arXiv subjects

B. Dromey

Publications and source records attributed to B. Dromey.

At least 19 recordsLinked to original sources

arXiv2025

Diagnosing the impact of relativistically intense prepulse on few-picosecond timeframes for short scale length laser-matter interactions

With the rapid proliferation of multi-petawatt (MPW) lasers globally, a new era of high-energy density science promises to emerge within the next decade. However, precise control over how light at these ultra-relativistic intensities interacts with matter (especially with solid-density targets) will be crucial to fully realize the cutting-edge scientific advancements and technological breakthroughs that the MPW regime promises to unlock. In this manuscript, we present experimental results, supported by numerical simulations, which show how intense prepulse activity on few-ps ($10^{-12}$ s) timescales leads to rapid shifts in the steepness of the preplasma generated on the surface of ultra-thin nanofoil targets. By combining a single-shot frequency resolved optical gating (FROG) autocorrelation device to diagnose on-shot incident laser pulse contrast, with coherent synchrotron emission (CSE) from relativistic laser plasmas as a probe for evolving plasma-scale length conditions, we provide an experimental benchmark for laser contrast on forthcoming MPW facilities, where high contrast on few-ps timescales will be essential for the next generation of laser-solid interactions.

physics.plasm-ph↗

arXiv2024

Real-time observation of frustrated ultrafast recovery from ionisation in nanostructured SiO2 using laser driven accelerators

Ionising radiation interactions in matter can trigger a cascade of processes that underpin long-lived damage in the medium. To date, however, a lack of suitable methodologies has precluded our ability to understand the role that material nanostructure plays in this cascade. Here, we use transient photoabsorption to track the lifetime of free electrons (t_c) in bulk and nanostructured SiO2 (aerogel) irradiated by picosecond-scale (10^-12 s) bursts of X-rays and protons from a laser-driven accelerator. Optical streaking reveals a sharp increase in t_c from < 1 ps to > 50 ps over a narrow average density (p_av) range spanning the expected phonon-fracton crossover in aerogels. Numerical modelling suggests that this discontinuity can be understood by a quenching of rapid, phonon-assisted recovery in irradiated nanostructured SiO_2. This is shown to lead to an extended period of enhanced energy density in the excited electron population. Overall, these results open a direct route to tracking how low-level processes in complex systems can underpin macroscopically observed phenomena and, importantly, the conditions that permit them to emerge.

physics.acc-ph↗

arXiv2021

Real-time Electron Solvation Induced by Bursts of Laser-accelerated Protons in Liquid Water

Understanding the mechanisms of proton energy deposition in matter and subsequent damage formation is fundamental to radiation science. Here we exploit the picosecond (10^-12 s) resolution of laser-driven accelerators to track ultra-fast solvation dynamics for electrons due to proton radiolysis in liquid water (H2O). Comparing these results with modelling that assumes initial conditions similar to those found in photolysis reveals that solvation time due to protons is extended by > 20 ps. Supported by magneto-hydrodynamic theory this indicates a highly dynamic phase in the immediate aftermath of the proton interaction that is not accounted for in current models.

physics.plasm-ph↗

arXiv2017

Enhanced laser-driven ion acceleration by superponderomotive electrons generated from near-critical-density plasma

We report on the experimental studies of laser driven ion acceleration from double-layer target where a near-critical density target with a few-micron thickness is coated in front of a nanometer thin diamond-like carbon foil. A significant enhancement of proton maximum energies from 12 to ~30 MeV is observed when relativistic laser pulse impinge on the double-layer target under linear polarization. We attributed the enhanced acceleration to superponderomotive electrons that were simultaneously measured in the experiments with energies far beyond the free-electron ponderomotive limit. Our interpretation is supported by two-dimensional simulation results.

physics.plasm-ph↗

arXiv2016

Single 100-terawatt attosecond X-ray light pulse generation

The birth of attosecond light sources is expected to inspire a breakthrough in ultrafast optics, which may extend human real-time measurement and control techniques into atomic-scale electronic dynamics. For applications, it is essential to obtain a single attosecond pulse of high intensity, large photon energy and short duration. Here we show that single 100-terawatt attosecond X-ray light pulse with intensity ${1\times10^{21}}\textrm{W}/\textrm{cm}^{ 2}$ and duration ${7.9} \textrm{as}$ can be produced by intense laser irradiation on a capacitor-nanofoil target composed of two separate nanofoils. In the interaction, a strong electrostatic potential develops between two nanofoils, which drags electrons out of the second foil and piles them up in vacuum, forming an ultradense relativistic electron nanobunch. This nanobunch exists in only half a laser cycle and smears out in others, resulting in coherent synchrotron emission of a single pulse. Such an unprecedentedly giant attosecond X-ray pulse may bring us to view a real attoworld.

physics.plasm-ph↗

arXiv2016

Attosecond Control of Relativistic Electron Bunches using Two-Colour Fields

Energy coupling during relativistically intense laser-matter interactions is encoded in the attosecond motion of strongly driven electrons at the pre-formed plasma-vacuum boundary. Studying and controlling this motion can reveal details about the microscopic processes that govern a vast array of light-matter interaction physics and applications. These include research areas right at the forefront of extreme laser-plasma science such as laser-driven ion acceleration1, bright attosecond pulse generation2,3 and efficient energy coupling for the generation and study of warm dense matter4. Here we demonstrate attosecond control over the trajectories of relativistic electron bunches formed during such interactions by studying the emission of extreme ultraviolet (XUV) harmonic radiation. We describe how the precise addition of a second laser beam operating at the second harmonic of the driving laser pulse can significantly transform the interaction by modifying the accelerating potential provided by the fundamental frequency to drive strong coherent emission. Numerical particle-in-cell code simulations and experimental observations demonstrate that this modification is extremely sensitive to the relative phase of the two beams and can lead to significant enhancements in the resulting harmonic yield. This work also reveals that the ability to control these extreme interactions with attosecond precision is an essential requirement for generation of ultra-bright, high temporal contrast attosecond radiation for atomic and molecular pump-probe experiments5,6.

physics.plasm-ph↗

arXiv2016

Temporal structure of attosecond pulses from laser-driven coherent synchrotron emission

The microscopic dynamics of laser-driven coherent synchrotron emission transmitted through thin foils are investigated using particle-in-cell simulations. For normal incidence interactions, we identify the formation of two distinct electron nanobunches from which emission takes place each half-cycle of the driving laser pulse. These emissions are separated temporally by 130 attoseconds and are dominant in different frequency ranges, which is a direct consequence of the distinct characteristics of each electron nanobunch. This may be exploited through spectral filtering to isolate these emissions, generating electromagnetic pulses of duration ~70 as.

physics.plasm-ph↗

arXiv2015

Generation of a neutral, high-density electron-positron plasma in the laboratory

We report on the laser-driven generation of purely neutral, relativistic electron-positron pair plasmas. The overall charge neutrality, high average Lorentz factor ($γ_{e/p} \approx 15$), small divergence ($θ_{e/p} \approx 10 - 20$ mrad), and high density ($n_{e/p}\simeq 10^{15}$cm$^{-3}$) of these plasmas open the pathway for the experimental study of the dynamics of this exotic state of matter, in regimes that are of relevance to electron-positron astrophysical plasmas.

physics.plasm-ph↗

arXiv2014

Ultrafast opacity in borosilicate glass induced by picosecond bursts of laser-driven ions

Direct investigation of ion-induced dynamics in matter on picosecond (ps, 10-12 s) timescales has been precluded to date by the relatively long nanosecond (ns, 10-9 s) scale ion pulses typically provided by radiofrequency accelerators1. By contrast, laser-driven ion accelerators provide bursts of ps duration2, but have yet to be applied to the study of ultrafast ion-induced transients in matter. We report on the evolution of an electron-hole plasma excited in borosilicate glass by such bursts. This is observed as an onset of opacity to synchronised optical probe radiation and is characterised by the 3.0 +/- 0.8 ps ion pump rise-time . The observed decay-time of 35 +/- 3 ps i.e. is in excellent agreement with modelling and reveals the rapidly evolving electron temperature (>10 3 K) and carrier number density (>10 17cm-3). This result demonstrates that ps laser accelerated ion bursts are directly applicable to investigating the ultrafast response of matter to ion interactions and, in particular, to ultrafast pulsed ion radiolysis of water3-5, the radiolytic decompositions of which underpin biological cell damage and hadrontherapy for cancer treatment6.

physics.plasm-ph↗

arXiv2014

Enhanced harmonic generation in relativistic laser plasma interaction

We report the enhancement of individual harmonics generated at a relativistic ultra-steep plasma vacuum interface. Simulations show the harmonic emission to be due to the coupled action of two high velocity oscillations -- at the fundamental $ω_L$ and at the plasma frequency $ω_P$ of the bulk plasma. The synthesis of the enhanced harmonics can be described by the reflection of the incident laser pulse at a relativistic mirror oscillating at $ω_L$ and $ω_P$.

physics.plasm-ph↗

arXiv2014

Scaling of laser-driven ion energies in the relativistic transparent regime

Laser-driven ions have compelling properties and their potential use for medical applications has attracted a huge global interest. One of the major challenges of these applications is generating beams of the required energies. To date, there has been no systematic study of the effect of laser intensity on the generation of laser-driven ions from ultrathin foils during relativistic transparency. Here we present a scaling for ion energies with respect to the on-target laser intensity and in considering target thickness we find an optimum thickness closely related to the experimentally observed relativistic transparency. A steep linear scaling with the normalized laser amplitude a0 has been measured and verified with PIC simulations. In contrast to TNSA, this scaling is much steeper and has been measured for ions with Z > 1. Following our results, ion energies exceeding 100MeV/amu are already accessible with currently available laser systems enabling realization of numerous advanced applications

physics.plasm-ph↗

arXiv2014

Relativistic plasma optics enabled by near-critical density nanostructured material

The nonlinear optical properties of a plasma due to the relativistic electron motion in an intense laser field are of fundamental importance for current research and the generation of brilliant laser-driven sources of particles and photons1-15. Yet, one of the most interesting regimes, where the frequency of the laser becomes resonant with the plasma, has remained experimentally hard to access. We overcome this limitation by utilizing ultrathin carbon nanotube foam16 (CNF) targets allowing the strong relativistic nonlinearities at near- critical density (NCD) to be exploited for the first time. We report on the experimental realization of relativistic plasma optics to spatio-temporally compress the laser pulse within a few micrometers of propagation, while maintaining about half its energy. We also apply the enhanced laser pulses to substantially improve the properties of an ion bunch accelerated from a secondary target. Our results provide first insights into the rich physics of NCD plasmas and the opportunities waiting to be harvested for applications.

physics.plasm-ph↗

arXiv2014

Bright subcycle XUV pulse from a single dense relativistic electron sheet

Relativistic electrons are prodigious sources of photons. Beyond classical accelerators, ultra-intense laser interactions are of particular interest as they allow the coherent motion of relativistic electrons to be controlled and exploited as sources of radiation. Under extreme laser conditions theory predicts that isolated free relativistic electron sheets (FRES) can be produced and exploited for the production of a new class of radiation - unipolar extreme ultraviolet(XUV) pulses. However, the combination of extremely rapid rise-time and highest peak intensity in these simulations is still beyond current laser technology. We demonstrate a route to isolated FRES with existing lasers by exploiting relativistic transparency to produce an ultra-intense pulse with a steep rise time. When such an FRES interacts with a second, oblique target foil the electron sheet is rapidly accelerated ('kicked'). The radiation signature and simulations demonstrate that a single, nanometer thick FRES was produced. The experimental observations together with our theoretical modeling suggest the production of the first unipolar (half-cycle) pulse in the XUV - an achievement that has so far only been realized in the terahertz spectral domain.

physics.plasm-ph↗

arXiv2013

160 MeV laser-accelerated protons from CH2 nano-targets for proton cancer therapy

Proton (and ion) cancer therapy has proven to be an extremely effective even supe-rior method of treatment for some tumors 1-4. A major problem, however, lies in the cost of the particle accelerator facilities; high procurement costs severely limit the availability of ion radiation therapy, with only ~26 centers worldwide. Moreover, high operating costs often prevent economic operation without state subsidies and have led to a shutdown of existing facilities 5,6. Laser-accelerated proton and ion beams have long been thought of as a way out of this dilemma, with the potential to provide the required ion beams at lower cost and smaller facility footprint 7-14. The biggest challenge has been the achievement of sufficient particle energy for therapy, in the 150-250 MeV range for protons 15,16. For the last decade, the maximum exper-imentally observed energy of laser-accelerated protons has remained at ~60 MeV 17. Here we the experimental demonstration of laser-accelerated protons to energies exceeding 150 MeV, reaching the therapy window. This was achieved through a dif-ferent acceleration regime rather than a larger laser, specifically a 150 TW laser with CH2 nano-targets in the relativistically transparent regime 18,19. We also demonstrate a clear scaling law with laser intensity based on analytical theory, computer simulations and experimental validation that will enable design of a pro-totype system spanning the full range of therapeutically desirable energies.

physics.plasm-ph↗

arXiv2013

Relativistic Frequency Synthesis of Light Fields

Waveform shaping and frequency synthesis based on waveform modulation is ubiquitous in electronics, telecommunication technology, and optics. For optical waveforms, the carrier frequency is on the order of several hundred THz, while the modulation frequencies used in conventional devices like electro- or acousto-optical modulators are orders of magnitude lower. As a consequence, any new frequencies are typically very close to the fundamental. The synthesis of new frequencies in the extreme ultraviolet (XUV), e.g. by using relativistic oscillating mirrors, requires modulation frequencies in the optical regime or even in the extreme ultraviolet. The latter has not been proven possible to date. Here we demonstrate that individual strong harmonics can indeed be generated by reflecting light off a plasma surface that oscillates at XUV frequencies. The strong harmonics are explained by nonlinear frequency mixing of near-infrared light and a laser-driven plasma oscillation in the extreme ultra-violet mediated by a relativistic non-linearity.

physics.plasm-ph↗

arXiv2013

A table-top laser-based source of femtosecond, collimated, ultra-relativistic positron beams

The generation of ultra-relativistic positron beams with short duration ($τ_{e^+} \leq 30$ fs), small divergence ($θ_{e^+} \simeq 3$ mrad), and high density ($n_{e^+} \simeq 10^{14} - 10^{15}$ cm$^{-3}$) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and $γ$-rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.

physics.acc-ph↗

arXiv2012

Soft X-ray harmonic comb from relativistic electron spikes

We demonstrate a new high-order harmonic generation mechanism reaching the `water window' spectral region in experiments with multi-terawatt femtosecond lasers irradiating gas jets. A few hundred harmonic orders are resolved, giving uJ/sr pulses. Harmonics are collectively emitted by an oscillating electron spike formed at the joint of the boundaries of a cavity and bow wave created by a relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by particle-in-cell simulations.

physics.plasm-ph↗

arXiv2010

Plasma surface dynamics and smoothing in the relativistic few-cycle regime

In laser-plasma interactions it is widely accepted that a non-uniform interaction surface will invariably seed hydrodynamic instabilities and a growth in the amplitude of the initial modulation. Recent experimental results [Dromey, Nat. Phys. 2009] have demonstrated that there must be target smoothing in femtosecond timescale relativistic interactions, contrary to prevailing expectation. In this paper we develop a theoretical description of the physical process that underlies this novel phenomena. We show that the surface dynamics in the few-cycle relativistic regime is dominated by the coherent electron motion resulting in a smoothing of the electron surface. This stabilization of plasma surfaces is unique in laser-plasma interactions and demonstrates that dynamics in the few-cycle regime differ fundamentally from the longer pulse regimes. This has important consequences for applications such as radiation pressure acceleration of protons and ions and harmonic generation from relativistically oscillating surfaces.

physics.plasm-ph↗