464 research outputs found
The impact of kinetic effects on the properties of relativistic electron-positron shocks
We assess the impact of non-thermally shock-accelerated particles on the
magnetohydrodynamic (MHD) jump conditions of relativistic shocks. The adiabatic
constant is calculated directly from first principle particle-in-cell
simulation data, enabling a semi-kinetic approach to improve the standard fluid
model and allowing for an identification of the key parameters that define the
shock structure. We find that the evolving upstream parameters have a stronger
impact than the corrections due to non-thermal particles. We find that the
decrease of the upstream bulk speed yields deviations from the standard MHD
model up to 10%. Furthermore, we obtain a quantitative definition of the shock
transition region from our analysis. For Weibel-mediated shocks the inclusion
of a magnetic field in the MHD conservation equations is addressed for the
first time
Exploring the nature of collisionless shocks under laboratory conditions
Collisionless shocks are pervasive in astrophysics and they are critical to
understand cosmic ray acceleration. Laboratory experiments with intense lasers
are now opening the way to explore and characterise the underlying
microphysics, which determine the acceleration process of collisionless shocks.
We determine the shock character - electrostatic or electromagnetic - based on
the stability of electrostatic shocks to transverse electromagnetic
fluctuations as a function of the electron temperature and flow velocity of the
plasma components, and we compare the analytical model with particle-in-cell
simulations. By making the connection with the laser parameters driving the
plasma flows, we demonstrate that shocks with different and distinct underlying
microphysics can be explored in the laboratory with state-of-the-art laser
systems
Three-dimensional simulations of laser-plasma interactions at ultrahigh intensities
Three-dimensional (3D) particle-in-cell (PIC) simulations are used to
investigate the interaction of ultrahigh intensity lasers (
W/cm) with matter at overcritical densities. Intense laser pulses are
shown to penetrate up to relativistic critical density levels and to be
strongly self-focused during this process. The heat flux of the accelerated
electrons is observed to have an annular structure when the laser is tightly
focused, showing that a large fraction of fast electrons is accelerated at an
angle. These results shed light into the multi-dimensional effects present in
laser-plasma interactions of relevance to fast ignition of fusion targets and
laser-driven ion acceleration in plasmas.Comment: 2 pages, 1 figur
Ion acceleration from laser-driven electrostatic shocks
Multi-dimensional particle-in-cell simulations are used to study the
generation of electrostatic shocks in plasma and the reflection of background
ions to produce high-quality and high-energy ion beams. Electrostatic shocks
are driven by the interaction of two plasmas with different density and/or
relative drift velocity. The energy and number of ions reflected by the shock
increase with increasing density ratio and relative drift velocity between the
two interacting plasmas. It is shown that the interaction of intense lasers
with tailored near-critical density plasmas allows for the efficient heating of
the plasma electrons and steepening of the plasma profile at the critical
density interface, leading to the generation of high-velocity shock structures
and high-energy ion beams. Our results indicate that high-quality 200 MeV
shock-accelerated ion beams required for medical applications may be obtained
with current laser systems.Comment: 33 pages, 12 figures, accepted for publication in Physics of Plasma
Melanin-concentrating hormone in peripheral circulation in the human
Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide with a well-characterised role in energy homeostasis and emergent roles in diverse physiologic functions such as arousal, mood and reproduction. Work to date has predominantly focused on its hypothalamic functions using animal models; however, little attention has been paid to its role in circulation in humans. The aims of this study were to (a) develop a radioimmunoassay for the detection of MCH in human plasma; (b) establish reference ranges for circulating MCH and (c) characterise the pattern of expression of circulating MCH in humans. A sensitive and specific RIA was developed and cross-validated by RP-HPLC and MS. The effective range was 19.5–1248 pg MCH/mL. Blood samples from 231 subjects were taken to establish a reference range of 19.5–55.4 pg/mL for fasting MCH concentrations. There were no significant differences between male and female fasting MCH concentrations; however, there were correlations between MCH concentrations and BMI in males and females with excess fat (P < 0.001 and P = 0.020) and between MCH concentrations and fat mass in females with excess fat (P = 0.038). Plasma MCH concentrations rose significantly after feeding in a group of older individuals (n = 50, males P = 0.006, females P = 0.023). There were no robust significant correlations between fasting or post-prandial MCH and resting metabolic rate, plasma glucose, insulin or leptin concentrations although there were correlations between circulating MCH and leptin concentrations in older individuals (P = 0.029). These results indicate that the role of circulating MCH may not be reflective of its regulatory hypothalamic role
One-to-one full scale simulations of laser wakefield acceleration using QuickPIC
We use the quasi-static particle-in-cell code QuickPIC to perform full-scale,
one-to-one LWFA numerical experiments, with parameters that closely follow
current experimental conditions. The propagation of state-of-the-art laser
pulses in both preformed and uniform plasma channels is examined. We show that
the presence of the channel is important whenever the laser self-modulations do
not dominate the propagation. We examine the acceleration of an externally
injected electron beam in the wake generated by 10 J laser pulses, showing that
by using ten-centimeter-scale plasma channels it is possible to accelerate
electrons to more than 4 GeV. A comparison between QuickPIC and 2D OSIRIS is
provided. Good qualitative agreement between the two codes is found, but the 2D
full PIC simulations fail to predict the correct laser and wakefield
amplitudes.Comment: 5 pages, 5 figures, accepted for publication IEEE TPS, Special Issue
- Laser & Plasma Accelerators - 8/200
Elimination of the numerical Cerenkov instability for spectral EM-PIC codes
When using an electromagnetic particle-in-cell (EM-PIC) code to simulate a
relativistically drifting plasma, a violent numerical instability known as the
numerical Cerenkov instability (NCI) occurs. The NCI is due to the unphysical
coupling of electromagnetic waves on a grid to wave-particle resonances,
including aliased resonances, i.e., , where and refer to the time and space
aliases and the plasma is drifting relativistically at velocity in the
-direction. Recent studies have shown that an EM-PIC code which uses a
spectral field solver and a low pass filter can eliminate the fastest growing
modes of the NCI. Based on these studies a new spectral PIC code for studying
laser wakefield acceleration (LWFA) in the Lorentz boosted frame was developed.
However, we show that for parameters of relevance for LWFA simulations in the
boosted frame, a relativistically drifting plasma is susceptible to a host of
additional unstable modes with lower growth rates, and that these modes appear
when the fastest growing unstable modes are filtered out. We show that these
modes are most easily identified as the coupling between modes which are purely
transverse (EM) and purely longitudinal (Langmuir) in the rest frame of the
plasma for specific time and space aliases. We rewrite the dispersion relation
of the drifting plasma for a general field solver and obtain analytic
expressions for the location and growth rate for each unstable mode, i.e, for
each time and space aliased resonances. We show for the spectral solver that
when the fastest growing mode is eliminated a new mode at the fundamental
resonance () can be seen. (Please check the whole abstract in the
paper).Comment: 36 pages, 12 figure
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