Target shape effects on monoenergetic GeV proton acceleration

When a circularly polarized laser pulse interacts with a foil target, there are three stages: pre-hole-boring, hole-boring and the light sail acceleration. We study the electron and ion dynamics in the first stage and find the minimum foil thickness requirement for a given laser intensity. Based on this analysis, we propose to use a shaped foil for ion acceleration, whose thickness varies transversely to match the laser intensity. Then, the target evolves into three regions: the acceleration, transparency and deformation regions. In the acceleration region, the target can be uniformly accelerated producing a mono-energetic and spatially collimated ion beam. Detailed numerical simulations are performed to check the feasibility and robustness of this scheme, such as the influence of shape factors and surface roughness. A GeV mono-energetic proton beam is observed in the three dimensional particle-in-cell simulations when a laser pulse with the focus intensity of 1022W=cm2 is used. The energy conversion efficiency of laser pulse to accelerated proton beam is more than 23%. Synchrotron radiation and damping effects are also checked in the interaction.Comment: 11 pages, 9 figure

Intramolecular Exciton Coupling and Induced Circular Dichroism From Bilirubin-Ephedrine Heteroassociation Complexes. Stereochemical Models for Protein Binding

Bichromophoric (4Z,15Z)-bilirubin-IXa, the cytotoxic and yel-: low-orange pigment of jaundice, prefers to adopt either of two enantiomeric intramolecularly hydrogeri-bonded conformations that are in dynamic equilibrium in solution. In the presence of optically active amino-alcohols, particularly ephedrines, the pigment solutions exhibit intense bisignate circular dichroism in the region of the bilirubin long wavelength UV-visible absorption band. The most intense circular dichroism Cotton effects, I !\u27lE; i --+ 200, are induced by O-methylephedrines, exceeding even those generally exhibited by bilirubin complexes with serum albumin and other proteins. Like serum albumin and other proteins, the optically active amino alcohols act as chiral templates, inducing an asymmetric transformation of bilirubin, whose induced bisignate circular dichroism Cotton effect originate from exciton sphtting of its two component pyrromethenone chromophores. The amines are thought to serve as agents for chiral molecular recognitlon by forming .diastereorneric salts with the pigment. And the complementary action of fJ-aryl and proximal hydroxyl and methoxyl group s provides insight into the binding forces important in bilirubinprotein binding

Injury assessment via stress analysis of the human knee joint

The largest articulation inside human body is the knee joint which is composed by hard components, soft tissues and surrounded muscles. The knee is a mobile hinge, and it permits flexion, extension, slight internal and external rotation of the leg. The knee joint is vulnerable to both sharp injury and chronic osteoarthritis. Once have been injured, the knee joint is not easily restored. This study employs separately the experimental measurement, reverse engineering and finite element analysis to investigate the dynamic characteristics of intricate knee joint. The three-dimensional geometric model of each component of knee joint includes hard tissues and soft tissues. The hard tissues have femur, tibia, fibula, patella and the soft tissues have meniscus, patellar ligament, medial and lateral collateral ligament, a pair of cruciate ligaments, etc. Then the model is imported into ANSYS software. Via modal, periodic excitation and impact analysis, the dynamic characteristics of each component and the whole knee model are received. The fundamental mode shapes, natural frequencies and stresses of all the components of knee are also obtained. These normal modes are essential when investigating the dynamic motion of the whole knee. The results show that after impact, the soft tissues have larger displacement than that of the hard tissues. Consequently, the fracture occurs when the stretch which is caused by external force excess ultimate strength of the component. It also explains why the athletes frequently injure the ligaments and tendons of the knee or ankle during the intensive exercise. Therefore, by reducing the motion of articulation, the professional player could not only reduce the generated internal stresses in the tissue but also consequently lessen the chance of injury

Acceleration of on-axis and ring-shaped electron beams in wakefields driven by Laguerre-Gaussian pulses

The acceleration of electron beams with multiple transverse structures in wakefields driven by Laguerre-Gaussian pulses has been studied through three-dimensional (3D) particle-in-cell simulations. Under different laser-plasma conditions, the wakefield shows different transverse structures. In general cases, the wakefield shows a donut-like structure and it accelerates the ring-shaped hollow electron beam. When a lower plasma density or a smaller laser spot size is used, besides the donut-like wakefield, a central bell-like wakefield can also be excited. The wake sets in the center of the donut-like wake. In this case, both a central on-axis electron beam and a ring-shaped electron beam are simultaneously accelerated. Further, reducing the plasma density or laser spot size leads to an on-axis electron beam acceleration only. The research is beneficial for some potential applications requiring special pulse beam structures, such as positron acceleration and collimation

Acceleration and evolution of a hollow electron beam in wakefields driven by a Laguerre-Gaussian laser pulse

We show that a ring-shaped hollow electron beam can be injected and accelerated by using a Laguerre-Gaussian laser pulse and ionization-induced injection in a laser wakefield accelerator. The acceleration and evolution of such a hollow, relativistic electron beam are investigated through three-dimensional particle-in-cell simulations. We find that both the ring size and the beam thickness oscillate during the acceleration. The beam azimuthal shape is angularly dependent and evolves during the acceleration. The beam ellipticity changes resulting from the electron angular momenta obtained from the drive laser pulse and the focusing forces from the wakefield. The dependence of beam ring radius on the laser-plasma parameters (e.g., laser intensity, focal size, and plasma density) is studied. Such a hollow electron beam may have potential applications for accelerating and collimating positively charged particles

Ultrasensitive amyloid β-protein quantification with high dynamic range using a hybrid graphene-gold surface-enhanced Raman spectroscopy platform.

Surface enhanced Raman spectroscopy (SERS) holds great promise in biosensing because of its single-molecule, label-free sensitivity. We describe here the use of a graphene-gold hybrid plasmonic platform that enables quantitative SERS measurement. Quantification is enabled by normalizing analyte peak intensities to that of the graphene G peak. We show that two complementary quantification modes are intrinsic features of the platform, and that through their combined use, the platform enables accurate determination of analyte concentration over a concentration range spanning seven orders of magnitude. We demonstrate, using a biologically relevant test analyte, the amyloid β-protein (Aβ), a seminal pathologic agent of Alzheimer's disease (AD), that linear relationships exist between (a) peak intensity and concentration at a single plasmonic hot spot smaller than 100 nm, and (b) frequency of hot spots with observable protein signals, i.e. the co-location of an Aβ protein and a hot spot. We demonstrate the detection of Aβ at a concentration as low as 10-18 M after a single 20 μl aliquot of the analyte onto the hybrid platform. This detection sensitivity can be improved further through multiple applications of analyte to the platform and by rastering the laser beam with smaller step sizes

Dense GeV electron–positron pairs generated by lasers in near-critical-density plasmas

Pair production can be triggered by high intensity lasers via the Breit-Wheeler process. However, the straightforward laser-laser colliding for copious numbers of pair creation requires light intensities several orders of magnitude higher than possible with the ongoing laser facilities. Despite the numerous proposed approaches, creating high-energy-density pair plasmas in laboratories is still challenging. Here we present an all-optical scheme for overdense pair production by two counter-propagating lasers irradiating near-critical-density plasmas at only ~1022W cm-2. In this scheme, bright γ-rays are generated by radiation-trapped electrons oscillating in the laser fields. The dense γ-photons then collide with the focused counter-propagating lasers to initiate the multi-photon Breit-Wheeler process. Particle-in-cell simulations indicate that one may generate a high-yield (1.05×1011) overdense (4×1022 cm-3 ) GeV positron beam using 10 PW scale lasers. Such a bright pair source has many practical applications and could be basis for future compact high luminosity electron-positron colliders

Generation of GeV positron and γ-photon beams with controllable angular momentum by intense lasers

Although several laser–plasma-based methods have been proposed for generating energetic electrons, positrons and γ-photons, manipulation of their microstructures is still challenging, and their angular momentum control has not yet been achieved. Here, we present and numerically demonstrate an all-optical scheme to generate bright GeV γ-photon and positron beams with controllable angular momentum by use of two counter-propagating circularly-polarized lasers in a near-critical-density plasma. The plasma acts as a 'switching medium', where the trapped electrons first obtain angular momentum from the drive laser pulse and then transfer it to the γ-photons via nonlinear Compton scattering. Further through the multiphoton Breit–Wheeler process, dense energetic positron beams are efficiently generated, whose angular momentum can be well controlled by laser–plasma interactions. This opens up a promising and feasible way to produce ultra-bright GeV γ-photons and positron beams with desirable angular momentum for a wide range of scientific research and applications