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We explore a novel simulation route for Plasma Wakefield Acceleration (PWFA) by using the computational method known as the Lattice Boltzmann Method (LBM). LBM is based on a discretization of the continuum kinetic theory while assuring the convergence toward hydrodynamics for coarse-grained fields (i.e., density, velocity, etc.). LBM is an established numerical analysis tool in computational fluid dynamics, able to efficiently bridge between kinetic theory and hydrodynamics, but its application in the context of PWFA has never been investigated so far. This paper takes a step forward to fill this gap. Results of LBM simulations for PWFA are discussed and compared with those of a code (Architect) implementing a Cold Fluid (CF) model for the plasma. In the hydrodynamic framework, we discuss the importance of regularization effects related to diffusion properties intrinsic of the LBM, allowing to go beyond the CF approximations. Issues on computational efficiency are also addressed. [ABSTRACT FROM AUTHOR]

Air gap defects in solid insulation of power equipment can cause partial discharge. Temperature, as an important characterization of partial discharge, can be used as a criterion for the deterioration degree of air gap defects to some extent. In this paper, the temperature characterization of air gap discharge is studied. A plasma-kinetics-based dielectric barrier discharge model is constructed. The calculation method of air gap discharge heat source under power frequency voltage is presented. Moreover, the influence of voltage amplitude and gap thickness on temperature is studied. Obtained results indicate that under the power frequency voltage, Joule (Ohmic) heat contributes 90% to the heat source, and O4+ and O2+ and electron e are the main contributors to Joule (Ohmic) heat. O4+ is also the dominant ion in the mixed gas. As the discharge progressed, O4+ almost all clustered near the instantaneous cathode. The O2+ density generally depends on the electron density, but compared with the electron, the spatial distribution of O2+ density is slightly shifted to the instantaneous cathode. The discharge heat source is positively correlated with the air gap thickness and the voltage amplitude. With the increase in air gap thickness, the rising rate of heat sources will also increase. However, when the voltage is 10 kV, the heat source increases first and then decreases with the increase in the air gap thickness. The discharge does not occur when the air gap thickness reaches above 4 mm. This study provides theoretical support for identifying the deterioration degree of air gap defects from the view of temperature. [ABSTRACT FROM AUTHOR]

In this paper, the frequency and instability variations under the influence of temperature effect in a cylindrical smooth-bore magnetron are investigated. To derive the eigenvalue equation, the Fourier transform of electrostatic flute perturbations together with the local approximation method along radial direction for perturbed density is applied to equations of the macroscopic fluid model and Poisson equation. The obtained eigenvalue equation is solved numerically by shooting to a fitting point method. The analysis of numerical results shows the change in frequency of second three azimuthal modes for the case when the perpendicular temperature is higher than the parallel temperature ( T ∥ < T ⊥ ), which is greater than the case when temperatures along azimuthal and radial directions are equal ( T ∥ = T ⊥ ) or T ∥ > T ⊥ . As the temperature rising, the frequency and growth rate instability increase except for Tr > Tθ that the growth rate instability is reduced until Tθ = 100 00k and then is increased. The minimum frequency variation is 0.002 GHz for the mode of l = 1 at 2 T ∥ = T ⊥ . The maximum change in frequency, in contrast, is 10.651 GHz for the mode of l = 5 at 4 T ∥ = T ⊥ . According to the obtained results, the temperature controlling could be help to frequency adjustment in magnetrons. [ABSTRACT FROM AUTHOR]

In this paper, the effect of surface recombination on N-atom production is discussed through a one-dimensional simulation of Townsend dielectric barrier discharge in pure N2 based on a fluid model. By comparison of the experimental results, the recommended value of the sticking coefficient of N–N surface recombination is 0.5–1. The spatial-temporal distribution of N-atom of simulation results in discharge and post-discharge agree with experimental results. When the sticking coefficient is 0.5, the primary active species include N, N2(A), and N2(a′). N4+ is the densest positive ion, which can reach 4.77 × 109 cm−3. N-atom can reach the saturation level within about 30 ms. The highest number density is 3.14 × 1014 cm−3 at the position 0.25 mm away from the surface. The numerical simulation results are very consistent with the experimental results. The contribution of surface recombination and three-body recombination for the decay of N-atom are roughly equal in the post-discharge region. [ABSTRACT FROM AUTHOR]

Proton imaging is a powerful tool for probing electromagnetic fields in a plasma, providing a path-integrated map of the field topology. However, in cases where the field structure is highly inhomogeneous, inferring spatial properties of the underlying field from proton images can be difficult. This problem is exemplified by recent experiments, which used proton imaging to probe the filamentary magnetic field structures produced by the Weibel instability in collisionless counterstreaming plasmas. In this paper, we perform analytical and numerical analyses of proton images of systems containing many magnetic filaments. We find that, in general, the features observed on proton images do not directly correspond to the spacing between magnetic filaments (the magnetic wavelength) as has previously been assumed and that they instead correspond to the filament size. We demonstrate this result by Fourier analysis of synthetic proton images for many randomized configurations of magnetic filaments. Our results help guide the interpretation of experimental proton images of filamentary magnetic structures in plasmas. [ABSTRACT FROM AUTHOR]

In this paper, a coaxial gyrotron with a double electron beam is investigated on the ohmic loss, starting current, mode competition, and beam wave interaction. Its beam-wave interaction equation and dependence of the starting current on the static magnetic field and parameter K (the ratio of the current of an electron beam to the total current of a double electron beam) are given. On this basis, a 4 MW 170 GHz coaxial gyrotron with a double electron beam is studied. By the numerical calculation, the influence of K on the starting current is analyzed under a given static magnetic field and at different guiding center radii of the double electron beam. Studies show that the ability of the gyrotron to suppress the mode competition has a relation with the selection of the guiding center radii of the two electron beams. Then, the investigation of a time-dependent multi-mode competition of the gyrotron is performed, which shows that the gyrotron can stably operate in the T E 38 , 18 − mode. The results show that by considering the ohmic loss, the gyrotron can operate at 170.4 GHz with the output power of 4.04 MW and the beam-wave interaction efficiency of 35.47% when B = 6.82 T, U = 88 KV, I = 129 A, and K = 0.4. Each of the two electron beams in the gyrotron has different beam-wave interaction efficiencies. Compared to the coaxial gyrotron with one electron beam, the beam-wave interaction efficiency of each electron beam in the coaxial gyrotron with a double electron beam has a different increase. [ABSTRACT FROM AUTHOR]

A one-dimensional, two-fluid, steady state model is used for the analysis of ion temperature effects to the plasma-wall transition. In this paper, the model is solved for a finite ratio e between the Debye and the ionization length, while in Part II [T. Gyergyek and J. Kovačič, Phys Plasmas 24, 063506 (2017)], the solutions for ε = 0 are presented. Ion temperature is treated as a given, independent parameter and it is included in the model as a boundary condition. It is shown that when the ion temperature larger than zero is selected, the ion flow velocity and the electric field at the boundary must be consistent with the selected ion temperature. A numerical procedure, how to determine such "consistent boundary conditions," is proposed, and a simple relation between the ion temperature and ion velocity at the boundary of the system is found. The effects of the ion temperature to the pre-sheath length, potential, ion temperature, and ion density drops in the pre-sheath and in the sheath are investigated. It is concluded that larger ion temperature results in a better shielding of the plasma from the wall. An attempt is made to include the ion heat flux qi into the model in its simplest form qi = -K' dTi/dx , where K' is a constant heat conduction coefficient. It is shown that inclusion of such a term into the energy transfer equation introduces an additional ion heating mechanism into the system and the ion flow then becomes isothermal instead of adiabatic even in the sheath. [ABSTRACT FROM AUTHOR]

Hamiltonian and action principle (HAP) formulations of plasma physics are reviewed for the purpose of explaining structure preserving numerical algorithms. Geometric structures associated with and emergent from HAP formulations are discussed. These include conservative integration, which exactly conserves invariants, symplectic integration, which exactly preserves the Hamiltonian geometric structure, and other Hamiltonian integration techniques. Basic ideas of variational integration and Poisson integration, which can preserve the noncanonical Hamiltonian structure, are discussed. Metriplectic integration, which preserves the structure of conservative systems with both Hamiltonian and dissipative parts, is proposed. Two kinds of simulated annealing, a relaxation technique for obtaining equilibrium states, are reviewed: one that uses metriplectic dynamics, which maximizes an entropy at fixed energy, and the other that uses double bracket dynamics, which preserves Casimir invariants. Throughout, applications to plasma systems are emphasized. The paper culminates with a discussion of geometric electromagnetic particle-in-cell [Kraus et al., J. Plasma Phys. (to be published); e-print arXiv:1609.03053v1 [math.NA]], a particle in cell code that incorporates Hamiltonian and geometrical structure preserving properties. [ABSTRACT FROM AUTHOR]

In this paper, we develop an innovative approach to study the shock wave propagation using the Sagdeev potential method. We also present an analytical solution for Korteweg de Vries Burgers (KdVB) and modified KdVB equation families with a generalized form of the nonlinearity term which agrees well with the numerical one. The novelty of the current approach is that it is based on a simple analogy of the particle in a classical potential with the variable particle energy providing one with a deeper physical insight into the problem and can easily be extended to more complex physical situations. We find that the current method well describes both monotonic and oscillatory natures of the dispersive-diffusive shock structures in different viscous fluid configurations. It is particularly important that all essential parameters of the shock structure can be deduced directly from the Sagdeev potential in small and large potential approximation regimes. Using the new method, we find that supercnoidal waves can decay into either compressive or rarefactive shock waves depending on the initial wave amplitude. Current investigation provides a general platform to study a wide range of phenomena related to nonlinear wave damping and interactions in diverse fluids including plasmas. [ABSTRACT FROM AUTHOR]

A plasmon-assisted channeling acceleration can be realized with a large channel, possibly at the nanometer scale. Carbon nanotubes (CNTs) are the most typical example of nano-channels that can confine a large number of channeled particles in a photon-plasmon coupling condition. This paper presents a theoretical and numerical study on the concept of high-field charge acceleration driven by photo-excited Luttinger-liquid plasmons in a nanotube [Z. Shi et al., Nat. Photonics 9, 515 (2015)]. An analytic description of the plasmon-assisted laser acceleration is detailed with practical acceleration parameters, in particular, with the specifications of a typical tabletop femtosecond laser system. The maximally achievable acceleration gradients and energy gains within dephasing lengths and CNT lengths are discussed with respect to laser-incident angles and CNT-filling ratios. [ABSTRACT FROM AUTHOR]

Eulerian simulations of the Vlasov-Poisson equations have been employed to analyze the excitation of slow electrostatic fluctuations (with phase speed close to the electron thermal speed), due to a beam-plasma interaction, and their propagation in linear and nonlinear regimes. In 1968, O'Neil and Malmberg [Phys. Fluids 11, 1754 (1968)] dubbed these waves "beam modes." In the present paper, previous analytical results on the beam modes in both linear and nonlinear regimes have been revisited numerically, pointing out that, when an electron beam is launched in a plasma of Maxwellian electrons and motionless protons and this initial equilibrium is perturbed by a monochromatic density disturbance, the electric field amplitude grows exponentially in time and then undergoes nonlinear saturation, associated with the kinetic effects of particle trapping and phase space vortex generation. Moreover, new numerical results give evidence that, when the initial density perturbation is setup in the form of a low amplitude random phase noise, the whole Fourier spectrum of wavenumbers is excited. As a result, the electric field profile appears as a train of isolated pulses, each of them being associated with a phase space vortex in the electron distribution function. At later times, these vortical structures tend to merge and, correspondingly, the electric pulses collapse, showing the tendency towards a time asymptotic configuration characterized by the appearance of electric soliton-like pulses. This dynamical evolution is driven by purely kinetic processes, possibly at work in many space and laboratory plasma environments. [ABSTRACT FROM AUTHOR]

The general dispersion relation of obliquely propagating magneto-sonic (MS) waves for the inhomogeneous and anisotropic plasmas is analyzed including the effect of wave-particle interaction. The numerical analysis is performed without expanding both the plasma dispersion and the modified Bessel functions to highlight the effects of density inhomogeneity and the temperature anisotropy. The obtained results are compared with the recent work [Naim et al., Phys. Plasmas 22, 062117 (2015)], where only drift mode near the magnetosonic frequency is investigated. In our paper, we additionally analyzed two related modes depicting that the drift effect leads to an asymmetric behavior in the dispersion properties of drift MS waves. The possible application to the solar coronal heating problem has also been discussed. [ABSTRACT FROM AUTHOR]

A one-dimensional hybrid model is presented in the paper to study the characteristics of ion energy and angular distributions on the rf-biased electrode in dual frequency capacitively coupled CF4 discharges. The hybrid model includes two parts: a fluid module and a Monte Carlo (MC) module. The fluid module determines the spatiotemporal evolutions of bulk plasma, and the MC module describes the ion-neutral collisions to predict the ion energy and angular distributions on rf-biased electrode. The discussion of this paper focuses on the influence of pressure, voltage, amplitude, and frequencies of the low frequency source on ion energy distributions (IEDs) and ion angular distributions (IADs) of CF3+ and F+ ions. For the CF3+ ions, the IEDs appear to have multiple-peak structures in the dual frequency capacitively coupled rf discharge, and the IADs have a significant peak at a small angle. With the increase of pressure, the maximum energy in IEDs decreases, and the IADs spread to a large angle region. With the decrease of the low frequency or increase of the LF voltage, more ions get a higher energy while they are going across the sheath region, the width and maximum ion energy of IEDs increase, and the peaks in IADs shift toward small angle regions. For the F+ ions, the IEDs are modulated strongly by the sheath field, and the peaks are more prominent than those of heavy ions. The width between different peaks is narrower than that of CF3+ ions, and the maximum energy is also higher. [ABSTRACT FROM AUTHOR]

Lower-hybrid drift and Buneman instabilities operate in current sheets with or without the guide field. The lower-hybrid drift instability is a universal instability in that it operates for all parameters. In contrast, the excitation of Buneman instability requires sufficiently thin current sheet. That is, the relative electron-ion drift speed must exceed the threshold in order for Buneman instability to operate. Traditionally, the two instabilities were treated separately with different mathematical formalisms. In a recent paper, an improved electrostatic dispersion relation was derived that is valid for both unstable modes [P. H. Yoon and A. T. Y. Lui, Phys. Plasmas 15, 072101 (2008)]. However, the actual numerical analysis was restricted to a one-dimensional situation. The present paper generalizes the previous analysis and investigates the two-dimensional nature of both instabilities. It is found that the lower-hybrid drift instability is a flute mode satisfying k·B=0 and k·∇n=0, where k represents the wave number for the most unstable mode, B stands for the total local magnetic field, and ∇n is the density gradient. This finding is not totally unexpected. However, a somewhat surprising finding is that the Buneman instability is a field-aligned mode characterized by k×B=0 and k·∇n=0, rather than being a beam-aligned instability. [ABSTRACT FROM AUTHOR]

In the conventional orotron (at least, in its classical configuration), a thickness of an electron beam propagating over the grating plate should be much smaller than a wavelength; also the clearance between the grating and the beam and possible misalignment should be smaller than this thickness. These requirements severely limit the choice of operating parameters and performance characteristics of this device. When a beam is slightly inclined to the grating surface, these limitations can be greatly mitigated: the operation can be more robust, beam thickness can be increased, so a higher microwave power can be generated. Such a configuration of the orotron with an inclined electron beam is studied in the present paper. The paper contains simplified equations describing nonlinear operation of such a device and results of numerical analysis of these equations with the electron velocity spread taken into account. The paper also contains a discussion of these results and their applicability to practical configurations, which can be designed for operation in the THz and sub-THz regions. [ABSTRACT FROM AUTHOR]

This paper revisits the relativistic limiting current in planar, cylindrical, and spherical diodes, with alternative analytic and numerical treatments which are easy to implement. Convenient, approximate expressions for the limited current are presented for gap voltages up to 10 MV. They are accurate to within 1% for planar diode, and to within 4% for both cylindrical and spherical diode in the range 10-5 < rc=ra < 500, where ra and rc are, respectively, the anode and cathode radius. [ABSTRACT FROM AUTHOR]

The present paper is devoted to the theoretical and numerical analysis of the spontaneously emitted electromagnetic fluctuations characterized by quasi-parallel wave vectors relative to the ambient magnetic field. The formulation is based upon the Klimontovich plasma kinetic theory. The comparative study is carried out between the spontaneously emitted field fluctuation spectrum constructed on the basis of a single Maxellian velocity distribution function (VDF) and the spectrum that arises from multi-component electron VDFs similar to those found in the solar wind. Typical solar wind electron VDF is composed of a Gaussian core and kappa distributions of halo and super-halo components. Of these, the halo and super-halo populations represent tenuous but energetic components. It is found that the energetic electrons make important contributions to the total emission spectrum. It is also found that the halo electrons are largely responsible for the emission spectrum in the whistler frequency range, whereas the more energetic super-halo electrons emit quasi-longitudinal fluctuations in the Langmuir frequency range, thus validating the recent quasi-steady state model of the solar wind electrons put forth by the present authors [Kim et al., Astrophys. J. 806, 32 (2015); Yoon et al., Astrophys. J. 812, 169 (2015)]. [ABSTRACT FROM AUTHOR]

Plasma acceleration research station is an electron beam driven plasma wakefield acceleration test stand proposed for CLARA facility in Daresbury Laboratory. In this paper, the interaction between the electron beam and the plasma is numerically characterised via 2D numerical studies by using VSIM code. The wakefields induced by a single bunch travelling through the plasma were found to vary from 200 MV/m to 3 GV/m for a range of bunch length, bunch radius, and plasma densities. Energy gain for the particles populating the bunch tail through the wakefields driven by the head of the bunch was demonstrated. After determining the achievable field for various beams and plasma configurations, a reference setting was determined for further studies. Considering this reference setting, the beam quality studies were performed for a two-bunch acceleration case. The maximum energy gain as well as the energy spread mitigation by benefiting from the beam loading was investigated by positioning the witness and driver bunches with respect to each other. Emittance growth mechanisms were studied considering the beam-plasma and beam-wakefield interactions. Eventually, regarding the findings, the initial commissioning plans and the aims for the later stages were summarised. [ABSTRACT FROM AUTHOR]

In this paper, the generation of high-quality and high-energy micro electron beam in vacuum by a chirped Gaussian laser pulse in the presence of an axial magnetic field is numerically investigated. The features of energy and angular spectra, emittances, and position distribution of electron beam are compared in two cases, i.e., in the presence and absence of an external magnetic field. The electron beam is accelerated with higher energy and qualified in spatial distribution in the presence of the magnetic field. The presence of an axial magnetic field improves electron beam spatial quality as well as its gained energy through keeping the electron motion parallel to the direction of propagation for longer distances. It has been found that a 64 μm electron bunch with about MeV initial energy becomes a 20 μm electron beam with high energy of the order of GeV, after interacting with a laser pulse in the presence of an external magnetic field. [ABSTRACT FROM AUTHOR]

Bump-on-tail instability of Langmuir waves propagating in unmagnetized Lorentzian plasma modeled by a Kappa velocity distribution with spectral index κ has been investigated in this paper. Growth rate of this microinstability has been determined analytically from the Langmuir wave dispersion relation. Change in the maximum growth rate with increasing κ has been numerically estimated for different number density and temperature ratios using solar wind data. [ABSTRACT FROM AUTHOR]

The fast rates of magnetic reconnection found in both nature and experiments are important to understand theoretically. Recently, it was demonstrated that two-fluid magnetic reconnection remains fast in the strong guide field regime, regardless of the presence of fast-dispersive waves. This conclusion is in agreement with recent results from kinetic simulations, and is in contradiction to the findings in an earlier two-fluid study, where it was suggested that fast-dispersive waves are necessary for fast reconnection. In this paper, we give a more detailed derivation of the analytic model presented in a recent letter and present additional simulation results to support the conclusions that the magnetic reconnection rate in this regime is independent of both collisional dissipation and system-size. In particular, we present a detailed comparison between fluid and kinetic simulations, finding good agreement in both the reconnection rate and overall length of the current layer. Finally, we revisit the earlier two-fluid study, which arrived at different conclusions, and suggest an alternative interpretation for the numerical results presented therein. [ABSTRACT FROM AUTHOR]

This paper presents a critical assessment of the theory of photo-detachment diagnostic method used to probe the negative ion density and electronegativity α=n/ne. In this method, a laser pulse is used to photo-detach all negative ions located within the electropositive channel (laser spot region). The negative ion density is estimated based on the assumption that the increase of the current collected by an electrostatic probe biased positively to the plasma is a result of only the creation of photo-detached electrons. In parallel, the background electron density and temperature are considered as constants during this diagnostics. While the numerical experiments performed here show that the background electron density and temperature increase due to the formation of an electrostatic potential barrier around the electropositive channel. The time scale of potential barrier rise is about 2?ns, which is comparable to the time required to completely photo-detach the negative ions in the electropositive channel (~3ns). We find that neglecting the effect of the potential barrier on the background plasma leads to an erroneous determination of the negative ion density. Moreover, the background electron velocity distribution function within the electropositive channel is not Maxwellian. This is due to the acceleration of these electrons through the electrostatic potential barrier. In this work, the validity of the photo-detachment diagnostic assumptions is questioned and our results illustrate the weakness of these assumptions. [ABSTRACT FROM AUTHOR]

This paper formulates the Tokamak Magneto-Hydrodynamics (TMHD), initially outlined by X. Li and L. E. Zakharov [Plasma Science and Technology 17(2), 97-104 (2015)] for proper simulations of macroscopic plasma dynamics. The simplest set of magneto-hydrodynamics equations, sufficient for disruption modeling and extendable to more refined physics, is explained in detail. First, the TMHD introduces to 3-D simulations the Reference Magnetic Coordinates (RMC), which are aligned with the magnetic field in the best possible way. The numerical implementation of RMC is adaptive grids. Being consistent with the high anisotropy of the tokamak plasma, RMC allow simulations at realistic, very high plasma electric conductivity. Second, the TMHD splits the equation of motion into an equilibrium equation and the plasma advancing equation. This resolves the 4 decade old problem of Courant limitations of the time step in existing, plasma inertia driven numerical codes. The splitting allows disruption simulations on a relatively slow time scale in comparison with the fast time of ideal MHD instabilities. A new, efficient numerical scheme is proposed for TMHD. [ABSTRACT FROM AUTHOR]

Localization of kinetic Alfvén waves (KAW) due to ponderomotive nonlinearity can be regarded as an important mechanism for heating the space plasmas. The present paper investigates the effect of background density fluctuations on the formation of large amplitude localized structures and turbulent spectrum of KAW applicable to magnetopause. The dynamical equations are derived, taking into account the ponderomotive nonlinearity of the KAW as well as the background fluctuations which are in the form of ion acoustic waves. The system is studied numerically as well as semi-analytically. The results reveal that the presence of density fluctuations affects the formation of localized structures. These fluctuations affecting the localization of KAW may also affect heating and acceleration of plasma. Respective turbulent scaling for the different amplitude of background fluctuations has also been studied. The relevance of the numerical results has been discussed with the THEMIS observations near the magnetopause [C. Chaston et al., Geophys. Res. Lett. 35, L17S08 (2008)]. [ABSTRACT FROM AUTHOR]

It is well known that electron cyclotron current drive (ECCD) around the o-point of magnetic island along the plasma current direction can stabilize neoclassical tearing modes (NTMs) in tokamak devices. The effects of the radial misalignment between the island and the driven current, the phase misalignment, and the on-duty ratio for modulated current drive on NTM stabilization are studied numerically in this paper. A small radial misalignment is found to significantly decrease the stabilizing effect. When a sufficiently large phase misalignment occurs for the modulated ECCD, the stabilization effect is also reduced a lot. The optimal on-duty ratio of modulated ECCD to stabilize NTMs is found to be in the range of 60%-70%. A larger on-duty ratio than 50% could also mitigate the effect of phase misalignment if it is not too large. There is no benefit from modulation if the phase misalignment is larger than a threshold. [ABSTRACT FROM AUTHOR]

In this paper, we present a study of anomalous diffusion using a Fokker-Planck description with fractional velocity derivatives. The distribution functions are found using numerical means for varying degree of fractionality of the stable Lévy distribution. The statistical properties of the distribution functions are assessed by a generalized normalized expectation measure and entropy in terms of Tsallis statistical mechanics. We find that the ratio of the generalized entropy and expectation is increasing with decreasing fractionality towards the well known so-called sub-diffusive domain, indicating a self-organising behavior. [ABSTRACT FROM AUTHOR]

Most of the investigated overmoded relativistic backward wave oscillators (RBWOs) are azimuthally symmetric; thus, they are designed through two dimensional (2-D) particle-in-cell (PIC) simulations. However, 2-D PIC simulations cannot reveal the effect of asymmetric modes on beam-wave interaction. In order to investigate whether asymmetric mode competition needs to be considered in the design of overmoded RBWOs, a numerical method of determining the composition of both symmetric and asymmetric modes in three dimensional (3-D) PIC simulations is introduced in this paper. The 2-D and 3-D PIC simulation results of an X-band overmoded RBWO are analyzed. Our analysis indicates that the 2-D and 3-D PIC simulation results of our device are quite different due to asymmetric mode competition. In fact, asymmetric surface waves, especially EH11 mode, can lead to serious mode competition when electron beam propagates near the surface of slow wave structures (SWSs). Therefore, additional method of suppressing asymmetric mode competition, such as adjusting the reflections at both ends of SWSs to decrease the Q-factor of asymmetric modes, needs to be utilized in the design of overmoded RBWOs. Besides, 3-D PIC simulation and modes decomposition are essential for designing overmoded RBWOs. [ABSTRACT FROM AUTHOR]

Three numerical techniques employing differentiation matrices are used to investigate the linear stability of drift-resistive-inertial ballooning mode for conditions that occur in tokamak edge plasmas. Hermite and Sinc spectral methods are applied to compute the ballooning mode eigenvalues and eigenvectors. In addition, a finite difference method is utilized to construct a differentiation matrix in order to verify results obtained using the spectral methods. It is shown that the spectral methods converge more rapidly than the finite difference method. Ballooning and strongly ballooning approximations are used to calculate the eigen-spectrum. The techniques that are utilized in this paper for calculating eigenvalues are quite general and are relevant to investigate other modes that use the ballooning mode formalism. [ABSTRACT FROM AUTHOR]

Experimental data compiled over five decades of dense plasma focus research are consistent with the snowplow model of sheath propagation, based on the hypothetical balance between magnetic pressure driving the plasma into neutral gas ahead and "wind pressure" resisting its motion. The resulting sheath velocity, or the numerically proportional "drive parameter," is known to be approximately constant for devices optimized for neutron production over 8 decades of capacitor bank energy. This paper shows that the validity of the snowplow hypothesis, with some correction, as well as the non-dependence of sheath velocity on device parameters, have their roots in local conservation laws for mass, momentum, and energy coupled with the ionization stability condition. Both upper and lower bounds on sheath velocity are shown to be related to material constants of the working gas and independent of the device geometry and capacitor bank impedance. [ABSTRACT FROM AUTHOR]

In this paper, a modified numerical method is used to investigate the mode composition of a Gigawatt-class Ka-band overmoded Cerenkov oscillator, which has been proposed and studied in our previous experiment. In the experiment, the measured angular distribution of radiation did not fit a single TM01 mode. So the particle in cell code calculations and the antenna radiation calculations are carried out, which show a consistent picture: the dominant modes are the TM01 mode and the TM03 mode, and their phase relationship is constant with time; therefore, a steady radiation pattern is produced, which matches the experimental data. As a conclusion, the comprehensive analysis shows that the existing modes of the output microwave in our experiment are the first five TM0n modes (n = 1-5), with corresponding power ratios of 36.64%, 0.78%, 56.26%, 5.70%, and 0.52%, and relative phase differences of 0°, 146°, 54°, 169°, and 133. [ABSTRACT FROM AUTHOR]

Experiments about the observations of stimulated Raman backscatter (SRS) and stimulated Brillouin backscatter (SBS) in Hohlraum were performed on Shenguang-III (SG-III) prototype facility for the first time in 2011. In this paper, relevant experimental results are analyzed for the first time with a one-dimension spectral analysis code, which is developed to study the coexistent process of SRS and SBS in Hohlraum plasma condition. Spectral features of the backscattered light are discussed with different plasma parameters. In the case of empty Hohlraum experiments, simulation results indicate that SBS, which grows fast at the energy deposition region near the Hohlraum wall, is the dominant instability process. The time resolved spectra of SRS and SBS are numerically obtained, which agree with the experimental observations. For the gas-filled Hohlraum experiments, simulation results show that SBS grows fastest in Au plasma and amplifies convectively in C5H12 gas, whereas SRS mainly grows in the high density region of the C5H12 gas. Gain spectra and the spectra of backscattered light are simulated along the ray path, which clearly show the location where the intensity of scattered light with a certain wavelength increases. This work is helpful to comprehend the observed spectral features of SRS and SBS. The experiments and relevant analysis provide references for the ignition target design in future. [ABSTRACT FROM AUTHOR]

The plasma scrape-off layer (SOL) in a tokamak is characterized by ion flow down a long narrow flux tube terminating on a solid surface. The ion flow velocity along a magnetic field line can be equal to or greater than sonic at the entrance of a Debye sheath or upstream in the presheath. This paper presents a numerical study of the transition between subsonic and supersonics flows. A quasineutral one-dimensional (1D) fluid code has been used for modeling of plasma transport in the SOL along magnetic field lines, both in steady state and under transient conditions. The model uses coupled equations for continuity, momentum, and energy balance with ionization, radiation, charge exchange, and recombination processes. The recycled neutrals are described in the diffusion approximation. Standard Bohm sheath criterion is used as boundary conditions at the material surface. Three conditions conducive for the generation of supersonic flows in SOL plasmas have been explored. It is found that in steady state high (attached) and low (detached) divertor temperatures cases, the role of particle, momentum, and energy loss is critical. For attached case, the appearance of shock waves in the divertor region if the incoming plasma flow is supersonic and its effect on impurity retention is presented. In the third case, plasma expansion along the magnetic field can yield time-dependent supersonic solutions in the quasineutral rarefaction wave. Such situations can arise in the parallel transport of intermittent structures such as blobs and edge localized mode filaments along field lines. [ABSTRACT FROM AUTHOR]

In this paper, a computational study of two counter-propagating helium plasma jets in ambient air is presented. A two-dimensional fluid model is applied to investigate the physical processes of the two plasma jets interaction (PJI) driven by equal and unequal voltages, respectively. In all studied cases, the PJI results in a decrease of both plasma bullets propagation velocity. When the two plasma jets are driven by equal voltages, they never merge but rather approach each other around the middle of the gas gap at a minimum approach distance, and the minimal distance decreases with the increase of both the applied voltages and initial electron density, but increases with the increase of the relative permittivity. When the two plasma jets are driven by unequal voltages, we observe the two plasma jets will merge at the position away from the middle of the gas gap. The effect of applied voltage difference on the PJI is also studied. [ABSTRACT FROM AUTHOR]

Continuum resonance damping is an important factor in determining the stability of certain global modes in fusion plasmas. A number of analytic and numerical approaches have been developed to compute this damping, particularly, in the case of the toroidicity-induced shear Alfvén eigenmode. This paper compares results obtained using an analytical perturbative approach with those found using resistive and complex contour numerical approaches. It is found that the perturbative method does not provide accurate agreement with reliable numerical methods for the range of parameters examined. This discrepancy exists even in the limit where damping approaches zero. When the perturbative technique is implemented using a standard finite element method, the damping estimate fails to converge with radial grid resolution. The finite elements used cannot accurately represent the eigenmode in the region of the continuum resonance, regardless of the number of radial grid points used. [ABSTRACT FROM AUTHOR]

A numerical study on microwave sustained argon discharge under atmospheric pressure is reported in this paper. The purpose of this study is to investigate both the process and effects of the conditions of microwave-excited gas discharge under atmospheric pressure, thereby aiding improvements in the design of the discharge system, setting the appropriate working time, and controlling the operating conditions. A 3D model is presented, which includes the physical processes of electromagnetic wave propagation, electron transport, heavy species transport, gas flow, and heat transfer. The results can be obtained by means of the fluid approximation. The maxima of the electron density and gas temperature are 4.96 X 1018 m-3 and 2514.8K, respectively, and the gas pressure remains almost unchanged for typical operating conditions with a gas flow rate of 20 l/min, microwave power of 1000 W, and initial temperature of 473 K. In addition, the conditions (microwave power, gas flow rate, and initial temperature) of discharge are varied to obtain deeper information about the electron density and gas temperature. The results of our numerical study are valid and clearly describe both the physical process and effects of the conditions of microwave-excited argon discharge. [ABSTRACT FROM AUTHOR]

The relativistic Weibel instability has received much attention because of its potential application to astrophysical problems. The purpose of this paper is to correct certain erroneous expressions in the author’s early paper on the subject [P. H. Yoon, Phys. Fluids B 1, 1336 (1989)], to discuss further properties of the relativistic Weibel instability, and to compare with its nonrelativistic counterpart. [ABSTRACT FROM AUTHOR]

This paper considers the asymptotic and numerical solution of a simple model for the expansion of a collisionless plasma into a plasma of lower density. The dependence on the density ratio of qualitative and quantitative features of solutions of the well-known cold-ion model is explored. In the cold-ion limit, we find that a singularity develops in the ion density in finite time unless the density ratio is zero or close to unity. The classical cold-ion model may cease to be valid when such a singularity occurs and we then regularize the model by the finite ion-temperature Vlasov-Poisson system. Numerical evidence suggests the emergence of a multi-modal velocity distribution. [ABSTRACT FROM AUTHOR]

Implosions of (quasi-)spherical loads with mass redistribution and displacement modification are investigated numerically. Both methods can theoretically counterbalance the nonuniformity of magnetic pressure along the load surface and realize quasi-spherical Z-pinch implosions. Mass redistribution is feasible for spherical loads with large radius and weight, while the displacement modification is more suitable for light loads, such as those composed of wire arrays. Simulation results suggest that, for mass redistributed spherical loads, wall instabilities induced by polar mass flows will deform the imploding shell. For prolate spherical loads, in which the wall instability cannot develop, the kinetic energy distribution is disturbed at high latitude. These passive behaviors and their possible mitigation methods, such as reshaping the electrode, are investigated numerically in this paper. [ABSTRACT FROM AUTHOR]

In this paper we extend the new techniques of W. Tierens and D. D. Zutter, J. Comput. Phys. 231, 5144 (2012) to include finite Larmor radius effects up to second order in the Larmor radius. We limit ourselves to the case of propagation perpendicular to the background magnetic field Bvector 0. We show that our time-domain technique is able to produce the lowest-order Bernstein wave (a wave believed to be useful for heating fusion devices [H. P. Laqua, Plasma Phys. Controlled Fusion 49, R1 (2007)]). The discrete equations retain many of the favourable properties described in W. Tierens and D. D. Zutter, J. Comput. Phys. 231, 5144 (2012), i.e., unconditional stability and a straightforward relation between the second-order accurate continuous dispersion relation and the dispersion relation of the discretized problem. The theory is illustrated by a place-independent and a place-dependent temperature numerical example. [ABSTRACT FROM AUTHOR]

Based on the set of nonlinear coupling equations, which has considered the relativistic effects of electrons, modulational instability by strong Langmuir waves has been investigated in this paper. Both the characteristic scale and maximum growth rate of the Langmuir field will enhance with the increase in the electron relativistic effect. The numerical results indicate that longitudinal perturbations induce greater instability than transverse perturbations do, which will lead to collapse and formation of the pancake-like structure. [ABSTRACT FROM AUTHOR]

The paper discusses the requirements for avalanche ionization in gas or gas mixtures initiated by REMPI or femtosecond-laser pre-ionization. Numerical examples of dependencies on partial composition for Ar:Xe gas mixture with REMPI of argon and subsequent classic avalanche ionization of Xe are presented. [ABSTRACT FROM AUTHOR]

The fully numerical nonlinear theory for a gyrotron with a special cross-section interaction cavity has been developed in this paper. In this theory, the analytical solution to different modes in the special cross-section interaction cavity is replaced by the numerical solution based on electromagnetic simulation results. A 0.4 THz third harmonic gyrotron with an azimuthally corrugated interaction cavity has been investigated by using this theory and simulation results show that this approach has a significant advantage of developing high harmonic terahertz gyrotrons. [ABSTRACT FROM AUTHOR]

Recent work investigating the interaction of magnetic islands with micro-turbulence has uncovered the striking observation of large scale vortex modes forming within the island structure [W. A. Hornsby et al., Phys. Plasmas 17, 092301 (2010)]. These electrostatic vortices are found to be the size of the island and are oscillatory. It is this oscillatory behaviour and the presence of turbulence that leads us to believe that the dynamics are related to the geodesic acoustic mode (GAM), and it is this link that is investigated in this paper. Here, we derive an equation for the GAM in the MHD limit, in the presence of a magnetic island modified three-dimensional axisymmetric geometry. The eigenvalues and eigenfunctions are calculated numerically and then utilised to analyse the dynamics of oscillatory large-scale electrostatic potential structures seen in both linear and non-linear gyro-kinetic simulations. [ABSTRACT FROM AUTHOR]

The wandering of magnetic field lines is an important topic in theoretical plasma physics and astrophysics. Previous analytical work, as well as computer simulations, is based on magnetostatic models to warrant mathematical and numerical tractability. Recently, we have studied the first time field line random walk in dynamical turbulence by using a field line tracing method. These calculations were performed for a slab model of the turbulence. It is the purpose of the present paper to use the latter method to compute the field line diffusion coefficient for dynamical two-dimensional turbulence. Two models for the dynamical correlation function are used, namely the damping model of dynamical turbulence and the nonlinear anisotropic dynamical turbulence model. It is shown that the largest scales of the turbulence and the choice of the dynamical turbulence model have a strong influence on the diffusivity of the field lines and the absolute value of the diffusion coefficient. [ABSTRACT FROM AUTHOR]

The purpose of inventing a double stage Hall thruster is to control the propellant ionization and ion acceleration independently. In order to better understand the physics involved in such a thruster, an improved particle-in-cell method is used in this paper to simulate the discharge process. It is shown that the numerical features in the ionization stage accord well with the experimental results. It is also indicated that the ionization process and the acceleration process cannot be separated completely, as a relatively important ionization still occurs in the acceleration stage. Furthermore, an optimal threshold of ionization voltage in the ionization stage is existed to obtain the most favorable distributions of plasma parameters in the whole discharge channel. [ABSTRACT FROM AUTHOR]

A novel large-orbit electron gun with a gradual reversal of the magnetic field for the 8 mm third-harmonic peniotron is numerically studied in this paper. In the permanent magnetic condition, an electron beam with axial velocity spread of 4.78%, guiding center deviation of 7.18%, and velocity ratio of 2.2 is generated in the gun, satisfying the special requirements of beam-wave interaction in the third-harmonic peniotron. Compared with the traditional three stage method, it is unnecessary here to generate a thin tubular electron beam before the reversal point and the mutational distribution of reversal magnetic field. In addition, the emission band can be placed in the area of axial magnetic field where its magnitude decreases gradually, which reduces the structure complexity and the difficulties in tube-making process greatly. Finally, the peniotron is predicted to yield an output power of 31.9 kW at 30 GHz, with its microwave conversion efficiency of up to 49.4%, which shows the perfect matching between its high frequency system and its electron-optical system. According to the numerical calculations, the device performance is very sensitive to the relative axial position between the magnetic system and the electrode system. If the magnetic system shifts 0.8 mm to the right, the device efficiency would decrease from 49.4% to 31.7%. This phenomenon provides meaningful information in the device hot-test. [ABSTRACT FROM AUTHOR]

In this paper, we discuss the formation of ring-shaped and gyro-phase restricted velocity distribution functions (VDFs) at the edges of a cloud of protons injected into non-uniform distributions of the electromagnetic field. The velocity distribution function is reconstructed using the forward test-kinetic method. We consider two profiles of the electric field: (1) a non-uniform E-field obtained by solving the Laplace equation consistent with the conservation of the electric drift and (2) a constant and uniform E-field. In both cases, the magnetic field is similar to the solutions obtained for tangential discontinuities. The initial velocity distribution function is Liouville mapped along numerically integrated trajectories. The numerical results show the formation of an energy-dispersed structure due to the energy-dependent displacement of protons towards the edges of the cloud by the gradient-B drift. Another direct effect of the gradient-B drift is the formation of ring-shaped velocity distribution functions within the velocity-dispersed structure. Higher energy particles populate the edges of the proton beam, while smaller energies are located in the core. Non-gyrotropic velocity distribution functions form on the front-side and trailing edge of the cloud; this effect is due to remote sensing of energetic particles with guiding centers inside the beam. The kinetic features revealed by the test-kinetic solutions have features similar to in-situ velocity distribution functions observed by Cluster satellites in the magnetotail, close to the neutral sheet. [ABSTRACT FROM AUTHOR]

This paper presents a numerical model for the inception of positive dc corona discharges in air near cylindrical anodes, which plays a bridge role between the classic positive corona onset criterion and the photoionization model considering the effective radiation wavelength. The predicted onset voltages agree well with Peek's experimental data in a wide range of conductor radii and relative air densities. The influence of the collisional quenching of emitting excited states on the surface onset field is significant with low air density or small conductor radius. Within the effective radiation wavelength, numerical expressions for the photon absorption coefficient in air and Townsend's second coefficient due to photoionization are deduced on the basis of the new model. A different perspective on the classic coefficients is given. [ABSTRACT FROM AUTHOR]

Experiments observed that the pulse duty cycle has effects on the plasma homogeneity in pulse-modulated radio frequency (rf) discharges. In this paper, pulse-modulated rf (13.56 MHz) helium discharges are theoretically investigated using a two dimensional fluid model. With the pulse period being fixed to 15 μs, it is found that when the pulse-on duration is over 4 μs, i.e., the duty cycle is larger than approximately 27%, the discharge transits from an inhomogeneous to a homogeneous mode in every specific part of each pulse cycle under currently-used simulation parameters. More quantitative analysis shows that the discharge becomes more homogeneous as the duty cycle is increased but does not reach complete homogeneity. Possible reasons for the homogeneity improvement are discussed. [ABSTRACT FROM AUTHOR]

This paper investigates hot spot characteristics generated by the superposition of multiple laser beams. First, properties of speckle statistics are studied in the context of only one laser beam by computing the autocorrelation function. The case of multiple laser beams is then considered. In certain conditions, it is shown that speckles have an ellipsoidal shape. Analytical expressions of hot spot radii generated by multiple laser beams are derived and compared to numerical estimates made from the autocorrelation function. They are also compared to numerical simulations performed within the paraxial approximation. Excellent agreement is found for the speckle width as well as for the speckle length. Application to the speckle patterns generated in the Laser MegaJoule configuration in the zone where all the beams overlap is presented. Influence of polarization on the size of the speckles as well as on their abundance is studied. [ABSTRACT FROM AUTHOR]