Palladium-catalysed enantioselective desymmetrisations

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Fast electron transport patterns in intense laser-irradiated solids diagnosed by modeling measured multi-MeV proton beams

The measured spatial-intensity distribution of the beam of protons accelerated from the rear side of a solid target irradiated by an intense (>10 Wcm) laser pulse provides a diagnostic of the two-dimensional fast electron density profile at the target rear surface and thus the fast electron beam transport pattern within the target. An analytical model is developed, accounting for rear-surface fast electron sheath dynamics, ionization and projection of the resulting beam of protons. The sensitivity of the spatial-intensity distribution of the proton beam to the fast electron density distribution is investigated. An annular fast electron beam transport pattern with filamentary structure is inferred for the case of a thick diamond target irradiated at a peak laser intensity of 6 × 10 Wcm

Influence of laser polarization on collective electron dynamics in ultraintense laser-foil interactions

The collective response of electrons in an ultrathin foil target irradiated by an ultraintense laser pulse is investigated experimentally and via 3D particle-in-cell simulations. It is shown that if the target is sufficiently thin that the laser induces significant radiation pressure, but not thin enough to become relativistically transparent to the laser light, the resulting relativistic electron beam is elliptical, with the major axis of the ellipse directed along the laser polarization axis. When the target thickness is decreased such that it becomes relativistically transparent early in the interaction with the laser pulse, diffraction of the transmitted laser light occurs through a so called 'relativistic plasma aperture', inducing structure in the spatial-intensity profile of the beam of energetic electrons. It is shown that the electron beam profile can be modified by variation of the target thickness and degree of ellipticity in the laser polarization

Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency

Control of the collective response of plasma particles to intense laser light is intrinsic to relativistic optics, the development of compact laser-driven particle and radiation sources, as well as investigations of some laboratory astrophysics phenomena. We recently demonstrated that a relativistic plasma aperture produced in an ultra-thin foil at the focus of intense laser radiation can induce diffraction, enabling polarization-based control of the collective motion of plasma electrons. Here we show that under these conditions the electron dynamics are mapped into the beam of protons accelerated via strong charge-separation-induced electrostatic fields. It is demonstrated experimentally and numerically via 3D particle-in-cell simulations that the degree of ellipticity of the laser polarization strongly influences the spatial-intensity distribution of the beam of multi-MeV protons. The influence on both sheath accelerated and radiation pressure accelerated protons is investigated. This approach opens up new routes to control laser-driven ion sources

Ion acceleration in ultra-thin foils undergoing relativistically induced transparency

This thesis reports on experimental and numerical investigations of ion acceleration and the underlying mechanisms of energy transfer in the interaction of intense laser pulses with ultra-thin foils undergoing relativistic induced transparency. The optimisation and optical control of the ion beam properties including the beam flux, maximum energy and energy spread is important for the development of applications of laser-driven ion beams. Multiple laser-ion acceleration mechanisms, driven by sheath fields, radiation pressure and transparency enhancement occur in intense laser pulse interactions with an ultra-thin foil. This is experimentally and numerically demonstrated in the work presented in this thesis. Results from an experimental investigation of ion acceleration from ultra-thin (nanometer-thick) foils using the Vulcan petawatt laser facility are presented. Spatially separating the multiple beam components arising from the differing acceleration mechanisms enables the underlying physics of the individual mechanisms to be investigated. In the case of foils undergoing relativistic induced transparency, it is shown that an extended channel and resulting jet is formed in the expanding plasma at the rear of the target, resulting in higher laser energy absorption into electrons and enhanced ion acceleration in a localised region. This results from volumetric heating of electrons by the laser pulse propagating within the channel. The measured maximum energy of the protons in the enhanced region of the jet is found to be highly sensitive to the laser pulse contrast and rising edge intensity profile of the laser. It is shown, using a controlled pre-expansion of the target, that an increase in the maximum proton energy by a factor two is achievable. Numerical investigations of the interaction, using particle-in-cell (PIC) simulations, show that an idealised sharp rising edge Gaussian laser intensity profile produces the highest proton energy, though this condition could not be achieved experimentally. The simulations show that controlled pre-expansion of the target, by variation of the rising edge intensity profile, enables better conditions for channel formation and energy coupling to electrons and thus protons. A detailed numerical (PIC) investigation of the mechanisms of laser energy transfer to electrons and ions in thin foils undergoing relativistically induced transparency is also presented. The role of streaming instabilities in the transfer of energy between particle species is investigated. It is found that in addition to the relativistic Buneman instability, which arises from streaming of the volumetrically heated relativistic electrons with the background ions during transparency, ionion streaming in the expanding plasma also plays a role in enhancing the final ion energy. Enhancement of proton maximum energies via ion-ion streaming from shock-accelerated aluminium ions is observed in 1D PIC simulations and the energy exchange is demonstrated to be sensitive to the plasma density. Energy transfer between co-directional ion species is also observed in higher dimension 2D simulations. The simulations show that the greatest enhancement in proton energy is due to streaming of electrons in the region of the plasma jet formed in the expanding plasma.This thesis reports on experimental and numerical investigations of ion acceleration and the underlying mechanisms of energy transfer in the interaction of intense laser pulses with ultra-thin foils undergoing relativistic induced transparency. The optimisation and optical control of the ion beam properties including the beam flux, maximum energy and energy spread is important for the development of applications of laser-driven ion beams. Multiple laser-ion acceleration mechanisms, driven by sheath fields, radiation pressure and transparency enhancement occur in intense laser pulse interactions with an ultra-thin foil. This is experimentally and numerically demonstrated in the work presented in this thesis. Results from an experimental investigation of ion acceleration from ultra-thin (nanometer-thick) foils using the Vulcan petawatt laser facility are presented. Spatially separating the multiple beam components arising from the differing acceleration mechanisms enables the underlying physics of the individual mechanisms to be investigated. In the case of foils undergoing relativistic induced transparency, it is shown that an extended channel and resulting jet is formed in the expanding plasma at the rear of the target, resulting in higher laser energy absorption into electrons and enhanced ion acceleration in a localised region. This results from volumetric heating of electrons by the laser pulse propagating within the channel. The measured maximum energy of the protons in the enhanced region of the jet is found to be highly sensitive to the laser pulse contrast and rising edge intensity profile of the laser. It is shown, using a controlled pre-expansion of the target, that an increase in the maximum proton energy by a factor two is achievable. Numerical investigations of the interaction, using particle-in-cell (PIC) simulations, show that an idealised sharp rising edge Gaussian laser intensity profile produces the highest proton energy, though this condition could not be achieved experimentally. The simulations show that controlled pre-expansion of the target, by variation of the rising edge intensity profile, enables better conditions for channel formation and energy coupling to electrons and thus protons. A detailed numerical (PIC) investigation of the mechanisms of laser energy transfer to electrons and ions in thin foils undergoing relativistically induced transparency is also presented. The role of streaming instabilities in the transfer of energy between particle species is investigated. It is found that in addition to the relativistic Buneman instability, which arises from streaming of the volumetrically heated relativistic electrons with the background ions during transparency, ionion streaming in the expanding plasma also plays a role in enhancing the final ion energy. Enhancement of proton maximum energies via ion-ion streaming from shock-accelerated aluminium ions is observed in 1D PIC simulations and the energy exchange is demonstrated to be sensitive to the plasma density. Energy transfer between co-directional ion species is also observed in higher dimension 2D simulations. The simulations show that the greatest enhancement in proton energy is due to streaming of electrons in the region of the plasma jet formed in the expanding plasma

Effects of target pre-heating and expansion on terahertz radiation production from intense laser-solid interactions

The first experimental measurements of intense (∼7×1019 W cm−2) laser-driven terahertz (THz) radiation from a solid target which is preheated by an intense pulse of laser-accelerated protons is reported. The total energy of the THz radiation is found to decrease by approximately a factor of 2 compared to a cold target reference. This is attributed to an increase in the scale length of the preformed plasma, driven by proton heating, at the front surface of the target, where the THz radiation is generated. The results show the importance of controlling the preplasma scale length for THz production

Tunable mega-ampere electron current propagation in solids by dynamic control of lattice melt

The influence of lattice-melt-induced resistivity gradients on the transport of mega-ampere currents of fast electrons in solids is investigated numerically and experimentally using laser-accelerated protons to induce isochoric heating. Tailoring the heating profile enables the resistive magnetic fields which strongly influence the current propagation to be manipulated. This tunable laser-driven process enables important fast electron beam properties, including the beam divergence, profile and symmetry, to be actively tailored, and without recourse to complex target manufacture

Azimuthal asymmetry in collective electron dynamics in relativistically transparent laser-foil interactions

Asymmetry in the collective dynamics of ponderomotively-driven electrons in the interaction of an ultraintense laser pulse with a relativistically transparent target is demonstrated experimentally. The 2D prole of the beam of accelerated electrons is shown to change from an ellipse aligned along the laser polarisation direction in the case of limited transparency, to a double-lobe structure aligned perpendicular to it when a significant fraction of the laser pulse co-propagates with the electrons. The temporally-resolved dynamics of the interaction are investigated via particle-in-cell simulations. The results provide new insight into the collective response of charged particles to intense laser fields over an extended interaction volume, which is important for a wide range of applications, and in particular for the development of promising new ultraintense laser-driven ion acceleration mechanisms involving ultrathin target foils

Associations between ambient PM\u3csub\u3e2.5\u3c/sub\u3e concentrations and respiratory symptoms in Melbourne, 1998-2005

Particulate matter (PM) has been widely associated with adverse effects on respiratory health, both overseas and in Australia. This study aimed to investigate the impacts of ambient particles of \u3c2.5 μm diameter (PM2.5) in Melbourne on adverse respiratory symptoms. Two cohorts of adults were recruited in 1992–1998, and completed detailed respiratory questionnaires in 1998–1999 and 2004–2005. The mean age at baseline was 37.2 years, 55% were female, and the mean time lapsed between the baseline and follow-up questionnaires was 5.2 years. PM2.5 exposure was assessed from gravimetric data and routine nephelometry at monitoring stations located centrally with respect to the residence of most participants. Daily exposures to PM2.5 were averaged over the previous 12 months and mean daily exposure was 6.8 μg/m3. Logistic regression models were used to examine associations between PM2.5 exposure and adverse respiratory symptoms. Adjustment was made for age, gender, current smoking status, and medication use, but further adjustment for atopy did not alter the results. There was insufficient variability in PM2.5 exposure among participants over the study period to provide convincing evidence for or against associations between PM2.5 and adverse respiratory symptoms