26 research outputs found
Conditions of structural transition for collisionless electrostatic shock
Collisionless shock acceleration, which transfers localized particle energies
to non-thermal energetic particles via electromagnetic potential, is ubiquitous
in space plasma. We investigate dynamics of collisionless electrostatic shocks
that appear at interface of two plasma slabs with different pressures using
one-dimensional particle-in-cell (PIC) simulations and find that the shock
structure transforms to a double-layer structure at the high density gradient.
The threshold condition of the structure transformation is identified as
density ratio of the two plasma slabs regardless of the
temperature ratio between them. We then update the collisionless shock model
that takes into account density expansion effects caused by a rarefaction wave
to improve the prediction of the critical Mach numbers. The new critical Mach
numbers are benchmarked by PIC simulations for a wide range of .
Furthermore, we introduce a semi-analytical approach to forecast the shock
velocity just from the initial conditions based on a new concept of the
accelerated fraction .Comment: 9 pages, 10 figures; accepted for publication on PR
Velocity Distributions and Density Profiles of Buffer-Gas Gooled One Component Plasma in Toroidal RF lon Trap
Abstract Velocity and spatial distributions of calcium (Ca*
Enhancement of collisionless shock ion acceleration by electrostatic ion two-stream instability in the upstream plasma
Ion acceleration in electrostatic collisionless shocks is driven by the interaction of the high-power laser with specially tailored near-relativistic critical density plasma. 2D EPOCH particle-in-cell simulations show that the ion acceleration is dependent on the target material used. In materials with low charge-to-mass ratio
Ion acceleration at two collisionless shocks in a multicomponent plasma
Intense laser-plasma interactions are an essential tool for the laboratory study of ion acceleration at a collisionless shock. With two-dimensional particle-in-cell calculations of a multicomponent plasma we observe two electrostatic collisionless shocks at two distinct longitudinal positions when driven with a linearly polarized laser at normalized laser vector potential a0 that exceeds 10. Moreover, these shocks, associated with protons and carbon ions, show a power-law dependence on a0 and accelerate ions to different velocities in an expanding upstream with higher flux than in a single-component hydrogen or carbon plasma. This results from an electrostatic ion two-stream instability caused by differences in the charge-to-mass ratio of different ions. Particle acceleration in collisionless shocks in multicomponent plasma are ubiquitous in space and astrophysics, and these calculations identify the possibility for studying these complex processes in the laboratory
Model experiment of magnetic field amplification in laser-produced plasmas via the Richtmyer-Meshkov instability
A model experiment of magnetic field amplification (MFA) via the Richtmyer-Meshkov instability (RMI) in supernova remnants (SNRs) was performed using a high-power laser. In order to account for very-fast acceleration of cosmic rays observed in SNRs, it is considered that the magnetic field has to be amplified by orders of magnitude from its background level. A possible mechanism for the MFA in SNRs is stretching and mixing of the magnetic field via the RMI when shock waves pass through dense molecular clouds in interstellar media. In order to model the astrophysical phenomenon in laboratories, there are three necessary factors for the RMI to be operative: a shock wave, an external magnetic field, and density inhomogeneity. By irradiating a double-foil target with several laser beams with focal spot displacement under influence of an external magnetic field, shock waves were excited and passed through the density inhomogeneity. Radiative hydrodynamic simulations show that the RMI evolves as the density inhomogeneity is shocked, resulting in higher MFA
Transonic Dislocation Propagation in Diamond
The motion of line defects (dislocations) has been studied for over 60 years
but the maximum speed at which they can move is unresolved. Recent models and
atomistic simulations predict the existence of a limiting velocity of
dislocation motions between the transonic and subsonic ranges at which the
self-energy of dislocation diverges, though they do not deny the possibility of
the transonic dislocations. We use femtosecond x-ray radiography to track
ultrafast dislocation motion in shock-compressed single-crystal diamond. By
visualizing stacking faults extending faster than the slowest sound wave speed
of diamond, we show the evidence of partial dislocations at their leading edge
moving transonically. Understanding the upper limit of dislocation mobility in
crystals is essential to accurately model, predict, and control the mechanical
properties of materials under extreme conditions
Highly radiative shock experiments driven by GEKKO XII
International audienceIn this paper, recent results obtained on highly radiative shocks generated in a xenon filled gas cell using the GEKKO XII laser facility are presented. Data show extremely high shock velocity (>=150 km/s) never achieved before in gas. Preliminary analyses based on theoretical dimensionless numbers and numerical simulations suggest that these radiative shocks reach a new radiative regime where the radiative pressure plays a role in the dynamics and structure of the shock. A major effect observed is a strong anisotropic emission in the downstream gas. This unexpected feature is discussed and compared to available 2D radiation hydrodynamic simulations