15 research outputs found
Instability of vortex array and transitions to turbulent states in rotating helium II
We consider superfluid helium inside a container which rotates at constant
angular velocity and investigate numerically the stability of the array of
quantized vortices in the presence of an imposed axial counterflow. This
problem was studied experimentally by Swanson {\it et al.}, who reported
evidence of instabilities at increasing axial flow but were not able to explain
their nature. We find that Kelvin waves on individual vortices become unstable
and grow in amplitude, until the amplitude of the waves becomes large enough
that vortex reconnections take place and the vortex array is destabilized. The
eventual nonlinear saturation of the instability consists of a turbulent tangle
of quantized vortices which is strongly polarized. The computed results compare
well with the experiments. Finally we suggest a theoretical explanation for the
second instability which was observed at higher values of the axial flow
Rotating inclined cylinder and the effect of the tilt angle on vortices
We study numerically some possible vortex configurations in a rotating
cylinder that is tilted with respect to the rotation axis and where different
numbers of vortices can be present at given rotation velocity. In a long
cylinder at small tilt angles the vortices tend to align along the cylinder
axis and not along the rotation axis. We also show that the axial flow along
the cylinder axis, caused by the tilt, will result in the Ostermeier-Glaberson
instability above some critical tilt angle. When the vortices become unstable
the final state often appears to be a dynamical steady state, which may contain
turbulent regions where new vortices are constantly created. These new vortices
push other vortices in regions with laminar flow towards the top and bottom
ends of the cylinder where they finally annihilate. Experimentally the inclined
cylinder could be a convenient environment to create long lasting turbulence
with a polarization which can be adjusted with the tilt angle.Comment: 10 pages, 10 figure
Hopping Transport in the Presence of Site Energy Disorder: Temperature and Concentration Scaling of Conductivity Spectra
Recent measurements on ion conducting glasses have revealed that conductivity
spectra for various temperatures and ionic concentrations can be superimposed
onto a common master curve by an appropriate rescaling of the conductivity and
frequency. In order to understand the origin of the observed scaling behavior,
we investigate by Monte Carlo simulations the diffusion of particles in a
lattice with site energy disorder for a wide range of both temperatures and
concentrations. While the model can account for the changes in ionic activation
energies upon changing the concentration, it in general yields conductivity
spectra that exhibit no scaling behavior. However, for typical concentrations
and sufficiently low temperatures, a fairly good data collapse is obtained
analogous to that found in experiment.Comment: 6 pages, 4 figure
The QCD Phase Structure at High Baryon Density
We consider the possibility that color deconfinement and chiral symmetry
restoration do not coincide in dense baryonic matter at low temperature. As a
consequence, a state of massive "constituent" quarks would exist as an
intermediate phase between confined nuclear matter and the plasma of deconfined
massless quarks and gluons. We discuss the properties of this state and its
relation to the recently proposed quarkyonic matter.Comment: 17 pages, 9 figure
Evolution of the electron temperature profile of ohmically heated plasmas in TFTR
Blackbody electron cyclotron emission was used to ascertain and study the evolution and behavior of the electron temperature profile in ohmically heated plasmas in the Tokamak Fusion Test Reactor (TFTR). The emission was measured with absolutely calibrated millimeter wavelength radiometers. The temperature profile normalized to the central temperature and minor radius is observed to broaden substantially with decreasing limiter safety factor q/sub a/, and is insensitive to the plasma minor radius. Sawtooth activity was seen in the core of most TFTR discharges and appeared to be associated with a flattening of the electron temperature profile within the plasma core where q less than or equal to 1. Two types of sawtooth behavior were identified in large TFTR plasmas (minor radius, a less than or equal to 0.8 m) : a typically 35 to 40 msec period ''normal'' sawtooth, and a ''compound'' sawtooth with 70 to 80 msec period