Search

The shock ignition approach to inertial confinement fusion offers a potential route to ignition and high gain. It proposes a low velocity fuel assembly on a low adiabat, and ignition through the launching of a late timed strong shock. Accurate descriptions of the coupling of laser energy into the capsule are required to model implosions, including driving the highly compressible fuel and the interaction of the shock launching spike with the coronal ablation plasma. Two well diagnosed experiments were performed on the Omega-60 laser facility that isolated key physics issues for the two steps of shock ignition. The first used a novel conical target to access for the first time the laser-plasma conditions relevant for full-scale shock ignition, in order to characterise the laser-plasma interactions and subsequent supra-thermal hot electrons. The dominant instability was identified as convective stimulated Raman scattering, producing hot electrons of ~40 keV with a laser energy conversion efficiency of 1-3%. This is unique and an essential measurement, as inclusion of hot electron generation and propagation in shock ignition simulations is crucial for constructing implosion designs that might be capable of reaching ignition. The second experiment investigated the implosion dynamics of warm deuterium filled capsules using shaped laser pulses that maintained a reduced shell adiabat and associated high fuel compressibility. A laser drive multiplier was tuned with trajectory measurements from a gated self-emission imager, a significant advancement in the ability to more accurately simulate reduced adiabat designs that are relevant for both shock ignition fuel and conventional central hot spot implosions. Despite significant low mode asymmetries that were identified during the in-flight fuel compression and within the late formed hot spot, the shell trajectory, hot spot morphology and peak neutron emission were well reproduced from one-dimensional simulations. More experiments coupled with predictive modelling are a necessity to determine whether inertial confinement fusion can be a future energy source.

The pressures required for high energy density physics are achieved experimentally using laser shock compression. This is a dynamic process, with picosecond time scales. Establishing a melt temperature in a dynamic system is made non trivia by overshoot of the melting point due to the kinetic barrier for the phase change. There is therefore a need to be able to model dynamic melt temperatures in such a way that a static melt curve can be predicted, and to link these simulations to an experimental method. By tracking liquid growth to create a numerical melt model, static melt curves of Cu and Ta (in the range 0-10 GPa) were obtained from dynamic heating simulated using molecular dynamics. These were in agreement, within an error of 1%, to literature melt curves and also to the values found using the established static coexistence method, with the computational cost reduced by two orders of magnitude. A typical shock target consists of an ablator, the target material and a tamper. A shock impedance method was developed to model the tamper as a mobile wall which returned the expected release wave through the bulk material. Different tamper materials could therefore be used to alter the cooling rate during release. With Cu as the test material, the literature melt curve (in the range 50-110 GPa) was recreated, with a 7% error in temperature. This method of modelling a tamper in molecular dynamics removes the need to know the potential of the tamper material. Both novel methods developed in this thesis have succeeded in reproducing established static melt curves, by modelling dynamic systems. These methods could be applied to laser shock experiments, in order to verify simulated melt curves at high pressures.

When high intensity lasers interact with solid targets, hot electrons are produced that can exit the material and leave behind a positive electric charge. As this accumulated charge is neutralised by a cold return current, radiation is emitted with characteristics dependent on laser and target properties. This thesis examines how electromagnetic radiation is emitted in experiments with long-pulse and short-pulse lasers. Radiofrequency electromagnetic pulses emitted during ps-duration laser interactions can disrupt scientific measurements and damage electronic equipment close to the target. A study of electromagnetic pulses produced by the Vulcan laser is presented. Strong fields exceeding 100kV/m and 0.1mT were measured 1.5m from the target using conducting probes. Scaling of the EMP field with laser and target parameters shows qualitative agreement with target charging models. A novel EMP mitigation scheme is presented using a dielectric spiral target holder. Experimental results are used to benchmark a frequency-domain dipole antenna model of EMP emission that connects charging physics to EMP fields measured at an arbitrary distance from the target. In a separate experiment, coil targets were driven with three ns laser beams from the Vulcan laser, generating multi-tesla quasi-static magnetic fields. Dual-axis proton deflectometry was used to measure electric and magnetic fields around the coils. Results suggest that wire electric fields of order 0.1GV/m develop on a 100ps timescale. Maximum currents of 10kA were observed towards the end of the laser drive for 1mm- and 2mm-diameter loop targets, corresponding to an axial magnetic field of B_0=12T in the 1mm loops. Deflectometry results agree well with a plasma diode model, whereas B-dot probe measurements of the magnetic field were approximately 10x larger. Analytic and computational modelling of charged particle motion in electric and magnetic fields is presented. Prospects for an all-optical platform for magnetized high energy density physics experiments are discussed.

Temperature measurement in opaque laser-shocked solids is a long-standing research problem. The diagnosis of temperature using x-ray diffraction, in particular the Debye-Waller effect, has long been proposed as a potential solution, though no experimental measurement has ever been published. This thesis investigates the obstacles present in such a temperature measurement scheme. A code was developed to efficiently analyse simulated x-ray diffraction patterns from molecular dynamics simulations so as to observe the Debye-Waller effect. This code was used to critically assess previous work performed in this area of research. It was found that material strength has a distinct impact on the behaviour of the Debye-Waller effect, though the impact on temperature measurement is minimal. It was also found that dislocations cause a significant error in temperature measurement, in conflict with the long-standing Wilkens theory of dislocations and x-ray diffraction. An experiment was performed at the Orion laser facility to assess the experimental limits of Debye-Waller temperature measurement. The diffuse nature of the x-ray diffraction signal from shocked samples inhibited efforts to remove background while leaving signal intact. Brighter x-ray sources with narrower energy spectra are recommended to treat this problem.

The key objectives of this thesis are to understand how to efficiently couple a high power laser to a target and produce bright X-ray and energetic ion sources. The approaches taken are to use plasma mirrors to improve the laser contrast, structure the target surface to increase the laser absorption and the use of ultra thin targets for ion acceleration. The experimental work is combined with computational calculations to further develop an understanding of the laser-target physics. The suppression of the early interaction is investigated using a double plasma mirror setup. This is studied by developing a model using geometric optics and ionisation thresholds of plasma mirrors. By using these calculations in Helios simulations it is inferred that the laser interaction with the target is delayed closer to the peak of the pulse. As a result, the target expansion is reduced to sub-micron lengths by the time the main laser peak interacts with the target. On an experiment using silicon targets with micro-structured surfaces we observe an enhancement in the X-ray emission. A conical crystal spectrometer is used for recording the emission centralised on K-alpha. The enhancement is interpreted from comparing the emission between micro-structured and flat surfaced targets. By combining the micro-structured targets with a double plasma mirror setup, a method for engineering plasma density scale length is achieved. By laser irradiating graphene we observe the generation of protons and carbon ions using radiochromic film and CR-39. The measurements are linked to EPOCH particle-in-cell simulations. From simulations it is inferred that the onset of relativistically induced transparency is important for producing energetic ions. The ion motion is determined by the emerging electric fields in experiment and simulations.

The primary focus of this thesis is understanding the production of magnetic fields during laser-plasma experiments. Each chapter investigates a different mechanism of producing magnetic fields. The first is from the by-product of launching asymmetric shocks which drive Biermann battery generated magnetic fields. The second looks at the reconnection of magnetic fields between two laser focal spots and the third is from fields produced around a current carrying loop target. Blast waves are investigated in the laboratory using a fast framing camera to capture multiple images on a single shot. In analysing the images, the blast wave's trajectory is compared to a Sedov-Taylor solution and the coupling of the laser energy into the shock wave is calculated to be 0.5-2%. The evolution of the blast wave's shape is characterised by fitting an ellipse to the outer edge and is observed to progress into a more symmetrical shape. Calculations show that two shocks produced in the interaction cause the change in ellipticity. We experimentally demonstrate that when two laser spots are placed in close proximity reconnection occurs. Diagnostics, including proton radiography, X-ray detectors and an optical probe, record and diagnose the existence of a semi-collisional reconnection event. The experimental data and simulations show that both Nernst and anisotropic pressure effects need to be taken into account for understanding and predicting the correct plasma dynamics observed. Magnetic fields are produced by driving a current through a loop attached to two plates and new measurements recording the voltages induced are presented in this thesis. It is found that the predicted values for the resistance, capacitance and inductance do not match those extracted from the experimental data and reasons for these are presented. Ideas for furthering this research to enhance our understanding in this area are given.

The primary focus in this thesis is on the production and detection of upstream-propagating waves in laser-laboratory plasmas. Upstream waves are a feature of shocks in plasmas; launched at or just ahead of the shock, they travel against the flow into the unshocked medium. Despite the importance of upstream waves in understanding the properties of the Earth’s bow shock, and the foreshock region just ahead of the shock, upstream waves have not previously been investigated in the laboratory. The laboratory shock waves are produced by laser-matter interaction, and allowed to propagate into a nitrogen atmosphere, immersed in a magnetic field. One-dimensional numerical simulations of the fluid enable the construction of the whistler dispersion function; a mode unstable to growth is identified. The mode is compared with observations taken by a magnetic field probe, understood by a wavelet analysis. Agreement between observation and prediction is found.

This thesis presents numerical investigations of the fast electron transport and discusses the fast electron heating of solid targets. Three areas have been investi- gated in this context: The first area introduces the concept of an ideal fast electron transverse confine- ment which is obtained when the transverse dimensions of the target are comparable to the laser spot size. This facilitates the heating of thick targets. This investigation also explores the angular dispersion phenomenon in the context of the fast electrons. This dispersion results in a longitudinal velocity spread of the fast electrons which adversely affects their penetration of the target, and this in turn impairs the heating. The work here shows that angular dispersion can not be avoided even when ideal fast electron transverse confinement is achieved. Moreover, this dispersion impedes fast electron penetration more significantly than does electric field inhibition. The results indicate the importance of taking the angular dispersion into account in fast electron transport calculations. The second area investigates the effect of grading the atomic number at the in- terface between the guide element and the solid substrate on resistive guide heating. The numerical results imply that this graded interface configuration improves the heating in large radius guide resembling that obtained in smaller guide. The larger radius guide with the graded interface configuration is more tolerant to laser point- ing stability than smaller radius. Further, this configuration increases the magnetic collimation of fast electrons since more powerful confining magnetic field is obtained. The last area studies numerically a Rayleigh-Taylor (RT) instability experiment driven in a fast-electron-heated solid target. It was found that it is possible to drive the RT instability in dense plasma isochoric heated by the fast electrons. The RT instability growth occurs in few picoseconds, after establishing strong radiative cooling. The curve growth rates depends on the type of atomic model used. Practi- calities of extracting RT instability data due to structure in the heating profile are described.

This thesis presents an investigation into the properties of relativistic electron beams generated by the interaction between an intense laser and a solid density target. Described are novel x-ray spectroscopic techniques for measuring the electron beam divergence, electron currents and to explore the electron distribution. A key aim of this work is the constraint of fast electron beam parameters by the comparison of well characterised experimental data to computational plasma models. Multi-dimensional imaging of a single spectral line allows insight into heating and inferred spatially resolved temperatures, which are around 300eV, are compared to simulation. By comparison of imaging data and simulation, the electron beam is found to diverge with a half angle of approximately 70◦ ± 10◦, and is found to have a diameter greater than that of the laser focal spot. Measured hollow ion spectra are shown, and calculations imply that their are two distinct ionisation mechanisms. The generation of hollow ion states is seen to be dominated by photoionisation in the the range of 7.2 ̊A−7.7 ̊A, and by collisional ionisation in the range 7.9 ̊A−8.1 ̊A. This implies the existence of an intense radiation field, and high fast electron flux within the target. The velocity distribution of electrons is explored by observation of polarised x- ray emission. Using a pair of HOPG crystal spectrometers, each of the polarised components of the x-ray spectra are measured independently to obtain the degree of polarisation. The anisotropic population of magnetic sublevels results in a net polarisation in the spectral emission, and the polarisation degree measured is indicative of the degree of anisotropy in the velocity distribution of the return current. The large, positive degree of polarisation found implies that polarised emission is caused by the return current, and that it contains the necessary energy to excite ions into a hydrogenic state.

Type-I ELMy H-mode is planned to be the reference inductive operational scenario for ITER. However, unmitigated type-I ELMs would cause unacceptable damage to ITER’s divertor, so a way of mitigating their effect must be found. This thesis focuses on the ergodization of the plasma edge using RMPs as a means of ELM control, and on related topics relevant to ergodic magnetic fields in MAST. Chapter 1 provides an introduction to nuclear fusion and chapter 2 provides an overview of ELMs. In chapter 3, the effect of ergodic fields on L-mode temperature profiles is calculated. It is found that no flattening of the temperature profile should be expected in MAST plasmas, which is in agreement with experimental results. In chapter 4, various metrics characterizing the degree of ergodicity in both L- and H-mode plasmas are calculated using vacuum modelling and compared with the amount of density pump-out observed experimentally in the those plasmas during the application of RMPs. The only parameter to show a correlation with the amount of density pump-out is the width of the laminar region. However, plasma response modelling provides a robust criterion for the occurrence of density pump-out that applies both to L- and H-mode plasmas. In chapter 5, the results of lower single-null H-mode ELM mitigation experiments using n=4 and n=6 RMPs are presented. ELM mitigation is achieved and refuelling of plasmas that had begun to undergo pump-out is successfully demonstrated. Both ELM frequency and density pump-out are found to increase with ELM coil current above a certain threshold. An analysis of an ergodic magnetic field formed by a current sheet on a rational surface is presented in chapter 6. Various properties of this ergodic field are calculated and compared with experimental effects observed in the plasma. Some agreement is found between the experimental data and the modelling. Finally, a summary of the thesis is given in chapter 7.

Using telescopes such as the Hubble Space Telescope, astronomers observe parsec-scale plasma jets emerging from Young Stellar Objects and colliding with the interstellar medium, generating shock structures. A set of mathematically rigorous scaling transformations exists allowing dynamic scaled models of radiative, yet otherwise ideal, hydrodynamic flows to be created in the laboratory. In this thesis two experiments are presented in which plasma jets were produced using high-intensity lasers. In the first of these, the effects on jet collimation of radiative emission and the presence of a gaseous ambient medium were studied, with a view to learning about the effects of these factors on YSO jets by studying their scaled laboratory counterparts. The second experiment was designed to be similar to the first in order to investigate applying the scaling relations to scaling between laboratory experiments. Previous work in this area has shown that jets made in vacuo from materials with a higher mean atomic number form narrower, better-collimated jets (e.g. Shigemori 2000). Also, for jets propagating into a gas, shock structures similar to those seen in simulations of the interaction of a YSO jet with the interstellar medium have been observed (e.g. Nicolai 2008). In this thesis, both of these results are replicated, and the collimation work extended with a study of the collimation of jets in 50 mb He gas. The energies of the laser pulses used in the first experiment were over an order of magnitude lower than those used in previous studies of cylindrically-symmetric jets. This gives the replication and development of previous work added significance, showing that the same physics can be used to describe the behaviour of a plasma on a smaller energy scale and demonstrating that jet modelling experiments can potentially be done using university-scale laser facilities.