Plasma Transport Simulations Research Articles

Numerical simulation of anomalous plasma transport in the presence of magnetic turbulence

The structure of plasma in the interplanetary space is briefly presented, and the problems related to the variability of solar activity are discussed. The features of magnetic turbulence in the solar wind are also described. Magnetic field fluctuations are one of the causes of enhanced transport both in laboratory and astrophysical plasmas. To a first approximation, the plasma particles follow the magnetic field lines, whose equations form a non-linear one and a half degrees of freedom system. Unless the fluctuation level is very low, numerical simulations are needed to study such a system. We review three-dimensional numerical simulations of field line transport in anisotropic magnetic turbulence. Several transport regimes are found: for low Kubo number, anomalous transport is obtained, featuring both subdiffusion, corresponding to trapping in cantori structures, and superdiffusion, corresponding to Levy flights in the stochastic layer. Increasing the Kubo number, and hence stochasticity, quasilinear, intermediate, and percolative regimes are found, in the order. An expression of the diffusion coefficient valid for generalized anisotropy is presented.

Effect of anisotropic D–D fusion neutron emission on counter calibration using activation techniques

Detailed plasma and neutron transport simulation is needed for accurate measurements of the total neutron emission by means of foil activation during deuterium neutral beam injection into toroidal fusion plasmas. Results from a dedicated package of computer codes for simulation of neutron production and neutron transport are presented. The simulation procedure comprises spatially resolved Fokker–Planck calculations of the deuteron velocity distribution, the calculation of absolute neutron spectra as functions of the direction of emission and, finally, a detailed Monte Carlo neutron transport simulation. The simulation procedure is used to calculate the influence of anisotropic neutron emission on activation foil measurements for neutral-beam-heated toroidal fusion plasmas due to the variation of the differential D–D neutron production with angle of emission. Theoretical results predict effects in the order of a few percent. This is consistent with experimental results which show that the total neutron yield is underestimated by about 5% if effects of anisotropic emission are being neglected.

Sensitivity of predictive tokamak plasma transport simulations

The sensitivity of our time-dependent simulations of low confinement (L-mode) discharges to variations in initial profiles and time-dependent boundary conditions has been explored. These time-dependent tokamak plasma simulations were performed using a theory-based Multi-mode transport model that includes ion temperature gradient (ITG) and trapped electron modes (TEM), kinetic and resistive ballooning modes and neoclassical modes. The density and temperature profiles predicted in our simulations of L-mode discharges are found to be robust, even with significant variations in the initial or boundary conditions. Although transport associated with a single mode can be strongly affected by local changes in plasma parameters resulting from changes in the boundary conditions, the total transport remains largely unchanged because of compensation by other transport modes. The sensitivity of the predicted temperature and density profiles to a variation in the Multi-mode model is also examined. When the Dominguez-Waltz theory of transport driven by ITG and TEM modes is replaced in the Multi-mode model by the Weiland description, we find that the predictions of the Weiland model more closely match the experimental data.

Rapid two-dimensional self-consistent simulation of inductively coupled plasma and comparison with experimental data

A methodology has been developed to achieve rapid two-dimensional self-consistent simulation of plasma transport and reaction in an inductively coupled source of arbitrary geometry and with arbitrary plasma and surface chemistries. In this modular finite element fluid simulation the reactor was divided into bulk plasma and sheath. The bulk plasma was assumed quasineutral and the electrons were assumed to be in Boltzmann equilibrium. Separate modules computed the power deposition into the plasma, electron temperature, charged species densities, and neutral species densities. Simulation results agreed favorably with available experimental data, taken in a chlorine plasma in a Gaseous Electronics Conference reference cell, without using any adjustable parameters. Rapid convergence makes the simulation tool especially attractive for technology computer-aided design (TCAD) applications.

Comparison between a two-dimensional simulation and a global conservation model for a compact ECR plasma source

We compare results from a two-dimensional simulation of plasma transport in a compact electron cyclotron resonance (ECR) source with predictions from a spatially averaged global model based on particle and energy conservation. Model predictions include plasma density, plasma potential and electron temperature. In addition, the models predict the fraction of input power lost to various processes in the plasma. The models agree to within about 10% over the conditions examined: 0.5-10 mTorr and 850-1500 W in a model argon gas. The global model can be used to establish scaling laws, and it also provides insight into the overall plasma behaviour in terms of general conservation principles. The two-dimensional simulation is essential when spatial profile information is crucial, such as for predictions of plasma uniformity.

Numerical simulation of plasma transport driven by the Io torus

The Rice Convection Model (RCM) has been modified to a form suitable for Jupiter (RCM‐J) to study plasma interchange motion in and near the Io plasma torus. The net result of the interchange is that flux tubes, heavily loaded with torus plasma, are transported outward, to be replaced by tubes containing little low‐energy (< 1 keV) plasma. The process is numerically simulated in terms of time evolution from an initial torus that is longitudinally asymmetric and with gradually decreasing density outward from Io's orbit. In the simulations, the nonlinear stage of the instability characteristically exhibits outreaching fingers of heavily‐loaded flux tubes that lengthen at an accelerating rate. Our principal finding is that the primary geometrical form of outward transport of torus plasma in Jupiter's magnetosphere is through long, outward‐moving fingers of plasma. In the simulations, the fingers mainly form in the active sector of the Io torus (the heavier side of the asymmetric torus), and they are spaced longitudinally roughly 20° apart.

Plasma transport simulation modelling for helical confinement systems

For a detailed prediction of the plasma parameters in the Large Helical Device (LHD), 3-D equilibrium/1-D transport simulations including empirical or drift wave turbulence models are performed which suggest that the global confinement time of the LHD is determined mainly by the electron anomalous transport in the plasma edge region rather than by the helical ripple transport in the core region. Even if the ripple loss can be eliminated, the increase in global confinement is 10%. However, the rise in the central ion temperature is more than 20%. If the anomalous loss can be reduced to half of the value used in the present scaling, as is the case in the H-mode of tokamak discharges, the neoclassical ripple loss through the ion channel becomes important even in the plasma core. The 5% radial inward shift of the plasma column with respect to the major radius improves the plasma confinement and increases the fusion product by more than 50% by reducing the neoclassical asymmetric ion transport loss and increasing the plasma radius (10%)

Back-averaging: An accelerated iterative method for simulating plasma diffusion

A fast accurate multi-stage numerical method, back-averaging, is developed, analyzed, and optimized. The technique is used to solve an alternating dimensional numerical simulation of fusion plasma transport based on nonlinear resistive magnetohydrodynamics (MHD) equations. The geometry of the model is a complicated time-dependent two-dimensional configuration of flux contours analogous to a doublet. Numerical convergence rate comparisons of the optimized back-averaging method with various other iterative techniques for solving the numerical problem show that optimized back-averaging is the fastest method of all those considered. Moreover, to accomplish convergence in a practical length of time for extremely peaked profiles and complicated time varying configurations, back-averaging is essential. Further, with a miminum of additional computation, optimized back-averaging yields extreme accuracy.

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