Jean-Noel Leboeuf

On the nature of radial transport across sheared zonal flows in electrostatic ion-temperature-gradient gyrokinetic tokamak plasma turbulence

It is argued that the usual understanding of the suppression of radial turbulent transport across a sheared zonal flow based on a reduction in effective transport coefficients is, by itself, incomplete. By means of toroidal gyrokinetic simulations of electrostatic, ion-temperature-gradient turbulence, it is found instead that the character of the radial transport is altered fundamentally by the presence of a sheared zonal flow, changing from diffusive to anticorrelated and subdiffusive. Furthermore, if the flows are self-consistently driven by the turbulence via the Reynolds stresses (in contrast to being induced externally), radial transport becomes non-Gaussian as well. These results warrant a reevaluation of the traditional description of radial transport across sheared flows in tokamaks via effective transport coefficients, suggesting that such description is oversimplified and poorly captures the underlying dynamics, which may in turn compromise its predictive capabilities.

Scattering of Electromagnetic Waves in the Presence of Wave Turbulence Excited by a Flow With Velocity Shear

It is well known that an incompressible sheared flow with an inflection point in the velocity profile will result in the formation of turbulent vortices. In the case of compressible plasma flow with velocity shear, ion-acoustic fluctuations in addition to vortices will be generated. We present detailed analysis of the excitation of such low-frequency oscillations in a compressible plasma flow with velocity shear. To examine the process of the excitation and nonlinear saturation of low-frequency oscillations in the presence of a flow shear, a nonlinear system of equations was derived. We employ a predictor-corrector method to solve this system numerically. Spectral analysis of obtained numerical solutions allows to calculate the turbulent density spectra for different velocity profiles. We find that the impact of this turbulence associated with ion-acoustic wave fluctuations is considerably more significant and dominant than that due to the turbulent vortices. Another goal for our research is to understand the influence of this turbulent flow on electromagnetic (EM) signals. Since the thickness of the flow can be fairly small in the case of hypersonic vehicles, we employed a single-scattering perturbation theory to study the scattering of the EM signals from the plasma sheath. We observe that the EM scattering from the turbulent density fluctuations of the flow results in shifted signal spectra above and below that of the source. Such shifts can have rather adverse effects on the sensor performance. For instance, shifts in temporal spectra will result in channel interference and crosstalk in communication systems. Because of the nature of the dispersion characteristics of the ion-acoustic waves, the shifts in spatial spectra of scattered EM waves can be very large. This will lead to large fluctuations in integrated phase shifts and hence, results in significant signal distortion. We carried out detailed theoretical analyses and numerical calculations to understand the nature of the influence of such hypersonic turbulent flow on EM signals. The complete loss of the EM signal because of the overdense condition of the plasma sheath is well known. We find that even in underdense conditions, GPS-based navigation can be significantly impaired because of the GPS signal distortion by the turbulence caused by such ion-acoustic wave fluctuations.

Nature of turbulent transport across sheared zonal flows: insights from gyrokinetic simulations*

The traditional view regarding the reduction of turbulence-induced transport across a stable sheared flow invokes a reduction of the characteristic length scale in the direction perpendicular to the flow as a result of the shearing and stretching of eddies caused by the differential pull exerted in the direction of the flow. A reduced effective transport coefficient then suffices to capture the reduction, that can then be readily incorporated into a transport model. However, recent evidence from gyrokinetic simulations of the toroidal ion-temperature-gradient mode suggests that the dynamics of turbulent transport across sheared flows changes in a more fundamental manner, and that the use of reduced effective transport coefficients fails to capture the full dynamics that may exhibit both subdiffusion and non-Gaussian statistics. In this contribution, after briefly reviewing these results, we propose some candidates for the physical mechanisms responsible for endowing transport with such non-diffusive characteristics, backing these proposals with new numerical gyrokinetic data.

Gyrokinetic particle-in-cell calculations of ion temperature gradient driven turbulence with parallel nonlinearity and strong flow corrections

Nonlinear gyrokinetic calculations have been performed with the three-dimensional, global, toroidal, nonlinear, particle-in-cell, delta-f or δf, massively parallel UCla-CANada (UCAN) code. Their purpose is to study the effects of the parallel nonlinearity and of strong (externally imposed) sheared flow corrections on ion temperature gradient (ITG) driven turbulence (ITGDT) in tokamaks. These calculations show that the strong flow corrections have a qualitative effect on the saturation level of the fluctuations and on the heat flux. The re-activated parallel nonlinearity, in combination with zonal flows, leads to a quantitative reduction in saturation level and heat flux. This reduction does however decrease with increasing system size.

Fusion Simulation Project: Integrated Simulation and Optimization of Magnetic Fusion Systems

This is the final report of a panel established as a subcommittee of the U. S. Department of Energy (DOE) Fusion Energy Sciences Committee (FESAC) on Integrated Simulation and Optimization of Magnetic Fusion Systems (ISOFS). The report was requested by the DOE in February 2002 and the approved report was transmitted to the DOE by the FESAC in December 2002. The report addresses the challenge of how to “develop fully integrated capability for predicting the performance of externally-controlled systems including turbulent transport, macroscopic stability, wave-particle physics, and multi-phase interfaces.”

High-resolution magnetohydrodynamic equilibrium code for unity beta plasmas

There is great interest in the properties of extremely high-s magnetohydrodynamic equilibria in axisymmetric toroidal geometry and the stability of such equilibria. However, few equilibrium codes maintain solid numerical behavior as beta approaches unity. The free-boundary algorithm presented herein utilizes a numerically stabilized multigrid method, current density input, position control, magnetic axis search, and dynamically adjusted simulated annealing. This approach yields numerically robust behavior in the spectrum of cases ranging from low to very high-s configurations. As the convergence time depends linearly on the total number of grid points, the production of extremely fine, low-error equilibria becomes possible. Such a code facilitates a variety of intriguing applications which include the exploration of the stability of extreme Shafranov shift equilibria.

Experimental Study of the Dynamics of Large- and Small-Scale Structures in the Plasma Column of Wire Array

The dynamics of large- and small-scale plasma structures is investigated in the precursor of 1-MA wire array Z-pinches by laser probing diagnostics. It is found that plasma streams from the wires induce density perturbations in the precursor. Small-scale perturbations and large-scale cells arise in the nonlinear stage before implosion. The spatial and temporal scales of the observed structures are in agreement with the theoretical investigation for current-driven excitation of electromagnetic flute modes.

Z

The development of instabilities in z-pinch plasmas has been studied with three-dimensional (3D) hybrid simulations1. Plasma equilibria without and with sheared axial flow have been considered. Results from the linear phase of the hybrid simulations compare well with linear Hall magnetohydrodynamics (MHD) calculations for sausage modes. The hybrid simulations show that sheared axial flow has a stabilizing effect on the development of both sausage and kink modes.

-Pinches

A novel approach to detect the existence of scale‐free transport in turbulent flows, based on the characterization of its Lagrangian characteristics, is presented and applied to two situations relevant for tokamak plasmas. The first one, radial transport in the presence of near‐critical turbulence, has been known for quite some time to yield scale‐free, superdiffusive transport. We use it to test the method and illustrate its robustness with respect to other approaches. The second situation, radial transport across radially‐sheared poloidal zonal flows driven by turbulence via the Reynold stresses, is examined for the first time in this manner. The result is rather surprising and different from the traditionally assumed diffusive behavior. Instead, radial transport behaves instead in a scale‐free, subdiffusive manner, which may have implications for the modeling of transport across transport barriers.

Hybrid simulations of current-carrying instabilities in z-pinch plasmas with sheared axial flow

Abstract Recently a toroidal plasma device with the confinement being provided by surface multipole magnetic cusps generated by permanent magnets has been operated at UCLA. The cusps are oriented in the poloidal direction and have a maximum field strength of 1.3 kG. A toroidal current is driven by a transformer; the loop voltage is about 100 V. It is found that for a hydrogen plasma a current of 3 to 6 kA is driven and that the current is carried by the ions. This gives direct ion heating to temperature of 40 to 100 eV in a 10 13 cm −3 density hydrogen plasma. The electron temperature is lower, being in the 20–40 eV range. The β of the plasma is high, in the 20–30% range. We have simulated the operation of this device using a 2 1 2 dimensional electromagnetic particle code. The model duplicates many of the features of the experiment, including the restraint of electron current by the cusps while the ions flow relatively freely, the focussing of the streaming ions to the center of the chamber by the magnetic lens effect of the cusps, and the pinch effect. A simple theoretical treatment explains both the experiments and the simulations, although the detailed simulation results indicate the need for a more complete theory.