25,200 research outputs found
Generation of Suprathermal Electrons by Collective Processes in Collisional Plasma
The ubiquity of high-energy tails in the charged particle velocity
distribution functions observed in space plasmas suggests the existence of an
underlying process responsible for taking a fraction of the charged particle
population out of thermal equilibrium and redistributing it to suprathermal
velocity and energy ranges. The present Letter focuses on a new and fundamental
physical explanation for the origin of suprathermal electron distribution
function in a highly collisional plasma. This process involves a newly
discovered electrostatic bremsstrahlung emission that is effective in a plasma
in which binary collisions are present. The steady-state electron velocity
distribution function dictated by such a process corresponds to a Maxwellian
core plus a quasi-inverse power-law tail, which is a feature commonly observed
in many space plasma environment. In order to demonstrate this, the system of
self-consistent particle- and wave- kinetic equations are numerically solved
with an initially Maxwellian electron velocity distribution and Langmuir wave
spectral intensity, which is a state that does not reflect the presence of
electrostatic bremsstrahlung process, and hence not in force balance. The
electrostatic bremsstrahlung term subsequently drives the system to a new
force-balanced steady state. After a long integration period it is demonstrated
the initial Langmuir fluctuation spectrum is modified, which in turn distorts
the initial Maxwellian electron distribution into a velocity distribution that
resembles the said core-suprathermal velocity distribution. Such a mechanism
may thus be operative at the coronal source region, which is characterized by
high collisionality.Comment: 7 pages, 2 figures. Published at: The Astrophysical Journal Letters,
Volume 849, Number 2, L30. url: https://doi.org/10.3847/2041-8213/aa956
N_pN_n dependence of empirical formula for the lowest excitation energy of the 2^+ states in even-even nuclei
We examine the effects of the additional term of the type on the recently proposed empirical formula for the lowest excitation
energy of the states in even-even nuclei. This study is motivated by the
fact that this term carries the favorable dependence of the valence nucleon
numbers dictated by the scheme. We show explicitly that there is not
any improvement in reproducing by including the extra
term. However, our study also reveals that the excitation energies
, when calculated by the term alone (with the mass number
dependent term), are quite comparable to those calculated by the original
empirical formula.Comment: 14 pages, 5 figure
Dynamic model for failures in biological systems
A dynamic model for failures in biological organisms is proposed and studied
both analytically and numerically. Each cell in the organism becomes dead under
sufficiently strong stress, and is then allowed to be healed with some
probability. It is found that unlike the case of no healing, the organism in
general does not completely break down even in the presence of noise. Revealed
is the characteristic time evolution that the system tends to resist the stress
longer than the system without healing, followed by sudden breakdown with some
fraction of cells surviving. When the noise is weak, the critical stress beyond
which the system breaks down increases rapidly as the healing parameter is
raised from zero, indicative of the importance of healing in biological
systems.Comment: To appear in Europhys. Let
Large Amplitude Dynamics of the Pairing Correlations in a Unitary Fermi Gas
A unitary Fermi gas has a surprisingly rich spectrum of large amplitude modes
of the pairing field alone, which defies a description within a formalism
involving only a reduced set of degrees of freedom, such as quantum
hydrodynamics or a Landau-Ginzburg-like description. These modes are very slow,
with oscillation frequencies well below the pairing gap, which makes their
damping through quasiparticle excitations quite ineffective. In atomic traps
these modes couple naturally with the density oscillations, and the
corresponding oscillations of the atomic cloud are an example of a new type of
collective mode in superfluid Fermi systems. They have lower frequencies than
the compressional collective hydrodynamic oscillations, have a non-spherical
momentum distribution, and could be excited by a quick time variation of the
scattering length.Comment: 4 pages, 3 figures, published version, updated figures and a number
of change
Dynamic model of fiber bundles
A realistic continuous-time dynamics for fiber bundles is introduced and
studied both analytically and numerically. The equation of motion reproduces
known stationary-state results in the deterministic limit while the system
under non-vanishing stress always breaks down in the presence of noise.
Revealed in particular is the characteristic time evolution that the system
tends to resist the stress for considerable time, followed by sudden complete
rupture. The critical stress beyond which the complete rupture emerges is also
obtained
Particle-in-cell and weak turbulence simulations of plasma emission
The plasma emission process, which is the mechanism for solar type II and
type III radio bursts phenomena, is studied by means of particle-in-cell and
weak turbulence simulation methods. By plasma emission, it is meant as a loose
description of a series of processes, starting from the solar flare associated
electron beam exciting Langmuir and ion-acoustic turbulence, and subsequent
partial conversion of beam energy into the radiation energy by nonlinear
processes. Particle-in-cell (PIC) simulation is rigorous but the method is
computationally intense, and it is difficult to diagnose the results. Numerical
solution of equations of weak turbulence (WT) theory, termed WT simulation, on
the other hand, is efficient and naturally lends itself to diagnostics since
various terms in the equation can be turned on or off. Nevertheless, WT theory
is based upon a number of assumptions. It is, therefore, desirable to compare
the two methods, which is carried out for the first time in the present paper
with numerical solutions of the complete set of equations of the WT theory and
with two-dimensional electromagnetic PIC simulation. Upon making quantitative
comparisons it is found that WT theory is largely valid, although some
discrepancies are also found. The present study also indicates that it requires
large computational resources in order to accurately simulate the radiation
emission processes, especially for low electron beam speeds. Findings from the
present paper thus imply that both methods may be useful for the study of solar
radio emissions as they are complementary.Comment: 21 pages, 9 figure
Electromechanical tuning of vertically-coupled photonic crystal nanobeams
We present the design, the fabrication and the characterization of a tunable
one-dimensional (1D) photonic crystal cavity (PCC) etched on two
vertically-coupled GaAs nanobeams. A novel fabrication method which prevents
their adhesion under capillary forces is introduced. We discuss a design to
increase the flexibility of the structure and we demonstrate a large reversible
and controllable electromechanical wavelength tuning (> 15 nm) of the cavity
modes.Comment: 11 pages, 4 figure
Nonlinear Development of Streaming Instabilities In Strongly Magnetized Plasmas
The nonlinear development of streaming instabilities in the current layers
formed during magnetic reconnection with a guide field is explored. Theory and
3-D particle-in-cell simulations reveal two distinct phases. First, the
parallel Buneman instability grows and traps low velocity electrons. The
remaining electrons then drive two forms of turbulence: the parallel
electron-electron two-stream instability and the nearly-perpendicular lower
hybrid instability. The high velocity electrons resonate with the turbulence
and transfer momentum to the ions and low velocity electrons.Comment: Accepted by PR
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