4,906 research outputs found
LOW-MASS X-RAY BINARIES AND THEIR RELATION TO THE NON-X-RAY SOURCES
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73252/1/j.1749-6632.1977.tb37032.x.pd
Turbulent transport in tokamak plasmas with rotational shear
Nonlinear gyrokinetic simulations have been conducted to investigate
turbulent transport in tokamak plasmas with rotational shear. At sufficiently
large flow shears, linear instabilities are suppressed, but transiently growing
modes drive subcritical turbulence whose amplitude increases with flow shear.
This leads to a local minimum in the heat flux, indicating an optimal E x B
shear value for plasma confinement. Local maxima in the momentum fluxes are
also observed, allowing for the possibility of bifurcations in the E x B shear.
The sensitive dependence of heat flux on temperature gradient is relaxed for
large flow shear values, with the critical temperature gradient increasing at
lower flow shear values. The turbulent Prandtl number is found to be largely
independent of temperature and flow gradients, with a value close to unity.Comment: 4 pages, 5 figures, submitted to PR
Kinetic Simulations of Magnetized Turbulence in Astrophysical Plasmas
This letter presents the first ab initio, fully electromagnetic, kinetic
simulations of magnetized turbulence in a homogeneous, weakly collisional
plasma at the scale of the ion Larmor radius (ion gyroscale). Magnetic and
electric-field energy spectra show a break at the ion gyroscale; the spectral
slopes are consistent with scaling predictions for critically balanced
turbulence of Alfven waves above the ion gyroscale (spectral index -5/3) and of
kinetic Alfven waves below the ion gyroscale (spectral indices of -7/3 for
magnetic and -1/3 for electric fluctuations). This behavior is also
qualitatively consistent with in situ measurements of turbulence in the solar
wind. Our findings support the hypothesis that the frequencies of turbulent
fluctuations in the solar wind remain well below the ion cyclotron frequency
both above and below the ion gyroscale.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
The interaction between transpolar arcs and cusp spots
Transpolar arcs and cusp spots are both auroral phenomena which occur when
the interplanetary magnetic field is northward. Transpolar arcs are associated
with magnetic reconnection in the magnetotail, which closes magnetic flux and
results in a "wedge" of closed flux which remains trapped, embedded in the
magnetotail lobe. The cusp spot is an indicator of lobe reconnection at the
high-latitude magnetopause; in its simplest case, lobe reconnection
redistributes open flux without resulting in any net change in the open flux
content of the magnetosphere. We present observations of the two phenomena
interacting--i.e., a transpolar arc intersecting a cusp spot during part of its
lifetime. The significance of this observation is that lobe reconnection can
have the effect of opening closed magnetotail flux. We argue that such events
should not be rare
Multiscale Gyrokinetics for Rotating Tokamak Plasmas: Fluctuations, Transport and Energy Flows
This paper presents a complete theoretical framework for plasma turbulence
and transport in tokamak plasmas. The fundamental scale separations present in
plasma turbulence are codified as an asymptotic expansion in the ratio of the
gyroradius to the equilibrium scale length. Proceeding order-by-order in this
expansion, a framework for plasma turbulence is developed. It comprises an
instantaneous equilibrium, the fluctuations driven by gradients in the
equilibrium quantities, and the transport-timescale evolution of mean profiles
of these quantities driven by the fluctuations. The equilibrium distribution
functions are local Maxwellians with each flux surface rotating toroidally as a
rigid body. The magnetic equillibrium is obtained from the Grad-Shafranov
equation for a rotating plasma and the slow (resistive) evolution of the
magnetic field is given by an evolution equation for the safety factor q.
Large-scale deviations of the distribution function from a Maxwellian are given
by neoclassical theory. The fluctuations are determined by the high-flow
gyrokinetic equation, from which we derive the governing principle for
gyrokinetic turbulence in tokamaks: the conservation and local cascade of free
energy. Transport equations for the evolution of the mean density, temperature
and flow velocity profiles are derived. These transport equations show how the
neoclassical corrections and the fluctuations act back upon the mean profiles
through fluxes and heating. The energy and entropy conservation laws for the
mean profiles are derived. Total energy is conserved and there is no net
turbulent heating. Entropy is produced by the action of fluxes flattening
gradients, Ohmic heating, and the equilibration of mean temperatures. Finally,
this framework is condensed, in the low-Mach-number limit, to a concise set of
equations suitable for numerical implementation.Comment: 113 pages, 3 figure
Zero-Turbulence Manifold in a Toroidal Plasma
Sheared toroidal flows can cause bifurcations to zero-turbulent-transport
states in tokamak plasmas. The maximum temperature gradients that can be
reached are limited by subcritical turbulence driven by the parallel velocity
gradient. Here it is shown that q/\epsilon (magnetic field pitch/inverse aspect
ratio) is a critical control parameter for sheared tokamak turbulence. By
reducing q/\epsilon, far higher temperature gradients can be achieved without
triggering turbulence, in some instances comparable to those found
experimentally in transport barriers. The zero-turbulence manifold is mapped
out, in the zero-magnetic-shear limit, over the parameter space (\gamma_E,
q/\epsilon, R/L_T), where \gamma_E is the perpendicular flow shear and R/L_T is
the normalised inverse temperature gradient scale. The extent to which it can
be constructed from linear theory is discussed.Comment: 5 Pages, 4 Figures, Submitted to PR
Transport Bifurcation in a Rotating Tokamak Plasma
The effect of flow shear on turbulent transport in tokamaks is studied
numerically in the experimentally relevant limit of zero magnetic shear. It is
found that the plasma is linearly stable for all non-zero flow shear values,
but that subcritical turbulence can be sustained nonlinearly at a wide range of
temperature gradients. Flow shear increases the nonlinear temperature gradient
threshold for turbulence but also increases the sensitivity of the heat flux to
changes in the temperature gradient, except over a small range near the
threshold where the sensitivity is decreased. A bifurcation in the equilibrium
gradients is found: for a given input of heat, it is possible, by varying the
applied torque, to trigger a transition to significantly higher temperature and
flow gradients.Comment: 4 pages, 4 figures, submitted to PR
Double Rosensweig instability in a ferrofluid sandwich structure
We consider a horizontal ferrofluid layer sandwiched between two layers of
immiscible non-magnetic fluids. In a sufficiently strong vertical magnetic
field the flat interfaces between magnetic and non-magnetic fluids become
unstable to the formation of peaks. We theoretically investigate the interplay
between these two instabilities for different combinations of the parameters of
the fluids and analyze the evolving interfacial patterns. We also estimate the
critical magnetic field strength at which thin layers disintegrate into an
ordered array of individual drops
Suppression of turbulence and subcritical fluctuations in differentially rotating gyrokinetic plasmas
Differential rotation is known to suppress linear instabilities in fusion
plasmas. However, even in the absence of growing eigenmodes, subcritical
fluctuations that grow transiently can lead to sustained turbulence. Here
transient growth of electrostatic fluctuations driven by the parallel velocity
gradient (PVG) and the ion temperature gradient (ITG) in the presence of a
perpendicular ExB velocity shear is considered. The maximally simplified case
of zero magnetic shear is treated in the framework of a local shearing box.
There are no linearly growing eigenmodes, so all excitations are transient. The
maximal amplification factor of initial perturbations and the corresponding
wavenumbers are calculated as functions of q/\epsilon (=safety factor/aspect
ratio), temperature gradient and velocity shear. Analytical results are
corroborated and supplemented by linear gyrokinetic numerical tests. For
sufficiently low values of q/\epsilon (<7 in our model), regimes with fully
suppressed ion-scale turbulence are possible. For cases when turbulence is not
suppressed, an elementary heuristic theory of subcritical PVG turbulence
leading to a scaling of the associated ion heat flux with q, \epsilon, velocity
shear and temperature gradient is proposed; it is argued that the transport is
much less stiff than in the ITG regime.Comment: 36 pages in IOP latex style; 12 figures; submitted to PPC
- …