Attoseconds and the exascale: on laser-plasma surface interactions

Laser peak powers rise inexorably higher, enabling the study of increasingly exotic high-energy-density plasmas. This thesis explores one such phenomenon, that of the interaction between a relativistically intense laser pulse and a solid-density plasma. The laser pulse is reflected. Both the reflected radiation and the electron bunches that induce the interaction have fascinating properties. Through the application of theory, simulation and experiment, this thesis strives to extend our understanding of this mechanism and thus direct the community towards potential applications for these sources. Of primary interest is the development of novel diagnostic tools. Theories have been developed and tested to describe the production of low emittance nano-Coulomb charge electron bunches. Such properties are comparable to forefront synchrotron sources but on a considerably more compact scale. These results have wide-reaching implications for future particle accelerator science and associated technologies. Furthermore, these electron bunches will initiate QED processes on next-generation laser facilities. The radiation they produce is composed of high harmonics of the incident laser pulse. This radiation can be coherently focused to unprecedented intensities and is of ultra-short duration, possibly even entering the zeptosecond regime. The intensity of X-ray harmonics has been measured on the ORION laser facility producing results consistent with theory and enabling the benchmarking of peak intensity simulations with real data. The work of this thesis has amassed interest within the community and in June 2024 its ideas will be tested on the GEMINI PW laser facility

Towards Pulsed Telecom Laser Ablation Loading for an Ytterbium Ion Trap

Trapped ion system is a promising platform for quantum computing, with long qubit coherence, high gate and measurement fidelities. To harness the full potential of the trapped ion system, it is imperative to employ an efficient ion loading scheme. While the conventional oven loading method has its drawbacks, such as slow loading time, resource wastage, and potential contamination that can compromise system longevity and introduce decoherence, the pulsed laser ablation loading offers a promising alternative. This method enables fast, resource efficient, and controllable generation of a neutral atomic flux. This thesis explores the development of a 1.57 μm pulsed laser ablation (PLA) loading scheme for an Ytterbium ion trap. With endeavors in theoretical simulations and experimental setups designed with a keen emphasis on laser-induced damage thresholds, we successfully generated a neutral flux from the ablation laser, acquired a comprehensive isotope spectrum, and estimated the velocity distribution in the atomic plume. A fiber-coupled ablation delivery module was also designed and proved feasible along with the main ablation experiments. This study establishes a solid groundwork for the future integration of pulsed laser ablation loading into our trap system and provides invaluable empirical insights into the use of high-power infrared (IR) pulsed lasers

The Physics of Laser Plasmas and Applications - Volume 2

This open access book (Volume 2) is part of the series "The Physics of Laser Plasmas and Applications." It serves as an introduction to the physics of compressible hydrodynamics, which is used to describe the temporal evolution of plasmas generated by intense laser irradiation of solid surfaces. For the benefit of students and young researchers, the book presents the fundamental equations and provides a comprehensible explanation of solutions to intricate fluid phenomena. It builds upon the concept of plasma generation through the heating of matter via the classical absorption of a laser, as expounded in Volume 1. The high-temperature plasma resulting from the laser interaction manifests in diverse hydrodynamic occurrences like shock waves and expansion waves. The initial sections of this book expound the essentials of compressible hydrodynamics, magnetohydrodynamics (MHD), and the physics of shock waves. The transfer of laser energy within an expanding plasma towards regions of higher density is achieved through electron and X-ray transport mechanisms. In both instances, conventional diffusion models prove inadequate, necessitating mathematical frameworks founded on the Boltzmann equation. The conveyed energy engenders ablation pressure, equivalent to tens of millions of atmospheres, on the solid surface. This pressure initiates powerful shock waves propagating through the solid material. The propagation of these shock waves is delineated for scenarios involving planar and spherical geometries. The text also introduces various solutions pertaining to convergent and divergent shocks in spherical geometries using self-similar models. The discourse then shifts towards ionization and related atomic processes, which govern the dynamics of plasmas created by laser irradiation of mid-Z and high-Z solids. The quantum mechanics of partially ionized atoms and their associated atomic processes are elucidated. Concluding the book is an exploration of the physics of warm dense matter (WDM) – an electron system characterized by quantum-mechanical, many-body interactions. The study of high-density plasmas featuring temperatures around 1 eV is undertaken through the lens of density functional theory (DFT). The theoretical breakdown of experimental data acquired via the X-ray free electron laser (X-FEL) is also provided. In essence, this second volume of the series amalgamates a comprehensive understanding of compressible hydrodynamics, shock wave physics, ionization processes, energy transfer, and the realm of warm dense matter. It equips readers to delve into the intricacies of plasma physics and laser interactions while utilizing modern theoretical frameworks and experimental methodologies. This is an open access book

Temperature and density measurements of plasmas

Diagnosing the temperatures and densities of plasmas is critical to the understanding of a wide variety of phenomena. Everything from equations of state for warm dense matter (WDM) found in Jovian planets and inertial confinement fusion (ICF) to turbulent and dissipative processes in laser-produced plasmas, rely on accurate and precise measurements of temperature and density. This work presents improvements on two distinct techniques for measuring temperatures and densities in plasmas: x-ray Thomson scattering (XRTS), and Langmuir probes (LPs). At the OMEGA laser facility, experiments on warm dense matter were performed by firing lasers at an ablator foil and driving a planar shock into cryogenically cooled liquid deuterium. XRTS in the collective scattering regime was implemented to probe the matter, measuring densities of ne ~ 4.3 × 1023 cm−3, temperatures of Te ~ 12 eV and ionizations of Z ~ 1.0. Through an extension to XRTS theory for inhomogeneous systems, it was possible to extract an additional parameter, the length scale of the shock, whose value of Λ ~ 1.33 nm was consistent with the predicted mean free path, and therefore the thickness of the shock. A unique triple Langmuir probe prototype was designed and tested at the Gregori group’s lab at the University of Oxford. This probe was designed for a high temporal resolution of ~ 200 MHz for probing laser-produced shocks. The probes were used to measure the shock formed from ablating carbon rods in an argon gas fill. The probe yielded plasma parameters of ne ~ 1.0 × 1017 cm−3 , and Te ~ 1.5 eV, consistent with measurements from interferometry and emission spectroscopy

Physics of intense light ion beams, production of high energy density in matter, and pulsed power applications. Annual report 1996/97

Physik intensiver Strahlen leichter Ionen, Erzeugung hoher Energiedichten in Materie und Anwendungen der Pulsed Power Technik Jahresbericht 1995 In dem Bericht werden die in 1995 erzielten Ergebnisse zum Arbeitsthema "Physik intensiver Ionenstrahlen und gepulster dichter Plasmen" dargestellt. Zus"tzlich wurden die neu hinzugekommenen Arbeiten zu industriellen Anwendungen der Pulsed Power Technik aufgenommen

Expansion dynamics in laser produced plasmas as function of laser parameters

Laser-produced plasmas have been extensively utilized in applications such as EUV lithography, laser-induced breakdown spectroscopy, inertial confinement fusion, and nanoparticle generation. Optimizing laser and plasma parameters in these applications is crucial for maximizing attributes like conversion efficiency, line emission, and nanoparticle generation. Achieving this optimization requires a detailed understanding of plasma property evolution during laser-material interaction and plasma expansion, which must be assessed using various plasma diagnostic tools. In this work, electrostatic probes, emission and absorption spectroscopy, interferometry, simulations, and analytical models were employed to investigate plasma property evolution under various conditions. Each method had unique advantages and limitations, and offered different insights into the various stages of the plasma lifecycle.The initial focus of this work was on understanding the appearance of different ion peaks observed in Faraday cup studies of LPP expansion into vacuum, categorized as the prompt, ultrafast, fast, and thermal ion peaks. Although previous studies had reported on the existence of these peaks, their physical origin was not well understood. In order to better understand their physical origin, a parametric study was conducted, exploring the variation in the peak properties as a function of distance, angle, material, and laser intensity. By comparing the results with different analytical models of plasma expansion, it was possible to attribute the properties of the ion peaks to different physical processes occurring during the laser-plasma interaction. More detailed investigations into the early evolution of the plasma properties during the laser-plasma interaction were conducted by using a combination of 2D FLASH simulations and Nomarski interferometry. The FLASH simulations provided the 2D spatial and temporal evolution of the electron density, temperature, and charge state of the plasma for times up to 20 ns after the arrival of the laser pulse. Comparison of the simulation results with measurements of electron density obtained using Nomarski interferometry showed excellent consistency, with slight deviations being attributed to experimental errors and uncertainty in equation of state parameters. Investigations of the dependence of the plasma properties on laser wavelength were also shown to be in partial agreement with analytical models, though it was found that the analytical models overpredicted the variation in temperature due to not taking into account radiation losses. Subsequent studies looked at the effects of ambient environments on the plasma dynamics, where the plasma became confined. In this case, interferometry and electrostatic probes could no longer be used due to the presence of shocks in the interferograms and due to ambient gas preventing the ions from reaching the electrostatic probe. Instead, measurements of plasma properties were performed using a combination of emission and absorption spectroscopy. Comparisons of the two methods indicated that small deviations were present in the measured temperatures. However, by modeling simulated spectra, it was shown that these deviations could be reduced by accounting for self-absorption effects in the plasma. The study also showed that confinement of the plasma plume resulted in the plasma reaching a state of local thermal equilibrium that persisted over long periods, even when the electron densities in the plasma should have fallen well below the McWhirter criterion. However, the exact mechanisms that allowed the plasma to remain in a state of local thermal equilibrium were not well understood. Limitations in the range of plasma properties that could be measured from emission and absorption spectroscopy alone indicated a need for an alternative diagnostic method. One such method was saturated absorption spectroscopy, with the main advantage of this method being its ability to provide Doppler-free linewidth measurements. However, very limited work had been performed on the application of saturated absorption spectroscopy to laser-produced plasma due to difficulties arising from poor signal to noise ratios caused by rapidly changing plasma conditions. In this study, it was shown that saturated absorption spectroscopy could be successfully applied to laser-produced plasma through careful optimization of the probe parameters, ambient conditions, and spectral line selection. Measurements of Doppler-free linewidths were used to obtain information on power, pressure, and natural line broadening present in the plasma. It was shown that such measurements can be used for obtaining information on various plasma properties, crowded spectral features, spectroscopic constants, and isotopic shifts

Laser ablation for sample introduction in inductively coupled plasma spectrometry.

The aim of this research was to investigate laser ablation as a means of sample introduction for the Inductively Coupled Plasma (ICP). This involved the configuration of an ICP spectrometer, a laser and a novel laser ablation chamber. The latter offered the facility of electrothermally heating samples before or during laser ablation.Several operating conditions associated with the ablation process were investigated and optimised. These optimisation studies utilised a variety of metallurgical materials. It was found that the Q-Switched mode of laser operation provided greater sensitivity than long pulsed operation. This research reports the first laser ablation ICP studies on paints and polymers. Ablation of paints was found to provide good precision. This was attributed to a combination of homogeneity and viscosity effect. Operating conditions associated with the ablation process were again examined and found to be independent of sample type. Calibration studies were performed using laser ablation of synthetic aqueous standards. This research includes the first reports of such an approach to calibration. The wide dynamic range associated with conventional ICP was also evident for laser ablation. Calibration followed by analysis of samples providedsemi-quantitative data for paints and polymers. Limits of detection were inferior to solutions nebulisation ICP

Development of a Laser-Spark Multicharged Ion System – Application in Shallow Implantation of Sic by Boron and Barium

A novel multicharged ion source, using laser ablation induced plasma coupled with spark discharge, has been investigated in this work. The designed and demonstrated ion source is cost-effective, compact and versatile. Experiments are described with the intention of demonstrating the practicability of ion implantation via laser ion source. Multicharged aluminum ions are generated by a ns Q-switched Nd:YAG laser pulse ablation of an aluminum target in an ultrahigh vacuum. The experiments are conducted using laser pulse energies of 45–90 mJ focused on the Al target surface by a lens with an 80-cm focal length to 0.0024 cm2 spot area and incident at 45° with the Al target surface. With the increase in the laser pulse energy, a slow increase in the number of ions generated is observed. The generation of ions with a higher charge state is also observed with the increase in the laser pulse energy. For 5 kV accelerating voltage applied to the Al target and using laser energy of 90 mJ, up to Al4+ charge is delivered to the detector which is located 140 cm away from the Al target. Raising accelerating voltage increases the charge extraction from the laser plasma and the energy of multicharged ions. The components of a transport line for a laser multicharged ion source are described. Aluminum and carbon multicharged ions are generated by a Q-switched, nanosecond Nd:YAG laser (wavelength λ = 1064 nm, pulse width τ = 7.4 ns, and pulse energy up to 82 mJ) ablation of a target in a vacuum chamber. Time-of-flight and three-grid retarding ion energy analyzers are used to determine the velocity and the charge state of the ions. A three-electrode cylindrical einzel lens is used to focus the ions. At 30 cm from the center of the focusing electrode of the einzel lens, Al1+ and Al2+ have a minimum beam diameter of ∼1.5 mm, while for Al3+ and Al4+ the minimum beam diameter is ∼2.5 mm. The simulation of the ion trajectories is done using SIMION 8.1. A high voltage pulse applied to a set of two parallel deflecting plates is used for the pickup of ions with different charge states according to their time-of-flight. An electrostatic cylindrical ion deflector is used for analysis and selection of charges with specific energy-to-charge ratio. The design of these transport line components and their operation are described. A spark discharge is coupled to a laser multicharged ion source to enhance ion generation. The laser plasma triggers a spark discharge with electrodes located in front of the ablated target. For an aluminum target, the spark discharge results in significant enhancement in the generation of multicharged ions along with higher charge states than observed with the laser source alone. When a Nd:YAG laser pulse (wavelength 1064 nm, pulse width 7.4 ns, pulse energy 72 mJ, laser spot area on target 0.0024 cm2) is used, the total multicharged ions detected by a Faraday cup is 1.0 nC with charge state up to Al3+. When the spark amplification stage is used (0.1 μF capacitor charged to 5.0 kV), the total charge measured increases by a factor of ∼9 with up to Al6+ charge observed. Using laser pulse energy of 45 mJ, charge amplification by a factor of ∼13 was observed for a capacitor voltage of 4.5 kV. The spark discharge in-creases the multicharged ion generation without increasing target ablation, which solely results from the laser pulse. This allows for increased multicharged ion generation with relatively low laser energy pulses and less damage to the surface of the target. Laser plasma generated by ablation of an Al target in vacuum is characterized by ion time-of-flight combined with optical emission spectroscopy. A Q-switched Nd:YAG laser (wavelength λ = 1064 nm, pulse width τ ∼ 7 ns, and fluence F ≤ 38 J/cm2) is used to ablate the Al target. Ions are accelerated according to their charge state by the double-layer potential developed at the plasma-vacuum interface. The ion energy distribution follows a shifted Coulomb-Boltzmann distribution. Optical emission spectroscopy of the Al plasma gives significantly lower plasma temperature than the ion temperature obtained from the ion time-of-flight, due to the difference in the temporal and spatial regions of the plasma plume probed by the two methods. Applying an external electric field in the plasma expansion region in a direction parallel to the plume expansion increases the line emission intensity. However, the plasma temperature and density, as measured by optical emission spectroscopy, remain unchanged. Aluminum multicharged ion generation from femtosecond laser ablation is studied. A Ti:sapphire laser (wavelength 800 nm, pulse width ∼100 fs, and maximum laser fluence of 7.6 J/cm2) is used. Ion yield and energy distribution of each charge state are measured. A linear relationship between the ion charge state and the equivalent acceleration energy of the individual ion species is observed and is attributed to the presence of an electric field within the plasma-vacuum boundary that accelerates the ions. The ion energy distribution follows a shifted Coulomb-Boltzmann distribution. For Al1+ and Al2+, the ion energy distributions have two components; the faster one can be attributed to multiphoton laser ionization, while the slower one is possibly due to collisional processes. Ion extraction from the plasma is increased with an applied external electric field, which is interpreted to be due to the retrograde motion of the plasma edge because of the external electric field. Multicharged ion generation by femtosecond laser ablation is compared to previously reported ion generation with nanosecond laser ablation and is shown to require significantly lower laser fluence and generates higher charge states and more energetic ions. Fully-stripped boron ions are generated by a nanosecond Nd:YAG laser (wave-length λ = 1064 nm, pulse width τ = 7 ns, and maximum laser pulse energy E = 175 mJ) ablation of a B target in vacuum. Higher charge states, along with the increase in the number of ions detected, are observed with the increase in the laser fluence. An external electric field between the end of the expansion chamber and a grounded grid is used to extract the ions and accelerate them according to their charge state. For 5 kV accelerating voltage applied to the B target and using a laser fluence of 115 J/cm2, ∼1.5 nC of total charge is delivered to the detector which is located ∼150 cm away from the B target. Ion deflection by an electrostatic field separates the ions from the neutrals and makes this geometry suitable for ion implantation. The developed multicharged ion deposition and implantation system was used to per-form interfacial treatment of the SiC/SiO2 interface using boron and barium ions. SRIM simulation was used to estimate the ion penetration depth in the SiC substrate. The multicharged ions were used for shallow ion implantation in 4H SiC. The optical bandgap of the 4H SiC was reduced due to boron ion implantation. Several MOSCAP devices were fabricated with a combination of boron and barium shallow implantation. High-low C-V measurements were used to characterize the MOSCAPs. Boron implantation affects the flatband voltage significantly, while the effect of barium ion implantation is negligible. Shallow boron implantation in the SiC/SiO2 interface reduces the flatband voltage from 4.5 V to 0.04 V

Kinetic-Hydrodynamic Modelling of Short-Pulse Doppler-Shift Spectroscopy Experiments, and Resistive Filamentation of Fast-Electron Transport

Three pump-3ω-probe Doppler-shift spectroscopy experiments are presented along with both 1D radiation-hydrodynamics modelling (HYADES) and 1D three-stage modelling process involving: HYADES radiation-hydrodynamics pre-pulse calculations; EPOCH kinetic particle-in-cell main-pulse calculation initialised from HYADES result; followed by hydrodynamic calculations, initialised from EPOCH result, of the evolution after the main-pulse. These investigations are aimed at exploring the formation of shocks at the front surface of targets after interaction with an ultra-short (30 fs), ultra-intense (10^18 W/cm^2 ) laser pulse. To this end a 3ω-probe is delayed then reflected from a 3ω critical surface on the front surface to obtain a temporal profile of the velocity of this surface. Two investigations use identical polished crown glass targets, but are performed with lasers systems with different contrast ratios (10^5 and 10^7 ). HYADES simulations match experimental results for the high contrast experiments except at early times. HYADES simulations of low contrast experiments do not agree. The three-step modelling process shows good agreement with experimental results in both cases, though with some adjustment to the pre-plasma scale-length for the low contrast case. The third Doppler-spectroscopy experiment uses a low density (over-dense) foam target with identical setup to high-contrast case described. Experimental results show a similar magnitude Doppler-shift evolution as in low-contrast case. HYADES simulations show similarities to experimental results but not overall trend. The three-step modelling process shows that the experimental response may be due to post-soliton formation as a result of SRS or photon acceleration plasma instabilities. This is supported by an additional 2D EPOCH simulation. A fourth theoretical investigation is presented into the transport of fast electrons produced 10^19−20 W/cm^2 laser pulses using the hybrid code ZEPHYROS. A low resistivity (< 5 µωm) at low temperatures (1 eV) is found to be of critical importance to suppressing filamentation of electron beams through low-Z targets