Physics of the dynamic ergodic divertor

The Dynamic Ergodic Divertor (DED) of TEXTOR is presently being installed. It consists of sixteen helically wound coils occupying about 30% of the wall at the HFS. The coils follow field lines on a “pre-selected” magnetic surface and are fed individually outside the vessel. A perturbation field is created by the electrical currents in the perturbation coils with Fourier components resonant to the magnetic surfaces. The stochastic boundary layer is generated in the outermost region of the plasma, which due to long and short connection lengths can be divided into ergodic and laminar regions. Field line tracing and mapping techniques were used to analyse properties of the TEXTOR-DED plasma boundary. The DED will operate with several frequencies (DC or AC up to 10 kHz). In the “dynamic” operation the convective heat flux is deposited to a large plasma-facing surface and forces are transferred to the plasma edge, what can introduce a differential rotation of the plasma

Bose-Einstein Condensate in Weak 3d Isotropic Speckle Disorder

The effect of a weak three-dimensional (3d) isotropic laser speckle disorder on various thermodynamic properties of a dilute Bose gas is considered at zero temperature. First, we summarize the derivation of the autocorrelation function of laser speckles in 1d and 2d following the seminal work of Goodman. The goal of this discussion is to show that a Gaussian approximation of this function, proposed in some recent papers, is inconsistent with the general background of laser speckle theory. Then we propose a possible experimental realization for an isotropic 3d laser speckle potential and derive its corresponding autocorrelation function. Using a Fourier transform of that function, we calculate both condensate depletion and sound velocity of a Bose-Einstein condensate as disorder ensemble averages of such a weak laser speckle potential within a perturbative solution of the Gross-Pitaevskii equation. By doing so, we reproduce the expression of the normalfluid density obtained earlier within the treatment of Landau. This physically transparent derivation shows that condensate particles, which are scattered by disorder, form a gas of quasiparticles which is responsible for the normalfluid component

Chaotic transport in Hamiltonian systems perturbed by a weak turbulent wave field

Chaotic transport in a Hamiltonian system perturbed by a weak turbulent wave field is studied. It is assumed that a turbulent wave field has a wide spectrum containing up to thousands of modes whose phases are fluctuating in time with a finite correlation time. To integrate the Hamiltonian equations a fast symplectic mapping is derived. It has a large time-step equal to one full turn in angle variable. It is found that the chaotic transport across tori caused by the interactions of small-scale resonances have a fractal-like structure with the reduced or zero values of diffusion coefficients near low-order rational tori thereby forming transport barriers there. The density of rational tori is numerically calculated and its properties are investigated. It is shown that the transport barriers are formed in the gaps of the density of rational tori near the low-order rational tori. The dependencies of the depth and width of transport barriers on the wave field spectrum and the correlation time of fluctuating turbulent field ( or the Kubo number) are studied. These numerical findings may have importance in understanding the mechanisms of transport barrier formation in fusion plasmas

Universal asymptotics of poloidal spectra of maagnetic perturbations of saddle coils in tokamaks

Universal and generic features of poloidal spectra of external magnetic perturbations created by a set of saddle coils in tokamak plasmas are studied in a vacuum approximation. It is found that the poloidal mode spectra with high-accuracy are described by a linear combination of three universal asymptotical formulas which depend only on the safety factor of the equilibrium plasma and the geometry of perturbation coils. The validity of these formulas is confirmed by numerical calculations of the mode spectra in the DIII-D plasma [G. L. Jackson et al., Europhys. Conf. Abstr. 27A, P-4.47 (2007)] and a spherical tokamak, National Spherical Tokamak Experiment [D. A. Gates, et al., Nucl. Fusion 49, 104016 (2009)], plasmas perturbed by external control coils. (C) 2011 American Institute of Physics. [doi:10.1063/1.3551756

Universal asymptotical behavior of poloidal spectra of resonant magnetic perturbations created by a set of saddle coils in tokamaks

Universal asymptotical behaviour of poloidal spectra of resonant magnetic perturbations created by a set of saddle coils in tokamak plasmas is studied in a vacuum approximation. It is shown that the poloidal mode spectra for a given toroidal mode can be presented by a linear combination of three universal asymptotical formulae corresponding to horizontal and vertical segments of a set of saddle coils. Each of the asymptotical formulae depends only on the safety factor of the equilibrium plasma and the geometry of the perturbation coils. The validity of the universal formulae is confirmed by numerical computations of the poloidal mode spectra for typical plasmas with large and small inverse aspect ratios, DIII-D-like plasmas with internal (I-) coils (Jackson et al 2003 Proc. 30th EPS Conf. on Controlled Fusion and Plasma Physics (St Petersburg, Russia, 7-11 July 2003) vol 27A (ECA) P-4.47) and spherical NSTX-like plasmas with error field coils (Gates et al 2009 Nucl. Fusion 49 104016)

New mechanism of runaway electron diffusion due to microturbulence in tokamaks

Chaotic transport of runaway electrons in a toroidal system in the presence of a weak small-scale magnetic turbulent field with a wide mode spectrum is studied. Using a fast running mapping, the radial profiles of turbulent diffusion coefficients are calculated. It is found that at large Kubo numbers the chaotic transport of the electrons is described by a fractal-like radial dependence of the diffusion coefficients with reduced or zero values near low-order rational drift surfaces which form transport barriers. The latter can be one of the main reasons of the improved confinement of runaway electrons in tokamaks. One can expect that this effect may lead to the formation of the nested beams of runaway electrons. [http://dx.doi.org/10.1063/1.4736718

On the accuracy of some mapping techniques used to study the magnetic field dynamics in tokamaks

The dynamics of magnetic field lines and of charged particles in toroidal chambers are commonly analysed by numerically solving the dynamical equations. They may also be analysed using deterministic reduced models, i.e. low-dimensional discrete time approximations (maps) of the Hamiltonian continuous time models. We report on the accuracy of the latter method by considering the mapping technique derived from the Hamilton-Jacobi equation. The optimum time stepping in some models for the study of the magnetic field in tokamaks is determined by using local criteria. Special attention is given to the analysis of stochasticity produced by time discretization. © 2008 IAEA, Vienna.SCOPUS: ar.jinfo:eu-repo/semantics/publishe