Increase In Electron Temperature Research Articles

Simulation of the Ohkawa-mechanism- dominated current drive of electron cyclotron waves using linear and quasi-linear models

Simulation on the Ohkawa-mechanism-dominated current drive (OKCD) of electron cyclotron (EC) waves is performed using TORAY-GA linear code, and the results are compared with those calculated by CQL3D quasi-linear code. It is found that the radial location of the OKCD profile is almost identical between the linear and the quasi-linear calculations. However, there are significant differences in the calculation of the total driven current and the peak value of the driven current profile between the two models. The calculated by the CQL3D code is at least 1.4 times larger than the results from the TORAY-GA code. For the calculation of , the results from CQL3D are at least 1.6 times larger than that calculated by TORAY-GA. With increasing electron temperature, the two models further enlarge the total driven current scaling factor and the peak driven current density scaling factor . This is mainly because the collision operator in TORAY-GA code adopts a high-speed model and does not retain the first-order Legendre expansion term for momentum conservation of electron self-collision. The quasi-linear effect does not have a significant influence on the total driven current of OKCD when the EC power level does not meet > 0.5. Therefore, in practical engineering, the TORAY-GA code can be used to calculate OKCD quickly and accurately by multiplying with appropriate scaling factors. The effect of momentum conservation is very important for OKCD and on-axis EC current drive (ECCD), but this effect is not important for off-axis ECCD. The results from this study show that the effects of electron trapping and the collision between resonant passing electrons and trapped electrons are responsible for the decrease in off-axis ECCD efficiency.

Effect of electron temperature in a magnetized plasma sheath using kinetic trajectory simulation

The understanding of the properties of magnetized plasma sheath has been various beneficial applications in surface treatment, electron emission gun, ion implantation, and nuclear fusion, etc. The effect of electron temperature on the magnetized plasma sheath has been studied for a fixed magnetic field and ion temperature. It has been observed that various plasma sheath parameters can be prominently altered by the varying temperature of the electron. The density of ion is influenced more by the change in electron temperature rather than the electron density. The temperature of the electron has a great effect at the wall, when electron temperature increases, the ion and electron densities at the wall decreases. This shows the potential at the wall also decreases follows the Poisson’s equation. Similarly, the electric field also decreases but total charge density increases when the electron temperature is increased.
 BIBECHANA 18 (2021) 58-66

Analysis of the Intensity Range for Self-Focusing of Bessel–Gauss Laser Beams in Plasma

We study the propagation dynamics of zeroth-order Bessel–Gauss laser beams in plasma in the relativistic ponderomotive regime of interaction. Based on the paraxial and Wentzel-Kramers-Brillouin approximations, we derive the nonlinear evolution equation governing the laser beam width parameter, in view of the parabolic equation approach. We predict the range of intensity for self-focusing of Bessel–Gauss laser beams with increase in plasma electron temperature. While analyzing the condition for self-trapping of Bessel–Gauss beams, we point out and discuss three regimes characterized by steady divergence, self-focusing, and oscillatory divergence of the laser-beam propagation in plasma. In particular, we consider the effect of the transverse component of the wave parameter in exploring the intensity range for self-focusing. The intensity range for self-focusing of Gaussian beam in the considered regime of interaction with plasma is also deduced as a particular case in our work.

The role of collisionless trapped electron mode turbulence on removal of helium ash and transport of deuterium-tritium ions

Removal of helium ash and the anomalous transport of deuterium (D) and tritium (T) ions driven by collisionless trapped electron mode (CTEM) turbulence in tokamak plasmas with weak magnetic shear are studied. We derive the eigenvalue of CTEM with helium ash, and calculate the quasi-linear turbulent fluxes of helium ash, D and T ions simultaneously. Based on the analytical results, the parametric dependence of CTEM instability as well as the anomalous transport of helium ash and D-T ions is investigated, in order to explore the parameter region that is favorable for expelling more helium ash than D and T ions. It is found that helium ash with higher temperature and steeper density profile plays a role of destabilizing CTEM instability, and has higher transport level than that of T ions. We also find that increasing electron temperature and flattening electron density profile are favorable for exhausting helium ash. Isotopic effects (i.e. increasing the fraction of T ions) enhance the transport of both helium ash and D-T ions. Moreover, the trend of stronger transport level of helium ash than that of D-T ions is enhanced by raising electron temperature and flattening electron density profile as well as isotopic effects. Besides, the diffusivity is much larger than the convection. This indicates that the CTEM turbulence driven helium ash transport is favorable for removing helium ash under the parameter region used in the present paper. The possible relevance of our theoretical results to experimental observations is also discussed.

Diagnostic of Ar-CO2 mixture plasma using a fine-structure resolved collisional radiative model

In the present work, we develop a collisional radiative (CR) model for the Ar-CO2 mixture plasma. The model is applied to the diagnostics of a recently reported low-pressure DC generated Ar-CO2 plasma of Rodríguez et al. [Phys. Plasmas 25, 053512 (2018)] utilizing their spectroscopic measurements who reported the plasma parameters at different concentrations of CO2 in Ar at various pressures and powers. Our CR model includes various population transfer mechanisms between the fine structure levels, viz., electron-impact excitation, radiative decay, and ionization, as well as their reverse processes, e.g., electron-impact de-excitation as well as two- and three-body recombination. The effects of radiation trapping and diffusion are also taken into account. The plasma parameters viz. electron density (ne) and electron temperature (Te) are obtained as a function of different pressures (0.2, 0.3, and 0.6 mbars) and discharge powers at 25 and 50% concentrations of CO2 in argon. These results are determined using measured intensities of seven intense emission lines out of 3p54p (2pi) → 3p54s (1si) fine-structure transitions. It is observed that both the electron density and electron temperature increase with the increase of CO2 concentration, which is in confirmation with experimental predictions. The populations obtained for 1si and 2pi levels from our CR model are also reported and compared respectively with the corresponding available values from the simple CR model and experiment of Rodríguez et al. [Phys. Plasmas 25, 053512 (2018)]. A significant difference in the populations is observed from the two models.

Driving frequency effect on discharge parameters and higher harmonic generation in capacitive discharges at constant power densities

Very high frequency (VHF) driven capacitive discharges are now being increasingly adopted for plasma-based materials processing due to their high processing rates and lower substrate damage. Past studies related to complex plasma dynamics and higher harmonics generation in such systems were limited to constant voltage/current conditions, whereas, industrial systems are mostly driven by constant power density sources. In the present study, using particle-in-cell (PIC) simulation, we explore the dynamics of collisionless symmetric capacitive discharges that is operated at constant power densities. Our focus is on the effect of the driving frequency on the discharge parameters like the electron density/temperature, the electron energy distribution function (EEDF), the ion energy distribution function (IEDF), and the generation of higher harmonics in the device. The simulations are performed for a driving frequency from 27.12 to 100 MHz in argon plasma at a gas pressure of 1 Pa and for two values of the power density, namely, 2 kW m−3 and 20 kW m−3. It is observed that the required discharge voltage for maintaining constant power density decreases and discharge current increases with an increase in the driving frequency. A transition frequency is observed at both power densities. The density decreases (electron temperature increases) before the transition frequency and the trend is reversed after crossing the transition frequency. The EEDF shows an enhancement in the population of the mid-energy range of electrons as the driving frequency increases up to the transition frequency thereby changing the shape of EEDF from bi-Maxwellian to nearly Maxwellian, and then transforms into a nearly bi-Maxwellian at higher driving frequencies. The IEDF at the electrode surface shows bimodal behaviour at a lower driving frequency, becoming more pronounced at a power density of 20 kW m−3, and then turning into a single energy peak. The corresponding maximum ion energy is found to decrease with driving frequency.

Absorption of a nanosecond laser pulse by a picosecond laser-induced preformed aluminum plasma

The LIBS (Laser-Induced Breakdown Spectroscopy) method has already demonstrated its reliability and its robustness in many situations for the multi-elemental composition determination of samples. However, certain conditions prevent a totally satisfactory determination. For instance, the method is weakly efficient to measure with accuracy the light elements concentration in metallic matrices. Since the laser pulse used to produce the plasma contributes to its heating, using an additional pulse (double pulse configuration) provides the increase in electron temperature and density without additional ablation. A better signal-to-noise ratio and a lower limit of detection can be reached. The present paper reports the results of different experiments performed to quantify the modifications induced (1) on the electron density by the second laser pulse in a preformed aluminum plasma, and (2) on the second laser pulse itself. The related experiments have been done in the case where the plasma is produced by a picosecond laser pulse and the second laser pulse is of the nanosecond type. The electron density reaches a maximum resulting from the total ionization of the aluminum plasma volume irradiated by the second laser pulse.

Dust charging and levitating in a magnetized plasma sheath containing superextensive electrons

A one-dimensional hydrodynamic model is developed for a magnetized plasma sheath, consisting of Maxwell electrons, q-non-extensive electrons, and thermal ions. A significant change is observed in the quantities characterizing isolated dust particles in the presence of superextensive electrons (q < 1). An examination is conducted on the effects of the q parameter, concentration, and temperature of superextensive electrons on the critical Bohm velocity, sheath potential drop, dust charging, and levitation in the sheath. As q decreases and as the superextensive electron concentration and temperature increase, the critical Bohm velocity, sheath potential drop, dust–charge number, and size of levitating dust particles increase. In addition, the dust particles mainly levitate near the sheath edge, and their size increases gradually from the sheath edge to the sheath space.

Investigation of effect of electron temperature on intensity and efficiency of terahertz generated by laser beating in inhomogeneous plasma

We report terahertz (THz) wave generation based on mixing of two radially polarized hollow Gaussian laser beams in periodic density plasma. We incorporate the effect of plasma electron temperature on THz intensity and efficiency. Our investigations reveal that emitted THz is radially polarized. Brightness and efficiency of THz significantly increase with plasma electron temperature. Our scheme predicts upto 2.4-fold enhancement in THz conversion efficiency with increase in plasma electron temperature from 0–10 KeV. We observe that effect of plasma electron temperature is more prominent when difference of laser frequency matches with plasma frequency i.e. The profile of THz depends on the laser parameters and it is independent of plasma electron temperature. A high THz field (∼10 MV cm−1) and high conversion efficiency (∼6%) is predicted for optimized pump laser and plasma parameters.

Plasma and Field Observations in the Magnetospheric Source Region of a Stable Auroral Red (SAR) Arc by the Arase Satellite on 28 March 2017

AbstractA stable auroral red (SAR) arc is an aurora with a dominant 630 nm emission at subauroral latitudes. SAR arcs have been considered to occur due to the spatial overlap between the plasmasphere and the ring‐current ions. In the overlap region, plasmaspheric electrons are heated by ring‐current ions or plasma waves, and their energy is then transferred down to the ionosphere where it causes oxygen red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetosphere associated with SAR arc. In this paper, we report the first quantitative evaluation of conjugate measurements of a SAR arc observed at 2204 UT on 28 March 2017 and investigate its source region using an all‐sky imager at Nyrölä (magnetic latitude: 59.4°N), Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring‐current ions and that electromagnetic ion cyclotron waves and kinetic Alfven waves were not observed above the SAR arc. The SAR arc was located at the ionospheric trough minimum identified from a total electron content map obtained by the GNSS receiver network. The Swarm satellite flying in the ionosphere also passed the SAR arc at ~2320 UT and observed a decrease in electron density and an increase in electron temperature during the SAR‐arc crossing. These observations suggest that the heating of plasmaspheric electrons via Coulomb collision with ring‐current ions is the most plausible mechanism for the SAR‐arc generation.

Interaction of an atmospheric pressure plasma jet with grounded and floating metallic targets: simulations and experiments

The interaction of kHz μs-pulsed atmospheric pressure He jets with metallic targets is studied through simulations and experiments, focusing on the differences between floating and grounded targets. It is shown that the electric potential of the floating target is close to grounded in the instants after the impact of the discharge, but rises to a high voltage, potentially more than half of the applied voltage, at the end of the 1 μs pulse. As a result, a return stroke takes place after the discharge impact with both grounded and floating targets, as a redistribution between the high voltage electrode and the low voltage target. Electric field, electron temperature and electron density in the plasma plume are higher during the pulse with grounded target than with floating target, as gradients of electric potential progressively dissipate in the latter case. Finally, at the fall of the pulse, another electrical redistribution takes place, with higher intensity with the highly-charged floating target than with the grounded target. It is shown that this phenomenon can lead to an increase in electric field, electron temperature and electron density in the plume with floating target.

Lanthanum Bromide (LaBr3(Ce)) based hard X-ray spectroscopic diagnostic for the study of runaway electrons at Aditya Tokamak

Study of hard X-ray (HX) spectrum is an important tool to understand the runaway electrons (RE) behavior present in the tokamak plasma as these energetic electrons, especially those generated during disruption, represent a major threat to tokamak plasma devices in terms of melting and damaging the plasma-facing component (PFC) and thereby reduced the lifetime of the first wall. A LaBr3(Ce) detector based spectroscopic diagnostic has been set up on the Aditya tokamak to measure the HX spectrum. This diagnostic consists of a photomultiplier tube and multi-channel analyzer along with the detector, viewing the whole plasma tangentially along a line of sight terminating on the limiter. The spectrum has been observed in the energy range of around ∼ 75 keV to 3 MeV and is continuum in nature. The spectrum is having the peaks within ∼ 85–150 keV and the estimated RE temperatures have been found to be in the range of 100–600 keV for the analyzed discharges. This temperature decreases with the increase of the thermal electron temperature of the discharges. The ratios of the estimated confinement time of runaway electron and energy confinement time of thermal particles are mostly inversely proportional to the plasma electron density.

Observations of Magnetic Field Line Curvature and Its Role in the Space Plasma Turbulence

Abstract Recent numerical simulations of plasma turbulence showed that magnetic field line curvature plays a key role in particle energization. Based on in situ high-resolution data provided by the four Magnetospheric Multiscale spacecraft, we investigate the magnetic field line curvature and its role in the turbulent magnetosheath plasma. Our analysis reveals that the curvature exhibits two power-law distributions: the low curvature follows the scaling as κ 0.33, and the large curvature has a scaling as κ −2.16. The curvature is anticorrelated with the magnitude of the magnetic field, but positively related to the normal force, the drift electric current, and the curvature drift acceleration term, indicating that intense energy dissipation due to the curvature drift occurs in the large curvature region. One typical example shows a localized increase of electron temperature that coincides with a peak in the curvature and the curvature drift acceleration term, which supports the role of the latter in local energization of electrons, in agreement with simulation results. These observations allow us to better understand the connection between magnetic field line curvature, energy dissipation, and particle energization in space and astrophysical plasmas.

Morphology transition from diffuse to diffuse-and-filamentary for an argon plume with varying sinusoidal frequency or voltage amplitude

With increasing sinusoidal frequency or its voltage amplitude, a central filament emerges in a diffuse plume downstream of an argon jet with a single-electrode geometry. The plume with a co-existing filament and diffuse part, also referred to as a diffuse-and-filamentary (DAF) plume, is formed after a sudden length increment from a diffuse plume. Only one negative discharge appears per voltage cycle for the diffuse plume. Besides the negative discharge, there is one positive discharge per voltage cycle for the DAF plume. By fast photography, negative discharge behaves as plump plasma bullets propagating along the argon flow, which results in diffuse plume or the diffuse part of the DAF plume. Comparatively, positive discharge behaves as slim bullets, leading to the filament formation. Based on optical emission spectroscopy, it is found that both electron density and electron temperature increase with increasing sinusoidal frequency or voltage amplitude for negative and positive discharges. Moreover, there are abrupt increments in them for negative discharge during the morphology transition. Compared with negative discharge, the positive one has higher electron density and electron temperature. All of the experimental phenomena mentioned above are qualitatively explained through considering the discharge enhancement by residual active species.

The role of fast electron energy transfer in the problem of shock ignition of laser thermonuclear target

The role of energy transfer by fast electrons, which is responsible for the positive effect of increasing the ablative pressure and the negative effect of preheating, on the implosion and thermonuclear gain of the target designed for shock ignition is investigated in comparison with the target designed for traditional spark ignition. On the base of one-dimensional hydrodynamic simulations with kinetic description of fast electron transfer it is shown, that depending on the characteristics of fast electron flux, in shock ignition target are manifested to varying degree both effects the positive and negative ones. This is a distinguishing feature of a shock ignition target compared to a traditional spark ignition target, in which only the negative effect of fast electron energy transfer takes place. In a shock ignition target up to temperatures of 50–60 keV, the positive effect is dominant and provides high gain. With an increase in the fast electron temperature, the role of the negative preheating effect increases, that ultimately leads to the ignition failure as temperature exceeding 90–100 keV.

Development of the MiXI Thruster with the ARCH Discharge

The Miniature Xenon Ion (MiXI) thruster with the Axial Ring-Cusp Hybrid ‘MiXI(ARCH)’ discharge was developed and operated with beam extraction at 1 kV. The thruster achieved 59% cathode-free total discharge efficiency at 23.7 mA xenon beam current with filament cathodes and low temperature operation, corresponding to a discharge loss of 226 W/A and propellant utilization of 72%. Thruster efficiency was observed to increase with increasing flow rate and decrease with increasing temperature up to thermal steady state. At thermal steady state, the thruster anode reached ∼320 °C due to the thermal isolation of the thruster head. Reducing the discharge chamber aspect ratio from 0.5 to 0.4 increased thermal steady state efficiency from 46% to 57% but required slow ramping of beam voltage and was limited to stable operation to above 0.5 sccm discharge propellant flow. In contrast to the 3-ring cusp configuration, MiXI(3-Ring), the performance is generally higher but is not able to achieve lower thrust levels and requires more complex start-up for stable operation. An analytical single-cell model was developed and applied to investigate internal processes of the MiXI(ARCH) discharge. The model emulated the effect of increasing flow on performance, indicating that the dominant loss mechanism is plasma electron current to the anode, in contrast to the 3-Ring geometry, which is dominated by primary electron losses. This model also matched trends reported in previous works of strongly increasing electron temperature and primary density with propellant utilization. Through this effort, the MiXI thruster’s highest achievable total efficiency has been increased, and several mechanisms for further improved efficiency have been identified.

Simulation of Discharge Characteristics for the Plasma Etching of Large Area SiO2 Substrates

The electron density distribution and average electron temperature distribution in reactive ion etching (RIE) chamber are of great significance for the ionization and excitation reaction rate. The uniformity of radial distributions of the electron density and average electron temperature in the discharge chamber greatly affects the uniformity of the etching, especially the plasma etching of large area SiO2 substrates. We study the effects of discharge conditions (including power and pressure) on the electron density and average electron temperature of plasma-etched 400 mm SiO2 substrates in the reactive ion etching chamber; the simulation results show that the allowable major discharge conditions strongly affect the characteristics of plasma in the large area reactive ion etching chamber. Specifically, with increase in the power both the electron density and average electron temperature increase, being accompanied by deteriorating uniformity of their radial distributions. As the pressure increases, the electron density increases but average electron temperature decreases, being accompanied by deteriorating uniformity of their radial distributions. These methods and conclusions can provide reference for the improvement of cavity structure and large area RIE equipment designing and selection of the process parameters.

Magnetospheric Multiscale (MMS) Observations of Magnetic Reconnection in Foreshock Transients

AbstractMagnetic reconnection is a fundamental process of energy conversion in plasmas between electromagnetic fields and particles. Magnetic reconnection has been observed directly in a variety of plasmas in the solar wind and Earth's magnetosphere. Most recently, electron magnetic reconnection without ion coupling was observed for the first time in the turbulent magnetosheath and within the transition region of Earth's bow shock. In the ion foreshock upstream of Earth's bow shock, there may also be magnetic reconnection especially around foreshock transients that are very turbulent and dynamic. With observations from the National Aeronautics and Space Administration's Magnetospheric Multiscale mission inside foreshock transients, we report two events of magnetic reconnection with and without a strong guide field, respectively. In both events, a super‐ion‐Alfvénic electron jet was observed within a current sheet with thickness less than or comparable to one ion inertial length. In both events, energy was converted from the magnetic field to electrons, manifested as an increase in electron temperature. Weak or no ion coupling was observed in either event. Results from particle‐in‐cell simulations of magnetic reconnection with and without a strong guide field are qualitatively consistent with observations. Our results imply that magnetic reconnection is another electron acceleration/heating process inside foreshock transients and could play an important role in shock dynamics.

Direct Observations of a Polar Cap Patch Formation Associated With Dayside Reconnection Driven Fast Flow

AbstractDayside solar‐produced concentrated F region plasma can be transported from the midlatitude region into the polar cap during geomagnetically disturbed period, creating plasma density irregularities like polar cap patches, which can cause scintillation and degrade performance of satellite communication and navigation at polar latitudes. In this paper, we observed and investigated a dynamic formation process of a polar cap patch during the 13 October 2016 intense geomagnetic storm. During the storm main phase, storm‐enhanced density (SED) was formed within an extended period of strong southward interplanetary magnetic field (IMF) Bz condition. Total electron content (TEC) map shows that a polar cap patch was segmented from the SED plume. The Sondrestrom Incoherent Scatter Radar (ISR) was right underneath the segmentation region and captured the dynamic process. It shows that the patch segmentation was related with a sudden northeastward flow enhancement reaching ~2 km/s near the dayside cusp inflow region. The flow surge was observed along with abrupt E region electron temperature increase, F region ion temperature increase, and density decrease. The upstream solar wind and IMF observations suggest that the flow enhancement was associated with dayside magnetic reconnection triggered by a sudden and short period of IMF By negative excursion. Quantitative estimation suggests that plasma density loss due to enhanced frictional heating was insufficient for the patch segmentation because the elevated F region density peaking at ~500 km made dissociative recombination inefficient. Instead, the patch was segmented from the SED by low‐density plasma transported by the fast flow channel from earlier local time.

Numerical study on the influence of electron temperature on void formation in dusty plasma

In the past, experiments have proven that the electron temperature in the void is lower than that in the dust cloud. Based on this point, the influence of the spatial distribution of the electron temperature on the void is studied by calculating the fluid equation with explicit ionization, and the images of the circular void and the ring void are obtained by simulation. It is found that when the electron temperature is greater than the threshold value, a stable circular void is generated, and as the electron temperature increases, the time required for the circular void to reach a steady state decreases and its size decreases. This law is not affected by electron temperature spatial distribution set in the center. Combined with the studies of circular void, we speculate that the electron temperature in the ring void is also lower than that in the dust cloud. Through the assumption of the spatial distribution of electron temperature, the simulation results in a ring-shaped void. And the influence of the intensity of the spatial variation of the electron temperature on the ring void is verified. The results show that when the electron temperature change is severe enough, a ring void is generated. And as the intensity increases, the time required for the ring void to reach a steady state decreases, and the size increases slightly.