898 research outputs found
Low Threshold Parametric Decay Back Scattering Instability in Tokamak ECRH Experiments
The experimental conditions leading to substantial reduction of
backscattering decay instability threshold in ECRH experiments in toroidal
devices are analyzed. It is shown that drastic decrease of threshold is
provided by non monotonic behavior of plasma density in the vicinity of
magnetic island and poloidal magnetic field inhomogeneity making possible
localization of ion Bernstein decay waves. The corresponding ion Bernstein wave
gain and the parametric decay instability pump power threshold is calculated.Comment: 7 pages, 4 figure
Spatial and Wavenumber Resolution of Doppler Reflectometry
Doppler reflectometry spatial and wavenumber resolution is analyzed within
the framework of the linear Born approximation in slab plasma model. Explicit
expression for its signal backscattering spectrum is obtained in terms of
wavenumber and frequency spectra of turbulence which is assumed to be radially
statistically inhomogeneous. Scattering efficiency for both back and forward
scattering (in radial direction) is introduced and shown to be inverse
proportional to the square of radial wavenumber of the probing wave at the
fluctuation location thus making the spatial resolution of diagnostics
sensitive to density profile. It is shown that in case of forward scattering
additional localization can be provided by the antenna diagram. It is
demonstrated that in case of backscattering the spatial resolution can be
better if the turbulence spectrum at high radial wavenumbers is suppressed. The
improvement of Doppler reflectometry data localization by probing beam focusing
onto the cut-off is proposed and described. The possibility of Doppler
reflectometry data interpretation based on the obtained expressions is shown.Comment: http://stacks.iop.org/0741-3335/46/114
Quasi-normal modes of superfluid neutron stars
We study non-radial oscillations of neutron stars with superfluid baryons, in
a general relativistic framework, including finite temperature effects. Using a
perturbative approach, we derive the equations describing stellar oscillations,
which we solve by numerical integration, employing different models of nucleon
superfluidity, and determining frequencies and gravitational damping times of
the quasi-normal modes. As expected by previous results, we find two classes of
modes, associated to superfluid and non-superfluid degrees of freedom,
respectively. We study the temperature dependence of the modes, finding that at
specific values of the temperature, the frequencies of the two classes of
quasi-normal modes show avoided crossings, and their damping times become
comparable. We also show that, when the temperature is not close to the avoided
crossings, the frequencies of the modes can be accurately computed by
neglecting the coupling between normal and superfluid degrees of freedom. Our
results have potential implications on the gravitational wave emission from
neutron stars.Comment: 16 pages, 7 figures, 2 table
Dissipation in relativistic superfluid neutron stars
We analyze damping of oscillations of general relativistic superfluid neutron
stars. To this aim we extend the method of decoupling of superfluid and normal
oscillation modes first suggested in [Gusakov & Kantor PRD 83, 081304(R)
(2011)]. All calculations are made self-consistently within the finite
temperature superfluid hydrodynamics. The general analytic formulas are derived
for damping times due to the shear and bulk viscosities. These formulas
describe both normal and superfluid neutron stars and are valid for oscillation
modes of arbitrary multipolarity. We show that: (i) use of the ordinary
one-fluid hydrodynamics is a good approximation, for most of the stellar
temperatures, if one is interested in calculation of the damping times of
normal f-modes; (ii) for radial and p-modes such an approximation is poor;
(iii) the temperature dependence of damping times undergoes a set of rapid
changes associated with resonance coupling of neighboring oscillation modes.
The latter effect can substantially accelerate viscous damping of normal modes
in certain stages of neutron-star thermal evolution.Comment: 25 pages, 9 figures, 1 table, accepted for publication in MNRA
Cooling of Akmal-Pandharipande-Ravenhall neutron star models
We study the cooling of superfluid neutron stars whose cores consist of
nucleon matter with the Akmal-Pandharipande-Ravenhall equation of state. This
equation of state opens the powerful direct Urca process of neutrino emission
in the interior of most massive neutron stars. Extending our previous studies
(Gusakov et al. 2004a, Kaminker et al. 2005), we employ phenomenological
density-dependent critical temperatures T_{cp}(\rho) of strong singlet-state
proton pairing (with the maximum T_{cp}^{max} \sim 7e9 K in the outer stellar
core) and T_{cnt}(\rho) of moderate triplet-state neutron pairing (with the
maximum T_{cnt}^{max} \sim 6e8 K in the inner core). Choosing properly the
position of T_{cnt}^{max} we can obtain a representative class of massive
neutron stars whose cooling is intermediate between the cooling enhanced by the
neutrino emission due to Cooper pairing of neutrons in the absence of the
direct Urca process and the very fast cooling provided by the direct Urca
process non-suppressed by superfluidity.Comment: 9 pages, 6 figures; accepted for publication in MNRA
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