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At least 1,729 records · Page 96Linked to original sources

PubMed2005

Broad energy spectrum of laser-accelerated protons for spallation-related physics.

A beam of MeV protons, accelerated by ultraintense laser-pulse interactions with a thin target foil, is used to investigate nuclear reactions of interest for spallation physics. The laser-generated proton beam is shown (protons were measured) to have a broad energy distribution, which closely resembles the expected energy spectrum of evaporative protons (below 50 MeV) produced in GeV-proton-induced spallation reactions. The protons are used to quantify the distribution of residual radioisotopes produced in a representative spallation target (Pb), and the results are compared with calculated predictions based on spectra modeled with nuclear Monte Carlo codes. Laser-plasma particle accelerators are shown to provide data relevant to the design and development of accelerator driven systems.

Journal Article↗

PubMed2005

Multi-GeV energy gain in a plasma-wakefield accelerator.

A plasma-wakefield accelerator has accelerated particles by over 2.7 GeV in a 10 cm long plasma module. A 28.5 GeV electron beam with 1.8 x 10(10) electrons is compressed to 20 microm longitudinally and focused to a transverse spot size of 10 microm at the entrance of a 10 cm long column of lithium vapor with density 2.8 x 10(17) atoms/cm3. The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.

Journal Article↗

PubMed2005

Whispering-gallery-mode resonances: a new way to accelerate charged particles.

Looking for future high energy accelerators we point at a very strong interaction between relativistic electrons and powerful electromagnetic fields existing in the vicinity of a dielectric cylinder in conditions of resonantly excited whispering gallery modes (WGM). A particular example of the WGM resonance, corresponding to angular index n=22, shows that the accelerating fields are almost 100 times stronger than these in the incident wave. That yields an acceleration rate of about 5 GeV/m with the incident microwave radiation beam of the wavelength lambda=1 cm and a moderately high intensity of P=1 MW/cm2.

Journal Article↗

PubMed2005

Laser-accelerated protons with energy-dependent beam direction.

The spatial distribution of protons, accelerated by intense femtosecond laser pulses interacting with thin target foils under oblique irradiation are investigated. Under certain conditions, the proton beams are directed away from the target normal. This deviation is towards the laser forward direction, with an angle that increases with the level and duration of the amplified spontaneous emission pedestal before the main laser pulse. In addition, for a given laser pulse, this beam deviation increases with proton energy. The observations are discussed in terms of different electron acceleration mechanisms and target normal sheath acceleration, in combination with a laser-controllable shock wave locally deforming the target rear surface.

Journal Article↗

PubMed2005

Pointlike gamma ray sources as signatures of distant accelerators of ultrahigh energy cosmic rays.

We discuss the possibility of observing distant accelerators of ultrahigh energy cosmic rays in synchrotron gamma rays. Protons propagating away from their acceleration sites produce extremely energetic electrons during photopion interactions with cosmic microwave background photons. If the accelerator is embedded in a magnetized region, these electrons will emit high energy synchrotron radiation. The resulting synchrotron source is expected to be pointlike, steady, and detectable in the GeV-TeV energy range if the magnetic field is at the nanoGauss level.

Journal Article↗

PubMed2006

Thomson-backscattered x rays from laser-accelerated electrons.

We present the first observation of Thomson-backscattered light from laser-accelerated electrons. In a compact, all-optical setup, the "photon collider," a high-intensity laser pulse is focused into a pulsed He gas jet and accelerates electrons to relativistic energies. A counterpropagating laser probe pulse is scattered from these high-energy electrons, and the backscattered x-ray photons are spectrally analyzed. This experiment demonstrates a novel source of directed ultrashort x-ray pulses and additionally allows for time-resolved spectroscopy of the laser acceleration of electrons.

Journal Article↗

PubMed2006

Stochastic acceleration in turbulent electric fields generated by 3D reconnection.

Electron and proton acceleration in three-dimensional electric and magnetic fields is studied through test particle simulations. The fields are obtained by a three-dimensional magnetohydrodynamic simulation of magnetic reconnection in slab geometry. The nonlinear evolution of the system is characterized by the growth of many unstable modes and the initial current sheet is fragmented with formation of small scale structures. We inject at random points inside the evolving current sheet a Maxwellian distribution of particles. In a relatively short time (less than a millisecond) the particles develop a power-law tail. The acceleration is extremely efficient and the electrons absorb a large percentage of the available energy in a small fraction of the characteristic time of the MHD simulation, suggesting that resistive MHD codes are unable to represent the full extent of particle acceleration.

Journal Article↗

PubMed2006

Laser-wakefield acceleration of monoenergetic electron beams in the first plasma-wave period.

Beam profile measurements of laser-wakefield accelerated electron bunches reveal that in the monoenergetic regime the electrons are injected and accelerated at the back of the first period of the plasma wave. With pulse durations ctau >or= lambda(p), we observe an elliptical beam profile with the axis of the ellipse parallel to the axis of the laser polarization. This increase in divergence in the laser polarization direction indicates that the electrons are accelerated within the laser pulse. Reducing the plasma density (decreasing ctau/lambda(p)) leads to a beam profile with less ellipticity, implying that the self-injection occurs at the rear of the first period of the plasma wave. This also demonstrates that the electron bunches are less than a plasma wavelength long, i.e., have a duration <25 fs. This interpretation is supported by 3D particle-in-cell simulations.

Journal Article↗

PubMed2006

Scalar-field-dominated cosmology with a transient acceleration phase.

A new cosmological scenario driven by a slow rolling homogeneous scalar field whose exponential potential V(Phi) has a quadratic dependence on the field Phi in addition to the standard linear term is discussed. The derived equation of state for the field predicts a transient accelerating phase, in which the Universe was decelerated in the past, began to accelerate at redshift z approximately 1, is currently accelerated, but, finally, will return to a decelerating phase in the future. This overall dynamic behavior is profoundly different from the standard evolution of the cold dark matter model with a cosmological constant, and may alleviate some conflicts in reconciling the idea of a dark-energy-dominated universe with observables in String or M theory. Some theoretical predictions for the present scalar field plus dark matter dominated stage are confronted with cosmological observations in order to test the viability of the scenario.

Journal Article↗

PubMed2006

Lagrangian measurements of inertial particle accelerations in grid generated wind tunnel turbulence.

We describe Lagrangian measurements of water droplets in grid generated wind tunnel turbulence at a Taylor Reynolds number of R(lambda)=250 and an average Stokes number (St) of approximately 0.1. The inertial particles are tracked by a high speed camera moving along the side of the tunnel at the mean flow speed. The standardized acceleration probability density functions of the particles have spread exponential tails that are narrower than those of a fluid particles (St approximately 0) and there is a decrease in the acceleration variance with increasing Stokes number. A simple vortex model shows that the inertial particles selectively sample the fluid field and are less likely to experience regions of the fluid undergoing the largest accelerations. Recent direct numerical simulations compare favorably with these first measurements of Lagrangian statistics of inertial particles in highly turbulent flows.

Journal Article↗

PubMed2006

Bubble acceleration in the ablative Rayleigh-Taylor instability.

The highly nonlinear evolution of the single-mode Rayleigh-Taylor instability (RTI) at the ablation front of an accelerated target is investigated in the parameter range typical of inertial confinement fusion implosions. A new phase of the nonlinear bubble evolution is discovered. After the linear growth phase and a short constant-velocity phase, it is found that the bubble is accelerated to velocities well above the classical value. This acceleration is driven by the vorticity accumulation inside the bubble resulting from the mass ablation and vorticity convection off the ablation front. While the ablative growth rates are slower than their classical values in the linear regime, the ablative RTI grows faster than the classical RTI in the nonlinear regime for deuterium and tritium ablators.

Journal Article↗

PubMed2005

The root tip and accelerating region suppress elongation of the decelerating region without any effects on cell turgor in primary roots of maize under water stress.

To identify the region in which a root perceives a decrease in the ambient water potential and changes its elongation rate, we applied two agar blocks (1 x 1 x 1 mm(3)) with low water potential bilaterally to primary roots of maize (Zea mays) at various positions along the root. When agar blocks with a water potential of -1.60 MPa (-1.60-MPa blocks) or lower were attached to a root tip, the rate of elongation decreased. This decrease did not result from any changes in the water status of elongating cells and was not reversed when the -1.60-MPa blocks were replaced by -0.03-MPa blocks. The rate decreased slightly and was unaffected, respectively, when -1.60-MPa blocks were applied to the so-called decelerating region of the elongating zone and the mature region. However, the rate decreased markedly and did not recover for several hours at least when such blocks were attached to the accelerating region. In this case, the turgor pressure of the elongating cells decreased immediately after the application of the blocks and recovered thereafter. The decrease in elongation rate caused by -1.60-MPa blocks applied to the root tip was unaffected by additional -0.03-MPa blocks applied to the accelerating region and vice versa. We concluded that a significant reduction in root growth could be induced by water stress at the root tip, as well as in the accelerating region of the elongating zone, and that transmission of some signal from these regions to the decelerating region might contribute to the suppression of cell elongation in the elongation region.

Osmotic Pressure↗

PubMed1998

Construction of 714 MHz HOM-free accelerating cavities.

A new 'higher-order-mode (HOM)-free' accelerating cavity has been developed which can provide an accelerating voltage of more than 400 kV per cavity at a frequency of 714 MHz. The harmful HOMs in the cavities, which can induce beam instabilities at high beam currents, were heavily damped by using four special waveguide ports and broadband microwave loads. Two cavities of this design were installed in the 1.54 GeV accelerator test facility (ATF) damping ring at KEK, and successfully stored beams. This cavity will also be very useful for synchrotron light sources. The basic design, characteristics of HOMs and construction of this cavity are reported.

Journal Article↗

PubMed2005

Robot-enhanced motor learning: accelerating internal model formation during locomotion by transient dynamic amplification.

When adapting to novel dynamic environments the nervous system learns to anticipate the imposed forces by forming an internal model of the environmental dynamics in a process driven by movement error reduction. Here, we tested the hypothesis that motor learning could be accelerated by transiently amplifying the environmental dynamics. A novel dynamic environment was created during treadmill stepping by applying a perpendicular viscous force field to the leg through a robotic device. The environmental dynamics were amplified by an amount determined by a computational learning model fit on a per-subject basis. On average, subjects significantly reduced the time required to predict the applied force field by approximately 26% when the field was transiently amplified. However, this reduction was not as great as that predicted by the model, likely due to nonstationarities in the learning parameters. We conclude that motor learning of a novel dynamic environment can be accelerated by exploiting the error-based learning mechanism of internal model formation, but that nonlinearities in adaptive response may limit the feasible acceleration. These results support an approach to movement training devices that amplify rather than reduce movement errors, and provide a computational framework for both implementing the approach and understanding its limitations.

Adaptation, Physiological↗

PubMed2004

Exogenous Smad3 accelerates wound healing in a rabbit dermal ulcer model.

Exogenous administration of transforming growth factor-beta (TGF-beta) improves wound healing by affecting cellular and molecular events involved in tissue repair. But mice with a deficiency of a key TGF-beta signaling intermediate, Smad3, paradoxically showed accelerated cutanenous wound healing, suggesting that endogenous Smad3 had inhibitory effect on cutaneous wound healing. Here we investigated the effect of exogenous expression of Smad3 in dermal fibroblasts on cutaneous wound healing. Subcutaneous injection of adenovirus-containing Smad3 complementary DNA (AdCMV-Smad3) targeting mainly dermal fibroblasts accelerated tissue repair following full-thickness dermal round wounds in rabbit ear as judged by the size of granulation tissue area, number of capillaries, and re-epithelialization rate of the wounds. Expressions of alpha-smooth muscle actin (alpha-SMA), vascular endothelial growth factor (VEGF), and fibroblast growth factor receptor were upregulated in the wounded area injected with AdCMV-Smad3. Consistent with the in vivo findings, overexpression of Smad3 induced alpha-SMA, VEGF, and TGF-beta1 expression and augmented chemotactic response in cultured dermal fibroblasts. Therefore, exogenous administration of Smad3 targeting dermal fibroblasts accelerated tissue repair in a rabbit dermal ulcer model by affecting fibroblast responses associated with wound healing. The results suggest that Smad3, when overexpressed in dermal fibroblasts, can promote wound healing.

Actins↗

PubMed2005

The sex steroid precursor DHEA accelerates cutaneous wound healing via the estrogen receptors.

Age-related impaired wound healing states lead to substantial morbidity and cost, with treatment in the USA resulting in an expenditure of over $9 billion per annum. Dehydroepiandrosterone (DHEA) is a ubiquitous adrenal hormone with immunomodulatory properties whose levels decline significantly with advanced age in humans. Conversion of DHEA locally to downstream steroid hormones leads to estrogenic and/or androgenic effects which may be important in age-related skin homeostasis, and which would avoid systemic adverse effects related to estrogen. We report that systemic DHEA levels are strongly associated with protection against chronic venous ulceration in humans. DHEA accelerated impaired healing in an impaired healing model (mice rendered hypogonadal) associated with increased matrix deposition and dampens the exaggerated inflammatory response. Such effects were mediated by local conversion of DHEA to estrogen, acting through the estrogen receptor, and vitro studies suggest a direct effect on specific pro-inflammatory cytokine production by macrophages via mitogen activated kinase (MAP) and phosphatidylinositol 3 (PI3) kinase pathways. In addition, we show that local injection of DHEA accelerates impaired healing in an ageing mouse colony. We suggest that exogenous application of DHEA accelerates impaired wound repair, results which may be applicable to the prophylaxis and treatment of human impaired wound healing states.

Aging↗

PubMedDate not supplied

K+ channel inhibition accelerates intestinal epithelial cell wound healing.

Restitution is the process by which superficial interruptions in the gastrointestinal mucosa are repaired by the flattening and spreading of epithelial cells surrounding the damage. During this process, mucosal epithelial cells undergo extensive reshaping and cytoskeletal remodeling. K(+) channels, located primarily on the basolateral surface of gut epithelial cells, are central to both actin polymerization, via their control of membrane potential, and cell volume regulation. We questioned whether K(+) channels are involved in restitution using an in vitro model of intestinal epithelium, monolayers of the human colon carcinoma cell line T84. We report that pharmacologic K(+) channel inhibition accelerates wound healing in T84 cell monolayers. Both Ca(++)-dependent and constitutively active channels are involved, as indicated by the sensitivity to clotrimazole, charybdotoxin, and barium. The ability of clotrimazole to accelerate wound resealing was also observed in Caco-2 cell sheets. Pharmacologic stimulation of K(+) channel activity had no effect on the repair rate. Analysis of the resealing process by time lapse and confocal microscopy revealed that K(+) channel inhibitors abolished the initial wound retraction, briefly accelerated the repair rate, and altered the shape of the cell sheet abutting the injury during the early phase of resealing. We hypothesize that K(+) channel inactivation interrupts the coregulation of f-actin polymerization and volume control that is initiated by the healing process.

Barium↗

PubMedDate not supplied

Acceleration of wound healing in healing-impaired db/db mice with a photocrosslinkable chitosan hydrogel containing fibroblast growth factor-2.

Application of ultraviolet light irradiation to a photocrosslinkable chitosan (Az-CH-LA) aqueous solution including fibroblast growth factor-2 (FGF-2) results within 30 seconds in an insoluble, flexible hydrogel. The FGF-2 molecules retained in the chitosan hydrogel remain biologically active and are released from the chitosan hydrogel upon in vivo biodegradation of the hydrogel. To evaluate the accelerating effect on wound healing of this hydrogel, full-thickness skin incisions were made in the backs of healing-impaired diabetic (db/db) mice and their normal (db/+) littermates. The mice were later killed, and histological sections of the wound were prepared. The degree of wound healing was evaluated using several histological parameters such as the rate of contraction, epithelialization, and tissue filling. Application of the chitosan hydrogel significantly advanced the rate of contraction on Days 0 to 2 in db/db and db/+ mice. Although the addition of FGF-2 into the chitosan hydrogel in db/+ mice had little effect, application of the chitosan hydrogel-containing FGF-2 further accelerated the adjusted tissue filling rate (Days 2 to 4 and Days 4 to 8) in db/db mice. Furthermore, the chitosan hydrogel-containing FGF-2 markedly increased the number of CD-34-positive vessels in the wound areas of db/db mice on Day 4. Thus, the application of chitosan hydrogel-containing FGF-2 onto a healing-impaired wound induces significant wound contraction and accelerates wound closure and healing.

Administration, Topical↗