Sheath Field Research Articles

Sheath formation around a dielectric droplet in a He atmospheric pressure plasma

Interactions at the interface between atmospheric pressure plasmas and liquids are being investigated to address applications ranging from nanoparticle synthesis to decontamination and fertilizer production. Many of these applications involve activation of droplets wherein the droplet is fully immersed in the plasma and synergistically interacts with the plasma. To better understand these interactions, two-dimensional modeling of radio frequency (RF) glow discharges at atmospheric pressure operated in He with an embedded lossy dielectric droplet (tens of microns in size) was performed. The properties of the sheath that forms around the droplet were investigated over the RF cycle. The electric field in the bulk plasma polarizes the dielectric droplet while the electron drift in the external electric field is shadowed by the droplet. The interaction between the bulk and sheath electric fields produces a maximum in E/N (electric field/gas number density) at the equator on one side of the droplet where the bulk and sheath fields are aligned in the same direction and a minimum along the opposite equator. Due to resistive heating, the electron temperature Te is maximum 45° above and below the equator of the droplet where power deposition per electron is the highest. Although the droplet is, on the average, negatively charged, the charge density on the droplet is positive on the poles and negative on the equator, as the electron motion is primarily due to diffusion at the poles but due to drift at the equator.

Dynamics of hot refluxing electrons in ultra-short relativistic laser foil interactions

We investigate the dynamics of hot refluxing electrons in the interaction of an ultra-short relativistic laser pulse with a thin foil target via particle-in-cell (PIC) simulations, which is governed by the multidimensional spatiotemporal evolution of a self-generated sheath field. The comparison of time-integrated energy spectra of refluxing and escaping electrons indicates the refluxing efficiency is higher than 95% in average for each bounce. The characteristics of wide transverse spatial distribution and energy-resolved angular distribution caused by the refluxing electrons show a direct correlation with the angular-dependent photon yield of bremsstrahlung emission, as verified by the hybrid simulations of coupling the PIC results with Monte Carlo particle transport code. We further clarify the energy dissipation mechanisms of refluxing electrons through the recirculation in the thin target under the electron-refluxing dominated regime and conclude that the self-generated sheath field plays a dominant role over the competing processes, such as the radiation loss, collisional stopping, and anomalous inhibition via the resistive field. The lifetime of recirculation is calculated to be a few hundred femtoseconds, that is, one order of magnitude shorter than the timescale of collisional dissipation, while it is one order of magnitude longer than the laser pulse duration. The results could provide useful insights to understand the hot electron transport and stopping, secondary radiation generation, and ion acceleration in the high energy density plasmas.

Enhanced Proton Acceleration from Laser Interaction with a Tailored Nanowire Target

Target normal sheath field acceleration via laser interaction with structured solid targets has been widely studied for its potential use in a wide range of applications. Here, a novel nanowire target with a corrugated front surface is proposed to improve the proton acceleration by a target normal sheath field. Two-dimensional particle-in-cell simulations demonstrated that with the existence of the corrugated surface, the cut-off energy of accelerated protons nearly doubles compared to the planar nanowire target. When interacting with the corrugated surface, the incident laser pulse is reflected multiple times, focused and reinforced in each cavity near the front surface, which leads to suppression of the reflectivity and an improvement in the absorption rate. Electrons are heated more efficiently and the sheath field at the target rear side is naturally enhanced. To further investigate the performance of this novel target, a series of simulations with various laser intensities and target sizes were also carried out. This simple target design may provide insights for experiments in the future and should arouse interest because of its wide application.

Insights into the Mechanism for Vertical Graphene Growth by Plasma-Enhanced Chemical Vapor Deposition.

Vertically oriented graphene (VG) has attracted attention for years, but the growth mechanism is still not fully revealed. The electric field may play a role, but the direct evidence and exactly what role it plays remains unclear. Here, we conduct a systematic study and find that in plasma-enhanced chemical vapor deposition, the VG growth preferably occurs at spots where the local field is stronger, for example, at GaN nanowire tips. On almost round-shaped nanoparticles, instead of being perpendicular to the substrate, the VG grows along the field direction, that is, perpendicular to the particles’ local surfaces. Even more convincingly, the sheath field is screened to different degrees, and a direct correlation between the field strength and the VG growth is observed. Numerical calculation suggests that during the growth, the field helps accumulate charges on graphene, which eventually changes the cohesive graphene layers into separate three-dimensional VG flakes. Furthermore, the field helps attract charged precursors to places sticking out from the substrate and makes them even sharper and turn into VG. Finally, we demonstrate that the VG-covered nanoparticles are benign to human blood leukocytes and could be considered for drug delivery. Our research may serve as a starting point for further vertical two-dimensional material growth mechanism studies.

Study of transition regime for amorphous to nano-crystalline silicon thin films using 27.12 MHz PECVD: Insight into plasma kinetics

In this article, we report the phase transition region of hydrogenated amorphous (a-Si:H) to nano-crystalline (nc-Si:H) silicon thin films deposited using 27.12 MHz assisted Plasma Enhanced Chemical vapor Deposition (PECVD) process with the approach of plasma diagnosis. This work presents for the first time a study of plasma characteristics using Impedance Analyser (V/I probe) at various applied power (4 W - 40 W), though till now this apparatus has been utilized only to analyse the applied delivered power during processing. On the basis of plasma diagnose, optimum bulk field (5 V/cm); sheath field (1376 V/cm) and minimum sheath width (7.4 x 10-4 cm) observed at 20 W power which provides a visible mark of transition from a-Si:H to nc-Si:H. On account of plasma properties, the deposition was carried out by considering the plasma-surface interaction during growth. The microstructure of the deposited films was characterized using Raman spectra, UV-Vis spectra and conductivity measurements and they were found to be well correlating with the evaluated plasma characteristics. In particular, it was found that at applied power near to the onset of transition regime i.e. at 10 W, preeminent properties of a-Si:H film was observed, predominantly in terms of highest photosensitivity (7.2x103), low photo-degradation and high deposition rate (~1.39 Å/s). On the other hand, volume fraction of crystallites (24 %), wider band gap (2.0 eV) and no photo-degradation observed for the film deposited at 20 W applied power which signifies the existence of crystallites in an amorphous matrix. Copyright © 2017 VBRI Press.

Transmission of an ICME Sheath Into the Earth's Magnetosheath and the Occurrence of Traveling Foreshocks

AbstractThe transmission of a sheath region driven by an interplanetary coronal mass ejection into the Earth's magnetosheath is studied by investigating in situ magnetic field measurements upstream and downstream of the bow shock during an ICME sheath passage on 15 May 2005. We observe three distinct intervals in the immediate upstream region that included a southward magnetic field component and are traveling foreshocks. These traveling foreshocks were observed in the quasi‐parallel bow shock that hosted backstreaming ions and magnetic fluctuations at ultralow frequencies. The intervals constituting traveling foreshocks in the upstream survive transmission to the Earth's magnetosheath, where their magnetic field, and particularly the southward component, was significantly amplified. Our results further suggest that the magnetic field fluctuations embedded in an ICME sheath may survive the transmission if their frequency is below ∼0.01 Hz. Although one of the identified intervals was coherent, extending across the ICME sheath and being long‐lived, predicting ICME sheath magnetic fields that may transmit to the Earth's magnetosheath from the upstream at L1 observations has ambiguity. This can result from the strong spatial variability of the ICME sheath fields in the longitudinal direction, or alternatively from the ICME sheath fields developing substantially within the short time it takes the plasma to propagate from L1 to the bow shock. This study demonstrates the complex interplay ICME sheaths have with the Earth's magnetosphere when passing by the planet.

Manipulation of laser-accelerated proton beam spatial distribution by laser machined microstructure targets

Experimental study of laser proton acceleration was carried out using laser machined line targets and cross targets with tens of micrometers scale. We have found that both the shape and material of the microstructure targets have significant influences on the distribution of the proton beam. For the aluminum line target, the proton beam spot expands in a direction perpendicular to the boundary of the target; while for the plastic line target, it expands parallel to the boundary of the target. Detailed PIC simulations of the aluminum target have been carried out, which show that due to the lateral transport of hot electrons and the sheath fields accumulated at the edge of the microstructure target, the divergence angle of the accelerated proton beam is changed accordingly, thereby modulating the spatial distribution: the elliptical beam spot can be obtained from the line target, and the quasi-square beam spot can be obtained from the cross target. Simulations of the plastic target indicate that the difference in the electron transport properties for the two types of targets may be the reason for the completely different beam spot shapes. This work shows that the microstructure targets can be a potential method to manipulate the spatial distribution and uniformity of the proton beam.

Commissioning experiment of the high-contrast SILEX-Ⅱ multi-petawatt laser facility

The results of a commissioning experiment on the SILEX-Ⅱ laser facility (formerly known as CAEP-PW) are reported. SILEX-Ⅱ is a complete optical parametric chirped-pulse amplification laser facility. The peak power reached about 1 PW in a 30 fs pulse duration during the experiment. The laser contrast was better than 1010 at 20 ps ahead of the main pulse. In the basic laser foil target interaction, a set of experimental data were collected, including spatially resolved x-ray emission, the image of the coherent transition radiation, the harmonic spectra in the direction of reflection, the energy spectra and beam profile of accelerated protons, hot-electron spectra, and transmitted laser energy fraction and spatial distribution. The experimental results show that the laser intensity reached 5 × 1020 W/cm2 within a 5.8 µm focus (FWHM). Significant laser transmission did not occur when the thickness of the CH foil was equal to or greater than 50 nm. The maximum energy of the accelerated protons in the target normal direction was roughly unchanged when the target thickness varied between 50 nm and 15 µm. The maximum proton energy via the target normal sheath field acceleration mechanism was about 21 MeV. We expect the on-target laser intensity to reach 1022 W/cm2 in the near future, after optimization of the laser focus and upgrade of the laser power to 3 PW.

Laser-accelerated protons using density gradients in hydrogen plasma spheres

The effect of different density profiles on micron-sized hydrogen plasma spheres is investigated when the plasma gets irradiated with an ultrashort circularly polarized laser. In this study, we show that significant improvement in the characteristics of the accelerated protonsviz.maximum proton energy, as well as their monoenergetic behaviour, is possible by using a plasma sphere having a tailored density profile. A linear-shaped density inhomogeneity is introduced in the plasma sphere such that the density is peaked at the centre and gradually dropping outwards. The density gradient is tuned by changing the peak density at the centre. The optimum regime of steepness is found for the maximum energy attained by the protons where the target is opaque enough for the radiation pressure to play its role, however not too opaque to inhibit efficient target heating. A novel Gaussian-shaped density profile is suggested which plays an important role in suppressing the sheath field. With a decreased rear-side field, a visible improvement of the monoenergetic feature of the protons is observed.

Enhanced laser-driven proton acceleration via improved fast electron heating in a controlled pre-plasma

The interaction of ultraintense laser pulses with solids is largely affected by the plasma gradient at the vacuum–solid interface, which modifies the absorption and ultimately, controls the energy distribution function of heated electrons. A micrometer scale-length plasma has been predicted to yield a significant enhancement of the energy and weight of the fast electron population and to play a major role in laser-driven proton acceleration with thin foils. We report on recent experimental results on proton acceleration from laser interaction with foil targets at ultra-relativistic intensities. We show a threefold increase of the proton cut-off energy when a micrometer scale-length pre-plasma is introduced by irradiation with a low energy femtosecond pre-pulse. Our realistic numerical simulations agree with the observed gain of the proton cut-off energy and confirm the role of stochastic heating of fast electrons in the enhancement of the accelerating sheath field.

Investigation of magnetic inhibition effect on ion acceleration at high laser intensities

The irradiation of a target with high laser intensity can lead to self-generation of an intense magnetic field (B-field) on the target surface. It has therefore been suggested that the sheath-driven acceleration of high-energy protons would be significantly hampered by the magnetization effect of this self-generated B-field at high enough laser intensities. In this paper, particle-in-cell simulations are used to study this magnetization effect on sheath-driven proton acceleration. It is shown that the inhibitory effect of the B-field on ion acceleration is not as significant as previously thought. Moreover, it is shown that the magnetization effect plays a relatively limited role in high-energy proton acceleration, even at high laser intensities when the mutual coupling and competition between self-generated electric (E-) and B-fields are considered in a realistic sheath acceleration scenario. A theoretical model including the v × B force is presented and confirms that the rate of reduction in proton energy depends on the strength ratio between B- and E-fields rather than on the strength of the B-field alone, and that only a small percentage of the proton energy is affected by the self-generated B-field. Finally, it is shown that the degraded scaling of proton energy at high laser intensities can be explained by the decrease in acceleration time caused by the increased sheath fields at high laser intensities rather than by the magnetic inhibitory effect, because of the longer growth time scale of the latter. This understanding of the magnetization effect may pave the way to the generation of high-energy protons by sheath-driven acceleration at high laser intensities.

The effects of laser pulse length and collisional ionization on the acceleration of titanium ions

The interaction of a relativistic laser pulse (1018 W cm−2 µm2) with foil targets can accelerate ions to energies of tens of MeV u−1 with optimized laser and target parameters. We report the results on simulations of the interaction of a 6.0 × 1020 W cm−2 laser pulse incident on ultrathin (10–500 nm) titanium foils to investigate the roles of laser pulse duration and ionization mechanisms in the acceleration of titanium ions. While holding peak intensity constant, two laser pulse durations were investigated, 140 and 650 fs. The optimum thickness is dependent on pulse duration, as it requires the concurrence of target transparency with the incidence of the peak laser intensity. The collisional processes do not play a significant role in Ti ion beam generation from the 140 fs laser pulse duration at the optimum thickness (30 nm). However, for the 650 fs laser optimum, collisions improve the conversion efficiency of highly energetic (10 MeV u−1), high charge titanium (Ti21 − 22 +) by a factor of 20, and the titanium ion cutoff energy by ∼15%. This improvement is due to the fact that collisional ionization increases the electron density of the plasma, which delays the time of relativistic transparency, causing collisions to decrease the optimum foil thickness from 150 to 100 nm. Additionally, collisional ionization increases the charge-to-mass ratio of the titanium, and injects more electrons into the accelerating sheath field. At the optimum thickness, target normal sheath acceleration is the dominant mechanism of acceleration, with additional contributions from radiation pressure and shock wave acceleration.

Interplanetary Origins of Some Intense Geomagnetic Storms During Solar Cycle 24 and the Responses of African Equatorial/Low‐Latitude Ionosphere to Them

AbstractThe interplanetary origins of eight selected intense geomagnetic storms that occurred during solar cycle 24 and the responses of total electron content (TEC) at eight African equatorial/low‐latitude stations to these storms were investigated. The responses of scintillations at Dakar and Addis Ababa to the 2011 and 2013 storms were also studied. Six of the eight storms were driven by interplanetary coronal mass ejection (ICME) transients: sheath fields, magnetic clouds, or both. The other two were driven by Co‐rotating interaction region (CIR). In the African equatorial/low‐latitude ionosphere, the TEC was particularly modified by the geomagnetic storms investigated. Our results showed that TEC responses to these storms' initial phases were generally negative. The ICME‐driven storms with daytime main phases caused pole‐ward transport of plasma from the magnetic equator toward the equatorial ionization anomaly (EIA) crests and equator‐ward movement of plasma from the daytime EIA crests’ locations, for those with local nighttime main phases, which we attributed to the effect of prompt penetration electric field diurnal orientations. We observed that all the phases of the two CIR‐driven storms majorly recorded negative ionospheric phases. Overall, the storm‐phase‐related equatorial/low‐latitude ionospheric responses were observed to be local time‐dependent and governed by the geomagnetic storms' interplanetary drivers. Comparatively, the CIR‐driven storms investigated were less geoeffective than the ICME transients‐driven storms investigated.

Driven dust vortex characteristics in plasma with external transverse and weak magnetic field

The two-dimensional hydrodynamic model for bounded dust flow dynamics in plasma is extended for analysis of driven vortex characteristics in presence of external transverse and weak magnetic field (B) in a planner setup and parametric regimes motivated by recent magnetized dusty plasma (MDP) experiments. This analysis has shown that shear in the B can produce a sheared internal field (E a ) in between electrons and ions due to the E × B and ∇B × B -drifts that cause rotation of dust cloud levitated in the plasma. The flow solution demonstrates that neutral pressure decides the dominance between the ions-drag and the E a -force. The shear ions-drag generates an anti-clockwise circular vortical structure, whereas the shear E a -force is very localized and gives rise to a clockwise D-shaped elliptical structure which turns into a meridional structure with decreasing B. Effect of the strength of B, shear mode numbers, and the sheath field are analyzed within the weak MDP regime, showing noticeable changes in the flow structure and its momentum. In the regime of high pressure and lower B, the E a -force becomes comparable or dominant over the ion drag and peculiar counter-rotating vortex pairs are developed in the domain. Further, when the B is flipped by 1800-degree, both the drivers act together and give rise to a single strong meridional structure, showing the importance of B-direction in MDP systems. Similar elliptical/meridional structures reported in several MDP experiments and relevant natural driven-dissipative flow systems are discussed.

Amplification of separated electric charge field due to the capture of laser-produced fast electrons oscillating near thin target

The effect of increasing strength of the electric field of separated charge due to the capture of laser-accelerated fast electrons in a thin solid target is theoretically substantiated. The target considered is so thin that a fast electron passes through the target during the time less than at least half of the applied laser pulse with the additional requirement that energy loss of a fast electron in the balk target is negligible compared to the energy transfer to ions accelerated in a sheath field. An analytical model is proposed of sheath field formation when fast electrons oscillate due to their reflections in the double layers at both boundaries of a thin target. It is shown that the increasing density of fast electrons inside the double layer and shortening of this layer are the reasons of sheath field strength amplification in a thin target in comparison with a thick target. The increase in the number and the energy of fast ions due to field amplification in a thin target is discussed.

Target normal sheath acceleration with a large laser focal diameter

The dependence of the laser-driven ion acceleration from thin titanium foils in the Target Normal Sheath Acceleration (TNSA) regime on target and laser parameters is explored using two dimensional particle-in-cell simulations. The oblique incidence (θL=45°) and large focal spot size (w0=40μm) are chosen to take an advantage of quasi one-dimensional geometry of sheath fields and effective electron heating. This interaction setup also reveals low and achromatic angular divergence of a proton beam. It is shown that the hot electron temperature deviates from the ponderomotive scaling for short laser pulses and small pre-plasmas. This deviation is mainly due to the laser sweeping, as the short duration laser pulse each moment in time effectively heats only a fraction of a focal spot on the foil. This instantaneous partial heating results in an electron temperature deviation from the ponderomotive scaling and, thus, lower maximum proton energies than it could have been expected from the TNSA theory.

Dynamic focusing of laser driven positron jets by self-generated fields

Focusing effect of laser-driven positron jets by self-generated target sheath fields has been observed for the first time experimentally and the results are supported by the computational studies. In the experiment, OMEGA EP short-pulse (0.7 ps, 500 J) irradiates mm-size gold targets with a concave back surface and reference flat-surface targets. Both targets exhibited positrons with quasi-monoenergetic energy peaks while targets with concave curvature also showed increased number of positrons at the detector. The data is consistent with hybrid-PIC simulations confirming that the time-varying electric fields driven by electrons escaping from the target significantly change the trajectories of positrons. These simulations show a small radius of curvature on the rear side increases the relative focusing effect and the positrons to electrons ratio in the escaping plasma. For the smallest radius of curvature, positron jets that are up to 10 times denser can be achieved.

Laser-driven proton acceleration via excitation of surface plasmon polaritons into TiO2 nanotube array targets

In this paper we report the measurement of laser-driven proton acceleration obtained by irradiating nanotube array targets with ultrashort laser pulses at an intensity in excess of 1020 W cm−2. The energetic spectra of forward accelerated protons show a larger flux and a higher proton cutoff energy if compared to flat foils of comparable thickness. Particle-In-Cell 2D simulations reveal that packed nanotube targets favour a better laser-plasma coupling and produce an efficient generation of fast electrons moving through the target. Due to their sub-wavelength size, the propagation of e.m. field into the tubes is made possible by the excitation of Surface Plasmon Polaritons, travelling down to the end of the target and assuring a continuous electron acceleration. The higher amount and energy of these electrons result in turn in a stronger electric sheath field on the rear surface of the target and in a more efficient acceleration of the protons via the target normal sheath acceleration mechanism.

Modified discrete element method (MDEM) as a numerical tool for cement sheath integrity in wells

Cement sheaths undergo extreme loading conditions in wells during subsurface operations. A damaged cement sheath may lead to fluid communication between different formation layers and fluid migration up to the surface, which can cause environmental, technical, and economic problems. In this study, we numerically analyze cement sheath stability using the modified discrete element method (MDEM). MDEM considers the cement sheath and rock formation as porous media and can model discontinuous fracturing of the materials. Analyses are performed based on a small-scale cement sheath integrity test and field cement pressure data measured by Cooke et al. (1983). Oil well class H cement was used, and its poroelastic properties were estimated using a micromechanics model and a multi-scale homogenization technique. First, the evolution of the stress state was approximated in the cement sheath and rock formation. Then, radial fracturing, shear failure, and interface debonding formation were studied under pressure increase/decrease operations. The effects of several parameters, such as the casing size, rock elastic parameters, and loading time, were also investigated. During the hydration of cement, the compressive and shear stresses evolved in the cement sheath, and the stresses in the surrounding formation also changed. The simulation results showed that a decrease in the casing pressure can lead to debonding of the casing-cement interface, and the calculated interface opening was within 1–50 μm. A pressure increase in the casing can lead to progressive shear and tensile failures in the cement sheath. A further pressure increase did not extend those failures into the rock formation; rather, it increased the number of radial fractures in the cement sheath. The analyses showed that a lower decrease and increase in the casing pressure is required to generate debonding and radial fracture in the cement sheath, respectively, in the early stages of hydration after the cement is set. This is also the case when the casing has a larger diameter and smaller thickness. When the cement sheath is bonded to a softer rock formation, the pressure increase required to create a fracture in the cement is lower compared with the case in which the sheath is bonded to a stiffer formation. However, in the case of softer formation, debonding was not observed.

Longitudinal Responses of the Equatorial/Low‐Latitude Ionosphere Over the Oceanic Regions to Geomagnetic Storms of May and September 2017

AbstractThis study presents the longitudinal dependence of responses of the equatorial/low‐latitude ionosphere over the oceanic regions to geomagnetic storms of 28 May and 8 September 2017. We investigated the interplanetary origins of the storms. Total electron content (TEC) data were obtained from Global Navigation Satellite System stations, located around the oceanic areas in the equatorial/low‐latitude regions. The Rate of change of TEC Index (ROTI) was used as a proxy for ionospheric irregularities over the study locations. Further, variations of the horizontal component of the Earth's magnetic fields, obtained from ground‐based magnetometers were studied. We used ionospheric disturbance currents, polar cap and auroral electrojet indices to monitor the storm time electric fields. The May 2017 storm was driven by sheath and magnetic cloud fields, while the September 2017 storm was driven by sheath fields. We observed a comparative dominance of TEC intensities over the Oceans than over the landlocked areas. Empirically, our results validated a theoretical suggestion of the existence of a dynamic ocean‐ionosphere coupling made by Godin et al. (2015, http://10.0.4.162/s40623-015-0212-4). Prompt Penetration Electric Fields (PPEF) was observed to be a key factor that controls TEC responses to storms. PPEFs caused TEC enhancements, mainly over the Pacific Ocean longitudes during the May 2017 storm and enhanced TEC over the Atlantic Ocean and the Pacific Oceans longitudes during the September 2017 storm. These PPEFs triggered irregularities over the Pacific Ocean longitudes, particularly during the main phase of May 2017 storm. Irregularities were generally inhibited by the September 2017 storm.