It can be converted to X-rays by irradiating the inside of a high-atomic-number e. The laser light is efficiently absorbed at the wall where material is heated sufficiently to radiate a large proportion of the absorbed energy as X-rays.
This heats other wall areas and these reradiate similarly, and so the X-ray field is partially trapped and smoothed, and can be used to drive an experimental package mounted from the wall. Spatial and temporal compression means that laser target sizes are small — mm typically and experimental durations are short — sec typically.
This makes target manufacture and diagnosis challenging. Complex simulation codes have been developed to model laser targets and the coupling between different physical processes. The workhorse codes are continuum fluid hydrocodes onto which numerous modular physics packages are added to model laser absorption, X-ray transport, etc. Many of the physics packages involve solving coupled non-linear partial differential equations, and for the fluid dynamics these are the three Euler hydrodynamic equations  that represent conservation of mass, momentum and energy:.
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These three equations in four unknowns, density , pressure , velocity and specific internal energy , are supplemented by material equations-of-state EoS that give from and to allow the system to be solved. Fluid viscosity is not represented in the above equations because it is usually small for HED plasmas.
Laser targets are designed using 1D, 2D and 3D radiation-hydrodynamic simulations like the example shown in Figure 3. Modelling is limited by computer power which has been increasing rapidly, with AWE resources increasing from 0. This now allows 2D and increasingly 3D simulations to be routine, which is important because most laser targets have significant 2D and 3D features. These large increases in computing power have been achieved through massively parallel processing with large simulations run over thousands of processors.
This requires codes to have good load balancing between processors with minimised processor-to-processor communications because this is relatively slow. Traditionally fluid dynamics is solved with either Lagrangian hydrodynamics where the computational mesh is anchored to the material Figure 3 , or Eulerian hydrodynamics where the mesh is fixed in space with material advected from cell to cell. Subzone interface reconstruction is very important for Eulerian hydrodynamics to ensure materials are advected accurately with little numerical diffusion, although some dissipation is inevitable.
Lagrangian hydrodynamics maintains material interfaces precisely but mesh distortion can become a problem leading to poor accuracy e. Lagrangian methods are more efficient with fine zoning focused in important regions to resolve laser and X-ray interactions and so it is frequently beneficial to model a laser target with Lagrangian hydrodynamics initially and then map to Eulerian at later times when the mesh starts to tangle. The distinction between Lagrangian and Eulerian is now blurring with the introduction of arbitrary Lagrangian—Eulerian ALE codes, where Lagrangian and Eulerian regions coexist with Eulerian hydrodynamics applied to areas where shear flows are high, while retaining the efficiency of Lagrangian hydrodynamics elsewhere.
An alternative approach to ALE is adaptive mesh refinement AMR where Eulerian hydrodynamics is used globally with patches of increasing mesh refinement applied based on local gradients to better resolve physical processes. Hydrodynamics is modelled explicitly with a global time step, governed by the fastest cell-to-cell communication based on the local sound speed to ensure stability i. Courant—Friedrichs—Lewy condition . Any element in a hot body emits X-ray radiation based on its temperature , density and its radiative opacity , where opacity is the opaqueness of the material to photons and sets transmission through an element of thickness as.
These photons then propagate and are absorbed or amplified by the surrounding material. This propagation obeys the radiation transport equation, which is a form of the Boltzmann transport equation ,. This can be solved using a range of methods. For optically thick systems such as a hohlraum wall, where there is continual strong absorption and isotropic re-emission of X-rays, the solution can be approximated by diffusion , with the flux of radiation energy proportional to the gradient in temperature, like thermal conduction , where is the diffusion coefficient, which is , with the Stefan—Boltzmann constant and is a weighted integral of the frequency-dependent opacity known as the Rosseland average .
Numerical diffusion solutions are fast and spatially smooth although the diffusion approximation is inappropriate for many situations of interest such as optically thin plasmas, where photons can free-stream or where an opaque wall e. In such cases a full transport solution is needed with the most common being statistical Monte Carlo methods where emission is approximated by particles, each representing a huge number of photons, and these are generated randomly based on the local material properties .
The most common alternative to Monte Carlo is deterministic SN , also known as discrete ordinate, where fixed quadrature sets e. Gauss—Legendre define discrete directions covering a unit sphere from each mesh cell along which the radiation transport equation is solved. Monte Carlo is typically less expensive than SN for modelling laser targets but suffers from statistical noise, which can make it unsuitable for modelling hydrodynamic instabilities, whereas SN can suffer from ray effects if the quadrature set is not of high enough order.
Monte Carlo and SN require efficient solutions for ray interceptions with the fluid mesh, and diffusion requires fast matrix inversions. An explicit radiation scheme would require a very small simulation time step e. This is impractical for multi-dimensional simulations so an implicit scheme is typically used which is universally stable, although this does not guarantee accuracy. For Monte Carlo the implicit feedback during a time step is achieved by treating absorption as pseudo-scattering , where the local conditions make this reasonable such as hot opaque material.
This assumes the absorbed energy is immediately reradiated isotropically with characteristics changed to those of the local material. Radiation-hydrodynamic modelling is fundamentally dependent on material properties data over a wide range of density and temperature space. The main inputs are equation-of-state i. These are typically interpolated from large grids generated offline by detailed atomic physics and thermodynamic codes. Much of the initial design work for laser targets can be done with analytical models, especially if the basic target evolution is 1D.
Hydrodynamics is well understood and analytical solutions of the Euler equations exist for a variety of phenomena of interest such as shock compression and adiabatic expansions , and these can be coupled together e. Here are some thoughts I have been considering since late The really short version is:. Plasma redshift heats the Inter Cluster Medium in the voids to such high temperatures, and with very low densities, that the resulting pressure corrals the denser, cooler Intra-Cluster Medium into its observed configuration - resembling bubble-like roughly spherical voids confining clusters of galaxies in sheets and filaments just as air bubbles confine soapy water.
Field, G. ApJ vol. The thermal bremsstrahlung from the model agrees with spectral measurements of the X-ray background XRB. It is shown that recent submillimeter measurements of the cosmic microwave background CMB are consistent with a spectrum distorted from blackbody by Compton scattering on the same IGG. It is also shown that the isotropy and intensity of the XRB rule out its origin from discrete gas clouds. Because of the large energy requirement to heat the IGG and other considerations, the existence of a cosmologically significant amount of hot IGG must be regarded as uncertain.
It is concluded that the amount of hot IGG corresponds to a critical density parameter of no more than 1. The spectrum of the extragalactic diffuse X-ray background has been measured with the GSFC Cosmic X-ray Experiment on HEAO 1 for regions of the sky away from known point sources and more than 20 deg from the galactic plane. A total exposure of 80 sq m-s-sr is available at present.
Free-free emission from an optically thin plasma of 40 plus or minus 5 keV provides an excellent description of the observed spectrum from 3 to 50 keV.
This spectral shape is confirmed by measurements from five separate layers of three independent detectors. Thus, the passage of starlight and also microwaves, UV, X-rays etc. Low density plasmas are totally unlike black-body radiators. Once they attain temperatures where all electrons are stripped from the nuclei of constituent atoms leaving mainly electrons, protons and helium nuclei their ability to radiate away their thermal energy gets less with increasing temperature.
How else could it get rid of that energy except by Free Free Emission thermal bremsstrahlung? As far as we know, it can't very well cool itself by physical contact with neighbouring matter, since the distances tens and hundreds of millions of light years are so vast and the IGM so low in density. Added New introductory paragraphs and important link added I suggest loading the "original data" and clicking "Play" and then "faster".
Then switch to the "enhanced structures" data and press "Play" and "faster" again. This latter set concentrates on the most dense areas.
To me, its really obvious that galaxies are not in the circular orbital patterns of gravitational motion as found in galaxies and stellar systems. The only obvious principle which could explain what we see is that the voids are the dominant force in the Universe - empty, but containing sufficient pressure to corral most or all galaxies. The location of quasars is hard to know, since a lot of their redshift is intrinsic to their local IGM.
The abovementioned images show only galaxy locations. Maybe quasars are not always confined like galaxies. This leads to two questions: How could the void, which is clearly of exceedingly low density, exert such pressure?
Assuming we don't invoke unconventional forces, particles etc. How could such a thin Void IGM, even if it was so hot and had sufficient pressure, corral things as heavy and hard to push as a star or a galaxy?
End of new intro. The bubble-like - or foam-like - structure observed in galaxy clusters and intergalactic voids is well established. It is deduced from observing galaxies according to their redshift. To a first approximation, I believe that redshift of galaxies, not quasars, BL Lacs etc.
Consider also the "Finger of God" aspect of galaxy redshift survey maps - I and Ari Brynjolfsson think this is caused by a cluster of galaxies having the rear ones appear more redshifted than the front ones, due to more plasma redshift, primarily in the denser plasma in the cluster. This foam-like arrangement is difficult or impossible to explain from a Big Bang perspective. One aspect of this is that it is a pattern which is a complete departure from the planetary structure of matter on smaller scales.
At the level of atoms, solar systems and galaxies we have the same paradigm - central mass with orbiting smaller masses, attracted by the electromagnetic force or gravity.
To some extent, I think we see the effects of gravity in clusters - but the clusters themselves do not seem to be circular or have a planetary structure. From all accounts, galaxy structures are elongated and irregular. Their really large-scale structure reveals them to be like the rubber in foam rubber - where the voids are like the empty bubbles in the foam. Consequently we must conclude tentatively, of course, since we don't know what we don't know that there is a more powerful force at work here than the gravitational attraction of galaxies at the very large inter-cluster distances.
The Void IGM has been established to be extraordinarily transparent, and relatively extremely free of atomic hydrogen. One estimate of the maximum possible density of H I atomic hydrogen in the IGM, based on the observed absence of atomic hydrogen absorption lines, is one atom of H per cubic kilometre. I have a reference for this somewhere. We may reasonably conclude that the Void IGM is extremely low density, since it does not seem to condense or collapse under its own gravity.