A leed analysis of the (2×1)H-Ni(110) structure

A monolayer of H atoms adsorbed on Ni(110) below 180 K forms a (2×1) structure. The unit cell exhibits a glide symmetry plane and contains two adsorbed atoms. Based on a quantitative comparison between experimental and calculated LEED I/V spectra using standard R-factors the following structure was derived: On the clean Ni(110) surface the separation between the first two atomic layers, d12, is contracted by 8.5%±1.5% with respect to the bulk value; those between the second and third and the third and fourth layer, d23 and d34, are expanded by 3.5%±1.5% and 1%±1.5%, respectively—in agreement with recent other results. In the presence of the H adlayer the contraction of d12 is reduced to 4.5%±1.5%, while the expansion of d23 is not affected within the limits of accuracy. The third interlayer spacing d34 returns to its bulk value. The H atoms occupy threefold-coordinated sites formed by two Ni atoms from the first layer and one Ni atom from the second layer which confirms previous more qualitative conclusions based on He diffraction and vibrational spectroscopy. The bond lengths between H and its neighbouring Ni atoms were determined to be equal, namely 1.72±0.1 Å

High Flux Femtosecond X-ray Emission from the Electron-Hose Instability in Laser Wakefield Accelerators

Bright and ultrashort duration X-ray pulses can be produced by through betatron oscillations of electrons during Laser Wakefield Acceleration (LWFA). Our experimental measurements using the \textsc{Hercules} laser system demonstrate a dramatic increase in X-ray flux for interaction distances beyond the depletion/dephasing lengths, where the initial electron bunch injected into the first wake bucket catches up with the laser pulse front and the laser pulse depletes. A transition from an LWFA regime to a beam-driven plasma wakefield acceleration (PWFA) regime consequently occurs. The drive electron bunch is susceptible to the electron-hose instability and rapidly develops large amplitude oscillations in its tail, which leads to greatly enhanced X-ray radiation emission. We measure the X-ray flux as a function of acceleration length using a variable length gas cell. 3D particle-in-cell (PIC) simulations using a Monte Carlo synchrotron X-ray emission algorithm elucidate the time-dependent variations in the radiation emission processes.Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev. Accel. Beam

The Effects of Trunk Muscle Training on Physical Fitness and Sport-Specific Performance in Young and Adult Athletes: A Systematic Review and Meta-Analysis.

Background The role of trunk muscle training (TMT) for physical ftness (e.g., muscle power) and sport-specifc performance measures (e.g., swimming time) in athletic populations has been extensively examined over the last decades. However, a recent systematic review and meta-analysis on the efects of TMT on measures of physical ftness and sport-specifc performance in young and adult athletes is lacking. Objective To aggregate the efects of TMT on measures of physical ftness and sport-specifc performance in young and adult athletes and identify potential subject-related moderator variables (e.g., age, sex, expertise level) and training-related programming parameters (e.g., frequency, study length, session duration, and number of training sessions) for TMT efects. Data Sources A systematic literature search was conducted with PubMed, Web of Science, and SPORTDiscus, with no date restrictions, up to June 2021. Study Eligibility Criteria Only controlled trials with baseline and follow-up measures were included if they examined the efects of TMT on at least one measure of physical ftness (e.g., maximal muscle strength, change-of-direction speed (CODS)/ agility, linear sprint speed) and sport-specifc performance (e.g., throwing velocity, swimming time) in young or adult competitive athletes at a regional, national, or international level. The expertise level was classifed as either elite (competing at national and/or international level) or regional (i.e., recreational and sub-elite). Study Appraisal and Synthesis Methods The methodological quality of TMT studies was assessed using the Physiotherapy Evidence Database (PEDro) scale. A random-efects model was used to calculate weighted standardized mean diferences (SMDs) between intervention and active control groups. Additionally, univariate sub-group analyses were independently computed for subject-related moderator variables and training-related programming parameters. Results Overall, 31 studies with 693 participants aged 11–37 years were eligible for inclusion. The methodological quality of the included studies was 5 on the PEDro scale. In terms of physical ftness, there were signifcant, small-to-large efects of TMT on maximal muscle strength (SMD =0.39), local muscular endurance (SMD =1.29), lower limb muscle power (SMD=0.30), linear sprint speed (SMD =0.66), and CODS/agility (SMD =0.70). Furthermore, a signifcant and moderate TMT efect was found for sport-specifc performance (SMD=0.64). Univariate sub-group analyses for subject-related moderator variables revealed signifcant efects of age on CODS/agility (p=0.04), with signifcantly large efects for children (SMD=1.53, p=0.002). Further, there was a signifcant efect of number of training sessions on muscle power and linear sprint speed (p≤0.03), with signifcant, small-to-large efects of TMT for>18 sessions compared to≤18 sessions (0.45≤SMD≤0.84, p≤0.003). Additionally, session duration signifcantly modulated TMT efects on linear sprint speed, CODS/agility, and sport-specifc performance (p≤0.05). TMT with session durations≤30 min resulted in signifcant, large efects on linear sprint speed and CODS/agility (1.66≤SMD≤2.42, p≤0.002), whereas session durations>30 min resulted in signifcant, large efects on sport-specifc performance (SMD=1.22, p=0.008). Conclusions Our fndings indicate that TMT is an efective means to improve selected measures of physical ftness and sportspecifc performance in young and adult athletes. Independent sub-group analyses suggest that TMT has the potential to improve CODS/agility, but only in children. Additionally, more (>18) and/or shorter duration (≤30 min) TMT sessions appear to be more efective for improving lower limb muscle power, linear sprint speed, and CODS/agility in young or adult competitive athletes.publishedVersio

Acute Effects of Massage or Active Exercise in Relieving Muscle Soreness: Randomized Controlled Trial

Andersen, LL, Jay, K, Andersen, CH, Jakobsen, MD, Sundstrup, E, Topp, R, and Behm, DG. Acute effects of massage or active exercise in relieving muscle soreness: randomized controlled trial. J Strength Cond Res 27(12): 3352–3359, 2013—Massage is commonly believed to be the best modality for relieving muscle soreness. However, actively warming up the muscles with exercise may be an effective alternative. The purpose of this study was to compare the acute effect of massage with active exercise for relieving muscle soreness. Twenty healthy female volunteers (mean age 32 years) participated in this examiner-blind randomized controlled trial (ClinicalTrials.gov NCT01478451). The participants performed eccentric contractions for the upper trapezius muscle on a Biodex dynamometer. Delayed onset muscle soreness (DOMS) presented 48 hours later, at which the participants (a) received 10 minutes of massage of the trapezius muscle or (b) performed 10 minutes of active exercise (shoulder shrugs 10 × 10 reps) with increasing elastic resistance (Thera-Band). First, 1 treatment was randomly applied to 1 shoulder while the contralateral shoulder served as a passive control. Two hours later, the contralateral resting shoulder received the other treatment. The participants rated the intensity of soreness (scale 0–10), and a blinded examiner took measures of pressure pain threshold (PPT) of the upper trapezius immediately before treatment and 0, 10, 20, and 60 minutes after treatment 48 hours posteccentric exercise. Immediately before treatment, the intensity of soreness was 5.0 (SD 2.2) and PPT was 138 (SD 78) kPa. In response to treatment, a significant treatment by time interaction was found for the intensity of soreness (p \u3c 0.001) and PPT (p \u3c 0.05). Compared with control, both active exercise and massage significantly reduced the intensity of soreness and increased PPT (i.e., reduced pain sensitivity). For both types of treatment, the greatest effect on perceived soreness occurred immediately after treatment, whereas the effect on PPT peaked 20 minutes after treatment. In conclusion, active exercise using elastic resistance provides similar acute relief of muscle soreness as compared with that using massage. Coaches, therapists, and athletes can use either active warm-up or massage to reduce DOMS acutely, for example, to prepare for competition or strenuous work, but should be aware that the effect is temporary, that is, the greatest effects occurs during the first 20 minutes after treatment and diminishes within an hour

High-Flux Femtosecond X-Ray Emission from Controlled Generation of Annular Electron Beams in a Laser Wakefield Accelerator

Annular quasimonoenergetic electron beams with a mean energy in the range 200-400 MeV and charge on the order of several picocoulombs were generated in a laser wakefield accelerator and subsequently accelerated using a plasma afterburner in a two-stage gas cell. Generation of these beams is associated with injection occurring on the density down ramp between the stages. This well-localized injection produces a bunch of electrons performing coherent betatron oscillations in the wakefield, resulting in a significant increase in the x-ray yield. Annular electron distributions are detected in 40% of shots under optimal conditions. Simultaneous control of the pulse duration and frequency chirp enables optimization of both the energy and the energy spread of the annular beam and boosts the radiant energy per unit charge by almost an order of magnitude. These well-defined annular distributions of electrons are a promising source of high-brightness laser plasma-based x rays