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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Positrons from accelerated particle interactions

Positron production from the decay of radioactive nuclei produced in nuclear interactions of accelerated particles is treated in detail. Laboratory data as well as theoretical considerations are used to construct energy-dependent cross sections for the production of a large number of radioactive positron emitters resulting from proton and alpha-particle interactions with ambient cosmic matter. Using these cross sections, positron production rates are calculated for a variety of energetic particle spectra, assuming solar abundances for both the energetic particles and the ambient medium. These results can be used for the study of astrophysical sites which emit annihilation radiation. In particular, the results have been applied to solar flares, where the observed 0.511 MeV line is shown to be due to positrons resulting from accelerated particle reactions.

Kozlovsky, B.↗
NASA NTRS1992

High-energy particle acceleration by explosive electromagnetic interaction in an accretion disk

By examining electromagnetic field evolution occurring in an accretion disk around a compact object, we arrive at an explosive mechanism of particle acceleration. Flux-freezing in the differentially rotating disk causes the seed and/or generated magnetic field to wrap up tightly, becoming highly sheared and locally predominantly azimuthal in orientation. We show how asymptotically nonlinear solutions for the electromagnetic fields may arise in isolated plasma blobs as a result of the driving of the fluid equations by the accretion flow. These fields are capable of rapidly accelerating charged particles from the disk. Acceleration through the present mechanism from AGN can give rise to energies beyond 10 exp 20 eV. Such a mechanism may present an explanation for the extragalactic origin of the most energetic observed cosmic rays.

Haswell, C. A.↗
NASA NTRS1979

Particle acceleration by propagating interplanetary shocks

The process of charged particle acceleration in interplanetary shocks has been simulated for typical parameters. Since space probe observations of charged particle fluxes are the principle means of inferring the action of an acceleration mechanism, the simulation was designed to follow particles backwards in time from a given observing point, usually 1 AU. This allowed a full diagnosis of which observed particles had interacted with an oncoming shock, where the interacting occurred, and by how much the energy had been changed by the shock interaction. Simple assumptions about the preshock energy spectrum allow the construction of a full temporal profile of the expected intensity, anisotropy, and energy spectrum. The simulations are apparently capable of reproducing the main features of the less than 10 MeV/nuc ion flux enhancements observed at interplanetary shocks using only a laminar interplanetary magnetic field, Archimidean spiral, and a spherical oblique shock with typical speed, strength, and shock normal-magnetic field angle.

Armstrong, T. P.↗
NASA NTRS1994

Particle acceleration in the dynamic magnetotail: Orbits in self-consistent three-dimensional MHD fields

The acceleration of protons in a dynamically evolving magnetotail is investigated by tracing particles in the fields obtained from a three-dimensional resistive magnetohydrodynamic (MHD) simulation. The MHD simulation, representing plasmoid formation and ejection through a near-Earth reconnection process, leads to cross-tail electric fields of up to approximately 4 mV/m with integrated voltages across the tail of up to approximately 200 kV. Energization of particles takes place over a wide range along the tail, due to the large spatial extent of the increased electric field together with the finite cross-tail extent of the electric field region. Such accelerated particles appear earthward of the neutral line over a significant portion of the closed field line region inside of the separatrix, not just in the vicinity of the separatrix. Two different acceleration processes are identified: a 'quasi-potential' acceleration, due to particle motion in the direction of the cross-tail electric field, and a 'quasi-betatron' effect, which consists of multiple energy gains from repeated crossings of the acceleration region, mostly on Speiser-type orbits, in the spatially varying induced electric field. The major source region for accelerated particles in the hundreds of keV range is the central plasma sheet at the dawn flank outside the reconnection site. Since this source plasma is already hot and dense, its moderate energization by a factor of approximately 2 may be sufficient to explain the observed increases in the energetic particle fluxes. Particles from the tail are the source of beams at the plasma sheet/lobe boundary. The temporal increase in the energetic particle fluxes, estimated from the increase in energy gain, occurs on a fast timescale of a few minutes, coincident with a strong increase in B(sub z), despite the fact that the inner boundary ('injection boundary') of the distribution of energized particles is fairly smooth.

Birn, Joachim↗
DOE OSTI2023

Collaborative Research: Effects of the magnetic field shear and flow shear on the kinetic physics and particle acceleration in relativistic magnetic reconnection (Final Report)

Magnetic reconnection is a fundamental plasma process that allows rapid changes of magnetic field topology and the conversion of magnetic energy into plasma kinetic energy. There has been a strong surge of interest in relativistic reconnection over the past few years in plasma astrophysics, but the rich physics of collisionless reconnection and its associated particle acceleration in the relativistic regime remain less studied compared to the non-relativistic counterparts. Magnetic reconnection in a realistic system is often accompanied with the magnetic field shear and shear flows. This setting is more general and likely the more common situation where most reconnection takes place. Within the past few decades, researchers had made significant progress in understanding how magnetic reconnection works under these conditions in the non-relativistic regime. However, how the effect of special relativity, among other effects, influences the dynamics of reconnection in the strongly magnetized astrophysical plasmas under these conditions is largely unknown. Our proposed research is to use fully kinetic simulations to understand kinetic physics and particle acceleration in relativistic magnetic reconnection in more realistic configurations and physical conditions. The primary goal of this project is to identify the fundamental effects caused by special relativity under the presence of field shear and flow shear.

43 PARTICLE ACCELERATORS↗
DOE OSTI2026

PIP-II Particle Accelerator Project

The PIP-II project is an essential upgrade of Fermilab’s particle accelerator complex. Groundbreaking occurred in March 2019. The upgrade will enable Fermilab’s accelerators to generate an unprecedented stream of neutrinos—subtle, subatomic particles that could hold the key to understanding the universe’s evolution—by creating the world’s most intense high-energy neutrino beams. This capability positions Fermilab to be the world leader in accelerator-based neutrino research. It enables the scientific program for the international, Fermilab-hosted Deep Underground Neutrino Experiment (DUNE) and Long-Baseline Neutrino Facility (LBNF). The key to PIP-II is power. When the PIP-II project is complete, Fermilab will be able to generate proton beams greater than 1 megawatt—60 percent higher than current capabilities. These powerful beams of protons will in turn create intense beams of neutrinos. Future PIP-II upgrades will triple the lab’s current beam power. PIP-II’s high-intensity proton beams will provide a flexible platform for the long-term future of the Fermilab accelerator complex and the U.S. accelerator-based particle physics program.

Fermilab, Fermilab↗
NASA NTRS2006

3-D RPIC Simulations of Relativistic Jets: Particle Acceleration, Magnetic Field Generation, and Emission

Recent PIC simulations using injected relativistic electron-ion (electro-positron) jets into ambient plasmas show that acceleration occurs in relativistic shocks. The Weibel instability created in shocks is responsible for particle acceleration, and generation and amplification of highly inhomogeneous, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection in relativistic jets. The "jitter" radiation from deflected electrons has different properties than the synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understand the complex time evolution and spectral structure in relativistic jets and gamma-ray bursts. We will present recent PIC simulations which show particle acceleration and magnetic field generation. We will also calculate associated self-consistent emission from relativistic shocks.

Nishikawa, K.-I.↗
NASA NTRS2011

Radiation from Accelerated Particles in Shocks and Reconnections

Plasma instabilities are responsible not only for the onset and mediation of collisionless shocks but also for the associated acceleration of particles. We have investigated particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic-like shock structure. In the leading shock, electron density increases by a factor of about 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. These magnetic fields contribute to the electrons transverse deflection and, more generally, relativistic acceleration behind the shock. We have calculated, self-consistently, the radiation from electrons accelerated in the turbulent magnetic fields. We found that the synthetic spectra depend on the Lorentz factor of the jet, its thermal temperature and strength of the generated magnetic fields. We are currently investigating the specific case of a jet colliding with an anti-parallel magnetized ambient medium. The properties of the radiation may be important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants.

Nishikawa, K.-I.↗
NASA NTRS2014

Synthesis of 3-D Coronal-Solar Wind Energetic Particle Acceleration Modules

Acute space radiation hazards pose one of the most serious risks to future human and robotic exploration. Large solar energetic particle (SEP) events are dangerous to astronauts and equipment. The ability to predict when and where large SEPs will occur is necessary in order to mitigate their hazards. The Coronal-Solar Wind Energetic Particle Acceleration (C-SWEPA) modeling effort in the NASANSF Space Weather Modeling Collaborative [Schunk, 2014] combines two successful Living With a Star (LWS) (http:lws.gsfc.nasa.gov) strategic capabilities: the Earth-Moon-Mars Radiation Environment Modules (EMMREM)[Schwadron et al., 2010] that describe energetic particles and their effects, with the Next Generation Model forthe Corona and Solar Wind developed by the Predictive Science, Inc. (PSI) group. The goal of the C-WEPA effort is to develop a coupled model that describes the conditions of the corona, solar wind, coronal mass ejections (CMEs) and associated shocks, particle acceleration, and propagation via physics-based modules. Assessing the threat of SEPs is a difficult problem. The largest SEPs typically arise in conjunction with X classflares and very fast (1000 kms) CMEs. These events are usually associated with complex sunspot groups(also known as active regions) that harbor strong, stressed magnetic fields. Highly energetic protonsgenerated in these events travel near the speed of light and can arrive at Earth minutes after the eruptiveevent. The generation of these particles is, in turn, believed to be primarily associated with the shock waveformed very low in the corona by the passage of the CME (injection of particles fromthe flare sitemay also playa role). Whether these particles actually reach Earth (or any other point) depends on their transport in theinterplanetary magnetic field and their magnetic connection to the shock.

exploration↗
NASA NTRS1988

Space Experiments with Particle Accelerators (SEPAC)

The purpose of Space Experiments with Particle Accelerators (SEPAC) on the Atmospheric Laboratory for Applications and Science (ATLAS 1) mission, is to carry out active and interactive experiments on and in the earth's ionosphere, atmosphere, and magnetosphere. The instruments to be used are an electron beam accelerator (EBA), plasma contactor, and associated instruments the purpose of which is to perform diagnostic, monitoring, and general data taking functions. Four major classes of investigations are to be performed by SEPAC. They are: beam plasma physics, beam-atmosphere interactions, the use of modulated electron beams as transmitting antennas, and the use of electron beams for remote sensing of electric and magnetic fields. The first class consists mainly of onboard plasma physics experiments to measure the effects of phenomena in the vicinity of the shuttle. The last three are concerned with remote effects and are supported by other ATLAS 1 investigations as well as by ground-based observations.

Obayashi, Tatsuzo↗
NASA NTRS1988

Particle acceleration and wave emissions associated with the formation of auroral cavities and enhancements

A unified model is presented which interrelates the various processes involved in auroral particle acceleration and the associated wave emissions, and it is shown how energy is coupled from the magnetosphere into the ionosphere. The phenomena in the magnetotail which can provide the source of free energy for the perpendicular electrostatic shocks are identified along with the associated potential structures in the nightside auroral region. The characteristics of the particle acceleration produced by the shock are described, including efficiency and energy transport between magnetospheric and ionospheric plasmas, the properties of the particle distributions throughout the shock region and return current regions, and the associated wave emissions. The source of free energy for the generation of ion conics is identified, as are the signatures for the formation of plasma cavities and/or enhancements.

Winglee, R. M.↗
DOE OSTI2026

Multiscale and multidimensional modeling of particle acceleration and transport in solar flares

Multi-messenger, multi-viewpoint, and time-resolved observations of solar flares are now providing unprecedented constraints on particle acceleration sites, energy conversion, and energy transport. The interpretation of current observations, including microwave imaging spectroscopy from EOVSA, hard x-ray (HXR) imaging from Solar Orbiter/STIX, gamma-ray diagnostics from Fermi, and in situ measurements from Parker Solar Probe and Solar Orbiter, collectively demands modeling frameworks that go beyond traditional spatially unresolved, one-zone models or single-mechanism descriptions. This review surveys multiscale and multidimensional modeling approaches, including kinetic, magnetohydrodynamic (MHD), and macroscopic particle models, that are being developed to meet the need. Kinetic simulations reveal that three-dimensional (3D) effects, including field-line chaos and self-generated turbulence, are essential for sustained power-law particle acceleration. MHD simulations now capture flux-rope eruptions, plasmoid-unstable current sheets, and turbulent flare regions in realistic magnetic topologies. Macroscopic models coupling MHD with energetic-particle models produce spatially resolved electron distributions and synthetic HXR and microwave emissions for direct comparison with observations. Despite these advances, outstanding challenges remain in bridging kinetic and global scales, improving MHD simulations and macroscopic particle models, and achieving quantitative model-observation closure.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Kinetic study of shock formation and particle acceleration in laser-driven quasi-parallel magnetized collisionless shocks

Quasi-parallel magnetized collisionless shocks are believed to be one of the most efficient accelerators in the universe. Compared to quasi-perpendicular shocks, quasi-parallel shocks are more difficult to form in the laboratory and to simulate because of their large spatial scales and long formation times. Our two-dimensional particle-in-cell simulations show that the early stages of quasi-parallel shock formation are achievable in experiments planned for the National Ignition Facility and that particles accelerated by diffusive shock acceleration (DSA) are expected to be observable in the experiment. Repetitive ion acceleration by crossings of the shock front, a key feature of DSA, is seen in the simulations. Other characteristic features of quasi-parallel shocks such as upstream wave excitation by energetic ions are also observed, and energy partition between the ions and the electrons in the downstream of the shock is briefly discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1988

Test particle acceleration in turbulent reconnecting magnetic fields

The effect of turbulence on particle acceleration in a MHD field was investigated by computing test particle trajectories in turbulent MHD reconnecting fields, including reconnection simulations at different magnetic Reynolds numbers. The dynamics of individual particles were investigated making it possible to examine the acceleration mechanism in great detail. It was found that turbulence influences the acceleration in two ways. It enhances the reconnection electric field while producing a stochastic electric field that gives rise to momentum diffusion; and it produces magnetic 'bubbles' and other irregularities that can temporarily trap test particles in the strong reconnection electric field for times comparable to the magnetofluid characteristic time.

Ambrosiano, John↗
NASA NTRS1993

Simulations of particle acceleration in parallel shocks: Direct comparison between Monte Carlo and one-dimensional hybrid codes

We have made a direct comparison between two different computer simulations of a plane, parallel, collisionless shock including particle acceleration to energies typical of those of diffuse ions observed at the earth bow shock. Despite the fact that the one-dimensional hybrid and Monte Carlo techniques employ entirely different algorithms, they give surprisingly close agreement in the overall shapes of the complete distribution functions for protons as well as heavier ions. Both methods show that energetic ions emerge smoothly from the background thermal plasma with approximately the same relative injection rate and that the fraction of the incoming plasma's energy flux that is converted into downstream enthalpy flux of the accelerated population (i.e., the acceleration efficiency) is similar in the two cases. The fraction of the downstream proton distribution made up of superthermal particles is quite large, with at least 10% of the energy flux going into protons with energies above 10 keV. In addition, an upstream precursor, produced by backstreaming energetic particles, is present in both shocks, although the Monte Carlo precursor is considerably longer than that produced in the hybrid shock. These results offer convincing evidence that, at least in these ways, the two simulations are consistent in their description of parallel shock structure and particle acceleration, and they lay the groundwork for development of shock models employing a combination of both methods.

Ellison, Donald C.↗
NASA NTRS1988

Computer modeling of test particle acceleration at oblique shocks

The present evaluation of the basic techniques and illustrative results of charged particle-modeling numerical codes suitable for particle acceleration at oblique, fast-mode collisionless shocks emphasizes the treatment of ions as test particles, calculating particle dynamics through numerical integration along exact phase-space orbits. Attention is given to the acceleration of particles at planar, infinitessimally thin shocks, as well as to plasma simulations in which low-energy ions are injected and accelerated at quasi-perpendicular shocks with internal structure.

Decker, Robert B.↗
DOE OSTI2026

Plasma flow generation and particle acceleration from expanding magnetic bubbles

Impulsive plasma dynamics in the laboratory are often driven by rising electric currents, yet their quantitative plasma response has not been well established. By means of fully kinetic particle-in-cell simulations and laser-driven capacitor-coil experiments, we show that a rising current expels plasma, forming an expanding magnetic bubble and accelerating particles. The expansion front velocity scales with the Alfvén speed determined by the magnetic field at its inner edge and the plasma density at its outer edge. This mechanism establishes impulsive current drive as a fundamental way that generates plasma flows and accelerates particles in laboratory plasmas, with potential relevance to astrophysics.

Zhang, Yang [Princeton University, NJ (United Stat↗