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At least 19 recordsLinked to original sources
PubMed2002

Measuring and optimizing the momentum aperture in a particle accelerator.

Particle motion in storage rings is confined by various aperture limits, the size of which restricts the performance of the ring in terms of injection efficiency, lifetime, etc. Intrabeam scattering makes particles sweep a large portion of the phase space, where their motion may eventually be resonantly or chaotically excited to large amplitudes leading to collision with the vacuum chamber. We report here the studies performed at the Advanced Light Source (ALS) on the on- and off-momentum particle motion that provides a good understanding of these limitations. Using off-momentum simulations and experiments together with frequency map analysis, we could precisely correlate beam loss areas with resonance locations. The very good agreement between simulations and experiments allowed us to provide guidance for avoiding these dangerous areas. This analysis results in predictive improvements of the momentum aperture, which actually led to a lifetime increase of 25% at the ALS for very high bunch charge.

Journal Article↗
PubMed2004

Particle acceleration in relativistic current sheets.

Relativistic current sheets have been proposed as the sites of dissipation in pulsar winds, jets in active galaxies, and other Poynting flux dominated flows. It is shown that the steady versions of these structures differ from their nonrelativistic counterparts because they do not permit transformation to a de Hofmann-Teller frame with zero electric field. Instead, their generic form is that of a true neutral sheet with no linking magnetic field component normal to the sheet. The maximum energy to which such structures can accelerate particles is derived, and used to compute the maximum frequency of the subsequent synchrotron radiation. This can be substantially in excess of standard estimates. In the magnetically driven gamma-ray burst scenario, acceleration of electrons is possible to energies sufficient to enable photon-photon pair production after an inverse Compton scattering event.

Journal Article↗
PubMed1998

Rolf Wideröe and the development of particle accelerators.

The development of particle accelerators is traced from the simple but ingenious table-top devices conceived during the late 1920s to the present day's large, complex machines which extend over tens of kilometers. The emphasis is on Rolf Wideröe and his seminal contributions to the field. Not only did Wideröe construct the first accelerator which accelerated charged particles to an energy higher than the maximum voltage difference is the accelerator proper, he also invented particle colliders, today's work horse in experimental particle physics.

History, 20th Century↗
PubMed2004

Cavitation inception on microparticles: a self-propelled particle accelerator.

Corrugated, hydrophilic particles with diameters between 30 and 150 microm are found to cause cavitation inception at their surfaces when they are exposed to a short, intensive tensile stress wave. The growing cavity accelerates the particle into translatory motion until the tensile stress decreases, and subsequently the particle separates from the cavity. The cavity growth and particle detachment are modeled by considering the momentum of the particle and the displaced liquid. The analysis suggests that all particles which cause cavitation are accelerated into translatory motion, and separate from the cavities they themselves nucleate. Thus, in the research of cavitation nuclei the link is established between developed cavitation bubbles and their origin.

Journal Article↗
PubMed2005

Accelerated particle electrophoretic motion and separation in converging-diverging microchannels.

Accelerated particle electrophoretic motions were visualized in converging-diverging microchannels on poly(dimethylsiloxane) chips. The accelerated particle electrophoretic separation is highly desirable in on-chip flow cytometry and high-speed electrophoresis. The effects of electric field, particle size, particle trajectory, and channel structure on the particle electrophoretic motion are examined. We find that the ratio of the particle velocity in the throat to that in the straight channel is significantly lower than their cross-sectional area ratio. This discrepancy may be attributed to the locally higher electric field around the two poles of a particle, as compared to other regions inside the microchannel. We also find that the particle velocity ratio is increased for smaller particles moving through symmetric converging-diverging channels under lower electric fields. These variations may be attributed to the negative dielectrophoretic force that is generated by the nonuniform electric field in the converging-diverging section. In addition, we find that particle trajectory has insignificant influences on the maximum velocity ratio obtained in the throat.

Journal Article↗
PubMed1989

[Dependence of the incidence of the occurrence of ovarian and uterine tumors in rats on the irradiation dosage from accelerated particles and gamma rays].

The incidence rate of ovarian and uterine tumours in rats after irradiation by protons with the energy of 645 MeV, 9 GeV and He2+ has been studied. The tumour rate is shown to increase with the radiation dose. The character of the dose-response for tumours of two sites is different. A relative risk of ovarian tumour incidence after He2+ exposure is twice as high as after other types of radiation.

Animals↗
PubMed2001

Fluid particle accelerations in fully developed turbulence.

The motion of fluid particles as they are pushed along erratic trajectories by fluctuating pressure gradients is fundamental to transport and mixing in turbulence. It is essential in cloud formation and atmospheric transport, processes in stirred chemical reactors and combustion systems, and in the industrial production of nanoparticles. The concept of particle trajectories has been used successfully to describe mixing and transport in turbulence, but issues of fundamental importance remain unresolved. One such issue is the Heisenberg-Yaglom prediction of fluid particle accelerations, based on the 1941 scaling theory of Kolmogorov. Here we report acceleration measurements using a detector adapted from high-energy physics to track particles in a laboratory water flow at Reynolds numbers up to 63,000. We find that, within experimental errors, Kolmogorov scaling of the acceleration variance is attained at high Reynolds numbers. Our data indicate that the acceleration is an extremely intermittent variable--particles are observed with accelerations of up to 1,500 times the acceleration of gravity (equivalent to 40 times the root mean square acceleration). We find that the acceleration data reflect the anisotropy of the large-scale flow at all Reynolds numbers studied.

Journal Article↗
PubMed2002

A neurophysiological approach to effects induced by accelerated particles on the central nervous system in mice.

This experiment was designed to study mechanisms of "phosphene vision". A beam of accelerated carbon particles (energy 100 and 300 MeV/n) was delivered into retina and sensorimotor cortex of anaesthetised mice bearing chronic electrodes. Cycles of light flashes were also administered. Following accelerated particles stimulation cycles to the retina, 500 to 5000 per stimulus, 30 stimuli per cycle, stimulus time-related brain waveforms in mice cortex were recorded. Their amplitude and latency appeared to be related to the energy delivered. The short waveforms latency (approximately 30 ms), and the lack of retinic potentials and oscillatory potentials following particles stimuli suggest that the impact of accelerated particles does not produce activation of retinic chemical structures.

Animals↗
PubMed1996

On residual dose rate within particle accelerator enclosures.

A mathematical proof of a theorem concerning the flux density of particles at any point interior to a simple closed convex surface in which the surface emits such particles uniformly and isotropically is presented. The relationship of this theorem, which is of rather general validity, to related observations of other workers is discussed. The ramifications of the theorem are considered with particular attention devoted to conditions commonly found at particle accelerators. It is concluded that results of this theorem are directly applicable to efforts to apply the principle of maintaining dose equivalents received by personnel as low as reasonably achievable.

Mathematics↗
PubMed2005

Energy spectrum of particles accelerated in relativistic collisionless shocks.

We analytically study diffusive particle acceleration in relativistic, collisionless shocks. We find a simple relation between the spectral index s and the anisotropy of the momentum distribution along the shock front. Based on this relation, we obtain s=(3beta(u)-2beta(u)beta(2)(d)+beta(3)(d))/(beta(u)-beta(d)) for isotropic diffusion, where beta(u) (beta(d)) is the upstream (downstream) fluid velocity normalized to the speed of light. This result is in agreement with previous numerical determinations of s for all (beta(u),beta(d)), and yields s=38/9 in the ultrarelativistic limit. The spectrum-anisotropy connection is useful for testing numerical studies and constraining anisotropic diffusion results. It suggests that the spectrum is highly sensitive to the form of the diffusion function for particles traveling along the shock front.

Journal Article↗
PubMed1977

On a simple method for routine check of the constancy of radiation quality of bremsstrahlung emitted by therapeutic particle accelerators.

The constancy of radiation quality of therapeutically employed particle accelerators has to be checked at weekly intervals. Any change in radiation quality may have considerable therapeutic effects owing to its influence on dose distribution. It can be recommended to make measurements instead of, or in addition to, the axial reference-point measurement at 5 and 15 cm depth in the phantom, at 5 cm depth in the beam axis and at a reference-point about 1 cm within the geometric edge of the field, for checking the constancy of the radiation quality of bremsstrahlung. Only then, if routine checks carried out for the axial and the lateral reference-point dose ratios do not show any deviations greater than e.g. +/- 2%, radiation quality is deemed to have remained sufficiently constant for radiotherapeutical applications.

Radiotherapy, High-Energy↗
PubMed2000

Particle acceleration and curvature TeV emission by rotating, supermassive black holes

It is shown that particles accelerating near the event horizon of a spinning supermassive black hole that is threaded by externally supported magnetic field lines suffer severe curvature losses that limit the maximum energy they can attain to values well below that imposed by the maximum voltage drop induced by the black hole dynamo. It is further shown that the dominant fraction of the rotational energy extracted from the black hole is radiated in the TeV band. The implications for vacuum breakdown and the observational consequences are discussed.

Journal Article↗
PubMed2006

Experimental observation of direct particle acceleration by stimulated emission of radiation.

We report the first experimental evidence for direct particle acceleration by stimulated emission of radiation. In the framework of this proof-of-principle experiment, a 45 MeV electron macrobunch was modulated by a high-power CO2 laser and then injected into an excited CO2 gas mixture. The emerging microbunches experienced a 0.15% relative change in the kinetic energy, in a less than 40 cm long interaction region. According to our experimental results, a fraction of these electrons have gained more than 200 keV each, implying that such an electron has undergone an order of magnitude of 2 x 10(6) collisions of the second kind.

Journal Article↗
PubMed2006

Particle acceleration by stimulated emission of radiation: theory and experiment.

The interaction of electromagnetic radiation with free electrons in the presence of an active medium has some appealing outcomes. Among them is particle acceleration by stimulated emission of radiation (PASER). In its framework, energy stored in an active medium (microscopic cavities) is transferred directly to an e -beam passing through. We have developed a two-dimensional analytic model for the evaluation of the energy exchange occurring as a train of electron microbunches traverses a dilute resonant medium. Efficient interaction occurs at resonance-namely, when the frequency of the train matches the resonance frequency of the medium. It is shown that the energy exchange is gamma independent for relativistic energies and it drops dramatically with an increase of the beam's radius. Based on this model, we have evaluated the relative change in the kinetic energy of a 0.1-nC 45-MeV macrobunch traversing an excited CO2 gas mixture-the former being modulated at the CO2 laser wavelength. Good agreement is found between the theoretical predictions and the results of the PASER experiment performed recently at Brookhaven National Laboratory.

Journal Article↗
PubMed2004

High-energy particle acceleration in the shell of a supernova remnant.

A significant fraction of the energy density of the interstellar medium is in the form of high-energy charged particles (cosmic rays). The origin of these particles remains uncertain. Although it is generally accepted that the only sources capable of supplying the energy required to accelerate the bulk of Galactic cosmic rays are supernova explosions, and even though the mechanism of particle acceleration in expanding supernova remnant (SNR) shocks is thought to be well understood theoretically, unequivocal evidence for the production of high-energy particles in supernova shells has proven remarkably hard to find. Here we report on observations of the SNR RX J1713.7 - 3946 (G347.3 - 0.5), which was discovered by ROSAT in the X-ray spectrum and later claimed as a source of high-energy gamma-rays of TeV energies (1 TeV = 10(12) eV). We present a TeV gamma-ray image of the SNR: the spatially resolved remnant has a shell morphology similar to that seen in X-rays, which demonstrates that very-high-energy particles are accelerated there. The energy spectrum indicates efficient acceleration of charged particles to energies beyond 100 TeV, consistent with current ideas of particle acceleration in young SNR shocks.

Journal Article↗
PubMed2003

[Comparison of risk of mammary tumors developing in rats exposed to high-energy accelerated particles and gamma-ray radiation].

The frequency of tumorigenesis was investigated in the mammary glands of noninbred female rats and Wistar rats, after total-body exposure to a single dose of 60CO gamma radiation, 645 MeV and 9 GeV protons and 4 GeV/nuclon ions of helium. It was found that the relative biological effects of accelerated charged particles was significantly higher. This finding was obtained from calculations of the frequency of mammary tumors.

Animals↗
PubMed2005

Randomly accelerated particle in a box: mean absorption time for partially absorbing and inelastic boundaries.

Consider a particle which is randomly accelerated by Gaussian white noise on the line segment 0<x<1 and is absorbed as soon as it reaches x=0 or x=1. The mean absorption time T(x,v), where x and v denote the initial position and velocity, was calculated exactly by Masoliver and Porrà in 1995. We consider a more general boundary condition. On arriving at either boundary, the particle is absorbed with probability 1-p and reflected with probability p. The reflections are inelastic, with coefficient of restitution, r. With exact analytical and numerical methods and simulations, we study the mean absorption time as a function of p and r.

Journal Article↗