Biomedical subjects

N A Tahir

Publications and source records attributed to N A Tahir.

9 recordsLinked to original sources

PubMed2006

Richtmyer-Meshkov flow in elastic solids.

Richtmyer-Meshkov flow is studied by means of an analytical model which describes the asymptotic oscillations of a corrugated interface between two perfectly elastic solids after the interaction with a shock wave. The model shows that the flow stability is due to the restoring effect of the elastic force. It provides a simple approximate but still very accurate formula for the oscillation period. It also shows that as it is observed in numerical simulations, the amplitude oscillates around a mean value equal to the post-shock amplitude, and that this is a consequence of the stress free conditions of the material immediately after the shock interaction. Extensive numerical simulations are presented to validate the model results.

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PubMed2005

Rayleigh-Taylor instability in elastic solids.

We present an analytical model for the Rayleigh-Taylor instability that allows for an approximate but still very accurate and appealing description of the instability physics in the linear regime. The model is based on the second law of Newton and it has been developed with the aim of dealing with the instability of accelerated elastic solids. It yields the asymptotic instability growth rate but also describes the initial transient phase determined by the initial conditions. We have applied the model to solid/solid and solid/fluid interfaces with arbitrary Atwood numbers. The results are in excellent agreement with previous models that yield exact solutions but which are of more limited validity. Our model allows for including more complex physics. In particular, the present approach is expected to lead to a more general theory of the instability that would allow for describing the transition to the plastic regime.

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PubMed2005

Proposal for the study of thermophysical properties of high-energy-density matter using current and future heavy-ion accelerator facilities at GSI Darmstadt.

The subject of high-energy-density (HED) states in matter is of considerable importance to numerous branches of basic as well as applied physics. Intense heavy-ion beams are an excellent tool to create large samples of HED matter in the laboratory with fairly uniform physical conditions. Gesellschaft für Schwerionenforschung, Darmstadt, is a unique worldwide laboratory that has a heavy-ion synchrotron, SIS18, that delivers intense beams of energetic heavy ions. Construction of a much more powerful synchrotron, SIS100, at the future international facility for antiprotons and ion research (FAIR) at Darmstadt will lead to an increase in beam intensity by 3 orders of magnitude compared to what is currently available. The purpose of this Letter is to investigate with the help of two-dimensional numerical simulations, the potential of the FAIR to carry out research in the field of HED states in matter.

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PubMed2005

The CERN Large Hadron Collider as a tool to study high-energy density matter.

The Large Hadron Collider (LHC) at CERN will generate two extremely powerful 7 TeV proton beams. Each beam will consist of 2808 bunches with an intensity per bunch of 1.15x10(11) protons so that the total number of protons in one beam will be about 3x10(14) and the total energy will be 362 MJ. Each bunch will have a duration of 0.5 ns and two successive bunches will be separated by 25 ns, while the power distribution in the radial direction will be Gaussian with a standard deviation, sigma=0.2 mm. The total duration of the beam will be about 89 mus. Using a 2D hydrodynamic code, we have carried out numerical simulations of the thermodynamic and hydrodynamic response of a solid copper target that is irradiated with one of the LHC beams. These calculations show that only the first few hundred proton bunches will deposit a high specific energy of 400 kJ/g that will induce exotic states of high energy density in matter.

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PubMed2003

Generation of a hollow ion beam: calculation of the rotation frequency required to accommodate symmetry constraint.

A hollow intense heavy ion beam with an annular focal spot has many important applications. The Gesellschaft für Schwerionenforschung, Darmstadt is planning to develop a radio frequency wobbler that will rotate the beam at extremely high frequencies and thus create an annular (ring shaped) focal spot. In this paper, we present an analytical model that determines the minimum rotation frequency of the wobbler in order to achieve a high degree of irradiation symmetry (an asymmetry of a few percent) of the target. Estimates for a typical heavy ion imploded target are also presented.

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PubMed2002

Implosion of multilayered cylindrical targets driven by intense heavy ion beams.

An analytical model for the implosion of a multilayered cylindrical target driven by an intense heavy ion beam has been developed. The target is composed of a cylinder of frozen hydrogen or deuterium, which is enclosed in a thick shell of solid lead. This target has been designed for future high-energy-density matter experiments to be carried out at the Gesellschaft für Schwerionenforschung, Darmstadt. The model describes the implosion dynamics including the motion of the incident shock and the first reflected shock and allows for calculation of the physical conditions of the hydrogen at stagnation. The model predicts that the conditions of the compressed hydrogen are not sensitive to significant variations in target and beam parameters. These predictions are confirmed by one-dimensional numerical simulations and thus allow for a robust target design.

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PubMed2001

Necessity of bunch compression for heavy-ion-induced hydrodynamics and studies of beam fragmentation in solid targets at a proposed synchrotron facility.

This paper presents two-dimensional numerical simulations of hydrodynamic response of a solid lead cylindrical target that is irradiated by an intense uranium beam having a particle energy of 1 GeV/u and that consists of 10(12) particles. Different time profiles have been considered for the beam power that include a case where the beam consists of five identical parabolic bunches with equal separation between neighboring bunches as well as a beam that consists of a single bunch. For the single bunch case we consider two different values for pulse length, namely, 1000 and 50 ns, respectively. Moreover we allow for two different values for the beam radius that is 0.5 and 1.0 mm, respectively. These calculations show that in order to achieve a high degree of beam-target coupling, it is absolutely essential to use a single bunched beam that has a reasonably short pulse length, which is 50 ns in this case. Such a large beam-target coupling efficiency is highly desirable for creating high-density strongly coupled plasmas as well as for studies that involve fragmentation of the projectile ions as the beam passes through solid matter. If the pulse length is assumed to be too long, substantial hydrodynamic expansion of the target material occurs during the early stages of irradiation that leads to significant reduction in the energy deposition by the ions that are delivered in the later part of the pulse. In case of the five-bunch configuration, heating caused by the first bunch is so strong that the target is completely distorted. As a result, the ions that are delivered in the later four bunches pass through the target without any interaction.

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PubMed2000

Metallization of hydrogen using heavy-ion-beam implosion of multilayered cylindrical targets.

Employing a two-dimensional simulation model, this paper presents a suitable design for an experiment to study metallization of hydrogen in a heavy-ion beam imploded multilayered cylindrical target that contains a layer of frozen hydrogen. Such an experiment will be carried out at the upgraded heavy-ion synchrotron facility (SIS-18) at the Gesellschaft für Schwerionenforschung, Darmstadt by the end of the year 2001. In these calculations we consider a uranium beam that will be available at the upgraded SIS-18. Our calculations show that it may be possible to achieve theoretically predicted physical conditions necessary to create metallic hydrogen in such experiments. These include a density of about 1 g/cm(3), a pressure of 3-5 Mbar, and a temperature of a few 0.1 eV.

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PubMed1999

Heavy-ion-beam-induced hydrodynamic effects in solid targets.

It is expected that after the completion of a new high current injector, the heavy-ion synchrotron (SIS) at the Gesellschaft für Schwerionforschung (GSI) Darmstadt will accelerate U(+28) ions to energies of the order of 200 MeV/u. The use of a powerful rf buncher will reduce the pulse length to about 50 ns, and employment of a multiturn injection scheme will provide 2 x 10(11) particles in the beam that correspond to a total energy of the order of 1 kJ. This upgrade of the SIS, hopefully, will be completed by the end of the year 2001. These beam parameters lead to a specific power deposition of the order of 1-2 TW/g in solid matter that will provide temperatures of about 10 eV. Such low specific power deposition will induce hydrodynamic effects in solid materials, and one may design appropriate beam-target interaction experiments that could be used to investigate the equation of state of matter under extreme conditions. The purpose of this paper is to propose suitable target designs with optimized parameters for the future GSI experiments with the help of one and two-dimensional hydrodynamic simulations. Cylindrical geometry is the natural geometry for highly focused ion beams, and therefore cylindrical targets are the most appropriate for this type of interaction experiments. The numerical simulations presented in this paper show that one can experimentally measure the characteristic sound speed in beam heated targets which is an important physical parameter. Moreover, one can study the propagation of ion-beam-induced shock waves in the solid materials. Different values for the specific power deposition, namely, 10, 25, 50, and 100 kJ/g, have been used. In some cases the pulse length is assumed to be 40 ns while in others it is considered to be 50 ns. Various materials including lead, aluminum, and solid neon have been used.

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