Search

In a recent paper [P. H. Yoon and G. Choe, Phys. Plasmas 28, 082306 (2021)], the weak turbulence theory for incompressible magnetohydrodynamics is formulated by employing the method customarily applied in the context of kinetic weak plasma turbulence theory. Such an approach simplified certain mathematical procedures including achieving the closure relationship. The formulation in the above-cited paper starts from the equations of incompressible magnetohydrodynamic (MHD) theory expressed via Elsasser variables. The derivation of nonlinear wave kinetic equation therein is obtained via a truncated solution at the second-order of iteration following the standard practice. In the present paper, the weak MHD turbulence theory is alternatively formulated by employing the pristine form of incompressible MHD equation rather than that expressed in terms of Elsasser fields. The perturbative expansion of the nonlinear momentum equation is carried out up to the third-order iteration rather than imposing the truncation at the second order. It is found that while the resulting wave kinetic equation is identical to that obtained in the previous paper cited above, the third-order nonlinear correction plays an essential role for properly calculating derived quantities such as the total and residual energies. [ABSTRACT FROM AUTHOR]

Magnetized liner inertial fusion (MagLIF) is a magneto-inertial-fusion concept that is studied on the 20-MA, 100-ns rise time Z Pulsed Power Facility at Sandia National Laboratories. Given the relative success of the platform, there is a wide interest in studying the scaled performance of this concept at a next-generation pulsed-power facility that may produce peak currents upward of 60 MA. An important aspect that requires more research is the instability dynamics of the imploding MagLIF liner, specifically how instabilities are initially seeded. It has been shown in magnetized 1-MA thin-foil liner Z-pinch implosion simulations that a Hall interchange instability (HII) effect [J. M. Woolstrum et al., Phys. Plasmas 29, 122701 (2022)] can provide an independent seeding mechanism for helical magneto-Rayleigh–Taylor instabilities. In this paper, we explore this instability at higher peak currents for MagLIF using 2D discontinuous Galerkin PERSEUS simulations, an extended magneto-hydrodynamics code [C. E. Seyler and M. R. Martin, Phys. Plasmas 18, 012703 (2011)], which includes Hall physics. Our simulations of scaled MagLIF loads show that the growth rate of the HII is invariant to the peak current, suggesting that studies at 20-MA are directly relevant to 60-MA class machines. [ABSTRACT FROM AUTHOR]

Discusses the computation of the local eigenmodes of field-reversed configurations using ideal magnetohydrodynamics. Comparison of the eigenmodes with earlier analytic models; Similarity of the analytic models to actual eigenmodes in structure and growth rate; Failure of the analytic models to predict features of the eigenmode behavior.

In hydrodynamics, Taylor's frozen-in hypothesis connects the wavenumber spectrum to the frequency spectrum of a time series measured in real space. In this paper, we generalize Taylor's frozen-in hypothesis to magnetohydrodynamic turbulence. We analytically derive one-point two-time correlation functions for Elsässer variables whose Fourier transform yields the corresponding frequency spectra, E ± (f). We show that for isotropic turbulence, E ± (f) ∝ | U 0 ∓ B 0 | 2 / 3 in the Kolmogorov-like model and E ± (f) ∝ (B 0 | U 0 ∓ B 0 |) 1 / 2 in the Iroshnikov–Kraichnan model, where U 0 and B 0 are the mean velocity and mean magnetic fields, respectively, and f ± = k | U 0 ∓ B 0 | / (2 π) are the respective frequencies for a wavenumber k. However, for anisotropic magnetohydrodynamic turbulence, E ± (f) ∝ B 0 2 / 3 when U 0 ≪ B 0 . These results are important for the analysis of solar wind, in particular, those measured by Parker Solar Probe. [ABSTRACT FROM AUTHOR]

A recent paper by Davies et al. [Phys. Plasmas 28, 012305 (2021)] presents rational interpolants for the resistive (α) and thermoelectric (β) transport coefficients as functions of the Hall parameter χ and the ion charge state Z. Here, this discussion is augmented by showing that, at least in the Lorentz limit Z → ∞ , the proposed rational interpolants for α ⊥ and β ⊥ can be made even more accurate over a larger range of χ by making slight modifications to ensure the exact asymptotic limits are preserved. Although the authors do not discuss the conductivity (κ) coefficients, this exercise can also be repeated for the rational interpolants of κ ⊥ and κ ∧ published in a similar work [Phys. Rev. Lett. 126, 075001 (2021)]. [ABSTRACT FROM AUTHOR]

Previously published calculations predict that the "staged z-pinch" (SZP) can achieve 400 MJ of fusion yield on a Z-class machine. The SZP is touted to need no external preheat mechanism and no external pre-magnetization method. Instead, it is claimed that the imploding liner can adequately "shock preheat" the fuel and magnetic field diffusion through the liner can adequately magnetize the fuel. In this paper, we analyze a number of published SZP calculations and demonstrate that the calculations have major errors—the computer code used to do the calculations does not appear to be accurately solving the physical model it is intended to solve. A variety of independent analyses lead to this conclusion. This conclusion is confirmed by detailed one-dimensional magnetohydrodynamic (MHD) calculations conducted on different computer codes using a variety of proposed SZP operating parameters. Although using parameters similar or identical to the published calculations, our MHD calculations do not reach fusion conditions; there is no conceivable modification of the parameters that would lead to high-gain fusion conditions using these other codes. Our analyses and a review of the magnetized target parameter space leads to further conclusion that the SZP should not be considered to be a potential high-gain fusion source. [ABSTRACT FROM AUTHOR]

Low-frequency hydromagnetic turbulence is thought to play an important role in charged particle energization in space and astrophysical environments. For understanding large-scale turbulence in magnetized plasmas, low-frequency electromagnetic turbulence has been widely investigated within the theoretical framework of incompressible magnetohydrodynamic (MHD) theory. Among the existing works is the weak turbulence formalism of incompressible MHD turbulence. The present paper revisits the existing formalism under the assumption of zero residual energy. Under the strict assumption of turbulence taking place in a two-dimensional plane, which can be interpolated to a three-dimensional situation with azimuthal symmetry, the well-known steady-state turbulent spectrum of k ⊥ − 2 is recovered, where k ⊥ denotes the wave number perpendicular to the ambient magnetic field. [ABSTRACT FROM AUTHOR]

The effort to obtain a set of MagnetoHydroDynamic (MHD) equations for a magnetized collisionless plasma was started nearly 60 years ago by Chew et al. [Proc. R. Soc. London, Ser. A 236(1204), 112-118 (1956)]. Many attempts have been made ever since. Here, we will show the derivation of a set of these equations from the gyrokinetic perspective, which we call it gyrokinetic MHD, and it is different from the conventional ideal MHD. However, this new set of equations still has conservation properties and, in the absence of fluctuations, recovers the usual MHD equilibrium. Furthermore, the resulting equations allow for the plasma pressure balance to be further modified by finite-Larmor-radius effects in regions with steep pressure gradients. The present work is an outgrowth of the paper on "Alfven Waves in Gyrokinetic Plasmas" by Lee and Qin [Phys. Plasmas 10, 3196 (2003)]. [ABSTRACT FROM AUTHOR]

This paper focuses on the numerical investigation of arc characteristics in an air direct current circuit breaker (air DCCB). Using magneto-hydrodynamics (MHD) theory, 3D laminar model and turbulencemodel are constructed and calculated. The standard k-epsilon model is utilized to consider the turbulence effect in the arc chamber of the DCCB. Several important phenomena are found: the arc column in the turbulence-model case is more extensive, moves much more slowly than the counterpart in the laminar-model case, and shows stagnation at the entrance of the chamber, unlike in the laminar-model case. Moreover, the arc voltage in the turbulence-model case is much lower than in the laminar-model case. However, the results in the turbulence-model case show a much better agreement with the results of the breaking experiments under DC condition than in the laminar-model case, which is contradictory to the previous conclusions from the arc researches of both the low-voltage circuit breaker and the sulfur hexafluoride (SF6) nozzle. First, in the previous air-arc research of the low-voltage circuit breaker, it is assumed that the air plasma inside the chamber is in the state of laminar, and the laminar-model application gives quite satisfactory results compared with the experiments, while in this paper, the laminar-model application works badly. Second, the turbulence-model application in the arc research of the SF6-nozzle performs much better and gives higher arc voltage than the laminar-model application does, whereas in this paper, the turbulence-model application predicts lower arc voltage than the laminar-model application does. Based on the analysis of simulation results in detail, the mechanism of the above phenomena is revealed. The transport coefficients are strongly changed by turbulence, which will enhance the arc diffusion and make the arc volume much larger. Consequently, the arc appearance and the distribution of Lorentz force in the turbulence-model case substantially differ from the arc appearance and the distribution of Lorentz force in the laminar-model case. Thus, the moving process of the arc in the turbulence-model case is slowed down and slower than in the laminar-model case. Moreover, the more extensive arc column in the turbulence-model case reduces the total arc resistance, which results in a lower arc voltage, more consistent with the experimental results than the arc voltage in the laminar-model case. Therefore, the air plasma inside this air DCCB is believed to be in the turbulence state, and the turbulencemodel is more suitable than the laminar model for the arc simulation of this kind of air DCCB. [ABSTRACT FROM AUTHOR]

The gyrokinetic magnetohydrodynamic (MHD) equations, related to the recent paper by W. W. Lee ["Magnetohydrodynamics for collisionless plasmas from the gyrokinetic perspective," Phys. Plasmas 23, 070705 (2016)], and their associated equilibria properties are discussed. This set of equations consists of the time-dependent gyrokinetic vorticity equation, the gyrokinetic parallel Ohm's law, and the gyrokinetic Ampere's law as well as the equations of state, which are expressed in terms of the electrostatic potential, ϕ, and the vector potential, A, and support both spatially varying perpendicular and parallel pressure gradients and the associated currents. The corresponding gyrokinetic MHD equilibria can be reached when ϕ→0 and A becomes constant in time, which, in turn, gives ∇.(J∥+J⊥)=0 and the associated magnetic islands, if they exist. Examples of simple cylindrical geometry are given. These gyrokinetic MHD equations look quite different from the conventional MHD equations, and their comparisons will be an interesting topic in the future. [ABSTRACT FROM AUTHOR]

It is typically assumed that the kinetic and magnetic helicities play a crucial role in the growth of large-scale dynamo. In this paper, we demonstrate that helicity is not essential for the amplification of large-scale magnetic field. For this purpose, we perform nonhelical magnetohydrodynamic (MHD) simulation, and show that the large-scale magnetic field can grow in nonhelical MHD when random external forcing is employed at scale 1/10 the box size. The energy fluxes and shell-to-shell transfer rates computed using the numerical data show that the large-scale magnetic energy grows due to the energy transfers from the velocity field at the forcing scales. [ABSTRACT FROM AUTHOR]

Electromagnetic gyrokinetic particle-in-cell simulations have been inhibited for long time by numerical problems. This paper discusses the origin of these problems. It also gives an overview and summary of the mitigation techniques. [ABSTRACT FROM AUTHOR]

In a previous paper by V. D. Pustovitov [Phys. Plasmas 24, 112513 (2017)], it is claimed that the proofs of the "Resistive-Wall-Mode theorem" by Pfirsch and Tasso [Nucl. Fusion 11, 259 (1971)] and extensions of that theorem for time dependent wall resistivity and equilibrium plasma flow are not detailed and that there are limitations restricting their applicability. In response, we provide here pertinent detailed derivations, showing that the proofs of the above-mentioned theorems are rigorous and complete, unlike the considerations of V. D. Pustovitov [Phys. Plasmas 24, 112513 (2017)], which ignore the self-adjointness of the operator ∇ × ∇ × and the fact that the force operator in the linearized ideal MHD momentum equation remains self-adjoint in the presence of equilibrium flows. [ABSTRACT FROM AUTHOR]

The mechanism of the transient pinch at low densities, outlined in an early paper by Rosenbluth, has been studied in detail. The collapse velocity is comparable to the Alfv en velocity although the theory of magnetohydrodynamics is not applicable to collision-free plasmas. The thickness of the surface current layer is found to be the electron inertial length (c/ωpe). The electron and ion trajectories have been calculated, the latter being essentially due to the electrostatic field which transfers the j x B force from the electrons to the positive ions. [ABSTRACT FROM AUTHOR]

The loss-of-equilibrium is a solar eruption mechanism whereby a sudden breakdown of the magnetohydrodynamic force balance in the Sun's corona ejects a massive burst of particles and energy into the heliosphere. Predicting a loss-of-equilibrium, which has more recently been formulated as the torus instability, relies on a detailed understanding of the various forces that hold the pre-eruption magnetic flux rope in equilibrium. Traditionally, idealized analytical force expressions are used to derive simplified eruption criteria that can be compared to solar observations and modeling. What is missing, however, is a validation that these idealized analytical force expressions can be applied to the line-tied, low-aspect-ratio conditions of the corona. In this paper, we address this shortcoming by using a laboratory experiment to study the forces that act on longlived, arched, line-tied magnetic flux ropes. Three key force terms are evaluated over a wide range of experimental conditions: (1) the upward hoop force; (2) the downward strapping force; and (3) the downward toroidal field tension force. First, the laboratory force measurements show that, on average, the three aforementioned force terms cancel to produce a balanced line-tied equilibrium. This finding validates the laboratory force measurement techniques developed here, which were recently used to identify a dynamic toroidal field tension force that can prevent flux rope eruptions [Myers et al., Nature 528, 526 (2015)]. The verification of magnetic force balance also confirms the low-β assumption that the plasma thermal pressure is negligible in these experiments. Next, the measured force terms are directly compared to corresponding analytical expressions. While the measured and analytical forces are found to be well correlated, the low-aspect-ratio, line-tied conditions in the experiment are found to both reduce the measured hoop force and increase the measured tension force with respect to analytical expectations. These two co-directed effects combine to generate laboratory flux rope equilibria at lower altitudes than are predicted analytically. Such considerations are expected to modify the loss-of-equilibrium eruption criteria for analogous flux ropes in the solar corona. [ABSTRACT FROM AUTHOR]

Helical magneto-Rayleigh–Taylor instability (MRTI) structures have been observed in z-pinch-driven liner implosion experiments with a pre-imposed axial magnetic field. We show that the formation of these helical structures can be described by a Hall magnetohydrodynamical (HMHD) model. We used the 3D extended magnetohydrodynamics simulation code PERSEUS (which includes Hall physics) [Seyler and Martin, Phys. Plasmas 18, 012703 (2011)] to study these helical instabilities and show that a Hall interchange instability in low-density coronal plasma immediately surrounding the dense liner is responsible for producing helically oriented effects in the magnetic field and current density within the coronal layer. This seeds the helical pitch angle of the MRTI even when other proposed helical seeding mechanisms are either not present in the experiments or not accounted for in the simulations. For example, this mechanism does not require low-density power-feed plasmas to be swept in from large radius or the development of electrothermal instabilities. The Hall Instability is, thus, a new, independent explanation for the origin of the helical instabilities observed in axially premagnetized liner experiments. Simulation results supporting this mechanism are presented. [ABSTRACT FROM AUTHOR]

Advanced tokamak scenarios are a possible approach to boosting reactor performances. Such schemes usually trigger current holes, a particular magnetohydrodynamics equilibrium where no current or pressure gradients exist in the core of the plasma. While such equilibria have large bootstrap fractions, flat pressure profiles in the plasma core may not be optimal for a reactor. However, moderate modifications of the equilibrium current profile can lead to diamagnetism where most of the pressure gradient is now balanced by poloidal currents and the toroidal magnetic field. In this paper, we consider the properties of diamagnetic current holes, also called “dual equilibria,” and demonstrate that fusion throughput can be significantly increased in such scenarios. Their stability is investigated using the DCON code. Plasmas with a beta peak of 30% and an average beta of 6% are found stable to both fixed and free-boundary modes with toroidal mode numbers n=1–4, as well as Mercier and high-n ballooning modes. This is not surprising as these scenarios have a normal beta close to 3. [ABSTRACT FROM AUTHOR]

The present discussion revisits the problem of nonresonant heating of ions by Alfvénic turbulence. It is shown that in the limit of weak Alfvénic turbulence it is appropriate to describe the nonresonant heating of protons as perpendicular pseudoheating. However, in a more general situation it is demonstrated that the more appropriate view of the nonresonant heating process is the pitch-angle scattering in the wave frame. The purpose of this paper is to generalize the earlier theory to the case in which the energy density of the turbulent Alfvén waves is not necessarily very low. For weakly turbulent situation the present analysis confirms the earlier finding by Wu and Yoon [Phys. Rev. Lett. 99, 075001 (2007)], according to whom the nonresonant Alfvén wave heating is described as leading to perpendicular pseudoheating of the protons. However, for more general situation the present paper demonstrates that pitch-angle scattering plays the principal role in the Alfvén wave pseudoheating process, and thereby shows that the perpendicular heating discussed by Wu and Yoon is kinetic in nature, not attributable to fluid motion. [ABSTRACT FROM AUTHOR]

It was shown in the first part to this paper how a simple magnetohydrodynamic model can be used to determine the stability of a tokamak plasma’s edge to peeling (external kink) modes. It was found that stability is determined by the value of Δ′, a normalized measure of the discontinuity in the radial derivative of the radial perturbation to the magnetic field at the plasma-vacuum interface. To avoid the possibility that numerical divergences near the X-point might lead to misleading conclusions about plasma stability, this paper calculates the value of Δ′ analytically. This is accomplished by showing that the method of conformal transformations can be applied to systems with a continuously varying nonzero boundary condition and using the technique to obtain analytical expressions for both the vacuum energy and Δ′. A conformal transformation is also used to obtain an equilibrium vacuum field surrounding a plasma with a separatrix and X-point. This allows the analytical expressions for the vacuum energy and Δ′ to be evaluated. The results here, combined with those in the first part of this paper, subsequently provide a quantitative description of the peeling mode’s growth rate as the plasma-vacuum boundary more closely approximates a separatrix with an X-point. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is thought to be the primary mode by which the solar wind couples to the terrestrial magnetosphere, driving phenomena such as magnetic storms and aurorae. While the theory of two-dimensional reconnection is well developed and has been applied with great success to axisymmetric and toroidal systems such as laboratory plasma experiments and fusion devices, it is difficult to justify the application of two-dimensional theory to nontoroidal plasma systems such as Earth’s magnetosphere. Unfortunately, the theory of three-dimensional magnetic reconnection is much less well developed, and even defining magnetic reconnection has turned out to be controversial. In this paper, recent progress in the use of magnetohydrodynamics (MHD) to address the physics of three-dimensional reconnection in Earth’s magnetosphere is reviewed. The paper consists of two parts. In the first part, various definitions of three-dimensional reconnection are reviewed, with the goal of mapping these definitions to sets of physical phenomena that have been identified as “reconnection” in various contexts. In the second part of the paper, MHD simulation results for the magnetosphere are presented, and two qualitatively distinct types of reconnection phenomena are identified: (1) Steady separator reconnection under generic northward interplanetary magnetic field (IMF) conditions, involving plasma flow across magnetic separatrices, and (2) time-dependent reconnection under generic southward IMF conditions, involving a locally detectable change in the magnetic field topology. It is concluded that magnetic reconnection phenomena at Earth’s dayside magnetopause are adequately captured by two distinct definitions: The Vasyliunas definition [V. M. Vasyliunas, Rev. Geophys 13, 303 (1975)], which identifies magnetic reconnection with plasma flow across magnetic separatrices, and the Greene definition [J. Greene, Phys. Fluids B 5, 2355 (1993)], which identifies magnetic reconnection with a violation of magnetic flux conservation. Further generalizations of the definition of magnetic reconnection—e.g., the Schindler–Hesse [K. Schindler and M. Hesse, J. Geophys. Res. 93, 5547 (1988)] definition, which identifies reconnection with spatially localized violations of ideal MHD—are, while potentially useful in characterizing reconnection phenomena in the absence of magnetic nulls, separators, or separatrices, unnecessary in the magnetospheric context. [ABSTRACT FROM AUTHOR]

The response to tearing perturbations of a resonant layer within a toroidal plasma often includes real frequencies. These real frequencies are of importance not only for the stability of the tearing mode but also for determining the response to an error field, including locking torques, and the stability properties when interacting with a resistive wall. Unfortunately, including the physics which drives these frequencies can make the model highly complicated and mask the root physics mechanisms driving the response, making it difficult to draw connections between the physics mechanisms occurring in different regimes. In this paper, we present a simple and intuitive method to derive the tearing mode layer properties in various regimes of resistive MHD with the key physics effects of pressure gradient, toroidal field line curvature and parallel dynamics, focusing on two important regimes: The resistive-inertial (RI) and the viscoresistive (VR) regimes. The usual Glasser effect, a toroidal effect which involves real frequencies, has been discussed only in the context of the RI regime. We find that it occurs in the VR regime as well. Based on these results and the presence of real frequencies in many other tearing regimes, we reach the conclusion that real frequencies in tearing modes are the rule rather than the exception. We also find that the existence of tearing modes with real frequencies in the VR and RI regimes is related to nearby electrostatic resistive interchange modes, themselves with real frequencies. Another important effect we find is that the threshold for the destabilization of resistive wall tearing modes is significantly lowered by plasma rotation in the presence of real frequency layers. [ABSTRACT FROM AUTHOR]

One-dimensional (1D) particle-in-cell (PIC; kinetic ions and electrons) and hybrid (kinetic ions; adiabatic and massless fluid electrons) simulations of highly oblique slow shocks (θBn=84° and β=0.1) [Yin et al., J. Geophys. Res., 110, A09217 (2005)] have shown that the dissipation from the ions is too weak to form a shock and that kinetic electron physics is required. The PIC simulations also showed that the downstream electron temperature becomes anisotropic (Te∥>Te⊥), as observed in slow shocks in space. The electron anisotropy results, in part, from the electron acceleration/heating by parallel electric fields of obliquely propagating kinetic Alfvén waves (KAWs) excited by ion-ion streaming, which cannot be modeled accurately in hybrid simulations. In the shock ramp, spiky structures occur in density and electron parallel temperature, where the ion parallel temperature decreases due to the reduction of the ion backstreaming speed. In this paper, KAW and electron physics in oblique slow shocks are further examined under lower electron beta conditions. It is found that as the electron beta is reduced, the resonant interaction between electrons and the wave parallel electric fields shifts to the tail of the electron velocity distribution, providing more efficient parallel heating. As a consequence, for βe=0.02, the electron physics is shown to influence the formation of a θBn=75° shock. Electron effects are further enhanced at a more oblique shock angle (θBn=84°) when both the growth rate and the range of unstable modes on the KAW branch increase. Small-scale electron and ion phase-space vortices in the shock ramp formed by electron-KAW interactions and the reduction of the ion backstreaming speed, respectively, are observed in the simulations and confirmed in homogeneous geometries in one and two spatial dimensions in the accompanying paper [Yin et al., Phys. Plasmas 14, 062104 (2007)]. Results from this study conclude that the inability of the hybrid method to model the Landau resonance of KAW with electrons and the resulting time-evolving electron parallel heating lead to differences between the PIC and hybrid simulations. [ABSTRACT FROM AUTHOR]

The ideal magnetohydrodynamic stability of a cylindrical screw pinch (i.e., a current-carrying plasma column embedded into an external axial magnetic field) has been considered in the past in great detail. However, the majority of these studies pertain, in fact, to an infinitely long pinch, where the axial eigenmodes can be represented as exp(ikz). The finite length is then accounted for by assigning a specific value to k, k=2π/L, with L being the distance between the electrodes; in this way, one recovers the familiar Kruskal–Shafranov (KS) stability condition. In the present paper it is emphasized that the solution of the exp(ikz) type cannot satisfy the boundary conditions at the conducting end plates. Previous papers on this subject are reviewed. An effective technique that allows one to analytically obtain stability criteria in the long-thin approximation is developed. Even in this (“long-thin”) case substantial deviations from the KS condition are found. In the general case, a convenient representation is obtained for the Green’s functions that express perturbations both inside and outside the plasma in terms of the radial displacement of the plasma boundary. These expressions are then used in combination with the energy principle to evaluate corrections to the long-thin approximation. [ABSTRACT FROM AUTHOR]

This paper describes the formulation of a hybrid model with fully gyrokinetic ions and a zero-inertia fluid model for the electrons. The electron fluid equations are derived from moments of the drift kinetic equation, taking the small mass ratio limit, but with finite electron temperature. This model eliminates the inertial Alfvén wave and any physics relating to electron transit motion, making it useful for studying low frequency, high β (β>m[sub e]/m[sub i]) electromagnetic turbulence as well as kinetic magnetohydradynamics (MHD) physics including kinetic ballooning and toroidal Alfvén eigenmodes. Electromagnetic effects (δB[sub ⊥]) are included through the parallel ion and electron current. A predictor-corrector scheme for the fluid part that is consistent with the gyrokinetic ion part has been developed. Here we derive the model equations, derive the linear kinetic-fluid theory in a three-dimensional shearless slab, and compare the simulation results with the linear theory. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In toroidal, magnetically confined plasmas, the heat and particle transport is strongly anisotropic, with transport along the field lines sufficiently strong relative to cross-field transport that the equilibrium pressure can generally be regarded as constant on the flux surfaces in much of the plasma. The regions near small magnetic islands, and those near the X-lines of larger islands, are exceptions, having a significant variation of the pressure within the flux surfaces. It is shown here that the variation of the equilibrium pressure within the flux surfaces in those regions has significant consequences for the pressure driven currents. It is further shown that the consequences are strongly affected by the symmetry of the magnetic field if the field is invariant under combined reflection in the poloidal and toroidal angles. (This symmetry property is called "stellarator symmetry.") In non-stellarator-symmetric equilibria, the pressure-driven currents have logarithmic singularities at the X-lines. In stellarator-symmetric MHD equilibria, the singular components of the pressure-driven currents vanish. These equilibria are to be contrasted with equilibria having B rp 0; where the singular components of the pressure-driven currents vanish regardless of the symmetry. They are also to be contrasted with 3D MHD equilibrium solutions that are constrained to have simply nested flux surfaces, where the pressure-driven current goes like 1=x near rational surfaces, where x is the distance from the rational surface, except in the case of quasi-symmetric flux surfaces. For the purpose of calculating the pressure-driven currents near magnetic islands, we work with a closed subset of the MHD equilibrium equations that involves only perpendicular force balance, and is decoupled from parallel force balance. It is not correct to use the parallel component of the conventional MHD force balance equation, B rp 0; near magnetic islands. Small but nonzero values of B rp are important in this region, and small non-MHD contributions to the parallel force balance equation cannot be neglected there. Two approaches are pursued to solve our equations for the pressure driven currents. First, the equilibrium equations are applied to an analytically tractable magnetic field with an island, obtaining explicit expressions for the rotational transform and magnetic coordinates, and for the pressure-driven current and its limiting behavior near the X-line. The second approach utilizes an expansion about the X-line to provide a more general calculation of the pressure-driven current near an X-line and of the rotational transform near a separatrix. The study presented in this paper is motivated, in part, by tokamak experiments with nonaxisymmetric magnetic perturbations, where significant differences are observed between the behavior of stellarator-symmetric and non-stellarator-symmetric configurations with regard to stabilization of edge localized modes by resonant magnetic perturbations. Implications for the coupling between neoclassical tearing modes, and for magnetic island stability calculations, are also discussed. [ABSTRACT FROM AUTHOR]

In this paper it is shown that in ideal magnetohydrodynamics (IMHD) a two-dimensional equilibrium with an X-shaped neutral point (X-point) configuration reacts to small and smooth perturbations of the plasma pressure by the development of sheets with singular current density while the X-point structure is replaced by a different kind of configuration containing either cusp-like or T-shaped singularities. This behavior is demonstrated by computing the time dependent response to a suitable initial perturbation using a relaxation method within ideal MHD. The two-dimensional configurations considered in this paper are motivated by models of space plasma systems like, e.g. the Earth's magnetotail containing a distant X-point where perturbations caused by the solar wind may force the formation of current sheets and the subsequent occurrence of magnetic reconnection. [ABSTRACT FROM AUTHOR]

The effects of m = 0 modes on equilibrium Z-pinch plasmas are studied in this paper using a drift-ideal magnetohydrodynamic (MHD) model. The model equations are an extension of ideal MHD to include finite-ion-inertial-length/cyclotron-frequency (Ωi) effects in Ohm's law and in the electron and ion heat transport equations. The linear modes contained in this model include the ideal interchange (sausage) mode and in the magnetized limit, Ω i τ i ≫ 1 with τi the ion collision time, nonideal entropy modes. It is well known that these two modes are decoupled in the k ρ s ≪ 1 limit, where k is the axial mode number and ρ s = c s / Ω i is the gyro-Bohm scale with cs the sound speed [B. Kadomtsev, Sov. Phys. JETP-USSR 10, 780 (1960)]. For Bennett equilibrium profiles, it is shown that the regions of stability for both modes are completely governed by the adiabatic coefficient γ in these limits. Equilibria with Bennett profiles are stable to entropy modes for γ < 2 but unstable to ideal modes and vice versa for γ > 2. However, these modes are no longer decoupled when k ρ s ≳ 1. The simulation results of the fully nonlinear set of equations in the magnetized limit show that seeded modes with k ρ s ≳ 1 and γ = 5/3 display the characteristics of both ideal and entropy modes. The general heat flux for both ions and electrons as a function of the species magnetization is retained in the model. Both the linear and nonlinear behaviors of seeded modes for k ρ s ≳ 1 display a strong dependence on the magnetization of the ions. The growth rate increases linearly with k at large kρs when the ions are magnetized but decreases with increasing k when Ω i τ i ≲ 1. [ABSTRACT FROM AUTHOR]

Measuring the equations of state of a compressed magnetized plasma is important for both advancing fusion experiments and understanding natural systems such as stellar winds. In this paper, we present results from our experiments on the thermodynamics of compressed magnetized plasmas; we call these studies "magnetothermodynamics." In these experiments, we generate parcels of relaxed, magnetized plasma at one end of the linear Swarthmore Spheromak eXperimental device and observe their compression in a closed conducting boundary installed at the other end. Plasma parameters are measured during compression. Instances of ion heating during compression are identified by constructing a pressure-volume diagram using the measured density, temperature, and volume of the magnetized plasma. An axial scan of the ion temperature at upstream locations suggests that the increase in ion temperature arises due to the compression of the magnetized plasma in the conducting boundary. The theoretically predicted magnetohydrodynamic (MHD) and double adiabatic equations of state are compared with experimental measurements to estimate the adiabatic nature of the compressed plasma. The equilibrium of our magnetized plasmas is well-described by magnetohydrodynamics; however, we find that the MHD equation of state is not supported by our data. Our results are more consistent with the parallel Chew-Goldberger-Low equation of state, suggesting that there is significant anisotropy in the ion distribution function. [ABSTRACT FROM AUTHOR]

The Sweet-Parker and Petschek models are well-established magnetohydrodynamics (MHD) models of steady magnetic reconnection. Recent findings on magnetic reconnection in high-Lundquist-number plasmas indicate that Sweet-Parker-type reconnection in marginally stable thin current sheets connecting plasmoids can produce fast reconnection. By contrast, it has proven difficult to achieve Petschek-type reconnection in plasmas with uniform resistivity because sustaining it requires localization of the diffusion region. However, Shibayama et al. [Phys. Plasmas 22, 100706 (2015)] recently noted that Petschek-type reconnection can be achieved spontaneously in a dynamical manner even under uniform resistivity through what they called dynamical Petschek reconnection. In this new type of reconnection, Petschek-type diffusion regions can be formed in connection with plasmoids. In this paper, we report the results of two-dimensional resistive MHD simulation with uniform resistivity, undertaken to determine the diffusion region localization mechanism under dynamical Petschek reconnection. Through this modeling, we found that the separation of the X-point from the flow stagnation point (S-point) plays a crucial role in the localization of the diffusion region because the motion of the X-point is restricted by the strong flow emanating from the flow stagnation point. This mechanism suggests that dynamical Petschek reconnection is possible even in large systems such as the solar corona. [ABSTRACT FROM AUTHOR]

The successful application of techniques inspired in Smoothed Particle Hydrodynamics (SPH) to magnetohydrodynamical (MHD) nonlinear simulations of magnetically confined plasmas requires the previous solution to a number of challenging issues that are still not fully resolved, namely, the construction of precise, arbitrary initial conditions in complicated geometries, the formulation of adequate boundary conditions for the magnetic field, and the correct treatment of three-dimensional toroidal boundaries of the arbitrary shape. In this paper, we present an SPH implementation of the nonlinear MHD equations that include our proposed solution to these issues and test its performance on a broad selection of nonlinear MHD problems: (1) the propagation of circularly polarized Alfvén waves, (2) the occurrence of magnetic reconnection for a Harris current-sheet, and (3) the nonlinear MHD stability properties of various cylindrical pinches. [ABSTRACT FROM AUTHOR]

The present paper provides a comprehensive description of various regimes involved in the two-fluid model of resistive tearing instability. These include two novel regimes of this instability, which correspond to the long-wave modes that can develop in a highly elongated current sheet. This issue is relevant to the study of fast magnetic reconnection and magnetic turbulence in magnetohydrodynamic objects with a large value of the Lundquist number. [ABSTRACT FROM AUTHOR]

This paper analyzes the coupling of magneto-Rayleigh-Taylor (MRT), sausage, and kink modes in an imploding cylindrical liner, using ideal MHD. A uniform axial magnetic field of arbitrary value is included in each region: liner, its interior, and its exterior. The dispersion relation is solved exactly, for arbitrary radial acceleration (-g), axial wavenumber (k), azimuthal mode number (m), liner aspect ratio, and equilibrium quantities in each region. For small k, a positive g (inward radial acceleration in the lab frame) tends to stabilize the sausage mode, but destabilize the kink mode. For large k, a positive g destabilizes both the kink and sausage mode. Using the 1D-HYDRA simulation results for an equilibrium model that includes a pre-existing axial magnetic field and a preheated fuel, we identify several stages of MRT-sausage-kink mode evolution. We find that the m = 1 kink-MRT mode has a higher growth rate at the initial stage and stagnation stage of the implosion, and that the m = 0 sausage-MRT mode dominates at the main part of implosion. This analysis also sheds light on a puzzling feature in Harris' classic paper of MRT [E. G. Harris, Phys. Fluids 5, 1057 (1962)]. An attempt is made to interpret the persistence of the observed helical structures [Awe et al., Phys. Rev. Lett. 111, 235005 (2013)] in terms of non-axisymmetric eigenmode. [ABSTRACT FROM AUTHOR]

The many sources of uncertainty in validation studies of plasma turbulence in magnetically confined fusion devices are well-known. In this paper, we investigate how to efficiently transform uncertainties in experimentally derived transport model inputs into model prediction uncertainties, using the quasilinear trapped-gyro-Landau-fluid (TGLF) turbulent transport model [Staebler et al., Phys. Plasmas 14, 055909 (2007)]. We use the rapidly converging and computationally inexpensive non-intrusive probabilistic collocation method (PCM) to propagate input parameter uncertainty probability distribution functions (PDFs) through TGLF, yielding PDFs of predicted transport fluxes. We observe in many cases that the flux PDFs exhibit significant non-normal features such as strong skewness, even when the input distributions were normal. To illustrate the utility of the PCM approach, we apply this methodology to transport predictions for a DIII-D ITER baseline plasma [Grierson et al., Phys. Plasmas 25, 022509 (2018)] in which the mix of neutral beam injection (NBI) and electron cyclotron heating (ECH) was varied. The model predictions show clear changes in the parametric dependencies and sensitivities of the turbulence between the two heating mixes. Specifically, when only NBI heating was used, the transport fluxes responded significantly only to the ion temperature gradient scale length. However, when both NBI and ECH were applied, the electron transport channels demonstrate a strong sensitivity to the electron temperature and density gradients not observed in the NBI-only case. Additional context for the PCM approach is provided by comparing its predictions with those obtained via a local flux-matching approach. A new set of validation metrics based on the Wasserstein distance is proposed for PDF-based comparisons. [ABSTRACT FROM AUTHOR]

Drift instabilities contribute to the formation of edge turbulence and zonal flows and thus the anomalous transport in tokamaks. Experiments often found micro-scale turbulence strongly coupled with large-scale magnetohydrodynamic (MHD) processes, whereas a general framework has been lacking that can cover both regimes, in particular, their coupling. In this paper, the linear resistive drift wave instability is investigated using a full 2-fluid MHD model, as well as its numerical implementation in the NIMROD code. Both analytical and numerical analyses reveal a macro-scale global drift wave eigenmode coupled with MHD dynamics and illustrate a non-monotonic dispersion relation with respect to both perpendicular and parallel wavenumbers. NIMROD results also reveal an edge-localized behavior in the radial mode structure as the azimuthal mode number increases, implying the dependence of the 2-fluid effects due to the inhomogeneous density profile. The edge-localization introduces a non-trivial dependence of the effective perpendicular wavenumber to the perpendicular mode number, which may explain the quantitative difference between the global dispersion relation and its local approximation from the conventional local theory. [ABSTRACT FROM AUTHOR]

Electron vortices are observed in the numerical simulation of current carrying plasmas on fast time scales where the ion motion can be ignored. In plasmas with nonuniform density n, vortices drift in the B × ▽n direction with a speed that is on the order of the Hall speed. This provides a mechanism for magnetic field penetration into a plasma. Here, we consider strong vortices with rotation speeds Vɸ close to the speed of light c where the vortex size δ is on the order of the magnetic Debye length λB = |B|/4Πen and the vortex is thus nonquasineutral. Drifting vortices are typically studied using the electron magnetohydrodynamic model (EMHD), which ignores the displacement current and assumes quasineutrality. However, these assumptions are not strictly valid for drifting vortices when δ ≈ λB. In this paper, 2D electron vortices in nonuniform plasmas are studied for the first time using a fully electromagnetic, collisionless fluid code. Relatively large amplitude oscillations with periods that correspond to high frequency extraordinary modes are observed in the average drift speed. The drift speed W is calculated by averaging the electron velocity field over the vorticity. Interestingly, the time-averaged W from these simulations matches very well with W from the much simpler EMHD simulations even for strong vortices with order unity charge density separation. [ABSTRACT FROM AUTHOR]

In this paper, we consider high-energy electron scattering and nonlinear trapping by oblique whistler waves via the Landau resonance. We use recent spacecraft observations in the radiation belts to construct the whistler wave model. The main purpose of the paper is to provide an estimate of the critical wave amplitude for which the nonlinear wave-particle resonant interaction becomes more important than particle scattering. To this aim, we derive an analytical expression describing the particle scattering by large amplitude whistler waves and compare the corresponding effect with the nonlinear particle acceleration due to trapping. The latter is much more rare but the corresponding change of energy is substantially larger than energy jumps due to scattering. We show that for reasonable wave amplitudes ~10-100 mV/m of strong whistlers, the nonlinear effects are more important than the linear and nonlinear scattering for electrons with energies ~10-50keV. We test the dependencies of the critical wave amplitude on system parameters (background plasma density, wave frequency, etc.). We discuss the role of obtained results for the theoretical description of the nonlinear wave amplification in radiation belts. [ABSTRACT FROM AUTHOR]

One of the main FAST (Fusion Advanced Studies Torus) goals is to have a flexible experiment capable to test tools and scenarios for safe and reliable tokamak operation, in order to support ITER and help the final DEMO design. In particular, in this paper, we focus on operation close to a possible border of stability related to low-q operation. To this purpose, a new FAST scenario has then been designed at Ip = 10 MA, BT = 8.5 T, q95 ≈ 2.3. Transport simulations, carried out by using the code JETTO and the first principle transport model GLF23, indicate that, under these conditions, FAST could achieve an equivalent Q ≈ 3.5. FAST will be equipped with a set of internal active coils for feedback control, which will produce magnetic perturbation with toroidal number n = 1 or n = 2. Magnetohydrodynamic (MHD) mode analysis and feedback control simulations performed with the codes MARS, MARS-F, CarMa (both assuming the presence of a perfect conductive wall and using the exact 3D resistive wall structure) show the possibility of the FAST conductive structures to stabilize n = 1 ideal modes. This leaves therefore room for active mitigation of the resistive mode (down to a characteristic time of 1 ms) for safety purposes, i.e., to avoid dangerous MHD-driven plasma disruption, when working close to the machine limits and magnetic and kinetic energy density not far from reactor values. [ABSTRACT FROM AUTHOR]

The gas dynamics properties of a stationary arc plasma flows are studied through the numerical simulations. A two dimensional axis-symmetric turbulent magneto-hydrodynamic plasma model is developed with the commercial code ANSYS FLUENT. The reliable κ-ε model is used to account for turbulence. In this paper, the plasma is assumed to be a fluid following Navier-Stokes equations, respecting local thermodynamic equilibrium, and described by only one temperature. Distributions of the pressure, velocity, temperature, density, and electric potential inside of thus cascaded arc are obtained for an arc current density of 106 A/m². The pressure inside the arc varies from 105 Pa to 100 Pa. The temperature at the arc axis can reach as high as 13 600 K. The electric potential drops uniformly along the axis with a magnitude of 160 V. In addition, distributions of the sonic velocity and Mach number are shown to describe supersonic behavior of thus cascaded arc, which have a good agreement with the analytical formula. [ABSTRACT FROM AUTHOR]

In this paper, we present experimental results on axially magnetized (Bz = 0.5 – 2.0 T), thin-foil (400 nm-thick) cylindrical liner-plasmas driven with ∼600 kA by the Michigan Accelerator for Inductive Z-Pinch Experiments, which is a linear transformer driver at the University of Michigan. We show that: (1) the applied axial magnetic field, irrespective of its direction (e.g., parallel or anti-parallel to the flow of current), reduces the instability amplitude for pure magnetohydrodynamic (MHD) modes [defined as modes devoid of the acceleration-driven magneto-Rayleigh-Taylor (MRT) instability]; (2) axially magnetized, imploding liners (where MHD modes couple to MRT) generate m = 1 or m = 2 helical modes that persist from the implosion to the subsequent explosion stage; (3) the merging of instability structures is a mechanism that enables the appearance of an exponential instability growth rate for a longer than expected time-period; and (4) an inverse cascade in both the axial and azimuthal wavenumbers, k and m, may be responsible for the final m = 2 helical structure observed in our experiments. These experiments are particularly relevant to the magnetized liner inertial fusion program pursued at Sandia National Laboratories, where helical instabilities have been observed. [ABSTRACT FROM AUTHOR]

A well-known Taylor problem in the theory of forced magnetic reconnection is investigated in the framework of the Hall-Magnetohydrodynamics. In the first part of the paper, we deal with the linear theory of the Hall-mediated forced reconnection. Then, in the second part, these results are used for demonstrating how the secondary tearing (plasmoid) instability can develop in the course of this process. [ABSTRACT FROM AUTHOR]

This paper reports experimental and theoretical investigations on plasma turbulence in the source plasma of a Large Volume Plasma Device. It is shown that a highly asymmetrical localized thin rectangular slab of strong plasma turbulence is excited by loss cone instability. The position of the slab coincides with the injection line of the primary ionizing energetic electrons. Outside the slab, in the core, the turbulence is weaker by a factor of 30. The plasma turbulence consists of oblique [θ = tan-1(k⊥/k∥) ≈ 87°] Quasi-Longitudinal (QL) electromagnetic whistlers in a broad band of 40kHz⊥ ~ 1.2 cm-1 and k∥ ~ 0.06cm-1. Experimental observations suggest that the primary agent for the turbulence is not driven by primary ionizing energetic electrons but by the loss cone feature in the velocity distribution of reflected energetic electrons. A magnetic mirror is formed in the Electron Energy Filter when it is energized. It is shown that it is this mirror which is responsible for both reflection of the energetic electrons and imposing loss cone feature on it. Theoretical framework is based upon Oblique whistler approximation by Sharma and Vlahos [Astrophys. J. 280, 405 (1984)] and Verkhoglyadova et al. [J. Geophys. Res. 115, A00F19 (2010)] and Quasi Longitudinal (QL) whistlers by Booker and Dyce [Radio Sci. J. Res 69D (1965)] for excitation of the plasma turbulence in the magnetosphere. [ABSTRACT FROM AUTHOR]

RFX-mod [Sonato et al., Fusion Eng. Des. 66, 161 (2003)] exploits its 192 active coils in both reversed-field pinch (RFP) and tokamak configurations with varying degrees of 3D shaping, providing also a test bed for validating stellarator codes and 3D nonlinear magnetohydrodynamic codes. This makes RFX-mod a unique and flexible facility for comparative studies on 3D shaping and control. The paper discusses how 3D fields allow access to RFP and tokamak advanced regimes. 3D fields are used to feedback control Single Helicity (SH) RFP equilibria with 1/7 helicity up to ∼2 MA. They also allow accessing SH regimes with higher density (Greenwald fraction up to 0.5), presently inaccessible in spontaneous SH regimes. Feedback on the 2/1 resistive-wall mode in RFX-mod tokamak plasmas allows for safe operation at q(a)<2, an almost unexplored promising regime. Forcing the 2/1 mode to saturate at finite but small level, a helical tokamak equilibrium with significant n = 1 modulation is produced and a new way to tailor sawteeth is found. The effects of different levels of 3D shaping on momentum transport in both RFP and tokamak helical states are discussed. [ABSTRACT FROM AUTHOR]

Curvature and diamagnetic effects are both recognized to have a stabilizing influence on tearing modes in the linear regime. In this paper, we investigate the impact of these effects on the linear stability of a (2, 1) magnetic island using non-linear two-fluid MHD simulations and we apply our results to Tore Supra experiments where its stability is not well understood from the single fluid MHD model. Simulations show an initial increase of the linear growth rate and then its reduction until full stability as diamagnetic frequency increases. This mechanism is therefore a plausible explanation for experimental observations where the (2, 1) mode was not observed although the single fluid model predicted its growth. Our simulations also show the importance of curvature for an efficient stabilization. A simple analytical model is derived to support the numerical results. [ABSTRACT FROM AUTHOR]

It is shown that some of the main results of the recent paper by Lakhin and Ilgisonis [Phys. Plasmas 18, 092103 (2011)], viz. the derivation of the equations for the continuous spectra of poloidally and toroidally rotating plasmas and their special solution for large aspect ratio tokamaks with large parallel flows were obtained before by Goedbloed, Beliën, van der Holst, and Keppens [Phys. Plasmas 11, 28 (2004)]. A further rearrangement of the system of equations for the coupled Alfvén and slow continuous spectra clearly exhibits: (a) coupling through a single tangential derivative, which is a generalization of the geodesic curvature; (b) the 'transonic' transitions of the equilibrium, which need to be carefully examined in order to avoid entering hyperbolic flow regimes where the stability formalism breaks down. A critical discussion is devoted to the implications of this failure, which is generally missed in the tokamak literature, possibly as a result of the wide-spread use of the sonic Mach number of gas dynamics, which is an irrelevant and misleading parameter in 'transonic' magnetohydrodynamics. Once this obstacle in understanding is removed, further application of the theory of trans-slow Alfvén continuum instabilities to both tokamaks, with possible implications for the L-H transition, and astrophysical objects like 'fat' accretion disks, with a possible new route to magnetohydrodynamic turbulence, becomes feasible. [ABSTRACT FROM AUTHOR]

In this paper, a three-dimensional (3d) magneto-hydro-dynamic (MHD) model of axial magnetic field vacuum arcs (AMFVAs) is established. Based on this model, AMFVAs are simulated and analyzed. Three-dimensional spatial distributions of many important plasma parameters and electric characteristics in AMFVAs can be obtained, such as ion number density, ion temperature, electron temperature, plasma pressure, current densities along different directions (x, y, and z), ion velocities along different directions, electric fields strength along different directions, and so on. Simulation results show that there exist significant spiral-shaped rotational phenomena in the AMFVAs, this kind of rotational phenomenon also can be verified by the many related experiments (AMFVAs photographs, especially for stronger AMF strength). For current simulation results of AMFVAs, the maximal rotational velocity at anode side is about 1100 m/s. Radial electric field is increased from arc center to arc edge; axial electric field is decreased from cathode side to anode side. Radial electric field at arc edge can be larger than axial electric field. Azimuthal electric field in most regions is much smaller than radial and axial electric field, but it can reach about 1.19 kV/m. Radial magnetic field is the smallest one compared with other components, it reaches to maximum value at the position near to anode, it can influence arc characteristics. [ABSTRACT FROM AUTHOR]

Recent counter-helicity spheromak merging experiments in the Swarthmore Spheromak Experiment (SSX) have produced a novel compact torus (CT) with unusual features. These include a persistent antisymmetric toroidal magnetic field profile and a slow, nonlinear emergence of the n = 1 tilt mode. Experimental measurements are inconclusive as to whether this unique CT is a fully merged field-reversed configuration (FRC) with strong toroidal field or a partially merged 'doublet CT' configuration with both spheromak- and FRC-like characteristics. In this paper, the SSX merging process is studied in detail using three-dimensional resistive MHD simulations from the Hybrid Magnetohydrodynamics (HYM) code. These simulations show that merging plasmas in the SSX parameter regime only partially reconnect, leaving behind a doublet CT rather than an FRC. Through direct comparisons, we show that the magnetic structure in the simulations is highly consistent with the SSX experimental observations. We also find that the n = 1 tilt mode begins as a fast growing linear mode that evolves into a slower-growing nonlinear mode before being detected experimentally. A simulation parameter scan over resistivity, viscosity, and line-tying shows that these parameters can strongly affect the behavior of both the merging process and the tilt mode. In fact, merging in certain parameter regimes is found to produce a toroidal-field-free FRC rather than a doublet CT. [ABSTRACT FROM AUTHOR]

In the past, the resonant cyclotron interaction between runaway electrons and lower hybrid waves via anomalous Doppler broadening was experimentally investigated, and it was shown to be able to create a barrier to the energy that could be reached by the runaway electrons [E. Li et al., Nucl. Instrum. Methods Phys. Res. A 621, 566 (2010)]. In this paper, to our knowledge for the first time, experimental evidence will be provided for a resonant cyclotron interaction between runaway electrons and magnetohydrodynamics modes in a stochastic magnetic field in the experimental advanced superconducting tokamak (EAST), which has been theoretically proposed as a mechanism able to limit the maximum attainable energy by runaway electrons in tokamak plasmas [J. R. Martín-Solís and R. Sánchez, Phys. Plasmas 15, 112505 (2008)]. [ABSTRACT FROM AUTHOR]

A recent publication [D. B. Sinars et al., Phys. Rev. Lett. 105, 185001 (2010)] describes the first controlled experiments measuring the growth of the magneto-Rayleigh-Taylor instability in fast (∼100 ns) Z-pinch plasmas formed from initially solid aluminum tubes (liners). Sinusoidal perturbations on the surface of these liners with wavelengths of 25-400 μm were used to seed single-mode instabilities. The evolution of the outer liner surface was captured using multiframe 6.151 keV radiography. The initial paper shows that there is good agreement between the data and 2-D radiation magneto-hydrodynamic simulations down to 50 μm wavelengths. This paper extends the previous one by providing more detailed radiography images, detailed target characterization data, a more accurate comparison to analytic models for the amplitude growth, the first data from a beryllium liner, and comparisons between the data and 3D simulations. [ABSTRACT FROM AUTHOR]

The Sweet–Parker layer in a system that exceeds a critical value of the Lundquist number (S) is unstable to the plasmoid instability. In this paper, a numerical scaling study has been done with an island coalescing system driven by a low level of random noise. In the early stage, a primary Sweet–Parker layer forms between the two coalescing islands. The primary Sweet–Parker layer breaks into multiple plasmoids and even thinner current sheets through multiple levels of cascading if the Lundquist number is greater than a critical value Sc≃4×104. As a result of the plasmoid instability, the system realizes a fast nonlinear reconnection rate that is nearly independent of S, and is only weakly dependent on the level of noise. The number of plasmoids in the linear regime is found to scales as S3/8, as predicted by an earlier asymptotic analysis [N. F. Loureiro et al., Phys. Plasmas 14, 100703 (2007)]. In the nonlinear regime, the number of plasmoids follows a steeper scaling, and is proportional to S. The thickness and length of current sheets are found to scale as S-1, and the local current densities of current sheets scale as S-1. Heuristic arguments are given in support of theses scaling relations. [ABSTRACT FROM AUTHOR]

Describing turbulent transport in fusion plasmas is a major concern in magnetic confinement fusion. It is now widely known that kinetic and fluid descriptions can lead to significantly different properties. Although more accurate, the kinetic calculation of turbulent transport is much more demanding of computer resources than fluid simulations. An alternative approach is based on a water-bag representation of the distribution function that is not an approximation but rather a special class of initial conditions, allowing one to reduce the full kinetic Vlasov equation into a set of hydrodynamics equations while keeping its kinetic character [P. Morel, E. Gravier, N. Besse et al., Phys. Plasmas 14, 112109 (2007)]. In this paper, the water-bag concept is used in a gyrokinetic context to study finite Larmor radius effects with the possibility of using the full Larmor radius distribution instead of an averaged Larmor radius. The resulting model is used to study the ion temperature gradient (ITG) instability. [ABSTRACT FROM AUTHOR]