We investigate in detail the initial susceptibility, magnetization curves, and microstructure of ferrofluids in various concentration and particle dipole moment ranges by means of molecular dynamics simulations. We use the Ewald summation for the long-range dipolar interactions, take explicitly into account the translational and rotational degrees of freedom, coupled to a Langevin thermostat. When the dipolar interaction energy is comparable with the thermal energy, the simulation results on the magnetization properties agree with the theoretical predictions very well.For stronger dipolar couplings, however, we find systematic deviations from the theoretical curves.We analyze in detail the observed microstructure of the fluids under different conditions. The formation of clusters is found to enhance the magnetization at weak fields and thus leads to a larger initial susceptibility. The influence of the particle aggregation is isolated by studying ferrosolids, which consist of magnetic dipoles frozen in at random locations but which are free to rotate.Due to the artificial suppression of clusters in ferro-solids the observed susceptibility is considerably lowered when compared to ferrofluids.

ASDEX Upgrade has successfully started the second experimental campaign with a full tungsten coverage of the plasma facing components and without using a boronisation for machine conditioning. The tungsten erosion at all relevant positions in the main chamber and the divertor was investigated. The outer divertor is by far the strongest source region, especially in discharges with high divertor temperature in-between ELMs. In the main chamber, the central column is usually the first limiting structure and produces then larger W erosion fluxes than the outboard limiters. Nevertheless, the tungsten influx from the outboard limiters has a much stronger effect on the tungsten content in the confined plasma. An increase of the available power from the fly-wheel generator allowed for improved H-mode operation at 1 MA, and H factors in the range of 1.2 could be achieved at acceptable W concentrations of about 2¢10 .

The complete magnetodissipative structure of ferrofluid dynamics is derived from general principles, without reference to the angular momentum of the ferromagnetic grains. The results are independent of most microscopic details, and easily interpret two previous experiments. Both the Debye theory and the effective-field theory by Shliomis are shown to be special cases of the new set of equations.

We present an analytic expression for the onset of the Faraday instability, which is applicable to a wide frequency range covering both shallow gravity and deep capillary waves. For sufficiently thin fluid layers, the surface oscillates in harmonic rather than subharmonic resonance with the forcing. An experimental confirmation of this result is reported. [S0031-9007(97)02704-X] PACS numbers: 47.20.Ma, 47.15.Cb, 47.20.Gv The observation of standing waves at the surface of a fluid layer subject to a vertical vibration dates back to Faraday [1]. For sufficiently strong driving, the plane surface undergoes an instabilty which gives rise to ordered wave patterns [2][3][4]. With a sinusoidal driving force, spatially periodic patterns [5][6][7], and quasiperiodic structures of eightfold or tenfold orientational order have been observed [8]. Faraday already recognized that the response of the surface is subharmonic; i.e., it appears with twice the period of the drive. The first investigation of the linear stability [9] showed that the problem can be reduced to a set of Mathieu oscillators. However, the analysis relies on the potential flow approximation which is restricted to inviscid fluids only. Viscous effects are usually treated by a heuristic damping in the Mathieu equation [10], proportional to the kinematic viscosity n. This approximation ignores viscous boundary layers along the container walls and beneath the surface, where additional dissipation occurs. Beyer and Friedrich [11] showed that the surface boundary layer gives rise to a memory kernel in the Mathieu equation. Since they used the idealized free slip boundary conditions, they did not catch the dominating damping in the bottom boundary layer. The most advanced theoretical investigation of the stability problem [12] is fully numerical, which renders a physical understanding difficult. An analytic approach which accounts for the different competing damping mechanisms is still missing. Recently, Kumar [13] presented an approximation for weakly damped Faraday waves based on a truncation of the numerical method [12]. Besides being rather implicit (a numerical minimization of the neutral stability curve is still required), his approach is not systematic since it is unclear to what order of viscosity the obtained result is valid [14]. He points out the very interesting possibility of a synchronous ( harmonic) rather than subharmonic surface resonance, if the wavelength l 2p͞k becomes comparable to the filling depth h. In this situation, the dissipation in the bottom boundary layer becomes dominant. Kumar could not determine the parameter region for the harmonic response by analytic means; instead he presented a numerical example.In the present Letter we develop a systematic perturbative treatment of the linear stability problem. We provide an analytic expression for the onset acceleration a c and the critical wave number k c . Also covered is the case of shallow water waves (l Ӎ h), where earlier approximations [10] fail. Our formula allows a direct inter...

Abstract. The intermittent character of turbulent transport is investigated with Langmuir probes in the scrape-off layer (SOL) and across the separatrix of ASDEX Upgrade Ohmic discharges. Radial profiles of plasma parameters are in reasonable agreement with results from other diagnostics. The probability density functions of ion-saturation current fluctuations exhibit a parabolic relation between skewness and kurtosis. Intermittent blobs and holes are observed outside and inside the nominal separatrix, respectively. They seem to be born at the edge of the plasma and are not the foothills of avalanches launched in the plasma core. A strong shear flow was observed 1 cm radially outside the location where blobs and holes seem to be generated. Blobs and holes in ASDEX Upgrade

We derive the complete set of macroscopic dynamic equations for ferrogels under an external magnetic field, including the magnetization as an independent dynamic degree of freedom. The magnetoelasticity comes in the form of magnetostriction and through the magnetic part of the Maxwell stress. Various dynamic couplings of the elastic degree of freedom with the magnetization and the magnetic field are found. We discuss static elongation, shear deformations, and the modified sound spectrum in the presence of an external magnetic field.

Operation with all tungsten plasma facing components has become routine in ASDEX Upgrade. The conditioning of the device is strongly simplified and short glow discharges are used only on a daily basis. The long term fuel retention was reduced by more than a factor of 5 as demonstrated in gas balance as well as in post mortem analyses. Injecting nitrogen Preprint submitted to Elsevier 23 November 2012for radiative cooling, discharges with additional heating power up to 23 MW have been achieved, providing good confinement (H98 y2 = 1), divertor power loads around 5 MW m −2 and divertor temperatures below 10 eV. ELM mitigation by pellet ELM pacemaking or magnetic perturbation coils reduces the deposited energy during ELMs, but also keeps the W density at the pedestal low. As a recipe to keep the central W concentration sufficiently low, central (wave) heating is well established and low density H-Modes could be re-established with the newly available ECRH power of up to 4 MW. The ICRH induced W sources could be strongly reduced by applying boron coatings to the poloidal guard limiters.

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