119 research outputs found
Spallative ablation of dielectrics by X-ray laser
Short laser pulse in wide range of wavelengths, from infrared to X-ray,
disturbs electron-ion equilibrium and rises pressure in a heated layer. The
case where pulse duration is shorter than acoustic relaxation time
is considered in the paper. It is shown that this short pulse may cause
thermomechanical phenomena such as spallative ablation regardless to
wavelength. While the physics of electron-ion relaxation on wavelength and
various electron spectra of substances: there are spectra with an energy gap in
semiconductors and dielectrics opposed to gapless continuous spectra in metals.
The paper describes entire sequence of thermomechanical processes from
expansion, nucleation, foaming, and nanostructuring to spallation with
particular attention to spallation by X-ray pulse
High shock release in ultrafast laser irradiated metals: Scenario for material ejection
We present one-dimensional numerical simulations describing the behavior of
solid matter exposed to subpicosecond near infrared pulsed laser radiation. We
point out to the role of strong isochoric heating as a mechanism for producing
highly non-equilibrium thermodynamic states. In the case of metals, the
conditions of material ejection from the surface are discussed in a
hydrodynamic context, allowing correlation of the thermodynamic features with
ablation mechanisms. A convenient synthetic representation of the thermodynamic
processes is presented, emphasizing different competitive pathways of material
ejection. Based on the study of the relaxation and cooling processes which
constrain the system to follow original thermodynamic paths, we establish that
the metal surface can exhibit several kinds of phase evolution which can result
in phase explosion or fragmentation. An estimation of the amount of material
exceeding the specific energy required for melting is reported for copper and
aluminum and a theoretical value of the limit-size of the recast material after
ultrashort laser irradiation is determined. Ablation by mechanical
fragmentation is also analysed and compared to experimental data for aluminum
subjected to high tensile pressures and ultrafast loading rates. Spallation is
expected to occur at the rear surface of the aluminum foils and a comparison
with simulation results can determine a spall strength value related to high
strain rates
Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
We propose a hybrid model to fit the X-ray spectra of atoll-type X-ray
transients in the soft and hard states. This model uniquely produces luminosity
tracks that are proportional to T^4 for both the accretion disk and boundary
layer. The model also indicates low Comptonization levels for the soft state,
gaining a similarity to black holes in the relationship between Comptonization
level and the strength of integrated rms variability in the power density
spectrum. The boundary layer appears small, with a surface area that is roughly
constant across soft and hard states. This result may suggestion that the NS
radius is smaller than its inner-most stable circular orbit.Comment: 15 pages, 15 figures, accepted for publication in the Ap
Nonlinear dynamics of magnetohydrodynamic flows of heavy fluid over an arbitrary surface in shallow water approximation
The magnetohydrodynamic equations system for heavy fluid over an arbitrary
surface in shallow water approximation is studied in the present paper. It is
shown that simple wave solutions exist only for underlying surfaces that are
slopes of constant inclination. All self-similar discontinuous and continuous
solutions are found. The exact explicit solutions of initial discontinuity
decay problem over a flat plane and a slope are found. It is shown that the
initial discontinuity decay solution is represented by one of five possible
wave configurations. For each configuration the necessary and sufficient
conditions for its realization are found. The change of dependent and
independent variables transforming the initial equations over a slope to those
over a flat plane is found.Comment: 43 pages, submitted to Theoretical and Computational Fluid Dynamic
Torsional Magnetic Oscillations in Type I X-Ray Bursts
Thermonuclear burning on the surface of a neutron star causes the expansion
of a thin outer layer of the star, . The layer rotates slower than
the star due to angular momentum conservation. The shear between the star and
the layer acts to twist the star's dipole magnetic field giving at first a
trailing spiral field. The twist of the field acts in turn to `torque up' the
layer increasing its specific angular momentum. As the layer cools and
contracts, its excess specific angular momentum causes it to {\it rotate
faster} than the star which gives a leading spiral magnetic field. The process
repeats, giving rise to torsional oscillations. We derive equations for the
angular velocity and magnetic field of the layer taking into account the
diffusivity and viscosity which are probably due to turbulence. The magnetic
field causes a nonuniformity of the star's photosphere (at the top of the
heated layer), and this gives rise to the observed X-ray oscillations. The fact
that the layer periodically rotates faster than the star means that the X-ray
oscillation frequency may ``overshoot'' the star's rotation frequency.
Comparison of the theory is made with observations of Chakrabarty et al. (2003)
of an X-ray burst of SAX J1808.4-3658.Comment: 7 pages, 6 figures, accepted for publication in the Ap
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