Atomic And Molecular Spectra Laser By Rajkumar Pdf 56
Donald
This talk will summarise recent work at Warwick on shock-ignition. This falls into two studies - the kinetic effects on laser-plasma interactions (LPI) and the associated hot-electron preheat. Both studies involve a coupling of the EPOCH particle-in-cell code and the Odin radhydro ALE code so the talk will begin with a brief introduction to shock-ignition and then a summary of the current status of EPOCH and Odin. In the high-power shock ignition spike of the laser profile the intensity is above the threshold for LPI and at the temperatures and scalelengths for ignition scale experiments kinetic effects can amplify these effects compared to fluid theory. Here we will discuss kinetic inflationary SRS (iSRS) and its consequences for shock-ignition. Hot-electrons from LPI are usually a problem as they pre-heat the core and inhibit compression. For shock-ignition however the hot-electrons may only become significant after the fuel is already compressed so it is possible that hot-electrons may benefit the path to ignition. The final part of the talk will describe the hot-electron model which has been implemented in Odin and discuss the initial results of their effect on implosion asymmetry and pre-heat.
Warm dense plasmas occur in astrophysical scenarios, and in the initial stages of laser-solid interaction. These plasma states are intrinsically difficult to model, at the intersection between classical plasmas and solid-state matter. Basic relations between radiation transport, ionisation stage, and density and temperature of electrons and ions, out of equilibrium, are challenging to predict. High harmonic generation (HHG), from IR laser-gas interactions, is a coherent source with photon energies in the XUV, capable of probing thin layers of solid density plasmas. HHG has been used for different imaging techniques, such as diffraction imaging and holography, by many groups including ours [1,2]. In recent years, we have gained better control over this laser-based source, from the control on its polarization [3], to wavefront [4], and yield. In this talk I will show how we are producing and using soft X-rays from High Harmonic Generation to probe 2D solid-density plasmas, in a single shot, for time and space resolved opacity measurements. After presenting the source, I will show a series of measurements of free-free opacities of highly correlated, warm dense plasmas, which highlight the impact of degeneracy on the plasma opacity [5]. Tapping into the potential of HHG sources as a probe, our ultimate goal is to better understand matter under extreme conditions, through ultrafast imaging of dense plasmas.
We use high power laser interactions and strong magnetic field generation in the laboratory to study accretion phenomena in young stellar objects. This coupling allows to model important processes related to matter accretion in order to better understand star and planetary system formation and evolution. Such scaled experimental approach can complement observations, by providing access to the underlying processes, with spatial and temporal resolution that is out of reach to observations. In my talk I will present our recent experimental results, including: (I) a study of the asymmetry induced upon accretion structures when columns of matter impact the surface of young stars with an oblique angle; and (II) a modelling of the scenario of matter accretion in the equatorial plane, claiming to explain the increased accretion rates of the EXor objects, supported by the demonstration of effective plasma propagation across B-field under scaled conditions relevant EXors. What we conclude that the latter study is that to the values of the astrophysical B-field in these objects, which is unknown at present, should be of the order of 100 Gauss.
Magnetic white dwarfs are part of some binary systems which accrete matter from their companion star as an accretion column. The accretion flow confined by the magnetic field lines falls at a supersonic velocity onto the magnetic poles of the white dwarf. At the impact, an accretion shock is generated and the post-shock region is structured as a result of the effects of complex radiative processes.
In this work, we present observational data, astrophysical numerical data, theoretical studies and finally experimental data obtained on different laser facilities as well as their associated numerical simulations. First, theoretical and numerical studies at the astrophysical scale describe the structure and the dynamics of the accretion column. In particular, we have studied the origin of rapid oscillations observed in the optical light curves of some objects. Then, we have completed these studies with an experimental approach to build laboratory millimetre scaled models of the radiation hydrodynamic processes occurred in the accretion column through powerful lasers. Experimental results obtained on the GEKKO XII laser facility and their interpretations are presented. Finally, we have optimized a new experimental design to achieve a similar regime on megajoule facilities in indirect drive. The data obtained from such experiments will provide new insights to improve astrophysical modelling.
The phenomenon of magnetic-field amplification due to the motion of turbulent plasma has been investigated in a series of experiments carried out at various high-energy laser facilities during the last five years. Plasma jets driven by intense laser irradiation pass through asymmetric grids, then collide head on, leading to developed turbulence. Thomson-scattering, soft-X-ray-imaging and proton-radiography diagnostics have allowed for a thorough characterisation of the plasma state, including measurements of temperature, flow velocities, turbulent spectra, and magnetic fields. Our key finding is that at sufficiently large magnetic Reynolds numbers, magnetic fields are amplified very efficiently, attaining dynamical strengths. The robustness of this conclusion has been confirmed subsequently via several extensions of the original experimental configuration. Our results lend support to theoretical expectations that plasma turbulence is responsible for the magnetic fields universally observed in various astrophysical environments, from stars to the intra-cluster medium.
The capability of Thomson Scattering (TS) to measure the local growth and saturation of the magnetic fields present in laboratory Ion-Weibel experiments has been developed and tested. Analysis of synthetic TS spectra generated from particle distribution functions in PIC simulations accurately reproduced simulated currents, validating the application of TS to magnetic field measurement. Applying this method to various shots at the OMEGA laser facility has yielded field measurements consistent with theoretical predictions as well as with other diagnostics [1,2].
The application of TS-based magnetic field measurement in other plasmas is also explored. Some TS field measurements on MAGPIE appear to be consistent with Faraday rotation measurements. The effects of scale lengths and plasma composition on this diagnostic are also explored [3].
Over the decades, multiscale modeling efforts have resorted to powerful methods, such as asymptotic/perturbative expansions and/or averaging techniques. As a result of these procedures, finer scale terms are typically discarded in the fundamental equations of motion.
Although this process has led to well consolidated plasma models, consistency issues may emerge in certain cases especially concerning the energy balance. This may lead to the presence of spurious instabilities that are produced by nonphysical energy sources. The talk proposes alternative techniques based on classical mechanics and its underlying geometric principles. Inspired by Littlejohn's guiding-center theory, the main idea is to apply physical approximations to the action principle (or the Hamiltonian structure) underlying the fundamental system, rather than operating directly on its equations of motion. Here, I will show how this method provides new energy-conserving variants of hybrid kinetic-MHD models, which suppress the spurious instabilities emerging in previous non-conservative schemes.
Ultrafast interactions of protons in matter have, until recently, been very difficult to study in real time. This has been due largely to an absence of sufficiently short pulse durations and pump-probe timing stability. Ultimately, this has precluded our ability to study the early stage ionisation dynamics leaving the conditions that underpin how these excited electrons transition from the continuum (i.e. interaction dimensionality etc) as largely an open question.
Did you ever want to make it rain with your laser? Are you tempted to create an indoor ski slope with the Vulcan Laser? This talk will explore some of the more exotic phenomena arising from laser filamentation, and the real physics underpinning these effects. We will even visit some of the more useful applications, their limitations and their potential. These include remote data transmission in foggy weather, atmospheric sensing and remote lasing of air and the triggering of lightning.
Deep inside planets, extreme density, pressure and temperature strongly modify the properties of the constituent materials. In conjunction with numerical simulations, experimental constraints on phase transformations and how they affect thermodynamic and transport properties at interior conditions are crucial to determine a planet's internal structure and evolution.
Laser-driven dynamic compression can easily reach the multi-megabar range typical of the pressure existing deep inside large planets and exoplanets, but large entropy creation during single-shock compression results in large shock-heating which limits the range of pressure achievable while keeping the temperature below 5000-10000 K. The versatility of large lasers can now be exploited to design advanced shock compression schemes that allow us to probe thermodynamic states other than the pressure/temperature conditions obtained in a single-shock experiment (Hugoniot), therefore opening to the possibility of tackling fundamental questions on the behavior of planetary relevant materials at extreme conditions.
b1e95dc632