Search results for "Stability"
showing 10 items of 3085 documents
Convective stability of hot matter in ultrarelativistic heavy-ion collisions
1992
Abstract The convective stability of strongly interacting matter undergoing hydrodynamic flow in ultrarelativistic heavy-ion collisions is studied in both the quark-gluon plasma and hadron gas phases. We find that this stability depends on the form of the initial conditions assumed for the hydrodynamic flow. In the case of initial conditions corresponding to partial transparency the flow of the quark-gluon plasma is stable whereas the flow of the hadron gas is convectively unstable. The timescale for hydrodynamic oscillations around the (stable or unstable) equilibrium state is found to be larger than the expected lifetime of the system, suggesting that the flow in either case is close to n…
Convection in the Surface Layers of Neutron Stars
1997
During some phases of a neutron star's evolution, the temperature gradient in the surface layers, calculated assuming only radiative and conductive transport, may exceed the adiabatic gradient. This superadiabatic gradient is the necessary (but not sufficient) condition for convective instability. The present paper examines the sufficiency condition for the onset of convection in neutron stars in the presence of a strong magnetic field. It is shown that the large fields typically found in neutron stars—about 1011 to 1013 G—stabilize the atmosphere against convection. Convective instability can arise only in neutron stars with very weak magnetic fields, ≤108-109 G. Convective motions in such…
SPH simulations of Shakura-Sunyaev instability at intermediate accretion rates
2003
We show that a standard Shakura-Sunyaev accretion disc around a black hole with an accretion rate lower than the critical Eddington limit does show the instability in the radiation pressure dominated zone. We obtain this result performing time-dependent simulations of accretion disks for a set of values of the viscosity parameter and accretion rate. In particular we always find the occurrence of the collapse of the disc: the instability develops always towards a collapsed gas pressure dominated disc and not towards the expansion. This result is valid for all initial configurations we tested. We find significant convective heat flux that increases the instability development time, but is not…
Gravitational Wave Echo of Relaxion Trapping
2021
To solve the hierarchy problem, the relaxion must remain trapped in the correct minimum, even if the electroweak symmetry is restored after reheating. In this scenario, the relaxion starts rolling again until the backreaction potential, with its set of local minima, reappears. Depending on the time of barrier reappearance, Hubble friction alone may be insufficient to retrap the relaxion in a large portion of the parameter space. Thus, an additional source of friction is required, which might be provided by coupling to a dark photon.The dark photon experiences a tachyonic instability as the relaxion rolls, which slows down the relaxion by backreacting to its motion, and efficiently creates a…
Number of metastable states of a chain with competing and anharmonicΦ4−like interactions
1993
We investigate the number of metastable configurations of a Φ 4 -like model with competing and anharmonic interactions as a function of an effective coupling constant η. The model has piecewise harmonic nearest-neighbor and harmonic next-nearerst-neighbor interactions. The number M of metastable states in the configuration space increases exponentially with the number N of particles: M∞exp(vN). It is shown numerically that, outside the previously considered range |η|<1/3, v is approximately linearly decreasing with η for |η|<1 and that v=0 for η≥1. These findings can be understood by describing the metastable configurations as an arrangement of kink solitons whose width creases with η
CONSTRUCTION OF METASTABLE STATES IN QUANTUM ELECTRODYNAMICS
2004
In this paper, we construct metastable states of atoms interacting with the quantized radiation field. These states emerge from the excited bound states of the non-interacting system. We prove that these states obey an exponential time-decay law. In detail, we show that their decay is given by an exponential function in time, predicted by Fermi's Golden Rule, plus a small remainder term. The latter is proportional to the (4+β)th power of the coupling constant and decays algebraically in time. As a result, though it is small, it dominates the decay for large times. A central point of the paper is that our remainder term is significantly smaller than the one previously obtained in [1] and as…
Stabilizing effect of driving and dissipation on quantum metastable states
2018
We investigate how the combined effects of strong Ohmic dissipation and monochromatic driving affect the stability of a quantum system with a metastable state. We find that, by increasing the coupling with the environment, the escape time makes a transition from a regime in which it is substantially controlled by the driving, displaying resonant peaks and dips, to a regime of frequency-independent escape time with a peak followed by a steep falloff. The escape time from the metastable state has a nonmonotonic behavior as a function of the thermal-bath coupling, the temperature, and the frequency of the driving. The quantum noise-enhanced stability phenomenon is observed in the investigated …
Coupling of density wave oscillations in parallel channels with high order modal kinetics: application to BWR out of phase oscillations
2000
Abstract In this paper, we study the behavior of a system formed by two parallel channels coupled to a multimodal kinetics. The first problem that arises is the calculation of the reactivity coefficients for the higher modes. This problem is solved by means of the introduction of distribution factors for a given reactor region which depend on the involved modes. We have also performed a detailed analysis of the different instability types which can be obtained from the model changing the boundary conditions and the feedback gains of the fundamental and first harmonic modes.
Stationary models of magnetized viscous tori around a Schwarzschild black hole
2020
We present stationary solutions of magnetized, viscous thick accretion disks around a Schwarzschild black hole. We assume that the tori are not self-gravitating, are endowed with a toroidal magnetic field, and obey a constant angular momentum law. Our study focuses on the role of the black hole curvature in the shear viscosity tensor and in their potential combined effect on the stationary solutions. Those are built in the framework of a causality-preserving, second-order gradient expansion scheme of relativistic hydrodynamics in the Eckart frame description which gives rise to hyperbolic equations of motion. The stationary models are constructed by numerically solving the general relativis…
Control of electron-cyclotron instability driven by strong ECRH in open magnetic trap
2018
We discuss the laboratory experiment on a controlled transition from the generation of periodic bursts of electromagnetic radiation into the continuous-wave regime of a cyclotron maser formed in a magnetically confined non-equilibrium plasma (Shalashov A. G. et al. , Phys. Rev. Lett. , 114 (2018) 205001). The kinetic cyclotron instability of the extraordinary wave of a weakly inhomogeneous magnetized plasma is driven by the anisotropic electron population resulting from electron cyclotron plasma heating in a MHD-stable minimum-B open magnetic trap. In the present communication we focus on a theoretical model that explains the existing data and motivates further experiments.