Study of the Cathode Sheath Dynamics After Arc Current Zero Crossing Using a Two-Dimensional Langmuir Probe System

Study of the Cathode Sheath Dynamics After Arc Current Zero Crossing Using a Two-Dimensional Langmuir Probe System

Thermal degradation kinetics study of polyvinyl chloride (PVC) sheath for new and aged cables

The thermal degradation dynamics of new and aged PVC sheaths was studied in detail. The results illustrated that compared to new PVC sheath, the onset decomposition of aged PVC sheath mainly happened at higher temperature with larger peak value of mass loss rate. Three model-free methods most commonly used were employed to estimate the activation energy values at different conversions. It was noted that the aged PVC sheath exhibited greater entire activation energy than new PVC sheath. Two thermal degradation regions were observed based on the activation energy variation with conversion. The threshold of conversion for two regions was 0.6 for new PVC sheath and 0.5 for aged PVC sheath. The possible reaction mechanism was predicted by generalized master-plots method. The reaction model corresponding to each region showed observed difference between new and aged PVC sheaths. The compensation effect was also used to calculate the related pre-exponential factor. The variation of thermal degradation behavior could be ascribed to the changes of chemical composition, molecular structure, composition proportion and various additives after thermal aging. Besides, the thermal degradation process was reconstructed by an ANN model and it indicated that the predicted data fitted well with the experimental data.