Showing papers by "David Neely published in 2021"

Scaling of laser-driven electron and proton acceleration as a function of laser pulse duration, energy, and intensity in the multi-picosecond regime

Abstract: A scaling study of short-pulse laser-driven proton and electron acceleration was conducted as a function of pulse duration, laser energy, and laser intensity in the multi-picosecond (ps) regime (∼0.8 ps–20 ps). Maximum proton energies significantly greater than established scaling laws were observed, consistent with observations at other multi-ps laser facilities. In addition, maximum proton energies and electron temperatures in this regime were found to be strongly dependent on the laser pulse duration and preplasma conditions. A modified proton scaling model is presented that is able to better represent the accelerated proton characteristics in this multi-ps regime.

Sources and space-time distribution of the electromagnetic pulses in experiments on inertial confinement fusion and laser-plasma acceleration

Abstract: When high-energy and high-power lasers interact with matter, a significant part of the incoming laser energy is transformed into transient electromagnetic pulses (EMPs) in the range of radiofrequencies and microwaves. These fields can reach high intensities and can potentially represent a significative danger for the electronic devices placed near the interaction point. Thus, the comprehension of the origin of these electromagnetic fields and of their distribution is of primary importance for the safe operation of high-power and high-energy laser facilities, but also for the possible use of these high fields in several promising applications. A recognized main source of EMPs is the target positive charging caused by the fast-electron emission due to laser-plasma interactions. The fast charging induces high neutralization currents from the conductive walls of the vacuum chamber through the target holder. However, other mechanisms related to the laser-target interaction are also capable of generating intense electromagnetic fields. Several possible sources of EMPs are discussed here and compared for high-energy and high-intensity laser-matter interactions, typical for inertial confinement fusion and laser-plasma acceleration. The possible effects on the electromagnetic field distribution within the experimental chamber, due to particle beams and plasma emitted from the target, are also described. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

Bremsstrahlung emission and plasma characterization driven by moderately relativistic laser-plasma interactions

Abstract: Relativistic electrons generated by the interaction of petawatt-class short laser pulses with solid targets can be used to generate bright X-rays via bremsstrahlung. The efficiency of laser energy transfer into these electrons depends on multiple parameters including the focused intensity and pre-plasma level. This paper reports experimental results from the interaction of a high intensity petawatt-class glass laser pulses with solid targets at a maximum intensity of $10^{19}$ W/cm$^2$. In-situ measurements of specularly reflected light are used to provide an upper bound of laser absorption and to characterize focused laser intensity, the pre-plasma level and the generation mechanism of second harmonic light. The measured spectrum of electrons and bremsstrahlung radiation provide information about the efficiency of laser energy transfer.

Experimental tests and signal unfolding of a scintillator calorimeter for laser-plasma characterization

Abstract: With the development of high-intensity and high-repetition rate laser systems, it has become crucial to be able to detect and characterize in real time the high-energy byproducts (mainly electrons and photons) of laser-generated plasma. A novel multi-purpose scintillator-based electromagnetic calorimeter focused on high-energy particle and photon measurements and capable of working on a shot-by-shot basis at high-repetition rate is being developed at the ELI Beamlines center. Preliminary tests of this device under photon and electron irradiation from conventional and laser-driven sources are summarized and the results are here presented. A corresponding signal unfolding technique which was ad-hoc developed to reconstruct energies of one or two thermal populations in short time is described in detail.

Selective Ion Acceleration by Intense Radiation Pressure.

Abstract: Accessing novel ion acceleration mechanisms, such as Radiation Pressure Acceleration (RPA), is a promising route to generate high energy beams of both light and heavy ions [1]. In particular, the Light Sail (LS) regime predicts high efficiency, mono-energetic beams and can be accessed with currently available high power laser facilities with the use of ultra-thin foils and circular polarisation [2-4]. In recent experiments at the GEMINI laser facility (RAL, UK), target bulk (carbon) ions were favourably accelerated in the LS-RPA regime up to 33MeV/nucleon at an optimal carbon foil thickness of 15nm, whereas protons only reached energies of 18 MeV. This result, which differs from what is typically observed in laser-solid interactions, where protons are always accelerated more efficiently than heavier ions, is interpreted with the support of multi-dimensional Particle in Cell (PIC) simulations. While the 40fs pulse was temporally cleaned by a double plasma mirror arrangement to increase the laser contrast to 10-14 at the ns timescale, it is shown that the limited preceding laser fluence incident on the target on the ps scale causes target expansion, with protons, being lighter, escaping from the interaction region. This leaves a pre-dominantly carbon plasma which, for circular polarization, is accelerated by RPA, with proton energies determined instead by plasma expansion and sheath effects. It is shown through simulations that controlling the laser temporal profile and plasma mirror activation opens up a promising route for controlling which ion species is preferentially accelerated in the RPA regime. This has particular importance as <1PW systems are coming online currently where these accelerations will begin to inherently dominate, and the preceding laser intensity will need to be suitably controlled.

A history of high-power laser research and development in the United Kingdom

Abstract: The first demonstration of laser action in ruby was made in 1960 by T. H. Maiman of Hughes Research Laboratories, USA. Many laboratories worldwide began the search for lasers using different materials, operating at different wavelengths. In the UK, academia, industry and the central laboratories took up the challenge from the earliest days to develop these systems for a broad range of applications. This historical review looks at the contribution the UK has made to the advancement of the technology, the development of systems and components and their exploitation over the last 60 years.

Kinematics of femtosecond laser-generated plasma expansion: Determination of sub-micron density gradient and collisionality evolution of over-critical laser plasmas

Abstract: An optical diagnostic based on resonant absorption of laser light in a plasma is introduced and is used for the determination of density scale lengths in the range of 10 nm to >1 μm at the critical surface of an overdense plasma. This diagnostic is also used to extract the plasma collisional frequency, allowing inference of the temporally evolving plasma composition on the tens of femtosecond timescale. This is found to be characterized by two eras: the early time and short scale length expansion (L 0.1λ); this is consistent with a hydrogen plasma decoupling from the bulk target material. Density gradients and plasma parameters on this scale are of importance to plasma mirror optical performance and comment is made on this theme.

Characterisation of a laser plasma betatron source for high resolution x-ray imaging

Abstract: We report on the characterisation of an x-ray source, generated by a laser-driven plasma wakefield accelerator. The spectra of the optimised source was consistent with an on-axis synchrotron spectrum with a critical energy of 13.8+2.2-1.9 keV and the number of photons per pulse generated above 1 keV was calculated to be 6+1.2-0.9 × 109. The x-ray beam was used to image a resolution grid placed 37 cm from the source, which gave a measured spatial resolution of 4 μm × 5 μm. The inferred emission region had a radius and length of 0.5 ± 0.2 μm and 3.2 ± 0.9 mm respectively. It was also observed that laser damage to the exit aperture of the gas cell led to a reduction in the accelerated electron beam charge and a corresponding reduction in x-ray flux due to the change in the plasma density profile.

Influence of target-rear-side short scale length density gradients on laser-driven proton acceleration

Abstract: The effects of a short plasma density scale length on laser-driven proton acceleration from foil targets is investigated by heating and driving expansion of a large area of the target rear surface. The maximum proton energy, proton flux and the divergence of the proton beam are all measured to decrease with increasing extent of the plasma expansion. Even for a small plasma scale length of the order of the laser wavelength (~1 µm), a significant effect on the generated proton beam is evident; a substantial decrease in the number of protons over a wide spectral range is measured. A combination of radiation-hydrodynamic and particle-in-cell simulations provide insight into the underlying physics. The results provide new understanding of the importance of even a small plasma density gradient, with implications for applications that require efficient laser energy conversion to ions, such as proton-driven fast-ignition of compressed fusion fuel.

Response of nuclear track detector CR-39 to low energy muons

Abstract: The effectiveness of the PolyAllyl Diglycol Carbonate (PADC) etched solid state nuclear track detector (SSNTD), commonly known as CR-39, as a muon detector is assessed. CR-39 is successfully used to detect higher rest mass particles such as neutrons and protons, and is, for example, widely used in neutron dosimetry applications. CR-39 is generally accepted as being less suitable to detect lower rest mass particles such as muons, and especially electrons, due mostly to their reduced momenta and consequently, reduced stopping power. However, there has been some evidence that CR-39 may have application in the detection of cosmic ray muons. Monte Carlo simulations indicate that CR-39 can detect muons with energies up to 2.8 MeV. Experimental data to demonstrate the ability of CR-39 to detect muons was acquired using the MuSR spectrometer station at the ISIS Neutron and Muon Source. Pits deposited in CR-39 generated by positive muons from the beamline have been characterised and compared with pits deposited by protons and neutrons from other sources. The extent to which a CR-39 SSNTD can discriminate muons from particles with different momenta and rest masses is discussed.

Design of activation based detection scheme for pulsed gamma ray emission from intense laser plasmas

Abstract: High intensity lasers (>1019 W/cm2) produce relativistic electrons when they interact with matter. The high energy electrons upon incidence on a solid target produce secondary emissions like protons, neutrons, positrons, x-ray emission and γ-rays1. Gamma rays produce from this interaction can be used to induce photoneutron reaction in a material, thereby producing short-lived isotopes or isomers2. The isotopes or isomers produced can be used for diagnosing the radiation flux and directionality3. Materials with short half-lives (in μs to ms time scale) are of interest as a diagnostic for shot to shot measurement of parameters in high repetition lasers (10 Hz), since they decay well before the incidence of the subsequent pulse on the material. For understanding the working of this diagnostic, systematic studies of decay of the isotopes/isomers produced and the attenuation of γ-rays in the material are necessary. The design and efficiency of a diagnostic for characterizing γ-rays using the method of nuclear activation for 10 Hz high repetition laser is presented.