Authors:Jim-Felix Lobsien; Michael Drevlak, Thomas Kruger, Samuel Lazerson, Caoxiang Zhu, Thomas Sunn Pedersen Abstract: Following up on earlier work which demonstrated an improved numerical stellarator coil design optimization performance by the use of stochastic optimization (Lobsien et al., Nucl. Fusion, vol. 58 (10), 2018, 106013), it is demonstrated here that significant further improvements can be made – lower field errors and improved robustness – for a Wendelstein 7-X test case. This is done by increasing the sample size and applying fully three-dimensional perturbations, but most importantly, by changing the design sequence in which the optimization targets are applied: optimization for field error is conducted first, with coil shape penalties only added to the objective function at a later step in the design process. A robust, feasible coil configuration with a local maximum field error of 3.66 % and an average field error of 0.95 % is achieved here, as compared to a maximum local field error of 6.08 % and average field error of 1.56 % found in our earlier work. These new results are compared to those found without stochastic optimization using the FOCUS and ONSET suites. The relationship between local minima in the optimization space and coil shape penalties is also discussed. PubDate: 2020-04-01T00:00:00.000Z DOI: 10.1017/S0022377820000227 Issue No:Vol. 86, No. 2 (2020)

Authors:Mohamad Shalaby; Avery E. Broderick, Philip Chang, Christoph Pfrommer, Ewald Puchwein, Astrid Lamberts Abstract: We study the longitudinal stability of beam–plasma systems in the presence of a density inhomogeneity in the background plasma. Previous works have focused on the non-relativistic regime where hydrodynamical models are used to evolve pre-existing Langmuir waves within inhomogeneous background plasmas. Here, for the first time we study the problem with kinetic equations in a fully relativistic way. We do not assume the existence of Langmuir waves, and we focus on the rate and the mechanism by which waves are excited in such systems from an initial perturbation. We derive the structure of the unstable modes and compute an analytical approximation for their growth rates. Our computation is limited to dilute and cold beams, and shows an excellent agreement with particle-in-cell simulations performed using the SHARP code. We show that, due to such an inhomogeneity, the virulent beam–plasma instabilities in the intergalactic medium are not suppressed but their counterparts in the solar wind can be suppressed as evidenced by propagating type-III solar radio bursts. PubDate: 2020-04-01T00:00:00.000Z DOI: 10.1017/S0022377820000215 Issue No:Vol. 86, No. 2 (2020)

Authors:Christopher G. Albert; Sergei V. Kasilov, Winfried Kernbichler Abstract: Accelerated statistical computation of collisionless fusion alpha particle losses in stellarator configurations is presented based on direct guiding-centre orbit tracing. The approach relies on the combination of recently developed symplectic integrators in canonicalized magnetic flux coordinates and early classification into regular and chaotic orbit types. Only chaotic orbits have to be traced up to the end, as their behaviour is unpredictable. An implementation of this technique is provided in the code SIMPLE (symplectic integration methods for particle loss estimation, Albert et al., 2020b, doi:10.5281/zenodo.3666820). Reliable results were obtained for an ensemble of 1000 orbits in a quasi-isodynamic, a quasi-helical and a quasi-axisymmetric configuration. Overall, a computational speed up of approximately one order of magnitude is achieved compared to direct integration via adaptive Runge–Kutta methods. This reduces run times to the range of typical magnetic equilibrium computations and makes direct alpha particle loss computation adequate for use within a stellarator optimization loop. PubDate: 2020-04-01T00:00:00.000Z DOI: 10.1017/S0022377820000203 Issue No:Vol. 86, No. 2 (2020)

Authors:Justin Ball; Stephan Brunner, Ajay C.J. Abstract: In this work, we highlight an issue that may reduce the accuracy of many local nonlinear gyrokinetic simulations – turbulent self-interaction through the parallel boundary condition. Given a sufficiently long parallel correlation length, individual turbulent eddies can span the full domain and ‘bite their own tails’, thereby altering their statistical properties. Such self-interaction is only modelled accurately when the simulation domain corresponds to a full flux surface, otherwise it is artificially strong. For Cyclone Base Case parameters and typical domain sizes, we find that this mechanism modifies the heat flux by approximately 40 % and it can be even more important. The effect is largest when using kinetic electrons, low magnetic shear and strong turbulence drive (i.e. steep background gradients). It is found that parallel self-interaction can be eliminated by increasing the parallel length and/or the binormal width of the simulation domain until convergence is achieved. PubDate: 2020-04-01T00:00:00.000Z DOI: 10.1017/S0022377820000197 Issue No:Vol. 86, No. 2 (2020)

Authors:H. Khalilpour; G. Foroutan Abstract: The effects of the electron energy distribution function (EEDF) on the structure of a dusty plasma sheath are investigated. Here, it is assumed that the electrons obey a Druyvesteyn-type distribution with a parameter $x$ controlling the shape of the EEDF. The Druyvesteyn-like distribution tends to a Maxwellian distribution as $x$ varies from 2 to 1. Using the orbital motion limited theory, the incident electron current on the dust is evaluated for a given $x$ . The results of numerical simulations are compared with those of a Maxwellian distribution. It was found that the sheath dynamics depends strongly on the magnitude of $x$ . The sheath thickness increases monotonically with increasing $x$ . However, the absolute dust charge decreases and, as a result, the accelerating ion drag force is weakened and thus the dust number density is enhanced. For a plasma with a Druyvesteyn-like distribution, the Bohm speed is a function of $x$ and increases with increasing $x$ . PubDate: 2020-04-01T00:00:00.000Z DOI: 10.1017/S0022377820000161 Issue No:Vol. 86, No. 2 (2020)

Authors:B. J. Frei; R. Jorge, P. Ricci Abstract: A gyrokinetic model is presented that can properly describe large and small amplitude electromagnetic fluctuations occurring on scale lengths ranging from the electron Larmor radius to the equilibrium perpendicular pressure gradient scale length, and the arbitrarily large deviations from thermal equilibrium that are present in the plasma periphery of tokamak devices. The formulation of the gyrokinetic model is based on a second-order accurate description of the single charged particle dynamics, derived from Lie perturbation theory, where the fast particle gyromotion is decoupled from the slow drifts assuming that the ratio of the ion sound Larmor radius to the perpendicular equilibrium pressure scale length is small. The collective behaviour of the plasma is obtained by a gyrokinetic Boltzmann equation that describes the evolution of the gyroaveraged distribution function. The collisional effects are included by a nonlinear gyrokinetic Dougherty collision operator. The gyrokinetic model is then developed into a set of coupled fluid equations referred to as the gyrokinetic moment hierarchy. To obtain this hierarchy, the gyroaveraged distribution function is expanded onto a Hermite–Laguerre velocity-space polynomial basis. Then, the gyrokinetic equation is projected onto the same basis yielding the spatial and temporal evolution of the Hermite–Laguerre expansion coefficients. A closed set of fluid equations for the lowest-order coefficients is presented. The Hermite–Laguerre projection is performed accurately at arbitrary perpendicular wavenumber values. Finally, the self-consistent evolution of the electromagnetic fields is described by a set of gyrokinetic Maxwell equations derived from a variational principle where the velocity integrals are explicitly evaluated. PubDate: 2020-04-01T00:00:00.000Z DOI: 10.1017/S0022377820000100 Issue No:Vol. 86, No. 2 (2020)

ab+initio
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+++++potential&rft.title=Journal+of+Plasma+Physics&rft.issn=0022-3778&rft.date=2020&rft.volume=86&rft.aulast=Terashkevich&rft.aufirst=V.&rft.au=V.+A.+Terashkevich&rft.au=V.+V.+Meshkov,+E.+A.+Pazyuk,+A.+V.+Stolyarov&rft_id=info:doi/10.1017/S0022377820000112">The collision cross-sections for proton–argon interaction based on ab initio $\text{ArH}^{+}$ potential

Authors:V. A. Terashkevich; V. V. Meshkov, E. A. Pazyuk, A. V. Stolyarov Abstract: The classical collision cross-sections of a proton with an argon atom as well as the thermal transport coefficients and rate constant of the colliding $\text{H}^{+}-\text{Ar}$ system are evaluated at the kinetic temperature $T\in [100,10\,000]~(\text{K})$ by means of the asymptotically correct analytical potential constructed for the ground $X^{1}\unicode[STIX]{x1D6F4}^{+}$ state of the ArH+ cation from the highly accurate ab initio data available in the entire range of internuclear distances (Terashkevich et al., J. Quant. Spectrosc. Radiat. Transfer, vol. 234, 2019, pp. 139–146). The results can be useful to estimate thermodynamic, transport and kinetic properties of the Ar/H2 plasma in a wide temperature range. PubDate: 2020-04-01T00:00:00.000Z DOI: 10.1017/S0022377820000112 Issue No:Vol. 86, No. 2 (2020)

Authors:Dov J. Rhodes; William M. Farrell Abstract: The problem of plasma expansion into a vacuum is revisited with the addition of a finite boundary condition; an electrically insulated surface. As plasma expands towards a charge-accumulating surface, the leading electron cloud charges the surface negatively, which in turn repels electrons and attracts ions. This plasma–surface interaction is shown to result in a feedback process which accelerates the plasma expansion. In addition, we examine the decrease in (negative) surface potential and associated near-surface electron density. To investigate this plasma coupling with an electrically floating surface, we develop an analytic model including four neighbouring plasma regions: (i) undisturbed plasma, (ii) quasi-neutral self-similar expansion, (iii) ion front boundary layer and (iv) electron cloud. A key innovation in our approach is a self-contained analytic approximation of the ion front boundary layer, providing a spatially continuous electric field model for the early phase of bounded plasma expansion. PubDate: 2020-04-01T00:00:00.000Z DOI: 10.1017/S0022377820000148 Issue No:Vol. 86, No. 2 (2020)

Authors:J. W. Connor; R. J. Hastie, K. Richards Abstract: The dependence of confinement on input power for a tokamak plasma with regions having a stiff temperature profile is explored. The resilience of the confinement of the core energy to increasing power loss by core radiation from impurities in such situations, as it is anticipated will be required in a demonstration fusion reactor (DEMO) design, is examined. PubDate: 2020-04-01T00:00:00.000Z DOI: 10.1017/S0022377820000136 Issue No:Vol. 86, No. 2 (2020)

Authors:Per Helander Abstract: The concept of the available energy of a collisionless plasma is discussed in the context of magnetic confinement. The available energy quantifies how much of the plasma energy can be converted into fluctuations (including nonlinear ones) and is thus a measure of plasma stability, which can be used to derive linear and nonlinear stability criteria without solving an eigenvalue problem. In a magnetically confined plasma, the available energy is determined by the density and temperature profiles as well as the magnetic geometry. It also depends on what constraints limit the possible forms of plasma motion, such as the conservation of adiabatic invariants and the requirement that the transport be ambipolar. A general method based on Lagrange multipliers is devised to incorporate such constraints in the calculation of the available energy, and several particular cases are discussed for which it can be calculated explicitly. In particular, it is shown that it is impossible to confine a plasma in a Maxwellian ground state relative to perturbations with frequencies exceeding the ion bounce frequency. PubDate: 2020-04-01T00:00:00.000Z DOI: 10.1017/S0022377820000057 Issue No:Vol. 86, No. 2 (2020)