Kinetic Electrons Research Articles

Large-scale parallel electric fields and return currents in a global simulation model

A new computational model, kglobal, is being developed to explore energetic electron production via magnetic reconnection in macroscale systems. The model is based on the discovery that the production of energetic electrons during reconnection is controlled by Fermi reflection in large-scale magnetic fields and not by parallel electric fields localized in kinetic scale boundary layers. Thus, the model eliminates these boundary layers. However, although the parallel electric fields that develop around the magnetic x-line and associated separatrices are not important in producing energetic electrons, there is a large scale electric field that kickstarts the heating of low-energy electrons and drives the cold-electron return current that accompanies escaping energetic electrons in open systems. This macroscale electric field is produced by magnetic-field-aligned gradients in the electron pressure. We have upgraded kglobal to include this large-scale electric field while maintaining energy conservation. The new model is tested by exploring the dynamics of electron acoustic modes which develop as a consequence of the presence of two electron species: hot kinetic and cold fluid electrons. Remarkably, the damping of electron acoustic modes is accurately captured by kglobal. Additionally, it has been established that kglobal correctly describes the dynamics of the interaction of the parallel electric field with escaping hot electrons through benchmarking simulations with the Particle-In-Cell code p3d.

A comparison of turbulent transport in a quasi-helical and a quasi-axisymmetric stellarator

Ion-temperature-gradient-driven (ITG) turbulence is compared for two quasi-symmetric (QS) stellarator configurations to determine the relationship between linear growth rates and nonlinear heat fluxes. We focus on the quasi-helically symmetric (QHS) stellarator HSX and the quasi-axisymmetric (QAS) stellarator NCSX. In normalized units, HSX exhibits higher growth rates than NCSX, while heat fluxes in gyro-Bohm units are lower in HSX. These results hold for simulations made with both adiabatic and kinetic electrons. The results show that HSX has a larger number of subdominant modes than NCSX and that eigenmodes are more spatially extended in HSX. We conclude that the consideration of nonlinear physics is necessary to accurately assess the heat flux due to ITG turbulence when comparing QS stellarator equilibria.

Gyrokinetic simulations of nonlinear interactions between magnetic islands and microturbulence

A conservative scheme of kinetic electrons for gyrokinetic simulations in the presence of magnetic islands has been implemented and verified in the gyrokinetic toroidal code, where zonal and non-zonal components of all perturbed quantities are solved together. Using this new conservative scheme, linear simulation of kinetic ballooning mode has been successfully benchmarked with the electromagnetic hybrid model. Simulations of nonlinear interactions between magnetic islands and the ion temperature gradient (ITG) mode in a tokamak show that the islands rotate at the electron diamagnetic drift velocity. The linear ITG structure shifts from the island O-point toward the X-point due to the pressure flattening effect inside the islands, and the nonlinear ITG structure peaks along the magnetic island separatrix because of the increased pressure gradient there.

Reversal of turbulent gyroBohm isotope scaling due to nonadiabatic electron drive

The influence of kinetic electrons on the isotope scaling of gyrokinetic turbulent energy flux is assessed. A simple framework is used to study the transition from ion-dominated turbulence regimes to regimes where electron and ion transport levels are comparable. In the ion-dominated regime, the turbulent ion energy flux increases as the ion mass increases, in agreement with simple gyroBohm scaling arguments. Conversely, in the latter regime for which the influence of electrons is significant, a strong reversal of the gyroBohm scaling is observed which cannot be captured by mixing length estimates. In this reversed regime, the turbulent ion energy flux decreases as ion mass increases. The reversal is controlled by the finite electron-to-ion mass-ratio dependence of the nonadiabatic electron response. This mass-ratio dependence is dominated by the parallel motion terms in the electron gyrokinetic equation and provides a correction to the bounce-averaged-electron limit which is independent of the mass ratio. The finite-mass correction is larger for light ions and explains the observed gyroBohm reversal for hydrogen plasmas. An implication is that isotope scaling may not be properly described by simplified fluid or bounce-averaged electron equations.

Influence of atomic kinetics on inverse bremsstrahlung heating and nonlocal thermal transport.

This paper describes a computational model that self-consistently combines physics of kinetic electrons and atomic processes in a single framework. The formulation consists of a kinetic Vlasov-Boltzmann-Fokker-Planck equation for free electrons and a non-Maxwellian collisional-radiative model for atomic state populations. We utilize this model to examine the influence of atomic kinetics on inverse bremsstrahlung (IB) heating and nonlocal thermal transport. We show that atomic kinetics affects nonlinear IB absorption rates by further modifying the electron distribution in addition to laser heating. We also show that accurate modeling of nonlocal heat flow requires a self-consistent treatment of atomic kinetics, because the effective thermal conductivity strongly depends on the ionization balance of the plasma.

The effect of elliptical polarization in MSXα calculations of the molecular-frame photoelectron angular distributions of CO C(1s) ionization

We present a theoretical investigation of the molecular-frame photoelectron angular distribution (MF-PAD) resulting from the inner-shell photoionization of fixed-in-space CO molecules with elliptically polarized light. The MSXα output program, originally written for linearly polarized light, was changed in order to calculate the MF-PAD for circularly and elliptically polarized light. The calculations were made for CO C(1s) inner-shell photoionization for 3, 10, and 30 eV kinetic energy electrons. The MF-PADs were calculated from 0° angle of ellipticity (equivalent to linear polarized light parallel to the molecular axis) in steps of 5° (corresponding to different types of elliptical light) up to 90° (equivalent to linear polarized light perpendicular to the molecular axis).

Global gyrokinetic simulation of microturbulence with kinetic electrons in the presence of magnetic island in tokamak

An electrostatic model has been formulated and implemented in the gyrokinetic toroidal code to study the nonlinear ion temperature gradient (ITG) turbulence in the presence of an n = 1, m = 2 magnetic island. The ions are described by the gyrokinetic equation while the electrons are treated with the drift-kinetic equation. In our simulation, an n = 1, m = 2 electrostatic mode is formed with the same vortex structure of the magnetic island. When the magnetic island flattening effect is turned on, the island vortex mode is well preserved and couples to the n = 0, m = 0 geodesic acoustic mode. Simulation shows that the magnetic island can suppress the ITG turbulence at the island O-point and strengthen it near the X-point. We show that the vortex mode can generate a substantial helical shear flow around the island. We also find that the turbulence and transport are suppressed inside the island and enhanced at the island X-point.

The Dominant Role of Energetic Ions in Solar Wind Interaction With the Moon

AbstractWe use the results of 3‐D electromagnetic hybrid (kinetic ions and fluid electrons) simulations and observations by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) /Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft to investigate the nature of the solar wind interaction with the moon and the resulting wake and tail structure. In the absence of a global magnetic field and an atmosphere at the moon, this interaction is due to the absorption of the solar wind plasma on the dayside and the means by which the resulting cavity in the tail is refilled. To explore this process, we conducted simulations of specific case studies well‐monitored by the ARTEMIS spacecraft. Preliminary runs revealed that when the solar wind core plasma is driving the interaction, the results deviate significantly from observations. When the small population of solar wind energetic ions is incorporated in the model, the observations are reproduced much better. Simulations with different density or energy of energetic ions show considerable changes in the results indicating their dominance in establishing the electromagnetic properties of the tail. In contrast, the results show less sensitivity to the solar wind flow speed. Here we conclude that the structure of the interaction region consists of compressional and rarefaction wakes and a central tail region with areas of enhanced and possibly reduced magnetic field level. The compressional wake diverts the flow away from the tail, and its presence is found to be highly time dependent while the rarefaction wake diverts the flow toward the tail and is found to be a semipermanent feature of the lunar interaction region.

Petascale particle-in-cell simulations of kinetic effects in inertial fusion energy plasmas

In this manuscript we describe new features in OSIRIS which allows us to simulate laser plasma interactions relevant to inertial fusion in 2D and quasi-3D. These features will allow for the simulation of laser plasma interactions with ever increasing realism and will finally allow for the quantitative comparison between large 2D/3D simulations and ongoing inertial fusion energy experiments with both kinetic electrons and ions in the near future.

SOLPS simulations with electron kinetic effects

Power exhaust is one of the critical issues for tokamak edge plasma research. Electron kinetic effects may play an important role in future fusion devices. The Kinetic Code for Plasma Periphery (KIPP) code was coupled to a 1D version of SOLPS with an iterative algorithm (Schneider R et al 2006 Contrib. Plasma Phys. 46 3; Braginskii S I 1965 Transport processes in a plasma Reviews of Plasma Physics vol 1 (New York: Consultants Bureau) p 205) to study the kinetic effects in a systematic way. The KIPP–SOLPS coupling algorithm allows us to incorporate kinetic electrons into the already sophisticated fluid model (B2) self-consistently. The KIPP–SOLPS coupling simulation results with pure deuterium and with carbon impurity in 1D geometry with stagnation point upstream and target downstream are presented. These results are then compared to the results of SOLPS simulations with different flux limiters. It shows that non-local electron parallel transport contributes to the non-Maxwellian tails of electrons near the target which reduces the electron target temperature. However, the non-Maxwellian tail has negligible impacts on deuterium ionization.

Gyrokinetic microinstability analysis of high-Ti and high-Te isotope plasmas in Large Helical Device

Transport and confinement characteristics, and microinstabilities for high- and high- isotope plasmas in Large Helical Device are explored by using the gyrokinetic Vlasov simulation GKV with hydrogen isotope ions and real-mass kinetic electrons. The experimental data indicate that the thermal diffusivity is reduced in the deuterium-dominated plasmas, where the deviation from the gyro-Bohm scaling in the overall tendency and the strong anomaly of the electron heat transport are identified. Linear gyrokinetic analyses identify that the growth rates of the ion temperature gradient and trapped electron mode (TEM) instabilities in the deuterium plasmas are reduced due to the change in the profile gradients and/or the isotope ion mass, and the radial dependence of the mixing-length diffusivity is qualitatively consistent with the experimental tendency. Also, TEM-like turbulent fluctuations are examined by the using the phase contrast imaging measurement and the linear gyrokinetic calculation for the high- deuterium plasma.

Contribution of kinetic electrons to GAM damping

We quantify the contribution of kinetic electrons to GAM damping. This contribution is not negligible due to a resonance between barely trapped/passing electrons bounce/transit frequency and the mode pulsation.

Neoclassical and turbulent E × B flows in flux-driven gyrokinetic simulations of Ohmic tokamak plasmas

The interplay of flows and turbulence in Ohmic FT-2 tokamak plasmas is analysed via gyrokinetic simulations with the flux-driven ELMFIRE code. The simulation predictions agree qualitatively with analytical estimates for the scaling of the neoclassical radial electric field as a function of collisionality for ad hoc parameters. For the experimental parameters, the global full-f modeling agrees well with the analytical estimates in a neoclassical setting, while including kinetic electrons and impurities has a small impact. Allowing turbulence to develop modifies the flow profile through relaxation of profiles caused by turbulent transport, non-adiabatic response of passing electrons around rational surfaces, and turbulent flow drive. Geodesic acoustic mode (GAM) is the main zonal flow component in the simulations, and its frequency and amplitude agree with theoretical predictions and experimental measurements. In the simulations, the non-linear energy transfer from the turbulence to the flows through the Reynolds force is balanced by the collisional flow dissipation. Temporal relationship between the oscillating flow, Reynolds force, and turbulent particle flux is consistent with the fundamental physics picture of GAM modulating turbulent transport on the time scale of the mode. Experimental evidence also suggests anti-correlation of GAM amplitude and turbulent fluctuations.

A fully implicit, conservative, non-linear, electromagnetic hybrid particle-ion/fluid-electron algorithm

The quasi-neutral hybrid model with kinetic ions and fluid electrons is a promising approach for bridging the inherent multi-scale nature of many problems in space and laboratory plasmas. Here, a novel, implicit, particle-in-cell based scheme for the hybrid model is derived for fully 3D electromagnetic problems with multiple ion species, which features global mass, momentum and energy conservation. The scheme includes sub-cycling and orbit-averaging for the ions, with cell-centered finite differences and implicit midpoint time advance. To reduce discrete particle noise, the scheme allows arbitrary-order shape functions for the particle-mesh interpolations and the application of conservative binomial smoothing. The algorithm is verified for a number of test problems to demonstrate the correctness of the implementation, the unique conservation properties, and the favorable stability properties of the new scheme. In particular, there is no indication of unstable growth of the finite-grid instability for a population of cold ions drifting through a uniform spatial mesh, in a set-up where several commonly used non-conservative schemes are highly unstable.

Oscillatory relaxation of zonal flows in a multi-species stellarator plasma

The low frequency oscillatory relaxation of zonal potential perturbations is studied numerically in the TJ-II stellarator (where it was experimentally detected recently for the first time in Alonso et al 2017 Phys. Rev. Lett. 118 185002). It is studied in full global gyrokinetic simulations of multi-species plasmas. The oscillation frequency obtained is compared with predictions based on single-species simulations using simplified analytical relations. It is shown that the frequency of this oscillation for a multi-species plasma can be accurately obtained from single-species calculations using extrapolation formulas. The damping of the oscillation and the influence of the different inter-species collisions is studied in detail. It is concluded that taking into account multiple kinetic ions and electrons with impurity concentrations realistic for TJ-II plasmas allows to account for the values of frequency and damping rate in zonal flows relaxations observed experimentally.

The occurrence of staircases in ITG turbulence with kinetic electrons and the zonal flow drive through self-interaction

Large scale structures in the E × B shearing rate, known as staircases, are shown to form in nonlinear gyro-kinetic turbulence simulations with kinetic electrons. However, in many cases, a small scale structure in the shearing rate is observed that appears to prevent the formation of staircases. The small scale structures are interpreted to be linked to the self-interaction of turbulent modes connected with the double periodic boundary conditions on the torus. The self-interaction is a newly discovered mechanism for zonal flow generation and is shown to scale proportional to the normalized Larmor radius. The mechanism is also affected by magnetic shear, being weaker at larger values.

Mechanistic Study of HafSOx Extreme Ultraviolet Inorganic Resists

Inorganic resists are promising for nanomanufacturing because of their potential for high-resolution and low line-edge roughness patterning and exceptional sensitivity to extreme ultraviolet (EUV) radiation. Hafnium oxide peroxide hydroxide sulfate (HafSOx) is a model EUV inorganic photoresist, wherein the EUV absorption coefficients for hafnium and O are much higher than for conventional polymer resists. Absorption of EUV radiation leads to electron emission that results in the HafSOx solubility change. We used desorption-based techniques to elucidate thermal- and radiation-induced processes that contribute to the HafSOx solubility switch. We found that low kinetic energy electrons render HafSOx insoluble and result in the desorption of molecular O2. Electron-stimulated desorption and postexposure Raman spectroscopy indicate similar kinetics for peroxide loss in HafSOx. Temperature-programmed desorption studies found that peroxide ligand desorption is best characterized as first order, with a broad distr...

Phase-dependent laser acceleration of electrons with symmetrically driven silicon dual pillar gratings.

We present the demonstration of phase-dependent laser acceleration and deflection of electrons using a symmetrically driven silicon dual pillar grating structure. We show that exciting an evanescent inverse Smith-Purcell mode on each side of a dual pillar grating can produce hyperbolic cosine acceleration and hyperbolic sine deflection modes, depending on the relative excitation phase of each side. Our devices accelerate sub-relativistic 99.0keV kinetic energy electrons by 3.0keV over a 15μm distance with accelerating gradients of 200MeV/m with 40nJ, 300fs, 1940nm pulses from an optical parametric amplifier. These results represent a significant step towards making practical dielectric laser accelerators for ultrafast, medical, and high-energy applications.

Global gyrokinetic simulations of intrinsic rotation in ASDEX Upgrade Ohmic L-mode plasmas

Non-linear, radially global, turbulence simulations of ASDEX Upgrade (AUG) plasmas are performed and the nonlinear generated intrinsic flow shows agreement with the intrinsic flow gradients measured in the core of Ohmic L-mode plasmas at nominal parameters. Simulations utilising the kinetic electron model show hollow intrinsic flow profiles as seen in a predominant number of experiments performed at similar plasma parameters. In addition, significantly larger flow gradients are seen than in a previous flux-tube analysis (Hornsby et al 2017 Nucl. Fusion 57 046008). Adiabatic electron model simulations can show a flow profile with opposing sign in the gradient with respect to a kinetic electron simulation, implying a reversal in the sign of the residual stress due to kinetic electrons. The shaping of the intrinsic flow is strongly determined by the density gradient profile. The sensitivity of the residual stress to variations in density profile curvature is calculated and seen to be significantly stronger than to neoclassical flows (Hornsby et al 2017 Nucl. Fusion 57 046008). This variation is strong enough on its own to explain the large variations in the intrinsic flow gradients seen in some AUG experiments. Analysis of the symmetry breaking properties of the turbulence shows that profile shearing is the dominant mechanism in producing a finite parallel wave-number, with turbulence gradient effects contributing a smaller portion of the parallel wave-vector.

A conservative scheme for electromagnetic simulation of magnetized plasmas with kinetic electrons

A conservative scheme has been formulated and verified for gyrokinetic particle simulations of electromagnetic waves and instabilities in magnetized plasmas. An electron continuity equation derived from the drift kinetic equation is used to time advance the electron density perturbation by using the perturbed mechanical flow calculated from the parallel vector potential, and the parallel vector potential is solved by using the perturbed canonical flow from the perturbed distribution function. In gyrokinetic particle simulations using this new scheme, the shear Alfvén wave dispersion relation in the shearless slab and continuum damping in the sheared cylinder have been recovered. The new scheme overcomes the stringent requirement in the conventional perturbative simulation method that perpendicular grid size needs to be as small as electron collisionless skin depth even for the long wavelength Alfvén waves. The new scheme also avoids the problem in the conventional method that an unphysically large parallel electric field arises due to the inconsistency between electrostatic potential calculated from the perturbed density and vector potential calculated from the perturbed canonical flow. Finally, the gyrokinetic particle simulations of the Alfvén waves in sheared cylinder have superior numerical properties compared with the fluid simulations, which suffer from numerical difficulties associated with singular mode structures.