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This paper presents a novel coupling of a kinetic description of neutrals with a fluid description of a fusion plasma. The code, plasma interacting super-atoms and molecules (PISAM), employs a grid-free Cartesian geometry and a direct simulation Monte Carlo approach to solve the kinetic equations of deuterium atoms and molecules. The grid-free geometry and the parallel nature of the neutral dynamics, in the absence of neutral–neutral interactions, allow for an unlimited and work-efficient parallelization of PISAM that always ensures a balanced workload. The highly optimized Python implementation obtains good performance while securing easy accessibility to new users. The coupling of PISAM with the edge turbulence model HESEL is outlined with emphasis on the technical aspects of coupling Message Passing Interface-parallelized Python and C++ codes. Furthermore, the paper presents and analyzes simulation results from running the coupled HESEL-PISAM model. These results demonstrate the impact of radial neutral transport and plasma–neutral dynamics perpendicular to the magnetic field. Specifically, they illustrate how the inward flow of neutral kinetic energy and the inhibition of radial electric shear, resulting from poloidal momentum transfer between atoms and ions, can affect the energy containment time. By comparing the results of the HESEL-PISAM model with those obtained from coupling HESEL with a diffusive-fluid-neutral model, the capabilities of diffusion models in predicting neutral transport in the plasma edge and scrape-off layer are elucidated. [ABSTRACT FROM AUTHOR]

The high-dimensional and multiscale nature of fusion plasma flows require the development of reduced models to be implemented in numerical codes capable of capturing the main features of turbulent transport in a sufficiently short time to be useful during tokamak operation. This paper goes further in the analysis of the dynamics of the κ − ε model based on the turbulent kinetic energy κ and its dissipation rate ε [Baschetti et al., Nucl. Fusion 61, 106020 (2021)] to improve the predictability of the transverse turbulent transport in simulation codes. The present 1D results show further capabilities with respect to current models (based on constant effective perpendicular diffusion) and on the standard quasi-linear approach. The nonlinear dependence of D in κ and ε estimated from two additional transport equations allows us to introduce some non-locality in the transport model. This is illustrated by the existence of parameter ranges with turbulence spreading. The paper also addresses another issue related to the uncertainties on the inherent free parameters of such reduced model. The study proposes a new approach in the fusion community based on a variational data assimilation involving the minimization of a cost function defined as the distance between the reference data and the calculated values. The results are good and show the ability of the data assimilation to reduce uncertainties on the free parameters, which remains a critical point to ensure the total reliability of such an approach. [ABSTRACT FROM AUTHOR]

In this paper, we present an electron acceleration model based on Bloch surface waves (BSWs). In our model, a dielectric multilayer deposited on a prism substrate is used to generate BSWs by a femtosecond laser pulse. It is found that the field enhancement factor of BSWs is larger than that of surface plasmon polaritons. We numerically solve Maxwell's equations to explain the phenomenon in space and time domain. Various aspects of the acceleration mechanism are discussed, including BSWs coupling and evanescent attenuation, the influence of electron injection time and angle, the kinetic energy spectrum, the angular distribution, and the angular-resolved energy spectrum. Such a model provides an all-optical method for electron acceleration. [ABSTRACT FROM AUTHOR]

In laser-driven inertial confinement fusion (ICF) specifically with high laser intensities, energetic (hot) electrons (HEs) can be generated via laser-plasma instabilities. HEs can significantly impact the target performance by modifying the implosion hydrodynamics. In this paper, the effects of moderate-energy (about 20 to 40 keV) HEs on the evolution of two-dimensional single-mode ablative Rayleigh–Taylor instability (ARTI) are studied through numerical simulations with a multigroup diffusion model in which the HE population is treated as a high-energy group launched from the boundary. With HEs present, it is found that ARTI linear growth rates are reduced even though the acceleration of the implosion shell is enhanced by HEs. The reduction in the linear growth rate is owing to the increase in the ablation velocity and the density scale length, and this stabilization effect is greater in the shorter-wavelength modes and/or higher-energy HE cases. The ARTI linear growth does not get mitigated monotonically as the HE number density increases for a given fixed HE kinetic energy. The HE number density minimizing the ARTI growth rate is found, likely due to the competition of the stabilizing and destabilizing hydrodynamic-parameter variations caused by HEs. [ABSTRACT FROM AUTHOR]

Dynamics of the implosion of the dense plasma focus play an essential role in converting electrical energy into the kinetic energy of the current sheath and subsequent production of accelerated electrons, ions, hard X-ray, and neutron emission. This paper presents the analysis of the implosion parameters, such as the implosion velocity and imploding mass, coupled with electrical parameters observed on the PF-1000 facility with a modified electrode system. The first two parameters are based on the 16-frame Mach–Zehnder interferometer, which provides the spatial distribution of electron density in a time sequence. Measurement of the total current, current derivative, and voltage enables us to evaluate the total inductance and kinetic energy driven by the capacitor bank. Then comparing the inductances and kinetic energies evaluated from the interferograms and electrical waveforms can provide more precise information on the current flowing in the imploding sheath. We present a possible way to deal with the fact that only part of the total current flows through the imploding layer. With the supposition that the rest of the current flows close to the insulator, we conclude that roughly 70% of the total current flows through the pinch, which is in good agreement with an input parameter of the Lee model, for example. [ABSTRACT FROM AUTHOR]

It is pointed out that the analysis of the shear flow and its different limiting cases presented in the paper, Phys. Plasmas 18, 102105 (2011), using kinetic approach is misleading. [ABSTRACT FROM AUTHOR]

Yoon and Fang [Phys. Plasmas 15, 122312 (2008)] formulated a second-order nonlinear kinetic theory that describes the turbulence propagating in directions parallel/anti-parallel to the ambient magnetic field. Their theory also includes discrete-particle effects, or the effects due to spontaneously emitted thermal fluctuations. However, terms associated with the spontaneous fluctuations in particle and wave kinetic equations in their theory contain proper dimensionality only for an artificial one-dimensional situation. The present paper extends the analysis and re-derives the dimensionally correct kinetic equations for three-dimensional case. The new formalism properly describes the effects of spontaneous fluctuations emitted in three-dimensional space, while the collectively emitted turbulence propagates predominantly in directions parallel/anti-parallel to the ambient magnetic field. As a first step, the present investigation focuses on linear wave-particle interaction terms only. A subsequent paper will include the dimensionally correct nonlinear wave-particle interaction terms. [ABSTRACT FROM AUTHOR]

This paper discusses a strategy to initialize a two-dimensional (2D) Reynolds-averaged Navier–Stokes model [LANL's Besnard–Harlow–Rauenzahn (BHR) model] in order to describe an unsteady transitional Richtmyer–Meshkov (RM)-induced flow observed in on-going high-energy-density ensemble experiments performed on the OMEGA-EP facility. The experiments consist of a nominal single-mode perturbation (initial amplitude a 0 ≈ 10 and wavelength λ = 100 μ m) with target-to-target variations in the surface roughness subjected to the RM instability with delayed Rayleigh–Taylor in a heavy-to-light configuration. Our strategy leverages high-resolution three-dimensional (3D) implicit large eddy simulations (ILES) simulations to initialize BHR-relevant parameters and subsequently validate the 2D BHR results against the 3D ILES simulations. A suite of five 3D ILES simulations corresponding to five experimental target profiles is undertaken to generate an ensemble dataset. Using ensemble averages from the 3D simulations to initialize the turbulent kinetic energy in the BHR model (K0) demonstrates the ability of the model to predict the time evolution of the interface as well as the density-specific-volume covariance, b. To quantify the sensitivity of the BHR results to the choice of K0 and the initial turbulent length scale, S0, we execute a parameter sweep spanning four orders of magnitude for both S0 and K0, generating a parameter space consisting of 26 simulations. The Pearson's correlation coefficient is used as a measure of discrepancy between the 2D BHR and 3D ILES simulations and reveals that the ranges 8 ≲ S 0 ≲ 20 μ m and 109 ≲ K 0 ≲ 10 10 cm2/s2 produce predictions that agree best with the 3D ILES results. [ABSTRACT FROM AUTHOR]

This paper presents the details of intermittent ion leakage from a Penning trap during potential barrier closure [H. Himura et al., Phys. Plasmas 24, 102129 (2017)]. A high-energy tail arises in the distribution functions of parallel kinetic energies of the ions after they pass through the upstream potential barrier ϕiu. Correlation between ion leakage and the slew rate of ϕiu is clearly verified. The total number of ions leaking out during the initial leakage is approximately predictable by considering the axial profile of the potential well of the Penning trap. The axial oscillation of leftover ions in the potential well agrees with the (1,0) mode of spheroidal plasmas in Dubin's notation, herein referred to as "Dubin's (1,0) mode," which strongly correlates with the intermittent ion leakage following the initial leakage. [ABSTRACT FROM AUTHOR]

As long suspected, low mode asymmetry in inertially confined fusion (ICF) implosions has been implicated as a performance limiting factor [Casey et al., "Evidence of three-dimensional asymmetries seeded by high-density carbon-ablator nonuniformity in experiments at the national ignition facility," Phys. Rev. Lett. 126, 025002 (2021)]. Recently a non-linear, but solvable, theory [Hurricane et al., "An analytic asymmetric-piston model for the impact of mode-1 shell asymmetry on ICF implosions," Phys. Plasmas 27, 062704 (2020)] based upon the simple picture of a pair of asymmetric pistons has generated new insights and provided some practical formulas for estimating the degradation of an implosion due to mode-1 asymmetry and demonstrated a previously unrecognized connection between measured hot-spot drift velocity, nuclear down-scatter ratio asymmetry, and the concept of residual kinetic energy (RKE). Asymmetry of the implosion "shell," as opposed to asymmetry of the hot-spot, was key to the classical mechanics model because the majority of the kinetic energy in an implosion is carried by the shell. Herein, the two-piston model is extended to a six-piston model in order to capture mode-2 asymmetry and coupling between mode-1 and mode-2. A key result of this new six-piston model is that the weighted harmonic mean of shell areal density is the fundamental quantity that determines the RKE and performance degradations for a three-dimensional implosion. Agreement is found between the scalings coming from the theory and ICF implosion data from the National Ignition Facility and to large ensembles of detailed simulations. The connection between the piston model's dependence upon the radius of peak velocity and coast-time is also highlighted in this paper. Finally, by extending the two-piston model to include time-dependent "swing," it is shown in the Appendix that the shell asymmetry at the time of stagnation dominates the solution for RKE. [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]

In laser-driven fusion, determining the ignition margin is an important prerequisite for evaluating the ignition robustness of a target design. The ignition threshold factor (ITF), defined as the shell kinetic energy at the time of maximum implosion velocity divided by the minimum ignition energy of the capsule, is widely adopted as a specific metric of the ignition margin. In this paper, in contrast to previous definitions of ITF, an additional quantity, i.e., the shell aspect ratio (Ar) at the maximum implosion velocity time, is found to have an important influence on the ignition margin. With including the quantity of Ar, we have obtained an extended ITF through the statistical modeling of following two steps with the help of a free available MULTI-IFE code and the PyMC3 Bayesian inference package: first, the sensitivity of the ignition cliff on implosion physical quantities at the maximum implosion velocity time is evaluated and the importance of Ar is revealed; second, an extended ITF that is proportional to A r α (α ≈ − 1.6) is obtained. Our simulations on fusion yields identify a definitive ignition cliff when the extended ITF equals unity. We then conclude that the shell aspect ratio Ar is an important quantity in our extended ITF expression that will be helpful for evaluating and optimizing the ignition target designs and practical implosion experiments. [ABSTRACT FROM AUTHOR]

Nonuniformities present in the laser illumination and target in laser-driven inertial confinement fusion experiments lead to an asymmetric compression of the target, resulting in an inefficient conversion of shell kinetic energy to thermal energy of the hot-spot plasma. In this paper, the effects of asymmetric compression of cryogenic deuterium tritium laser-direct-drive implosions are examined using a suite of nuclear and x-ray diagnostics on the OMEGA laser. The neutron-averaged hot-spot velocity ( u → hs ) and apparent ion temperature ( T i ) asymmetry are determined from neutron time-of-flight measurements of the primary deuterium tritium fusion neutron energy spectrum, while the areal density (ρR) of the compressed fuel surrounding the hot spot is inferred from measurements of the scattered neutron energy spectrum. The low-mode perturbations of the hot-spot shape are characterized from x-ray self-emission images recorded along three quasi-orthogonal lines of sight. Implosions with significant mode-1 laser-drive asymmetries show large hot-spot velocities (>100 km/s) in a direction consistent with the hot-spot elongation observed in x-ray images, measured T i asymmetry, and ρR asymmetry. Laser-drive corrections have been applied through shifting the initial target location in order to mitigate the observed asymmetry. With the asymmetry corrected, a more-symmetric hot spot is observed with reduced u → hs , T i asymmetry, ρR asymmetry, and a 30% increase in the fusion yield. [ABSTRACT FROM AUTHOR]

In collision-poor space plasmas, protons with an excess of kinetic energy or temperature in the direction perpendicular to the background magnetic field can excite the electromagnetic ion cyclotron (EMIC) instability. This instability is expected to be highly sensitive to suprathermal protons, which enhance the high-energy tails of the observed velocity distributions and are well reproduced by the (bi-)Kappa distribution functions. In this paper, we present the results of a refined quasi-linear approach, able to describe the effects of suprathermal protons on the extended temporal evolution of EMIC instability. It is, thus, shown that suprathermals have a systematic stimulating effect on the EMIC instability, enhancing not only the growth rates and the range of unstable wavenumbers but also the magnetic fluctuating energy density reached at the saturation. In effect, the relaxation of anisotropic temperature also becomes more efficient, i.e., faster in time and closer to isotropy. [ABSTRACT FROM AUTHOR]

This paper studies the turbulent kinetic energy ( k ⊥ ) in 2D isothermal electrostatic interchange-dominated E × B drift turbulence in the scrape-off layer and its relation to particle transport. An evolution equation for the former is analytically derived from the underlying turbulence equations. Evaluating this equation shows that the dominant source for the turbulent kinetic energy is due to interchange drive, while the parallel current loss to the sheath constitutes the main sink. Perpendicular transport of the turbulent kinetic energy seems to play a minor role in the balance equation. Reynolds stress energy transfer also seems to be negligible, presumably because no significant shear flow develops under the given assumptions of isothermal sheath-limited conditions in the open field line region. The interchange source of the turbulence is analytically related to the average turbulent E × B energy flux, while a regression analysis of TOKAM2D data suggests a model that is linear in the turbulent kinetic energy for the sheath loss. A similar regression analysis yields a diffusive model for the average radial particle flux, in which the anomalous diffusion coefficient scales with the square root of the turbulent kinetic energy. Combining these three components, a closed set of equations for the mean-field particle transport is obtained, in which the source of the turbulence depends on mean flow gradients and k ⊥ through the particle flux, while the turbulence is saturated by parallel losses to the sheath. Implementation of this new model in a 1D mean-field code shows good agreement with the original TOKAM2D data over a range of model parameters. [ABSTRACT FROM AUTHOR]

Simulations predict that directly driven multi-shell targets can provide a robust alternative to conventional high-convergence implosion concepts by coupling two to three times more energy into the final igniting thermonuclear fuel assembly than indirect-drive concepts. The three-shell directly driven Revolver concept [K. Molvig, M. J. Schmitt, B. J. Albright, E. S. Dodd, N. M. Hoffman, G. H. McCall, and S. D. Ramsey, Phys. Rev. Lett. 116, 255003 (2016)] utilizes a design that maximizes laser energy conversion into inward kinetic energy of the outermost ablator shell (∼9%) while minimizing the DT fuel convergence (∼9) to reduce the mixing of material from the innermost shell into the fuel. Inherent in this design concept is the use of 192 narrow beams (with a 1/e laser beam-to-capsule diameter ratio of 0.33) from the National Ignition Facility laser pointed in a polar direct drive laser configuration. In this paper, we demonstrate that low average laser intensity at the capsule surface (≤300 TW/cm2) limits the measured laser backscatter, indicating that a greater amount of laser energy is coupled into the target. Omega experiments have been performed to determine the coupling of laser energy to the outermost shell of a scaled Revolver target (i.e., the ablator shell) by measuring capsule implosion trajectories and scattered-light fractions for two different drive configurations. Comparisons of simulated shell trajectory and velocity profiles with experimental data obtained from self-emission images show good agreement and are consistent with measured scattered light data. Moreover, the low levels of scattered light measured are consistent with post-shot simulation results that show high hydro-coupling efficiency. These results strengthen the case for using narrow beams at low intensity to drive large ablator capsules for future direct-drive, multi-shell ignition concepts. [ABSTRACT FROM AUTHOR]

Low-mode asymmetry is known to be a main source of yield degradation in implosion experiments performed at the National Ignition Facility at the Lawrence Livermore National Laboratory. In this paper, we present a theoretical model of the deceleration phase to investigate low-mode asymmetries, which is derived by considering the main fuel layer to be composed of thin shell pieces and neglecting the interaction between these pieces in the longitudinal direction. The model is able to characterize the evolution of low-mode asymmetries and assess the corresponding performance degradation, as validated numerically using the radiation hydrodynamics code LARED-S. The deceleration phases of implosions modulated separately by P2 (in Legendre polynomials) asymmetries in the shell mass, shell velocity, and hot-spot radius are studied using this model. It is found that asymmetries in the shell velocity and hot-spot radius have more pronounced effects than shell mass asymmetry on capsule distortion, resulting in greater yield degradation. The results obtained using this model indicate that yield degradation is mainly caused by the increase in residual kinetic energy at stagnation time, which is identical for all three types of asymmetries. [ABSTRACT FROM AUTHOR]

The two-dimensional Richtmyer-Meshkov instability occurs as shock waves pass through a perturbed material interface, triggering transition to an inhomogeneous turbulence variable density flow. This paper presents a series of large-eddy-simulations of the two dimensional turbulent RM instability and compares the results to the fully three dimensional simulations. There are two aims for this paper, the first is to explore what numerical resolution is required for a statistically converged solution for a two dimensional inhomogeneous flow field. The second aim is to elucidate the key differences in flow physics between the two dimensional and three dimensional Richtmyer-Meshkov instabilities, particularly their asymptotic self-similar regime. Convergence is achieved using 64 independent realisations and grid resolutions up to 4096² in the plane. It is shown that for narrowband cases the growth rate θ = 0.48 which is substantially higher than the three-dimensional equivalent. Mix measures are consistently lower compared to three-dimensional, and the kinetic energy distribution is homogeneous at late time. The broadband case has a similar initial growth rate as the three-dimensional case, with a marginally lower θ = 0.63. Mix is similar in magnitude, but is reducing at late time. The spectra in both cases exhibit the dual-cascade expected from two-dimensional turbulence. [ABSTRACT FROM AUTHOR]

This paper investigates nonlinear evolutions of large amplitude oblique whistler waves (LOWWs) and the interaction with electrons using one-dimensional electromagnetic kinetic simulations. The present research is motivated by recent studies about the nonlinear phenomena of LOWWs. When the propagation angle is not close to the resonance cone angle, the trapping of electrons in the electric potential of LOWWs leads to a moderate damping and a mild acceleration of the electrons via the O'Neil-type damping. In contrast, when the propagation angle of LOWWs is close to the resonance cone angle, the LOWWs undergo a heavy damping accompanied by the stochastic thermalization of the electrons, especially in the perpendicular direction. It is found that the stochastic parameter S, defined by S = 16 k ∥ 2 ( e m e ) Φ 0 | J 0 ( k ⊥ ρ ) | ω c e 2 , is a crucial factor determining the damping process. This result demonstrates the importance of self-consistent electron kinetic effects, which are not included in the previous single-particle or fluid approach. The implications of the present findings are discussed. [ABSTRACT FROM AUTHOR]

In this paper, we show that in some parameter range in gyrokinetic simulations, it is possible to apply a control method to stimulate the appearance of zonal flows while minimizing the duration of the control process and the impact on plasma parameters. For this purpose, a gyrokinetic code considering only trapped particles is used. The starting point of our work is a situation where zonal flows transiently appear after the nonlinear phase of saturation of trapped electron modes or trapped ion modes' micro-instabilities. These are observed to be strongly reduced in a later phase, permitting streamers to govern the plasma behavior in the steady-state. By intervening during this latter state (after this transient growth and decay of zonal flow), i.e., by increasing the ion/electron temperature ratio for a short time, it is found to be possible to bifurcate to a new steady-state, in which zonal flows are strongly present and are maintained indefinitely, thereby allowing a significant reduction in radial heat fluxes. [ABSTRACT FROM AUTHOR]

The neoclassical polarization current, which can be generated by a time-dependent electric field resulting from magnetic island rotation, is believed to play an important role in the initial stage of the neoclassical tearing mode (NTM) evolution in tokamak plasmas. In the previous analytical description of the neoclassical polarization current contribution to the evolution of NTMs in the limit of low collision frequency (νii ≪ εω, νii is ion collision frequency, ε is the inverse aspect ratio, and ω is the island propagation frequency in the plasma rest frame), the width of magnetic islands has been assumed to be much larger than the finite-banana-width (FBW) of the trapped ions in order to solve the drift-kinetic equation of ions by using the perturbation method. In this paper, we introduce a new analytical approach to investigate the neoclassical polarization current contribution to the NTM evolution without the assumption of the large island width by solving the drift-kinetic equation in a so-called ion-banana-center coordinate system. The results show that, when the island width is comparable to the FBW of the thermal ion, the neoclassical polarization current term in the equation of the NTM evolution is much smaller than the previous analytical expression but matches well with the empirical anticipation commonly adopted in experiments. [ABSTRACT FROM AUTHOR]

The study presented in this paper has shown that the generation of hard x rays and high-energy ions, which are detected in pinch implosion experiments, may be associated with the Coulomb explosion of the hot spot that is formed due to the outflow of the material from the pinch cross point. During the process of material outflow, the temperature of the hot spot plasma increases, and conditions arise for the plasma electrons to become continuously accelerated. The runaway of electrons from the hot spot region results in the buildup of positive space charge in this region followed by a Coulomb explosion. The conditions for the hot spot plasma electrons to become continuously accelerated have been revealed, and the estimates have been obtained for the kinetic energy of the ions generated by the Coulomb explosion. [ABSTRACT FROM AUTHOR]

This paper reports on a simulation of reflectometry measurement in Tore Supra ohmic discharges, for which the experimental observations as well as gyrokinetic non-linear computations predict a modification of turbulence spectrum between the linear (LOC) and the saturated ohmic confinement (SOC) regimes. Synthetic reflectometry simulations coupling full-wave computations with gyrokinetic data are carried out. This allows a direct comparison between the gyrokinetic non-linear predictions and experimental observations. The synthetic diagnostic results are found in a good agreement with the experimental findings; in particular, they reproduce well the quasi-coherent peak in the fluctuation spectrum of LOC regimes dominated by a trapped electron mode turbulence. It is also shown that such synthetic tools are valuable for (i) an enhanced interpretation of the reflectometry measurement (for instance, through the investigation of the 2D effects) and (ii) a better understanding of the turbulence properties (for instance, via the analysis of its poloidal asymmetry). [ABSTRACT FROM AUTHOR]

Magnetic flux ropes (MFR) are universal magnetoplasma structures (similar to cylindrical screw pinches) formed in reconnecting current sheets. In particular, MFR with scales from about the ion inertial length to MHD range are widely observed in the Earth magnetosphere. Typical MFR have force-free configuration with the axial magnetic field peaking on the MFR axis, whereas bifurcated MFR with an off-axis peak of the axial magnetic field are observed as well. In the present paper, we develop kinetic models of force-free and bifurcated MFR and determine consistent ion and electron distribution functions. The magnetic field configuration of the force-free MFR represents well-known Gold-Hoyle MFR (uniformly twisted MFR). We show that bifurcated MFR are characterized by the presence of cold and hot current-carrying electrons. The developed models are capable to describe MFR observed in the Earth magnetotail as well as MFR recently observed by Magnetospheric Multiscale Mission at the Earth magnetopause. [ABSTRACT FROM AUTHOR]

The bi-modal nature of charge integrated ion kinetic energy distributions, which result from ultrafast laser produced plasmas, is discussed in this paper. A negatively biased Faraday cup was used as a charge collector to measure ion distributions from three different solid targets that had been irradiated with an ultrafast laser in the fluence range 0:1-1 J/cm². A bi-modal time of flight distribution is found for all three targets (C, Al, and Cu). In the case of the metallic targets (Al and Cu), high- and low-kinetic energy peaks exhibit quite different dependencies on laser fluence, whereas for the semi-metallic target (C), both peaks scale similarly with ultrafast laser fluence. The results are discussed within the framework of a one dimensional capacitor model resulting in ion acceleration. [ABSTRACT FROM AUTHOR]

In this paper, we perform a series of high-resolution 3D simulations of an OMEGA-type inertial confinement fusion (ICF) capsule implosion with varying levels of initial long-wavelength asymmetries in order to establish the physical energy loss mechanism for observed yield degradation due to longwavelength asymmetries in symcap (gas-filled capsule) implosions. These simulations demonstrate that, as the magnitude of the initial asymmetries is increased, shell kinetic energy is increasingly retained in the shell instead of being converted to fuel internal energy. This is caused by the displacement of fuel mass away from and shell material into the center of the implosion due to complex vortical flows seeded by the long-wavelength asymmetries. These flows are not fully turbulent, but demonstrate mode coupling through non-linear instability development during shell stagnation and late-time shock interactions with the shell interface. We quantify this effect by defining a separation lengthscale between the fuel mass and internal energy and show that this is correlated with yield degradation. The yield degradation shows an exponential sensitivity to the RMS magnitude of the longwavelength asymmetries. This strong dependence may explain the lack of repeatability frequently observed in OMEGA ICF experiments. In contrast to previously reported mechanisms for yield degradation due to turbulent instability growth, yield degradation is not correlated with mixing between shell and fuel material. Indeed, an integrated measure of mixing decreases with increasing initial asymmetry magnitude due to delayed shock interactions caused by growth of the long-wavelength asymmetries without a corresponding delay in disassembly. [ABSTRACT FROM AUTHOR]

In evaluating neoclassical transport by radially local simulations, the magnetic drift tangential to a flux surface is usually ignored in order to keep the phase-space volume conservation. In this paper, effect of the tangential magnetic drift on the local neoclassical transport is investigated. To retain the effect of the tangential magnetic drift in the local treatment of neoclassical transport, a new local formulation for the drift kinetic simulation is developed. The compressibility of the phasespace volume caused by the tangential magnetic drift is regarded as a source term for the drift kinetic equation, which is solved by using a two-weight δf Monte Carlo method for non-Hamiltonian system [G. Hu and J. A. Krommes, Phys. Plasmas 1, 863 (1994)]. It is demonstrated that the effect of the drift is negligible for the neoclassical transport in tokamaks. In non-axisymmetric systems, however, the tangential magnetic drift substantially changes the dependence of the neoclassical transport on the radial electric field Er. The peaked behavior of the neoclassical radial fluxes around Er=0 observed in conventional local neoclassical transport simulations is removed by taking the tangential magnetic drift into account. [ABSTRACT FROM AUTHOR]

In this paper, the effect of an external inhomogeneous magnetic field on the high intensity laser absorption rate in a sub-critical plasma has been investigated by employing a relativistic electromagnetic 1.5 dimensional particle-in-cell code. Relying on the effective nonlinear phenomena such as phase-mixing and scattering, this study shows that in a finite-size plasma the laser absorption increases with inhomogeneity of the magnetic field (i.e., reduction of characteristic length of inhomogeneous magnetic field, λp) before exiting a considerable amount of laser energy from the plasma due to scattering process. On the other hand, the presence of the external inhomogeneous magnetic field causes the maximum absorption of laser to occur at a shorter time. Moreover, study of the kinetic results associated with the distribution function of plasma particles shows that, in a special range of the plasma density and the characteristic length of inhomogeneous magnetic field, a considerable amount of laser energy is transferred to the particles producing a population of electrons with kinetic energy along the laser direction. [ABSTRACT FROM AUTHOR]

In this paper, we have developed the kinetics of E-region ionospheric plasma comprising of fine dust grains and shown that the electron density in E-layer can purposely be reduced/enhanced up to desired level by inserting fine dust particles of appropriate physical/material properties; this may certainly be promising for preferred rf-signal processing through these layers. The analytical formulation is based on average charge theory and includes the number and energy balance of the plasma constituents along with charge balance over dust particles. The effect of varying number density, work function, and photo-efficiency of dust particles on ionospheric plasma density at different altitude in E-layer has been critically examined and presented graphically. [ABSTRACT FROM AUTHOR]

Experiments conducted at DIII-D investigate the role of drift kinetic damping and fast neutral beam injection (NBI)-ions in the approach to the no-wall βN limit. Modelling results show that the drift kinetic effects are significant and necessary to reproduce the measured plasma response at the ideal no-wall limit. Fast neutral-beam ions and rotation play important roles and are crucial to quantitatively match the experiment. In this paper, we report on the model validation of a series of plasmas with increasing βN, where the plasma stability is probed by active magnetohydrodynamic (MHD) spectroscopy. The response of the plasma to an externally applied field is used to probe the stable side of the resistive wall mode and obtain an indication of the proximity of the equilibrium to an instability limit. We describe the comparison between the measured plasma response and that calculated by means of the drift kinetic MARS-K code [Liu et al., Phys. Plasmas 15, 112503 (2008)], which includes the toroidal rotation, the electron and ion drift-kinetic resonances, and the presence of fast particles for the modelled plasmas. The inclusion of kinetic effects allows the code to reproduce the experimental results within ~13% for both the amplitude and phase of the plasma response, which is a significant improvement with respect to the undamped MHD-only model. The presence of fast NBI-generated ions is necessary to obtain the low response at the highest βN levels (~90% of the ideal no-wall limit). The toroidal rotation has an impact on the results, and a sensitivity study shows that a large variation in the predicted response is caused by the details of the rotation profiles at high βN . [ABSTRACT FROM AUTHOR]

In this paper, we revisit the hydrodynamic limit of the Langmuir wave dispersion relation based on the Wigner-Poisson model in connection with that obtained directly from the original Lindhard dielectric function based on the random-phase-approximation. It is observed that the (fourth-order) expansion of the exact Lindhard dielectric constant correctly reduces to the hydrodynamic dispersion relation with an additional term of fourth-order, beside that caused by the quantum diffraction effect. It is also revealed that the generalized Lindhard dielectric theory accounts for the recently discovered Shukla-Eliasson attractive potential (SEAP). However, the expansion of the exact Lindhard static dielectric function leads to a k4 term of different magnitude than that obtained from the linearized quantum hydrodynamics model. It is shown that a correction factor of 1/9 should be included in the term arising from the quantum Bohm potential of the momentum balance equation in fluid model in order for a correct plasma dielectric response treatment. Finally, it is observed that the long-range oscillatory screening potential (Friedel oscillations) of type cos(2kFr)/r³, which is a consequence of the divergence of the dielectric function at point k = 2kF in a quantum plasma, arises due to the finiteness of the Fermi-wavenumber and is smeared out in the limit of very high electron number-densities, typical of white dwarfs and neutron stars. In the very low electron number-density regime, typical of semiconductors and metals, where the Friedel oscillation wavelength becomes much larger compared to the interparticle distances, the SEAP appears with a much deeper potential valley. It is remarked that the fourth-order approximate Lindhard dielectric constant approaches that of the linearized quantum hydrodynamic in the limit if very high electron number-density. By evaluation of the imaginary part of the Lindhard dielectric function, it is shown that the Landau-damping region in ω-k plane increases dramatically by increase of the electron number-density. [ABSTRACT FROM AUTHOR]

In hydrodynamic turbulence, the kinetic energy injected at large scales cascades to the inertial range, leading to a constant kinetic energy flux. In contrast, in magnetohydrodynamic (MHD) turbulence, a fraction of kinetic energy is transferred to the magnetic energy. Consequently, for the same kinetic energy injection rate, the kinetic energy flux in MHD turbulence is reduced compared to its hydrodynamic counterpart. This leads to relative weakening of the nonlinear term ⟨ | (u · ∇) u | ⟩ , (where u is the velocity field) and turbulent drag, but strengthening of the velocity field in MHD turbulence. We verify the above using shell model simulations of hydrodynamic and MHD turbulence. Quasi-static MHD turbulence too exhibits turbulent drag reduction similar to MHD turbulence. [ABSTRACT FROM AUTHOR]

Direct measurement of axial magnetic field in the PF-1000 dense plasma focus (DPF), and its reported correlation with neutron emission, call for a fresh look at previous reports of existence of axial magnetic field component in the DPF from other laboratories, and associated data suggesting toroidal directionality of fast ions participating in fusion reactions, with a view to understand the underlying physics. In this context, recent work dealing with application of the hyperbolic conservation law formalism to the DPF is extended in this paper to a curvilinear coordinate system, which reflects the shape of the DPF current sheath. Locally unidirectional shock propagation in this coordinate system enables construction of a system of 7 one-dimensional hyperbolic conservation law equations with geometric source terms, taking into account all the components of magnetic field and flow velocity. Rankine-Hugoniot jump conditions for this system lead to expressions for the axial magnetic field and three components of fluid velocity having high ion kinetic energy. [ABSTRACT FROM AUTHOR]

During transient instabilities in a 2 eV, highly collisional MHD-driven plasma jet experiment, evidence of a 6 keV electron tail was observed via x-ray measurements. The cause for this unexpected high energy tail is explored using numerical simulations of the Rutherford scattering of a large number of electrons and ions in the presence of a uniform electric field that is abruptly turned on as in the experiment. When the only active processes are Rutherford scattering and acceleration by the electric field, contrary to the classical Fokker–Planck theory of plasma resistivity, it is found that no steady state develops, and instead, the particle kinetic energy increases continuously. However, when a power loss mechanism is introduced mimicking atomic line radiation, a near steady state can develop and, in this case, an energetic electron tail similar to that observed in the experiment can develop. The reasons underlying this behavior are analyzed, and it is shown that an important consideration is that Rutherford scattering is dominated by the cumulative effect of grazing collisions, whereas atomic line radiation requires an approximately direct rather than a grazing collision. [ABSTRACT FROM AUTHOR]

In the double-cone ignition (DCI) inertial confinement fusion (ICF) scheme, head-on collision of high density plasma jets is one of the most distinguished feature when compared with the traditional central ignition and fast ignition of ICF. However, the application of traditional hydrodynamic simulation methods becomes limited, due to serious plasma penetrations, mixing, and kinetic physics that might occur in the collision process. To overcome such limitations, we propose a new simulation method for large-scale high density plasmas. This method takes advantages of modern particle-in-cell simulation techniques and binary Monte Carlo collisions, including both long-range collective electromagnetic fields and short-range particle–particle interactions. Especially, in this method, the restrictions of simulation grid size and time step, which usually appear in a fully kinetic description, are eliminated. In addition, collisional coupling and state-dependent coefficients, which are usually approximately used with different forms in fluid descriptions, are also removed in this method. Energy and momentum exchanges among particles and species, such as thermal conductions and frictions, are modeled by "first principles" kinetic approaches. The correctness and robustness of the new simulation method are verified, by comparing with fully kinetic simulations at small scales and purely hydrodynamic simulations at large scale. Following the conceptual design of the DCI scheme, the colliding process of two plasma jets with initial density of 100 g/cc, initial thermal temperature of 65 eV, and counter-propagating velocity at 300 km/s is investigated using this new simulation method. Quantitative values, including density increment, increased plasma temperature, confinement time at stagnation, and conversion efficiency from the colliding kinetic energy to thermal energy, are obtained with a density increment of about three times, plasma temperature of 400 eV, confinement time at stagnation of 50 ps, and conversion efficiency of 85%. These values agree with the recent experimental measurements at a reasonable range. [ABSTRACT FROM AUTHOR]

To comprehensively understand the saturation of two-dimensional (2D) magnetized Kelvin–Helmholtz-instability-driven turbulence, energy transfer analysis is extended from the traditional interaction between scales to include eigenmode interactions, by using the nonlinear couplings of linear eigenmodes of the ideal instability. While both kinetic and magnetic energies cascade to small scales, a significant fraction of turbulent energy deposited by unstable modes in the fluctuation spectrum is shown to be re-routed to the conjugate-stable modes at the instability scale. They remove energy from the forward cascade at its inception. The remaining cascading energy flux is shown to attenuate exponentially at a small scale, dictated by the large-scale stable modes. Guided by a widely used instability-saturation assumption, a general quasi-linear model of instability is tested by retaining all nonlinear interactions except those that couple to the large-scale stable modes. These complex interactions are analytically removed from the magnetohydrodynamic equations using a novel technique. Observations are an explosive large-scale vortex separation instead of the well-known merger of 2D, a dramatic enhancement in turbulence level and spectral energy fluxes, and a reduced small-scale dissipation length scale. These show the critical role of the stable modes in instability saturation. Possible reduced-order turbulence models are proposed for fusion and astrophysical plasmas, based on eigenmode-expanded energy transfer analyses. [ABSTRACT FROM AUTHOR]

The ionization effect on both the evolution and internal structure of a blast wave (BW) is determined in laboratory conditions. In a first step, the Rankine–Hugoniot equations describing the structure of the shock front together with the Saha equation modeling ionization are solved analytically in a consistent way for the conditions of a cold initial atomic hydrogen gas. In a second step, a simplified approach is used by introducing an effective adiabatic index γ * that takes into account ionization arising at the shock front. Finally, γ * is used as input data in the self-similar model derived formerly by Barenblatt to describe the structure and the dynamics of the ionizing BW. For the typical laboratory conditions of blast wave experiments, ionization achieves a hydrogen gas compression up to about 11 times at the shock front of the blast wave where a thin and dense shell forms. For such a compression, the value of the effective adiabatic index is γ * ≃ 1.2 leading to a self-similar evolution of the BW where its radius R(t) varies according to R (t) ∝ t α * with α * ≃ 0.33. This value of α * is lower than the adiabatic expansion stage α = 2 / 5 , where the total energy of the BW is conserved. Thus, ionization is found to act as a cooling effect at the shock front where a fraction of kinetic energy is absorbed to ionize the gas. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is a key fundamental process in collisionless plasmas, which converts magnetic energy to plasma kinetic energy. Past observation and simulation studies suggested that this process causes an efficient energy conversion through the formation and coalescence of multiple magnetic islands. In this study, based on a large-scale two-dimensional fully kinetic simulations of coalescing multiple islands with a moderate guide magnetic field, we first examined the spatial dimensions of the internal structures of the coalescing islands. The results show that the dimensions of the structures in the directions normal to and along the initial current sheet depend on the initial thickness of the current sheet and the number of coalescing islands. We then found that the horizontal dimension of the structures controls the evolution time scale of the island coalescence process. We further found that when the vertical dimension of the structures, which corresponds to the length of the reconnection X-line in the reconnection outflow direction at the merging point between the two coalescing islands, is sufficiently longer than the ion inertial length, reconnection in the merging current sheet can well mature and both ions and electrons can be effectively heated around the merging X-line. The obtained scaling predicts that such a strong heating by well-matured reconnection in the island coalescence process would be seen in various plasma environments, such as the Earth's magnetotail and solar flares. [ABSTRACT FROM AUTHOR]

Nonlinear resistive-magnetohydrodynamics (MHD) computation with heating and anisotropic transport is applied to examine the interaction between thermal energy and magnetic fluctuations in inductively driven reversed-field pinches (RFPs). The magnetic fluctuations underlie magnetic field reversal through dynamo-like correlations, and they enhance thermal energy transport through fluctuations of parallel heat flux density. With the unfavorable magnetic curvature that exists across the RFP profile, thermal energy also affects the magnetic fluctuations. Computations with the NIMROD code [Sovinec et al., J. Comput. Phys. 195, 355–386 (2004)] integrate nonlinear MHD dynamics with energy transport and reproduce an RFP state with experimentally relevant values of plasma-β. Equilibria constructed from results of the 3D computations are analyzed to assess the sources of free energy in the saturated nonlinear state. Linear computations for these profiles show unstable modes of tearing parity. Their eigenfunctions are used to evaluate and compare stabilizing and destabilizing contributions to the kinetic energy integral. An assessment of the drives in the integral reveals that the pressure gradient drive is of comparable magnitude to the parallel current drive, and only the sum of the two surpasses the stabilizing contributions. Correlation of magnetic and parallel heat flux density fluctuations in the nonlinear computations shows that fluctuation-induced thermal conduction is the dominant mode of energy loss, as expected from experimental evidence. Decomposition of the fluctuating heat flux density shows that second-order correlations, alone, do not explain the total energy transport. Higher-order correlations are also important. [ABSTRACT FROM AUTHOR]

A two-dimensional complex plasma containing active Janus particles was experimentally studied. A single layer of micrometer-sized plastic microspheres was suspended in the plasma sheath of a radio frequency discharge in argon at low pressure. The particle sample used was a mixture of regular particles and Janus particles, which were coated on one side with a thin layer of platinum. Unlike a suspension consisting of regular particles only, the suspension with the inclusion of Janus particles did not form an ordered lattice in the experimental conditions used. Instead, the particles moved around with high kinetic energy in a disordered suspension. Unexpectedly, the mean kinetic energy of the particles declined as the illumination laser power was increased. This is explained by the competition of two driving forces: the photophoretic force and the oppositely directed ion drag force. The mean-squared displacement of the particles scaled as t α with α = 2 at small times t indicating ballistic motion and α = 0.56 ± 0.27 at longer times due to the combined effect of the Janus particle propensity to move in circular trajectories and external confinement. [ABSTRACT FROM AUTHOR]

We present results from a combined experimental and numerical simulation study of the anisotropy of the expansion of a laser-produced plasma into vacuum. Plasma is generated by nanosecond Nd:YAG laser pulse impact (laser wavelength λ = 1.064 μ m) onto tin microdroplets. Simultaneous measurements of ion kinetic energy distributions at seven angles with respect to the direction of the laser beam reveal strong anisotropic emission characteristics, in close agreement with the predictions of two-dimensional radiation-hydrodynamic simulations. Angle-resolved ion spectral measurements are further shown to provide an accurate prediction of the plasma propulsion of the laser-impacted droplet. [ABSTRACT FROM AUTHOR]

Ultracold plasmas are a promising candidate for the creation of strongly coupled Coulomb systems. Unfortunately, the values of the coupling parameter Γ e actually achieved after photoionization of the neutral atoms remain relatively small because of the considerable intrinsic heating of the electrons. A conceivable way to get around this obstacle might be to utilize a spontaneous ionization of the ultracold Rydberg gas, where the initial kinetic energies could be much less. However, the spontaneous avalanche ionization will result in a very inhomogeneous distribution (clusterization) of the ions, which can change the efficiency of the electron relaxation in the vicinity of such clusters substantially. In the present work, this hypothesis is tested by an extensive set of numerical simulations. As a result, it is found that despite a less initial kinetic energy, the subsequent relaxation of the electron velocities in the clusterized plasmas proceeds much more violently than in the case of the statistically uniform ionic distribution. The electron temperature, first, experiences a sharp initial jump (presumably, caused by the "virialization" of energies of the charged particles) and, second, exhibits a gradual subsequent increase (presumably, associated with a multi-particle recombination of the electrons at the ionic clusters). As a possible tool to reduce the anomalous temperature increase, we also considered a two-step plasma formation, involving the blockaded Rydberg states. This leads to a suppression of the clusterization due to a quasi-regular distribution of ions. In such a case, according to the numerical simulations, the subsequent evolution of the electron temperature proceeds more gently, approximately with the same rate as in the statistically uniform ionic distribution. [ABSTRACT FROM AUTHOR]

We present test-particle simulations of electrons during a nonlinear magnetohydrodynamic (MHD) simulation of a type-I edge localized mode to explore the effect of an eruptive plasma filament on the kinetic level. The electrons are moderately heated and accelerated during the filamentary eruption on a fast timescale of the order of 0.5 ms. A clearly non-thermal tail is formed in the distribution of the kinetic energy that is of power-law shape and reaches 90 keV for some particles. The acceleration is exclusively observed in the direction parallel to the magnetic field, i.e., with a clear preference in countercurrent direction, and we show that the parallel electric field is the cause of the observed acceleration. Most particles that escape from the system leave at one distinct strike-line in the outer divertor leg at some time during their energization. The escaping high-energy electrons in the tail of the energy distribution are not affected by collisions; thus, they show characteristics of runaway electrons. The mean square displacement indicates that transport in energy space clearly is superdiffusive, and interpreting the acceleration process as a random walk, we find that the distributions of energy-increments exhibit exponential tails, and transport in energy space is equally important of convective (systematic) and diffusive (stochastic) nature. By analyzing the MHD simulations per se, it turns out that the histograms of the parallel electric field in the edge region exhibit power-law shapes, and this clearly non-Gaussian statistics is ultimately one of the reasons for the moderately anomalous phenomena of particle transport that we find in energy space. [ABSTRACT FROM AUTHOR]

E x B shear and trapped electron precession shear induced suppression of micro-turbulence is studied in general tokamak geometry. A systematic derivation of a two-point equation for trapped electron related turbulence based on modern bounce kinetic formalism is performed. A two-point nonlinear analysis yields the new criterion for the turbulence suppression |ωExB + ωPS| > ΔωT, where DxT is decorrelation rate of the ambient turbulence, ωExB is E x B shearing rate in general tokamak geometry [Hahm and Burrell, Phys. Plasmas 2, 1648 (1995)], and xPS is precession shearing rate. Therefore, both E x B shear and trapped electron precession shear can contribute to suppression of turbulence, and these effects can either add up or subtract depending on their relative sign. This result provides a better understanding of electron thermal internal transport barrier formation under various conditions. [ABSTRACT FROM AUTHOR]

The thermal particles contributed neoclassical toroidal viscosity (NTV) have been successfully developed and explored by many impressive works such as the study by Shaing et al. [Phys. Plasmas 10, 1443 (2003)] and Zhu et al. [Phys. Rev. Lett. 96, 225002 (2006)]. In this work, the scope of the NTV study is extended to explore the contribution of energetic particles (EPs) through both theory and experiments. In theory, the existence of the NTV torque due to the precessional drift resonance of trapped EPs is identified based on the equivalence between the NTV torque and the perturbed drift kinetic energy [J. Park, Phys. Plasmas 18, 110702 (2011)]. Toroidal modeling with the Magneto Resistive Spectrum - drift Kinetic code [Y. Liu, Phys. Plasmas 15, 112503 (2008)], based on this equivalence, indicates that trapped EPs can contribute a significant amount of the NTV torque. Meanwhile, this work also focuses on developing the dedicated DIII-D experiments in the presence of the n = 2 external magnetic perturbation to verify the EP induced NTV (EP-NTV) by measuring the change of the NTV torque while varying the angle and the voltage of the neutral beam injection. However, the developed experiments have been unable to create conditions necessary to clearly demonstrate the presence of EP-NTV. The main challenge is separating the resonant and non-resonant momentum transport responses in the plasma. The experience, gained from this study, can help the further exploration of EP-NTV in the future experiments. [ABSTRACT FROM AUTHOR]

In inertial confinement fusion, the ignition threshold factor (ITF), defined as the ratio of the available shell kinetic energy to the minimum ignition energy, is an important metric for quantifying how far an implosion is from its performance cliff. Traditional ITF research is based on analytical theories with explicit scaling laws and parameters obtained by numerically fitting simulation data. This present study uses machine learning (ML) methods to train implicit but more reliable ITF expressions. One-dimensional numerical simulations are used to develop a dataset with 20 000 targets, in which alpha particle heating magnifies the fusion yield by a factor of 6.5. These targets are defined as marginal ignition targets whose ITF equals unity. ML models such as neural networks, support vector machines, and Gaussian processes are trained to connect the minimum ignition velocity vigt with other implosion parameters, yielding an ML-based ITF of (v imp / v igt) 7.5 , where vimp represents the implosion velocity. Then, these ML models are used to obtain curves of the ignition probability vs the ITF and improved ignition cliffs that show considerably better accuracy than traditional scaling laws, which are observed. The results demonstrate that ML methods have promising application prospects for quantifying ignition margins and can be useful in optimizing ignition target designs and practical implosion experiments. [ABSTRACT FROM AUTHOR]

New field solvers are developed in the gyrokinetic code GEM [Chen and Parker, J. Comput. Phys. 220, 839 (2007)] to simulate low-n modes. A novel discretization is developed for the ion polarization term in the gyrokinetic vorticity equation. An eigenmode analysis with finite Larmor radius effects is developed to study the linear resistive tearing mode. The mode growth rate is shown to scale with resistivity as γ~η1/3, the same as the semi-collisional regime in previous kinetic treatments [Drake and Lee, Phys. Fluids 20, 1341 (1977)]. Tearing mode simulations with gyrokinetic ions are verified with the eigenmode calculation. [ABSTRACT FROM AUTHOR]

A rotating magnetic island is imposed in the gyrokinetic code GKW, when finite differences are used for the radial direction, in order to develop the predictions of analytic tearing mode theory and understand its limitations. The implementation is verified against analytics in sheared slab geometry with three numerical tests that are suggested as benchmark cases for every code that imposes a magnetic island. The convergence requirements to properly resolve physics around the island separatrix are investigated. In the slab geometry, at low magnetic shear, binormal flows inside the island can drive Kelvin-Helmholtz instabilities which prevent the formation of the steady state for which the analytic theory is formulated. [ABSTRACT FROM AUTHOR]

Implosions of Hohlraum-driven double shell targets as an alternative inertial confinement fusion concept are underway at the National Ignition Facility. The double shell system relies on a series of energy transfer processes starting from thermal x-ray absorption by the outer shell, followed by collisional transfer of kinetic energy to a heavy metal inner shell, and finally, conversion to the internal energy of the deuterium-tritium fuel. During each of these energy transfer stages, low-mode asymmetries can act to reduce the ideal transfer efficiency degrading double shell performance. Mechanisms, such as hard x-ray preheat from the Hohlraum, not only decrease the efficiency of kinetic energy transfer but may also be a source of low-mode asymmetry. In this article, we evaluate the shape transfer processes through the time of shell collision using two-dimensional integrated Hohlraum and capsule computations. We find that the dominant mode of the shape transfer is well described using a "radial impulse" model from the shape of the foam pressure reservoir. To evaluate the importance of preheat on inner shell shape, we also report on first measurements of Au L-shell preheat asymmetry in a double shell with a tungsten pusher. These measurements showed a 65% higher preheat velocity at the pole of the capsule relative to the equator. We also found that the experiments provided rigorous constraints by which to test the Hohlraum model settings that impact the amount and symmetry of Au L-shell preheat via the plasma conditions inside the outer cone Au bubble. [ABSTRACT FROM AUTHOR]

In an x-ray driven cavity experiment, an intense flux of soft x rays on the emitting surface produces significant emission of photoelectrons having several kiloelectronvolts of kinetic energy. At the same time, rapid heating of the emitting surface occurs, resulting in the release of adsorbed surface impurities and subsequent formation of an impurity plasma. This numerical study explores a simple model for the photoelectric currents and the impurity plasma. Attention is given to the effect of varying the composition of the impurity plasma. The presence of protons or hydrogen molecular ions leads to a substantially enhanced cavity current, while heavier plasma ions are seen to have a limited effect on the cavity current due to their lower mobility. Additionally, it is demonstrated that an additional peak in the current waveform can appear due to the impurity plasma. A correlation between the impurity plasma composition and the timing of this peak is elucidated. [ABSTRACT FROM AUTHOR]