1,721,031 research outputs found
Magnetohydrodynamic modelling of supersonic jets and colliding blast waves for laboratory astrophysics investigation
No full textThe thesis is related to laboratory astrophysics, and investigates with this technique, the launching mechanism for young stellar object jets and the interaction of two supernovae remnant in the Sedov-Taylor regime. Recent experiments performed at Imperial College on the pulsed-power magpie facility have successfully shown the formation of magnetically driven radiatively cooled plasmas jets formed from radial wire arrays, which are relevant to studying the launching mechanisms of astrophysical jets [A. Ciardi, et al. Phys. Plasmas 14, p056501 (2007)]. The experiments have been now extended to study episodic mass ejection ( 25 ns [F. A. Suzuki-Vidal, et al. 49th Annual Meeting of the Division of Plasma Physics, UO4.00007 (2007)]) and the interaction of jets and magnetic bubbles with an ambient gas. The dynamics of the interaction is investigated through three-dimensional resistive magneto-hydrodynamic simulations using the code gorgon [A. Ciardi, et al. Phys. Plasmas 14, p056501 (2007) – J.P. Chittenden, et al. Plasma Phys. Control. Fusion 46 B457 (2004)]. In particular ablation of the cathode is investigated numerically to explain the periodicity and subsequent formation of multiple bubbles. Comparison with experiments is offered to validate the results. The complex structure of the magnetic field is investigated, the conservation of the magnetic flux is explained and the consequent confinement offered to the central jet. Furthermore the interaction of the plasma outflows with an ambient gas is investigated. The formation of shocks in the ambient gas, as well as the formation of three-dimensional Mach stems is analyzed. In addition, recent experiment at Imperial College performed by the QOLS group, by laser-heating a medium of atomic clusters [R. A. Smith, et al. 2007 Plasma Phys. Control. Fusion 49 B117-B124 (2007)], shows the capability to create plasmas with sufficiently high energy densities to launch strong shocks. Interactions between high-Mach number shock waves are believed to be responsible for many of the complex, turbulent structures seen in astrophysical objects including supernova remnants. The experiment of two colliding Sedov-Taylor regime blast-waves is modelled. Detailed 3D numerical modeling is performed in order to study the importance of thermal conduction, rarefaction waves, refractive shock waves and complex three-dimensional mach stem formation. The simulated data are benchmark against a three-dimensional tomography image (newly developed experimental technique). The collision of two blast-waves should reproduce the non uniform interstellar medium where supernovas normally expand.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Shock ignition: a brief overview and progress in the design of robust targets
No full textShock ignition is a laser direct-drive inertial confinement fusion (ICF)
scheme in which the stages of compression and hot spot formation are partly
separated. The fuel is first imploded at a lower velocity than in conventional ICF,
reducing the threats due to Rayleigh-Taylor instability (RTI). Close to stagnation,
an intense laser spike drives a strong converging shock, which contributes to hot
spot formation. This paper starts with a brief overview of theoretical studies,
target design and experimental results on shock ignition. The second part of
the paper illustrates original work aiming at the design of robust targets and
computation of the relevant gain curves. Following Chang et al. [Phys. Rev.
Lett. 104 135002 (2010)] a safety factor for high gain, IFT* (analogous to the
ignition threshold factor ITF [Clark et al., Phys. Plasmas 15, 056305 (2008)]) ,
is evaluated by means of parametric one-dimensional simulations with artificially
reduced reactivity. SI designs scaled as in Atzeni et al. [New J. Phys. 15, 045004
(2013)] are found to have nearly the same ITF*. For a given target, such ITF*
increases with implosion velocity and laser spike power. A gain curve with a
prescribed ITF* can then be simply generated by upscaling a reference target
with that value of ITF*. An interesting option is scaling in size by reducing the
implosion velocity to keep the ratio of implosion velocity to self-ignition velocity
constant. At given total laser energy, targets with higher ITF* are driven to higher
implosion velocity and achieve somewhat lower gain. However, 1D gain higher
than 100 is achieved at (incident) energy below 1 MJ, implosion velocity below
300 km/s, and peak incident power below 400 TW. Two-dimensional simulations
of mispositioned targets show that targets with higher ITF* indeed tolerate larger
displacements
Magnetic field generation and diffusion by a laser-produced blast wave propagating in non-homogenous plasma
Full text linkIn this paper we discuss the magnetic field self generation, via the so-called Biermann battery effect, and its diffusion for a blast wave (BW) expanding in a perturbed background medium. A series of simulations verify the bi-linear behavior of the Biermann battery source term both in amplitude and in wavenumber. Such a behavior is valid in the limit of no diffusivity. When diffusivity is also considered, we observe an inverse proportionality with the wavenumber: for large wavenumber perturbation magnetic diffusivity plays a key role. Writing the induction equation in a dimensionless form we discuss how, in terms of magnetic properties, the BW can be subdivided into three main regions: the remnant where the frozen-in-flow approximation holds, the thin shell where the magnetic field is in fact generated but at the same time begins to diffuse, and the shock front where the magnetic field diffuses away. A possible experimental scenario that could induce magnetic fields of about 100 gauss is finally investigated. Simulations have been performed with the code DUED
Improved robustness study of a shock ignited target, with DUED code including non-local electron transport and 3D laser ray-tracing
Full text linkAccurate descriptions of laser power coupling to the plasma and electron energy
transport are crucial for designing shock-ignition targets and assessing their robustness (in
particular with regard to laser and positioning errors). To this purpose, the 2D DUED laser
fusion code has been improved with the inclusion of a 3D laser ray-tracing scheme and a model
for non-local electron transport. 2D simulations with the upgraded code are presented; the
dependence of the fusion yield vs target displacement is studied. Two different irradiation
configurations are considered
Estimation of yield stability for repetition rate HiPER shock-ignition targets using start-to-end hydrodynamic simulations
No full textWe present results of a numerical investigation campaign for HiPER
baseline targets driven by a shock-ignition laser pulse[1]. High-gain
1D hydrodynamic implosions were studied in order to identify a
reference point design for this ignition scheme. A parametric scan of
capsule illumination pattern has been conducted in order to maximise
laser intensity uniformity, while keeping to a minimum shot-to-shot
fluctuations and total drive energy[2].
The irradiation stability requirements, i.e. laser beams with a wide
and flat radial profile, are in contrast with cross-beam energy transfer
requirements, e.g. non-overlapping beams with narrow beam waist
[3]. This issue is presented and discussed in the framework of HiPER
project.
Start-to-end 2D hydrodynamic simulations coupled to fully 3D laser
raytracing were performed for selected cases in order to assess fusion
yield dependance on main irradiation parameters. The importance
and accuracy of transport mechanisms currently implemented in
hydrocodes is discussed. A caveat on the use and interpretation of
start-to-end simulations of capsule implosion is proposed.
References
[1] Atzeni S. et al, PPCF 53 035010 (2011).
[2] Schiavi A. et al., EPL 94 35002 (2011)
[3] Froula D.H. et al., PRL 108, 125003 (2012
Effects of non-local electron transport in one-dimensional and two-dimensional simulations of shock-ignited inertial confinement fusion targets
No full textIn some regions of a laser driven inertial fusion target, the electron mean-free path can become comparable to or even longer than the electron temperature gradient scale-length. This can be particularly important in shock-ignited (SI) targets, where the laser-spike heated corona reaches temperatures of several keV. In this case, thermal conduction cannot be described by a simple local conductivity model and a Fick's law. Fluid codes usually employ flux-limited conduction models, which preserve causality, but lose important features of the thermal flow. A more accurate thermal flow modeling requires convolution-like non-local operators. In order to improve the simulation of SI targets, the non-local electron transport operator proposed by Schurtz-Nicolai-Busquet [G. P. Schurtz el al., Phys. Plasmas 7, 4238 (2000)1 has been implemented in the DUED fluid code. Both one-dimensional (1D) and two-dimensional (2D) simulations of SI targets have been performed. 1D simulations of the ablation phase highlight that while the shock profile and timing might be mocked up with a flux-limiter; the electron temperature profiles exhibit a relatively different behavior with no major effects on the final gain. The spike, instead, can only roughly be reproduced with a fixed flux-limiter value. 1D target gain is however unaffected, provided some minor tuning of laser pulses. 2D simulations show that the use of a non-local thermal conduction model does not affect the robustness to mispositioning of targets driven by quasi-uniform laser irradiation. 2D simulations performed with only two final polar intense spikes yield encouraging results and support further studies. (C) 2014 AIP Publishing LLC
Electromagnetic self-consistent field initialization and fluid advance techniques for hybrid-kinetic PWFA code Architect
No full textThe realization of Plasma Wakefield Acceleration experiments with high quality of the accelerated bunches requires an increasing number of numerical simulations to perform first-order assessments for the experimental design and online-analysis of the experimental results. Particle in Cell codes are the state-of-the-art tools to study the beam-plasma interaction mechanism, but due to their requirements in terms of number of cores and computational time makes them unsuitable for quick parametric scans. Considerable interest has been shown thus in methods which reduce the computational time needed for the simulation of plasma acceleration. Such methods include the use of hybrid kinetic-fluid models, which treat the relativistic bunches as in a PIC code and the background plasma electrons as a fluid. A technique to properly initialize the bunch electromagnetic fields in the time explicit hybrid kinetic-fluid code Architect is presented, as well the implementation of the Flux Corrected Transport scheme for the fluid equations integrated in the code
Perspectives for inertial fusion
No full textThe quest for demonstrating ignition and gain in Inertial Confinement Fusion [1] has come to a crucial point, where the long awaited
proof of principle of this approach to fusion still fails to arrive. We present the status of this field of research, both at the international
[2] and european level [3], focussing on the standard approach and on the new strategies in laser fusion.
The path ahead is also indicated, highlighting some of the key points in the design of a fusion reactor. The recent results obtained by
our group on target design for shock ignition [4], optimization of direct-drive irradiation schemes [5], and energy and laser
wavelength scaling models are illustrated [6].
[1] J.D. Lindl, Physics of Plasmas 2, 3933 (1995).
[2] J.D. Lindl, E.I. Moses, Physics of Plasmas 18, 050901 (2011).
[3] M. Dunne, Nature Physics 2, 2 (2006).
[4] S. Atzeni, A. Schiavi, A. Marocchino, Plasma Physics and Controlled Fusion 53, 35010 (2011).
[5] A. Schiavi, S. Atzeni, A. Marocchino, Europhysics Letters 94, 35002 (2011).
[6] S. Atzeni, A. Marocchino, A. Schiavi, G. Schurtz, New J. of Physics 15, 045004 (2013)
Non-local electron transport: models and impact on direct-drive shock-ignition targets
No full textShock Ignition schemes require laser pulses with intensities exceeding
10^15 W/cm^2. At such intensities a few percent of electrons have
a mean free path longer than the characteristic temperature gradient
scale length. The classical Spitzer-Harm thermal conduction operator
(even including flux-limitation) is no longer appropriate, non-local
transport models need to be used to catch the underlying physics.
We will first briefly compare two non-local electron transport models:
Schurtz-Nicolai-Busquet [1] and the Colombant-Manheimer-Goncharov
[2,3] model. We will highlight few key numerical aspects and discuss
their use in inertial fusion hydrodynamic codes.
We will then focus on the use such models in shock ignition target
simulations. In the first place we will discuss the effect of non-local
transport during the ablation phase where electron transport affects
temperature and density evolution and generation of the ablation
pressure. In the second place we will discuss how the models can also
treat preheating of the target payload.
References
[1] G. Schurtz et al. Physics of Plasmas 7 10 (2000) pp. 4238-4249.
[2] W. Manheimer et al. Physics of Plasmas 15 8 (2008) pp. 083103
[3] D. Colombant et al. Journal of Computational Physics 229 11 (2010)
pp. 4369-438
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