Resonant Acceleration Research Articles

Electron acceleration by kinetic Alfvén waves

As Alfvén waves with finite extent perpendicular to the magnetic field propagate from the magnetosphere to the ionosphere, there is a region of parallel electric field in the “wave front” of the propagating wave. For short perpendicular wavelengths this parallel electric field can be large enough to accelerate electrons to auroral energies. This problem is solved for the case of uniform plasma density and background magnetic field. The parallel electric field solution is then applied to a background Maxwellian plasma to study the effects of the acceleration due to this field on the electron distribution function. Two effects are found: (1) the relatively modest acceleration of the bulk of the background electrons and (2) Fermi‐like resonant acceleration of a small component of the electrons up to velocities of the order of twice the Alfvén speed. Although both effects always occur, the response of the background electrons is a sensitive function of the magnitude, wavelength, and timescale associated with the driving perpendicular electric field. In particular, the latter effect does not produce a significant signature for all conditions. However, for reasonable values of perpendicular electric field magnitude and scale size, and plasma parameters appropriate for auroral field lines at altitudes around 7000 km near where the Alfvén speed peaks, the effect can be significant.

Solar particle acceleration by slow magnetosonic waves

Particle acceleration by slow magnetosonic waves (SMW) has been systematically disregarded. We examine the real possibility of this mode for acceleration of solar electrons on basis to a quantitative analysis. Evaluations of the times scales involved in the phenomenon allow us to determine the conditions in the solar atmosphere where resonant acceleration by the slow MHD mode may occur.

Nonlinear evolution of a strongly sheared cross-field plasma flow

A study is presented of the nonlinear evolution of a magnetized plasma in which a localized electron cross-field flow is present. The peak velocity of the flow is denoted by V0; LE represents the flow’s shear scale length; and the regime ρe<LE<ρi is considered, where ρi and ρe denote the ion and electron Larmor radii, respectively. It is shown that if the shear frequency ωs=V0/LE is larger than the lower-hybrid frequency, ωLH, then the system dynamics is dominated by the onset of the electron–ion-hybrid (EIH) mode which leads to the formation of coherent (vortexlike) structures in the electrostatic potential of the ensuing lower-hybrid waves. The wavelength of these structures is on the order of LE, and correlates well with that predicted by the linear theory of the EIH mode. Since the characteristic wavelength is longer than ρe, the corresponding phase velocity is low enough that there results significant direct resonant ion acceleration perpendicular to the confining magnetic field. When ωs≳3ωLH, the system exhibits significant anomalous viscosity (typically an order of magnitude larger than that due to Coulomb collisions), which increases as the shear frequency is increased. As ωs is reduced below ωLH, shear effects are no longer dominant and a smooth transition takes place in which the system dynamics is governed by the short wavelength (on the order of ρe) lower-hybrid drift instability.

Lower hybrid resonance acceleration of electrons and ions in solar flares and the associated microwave emission

view Abstract Citations (52) References (67) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Lower Hybrid Resonance Acceleration of Electrons and Ions in Solar Flares and the Associated Microwave Emission McClements, K. G. ; Bingham, R. ; Su, J. J. ; Dawson, J. M. ; Spicer, D. S. Abstract The particle acceleration processes here studied are driven by the relaxation of unstable ion ring distributions; these produce strong wave activity at the lower hybrid resonance frequency which collapses, and forms energetic electron and ion tails. The results obtained are applied to the problem posed by the production of energetic particles by solar flares. The numerical simulation results thus obtained by a 2 1/2-dimensional particle-in-cell code show a simultaneous acceleration of electrons to 10-500 keV energies, and of ions to as much as the 1 MeV range; the energy of the latter is still insufficient to account for gamma-ray emission in the 4-6 MeV range, but furnish a seed population for further acceleration. Publication: The Astrophysical Journal Pub Date: May 1993 DOI: 10.1086/172679 Bibcode: 1993ApJ...409..465M Keywords: Electron Acceleration; Ion Temperature; Microwave Emission; Resonant Frequencies; Solar Electrons; Solar Flares; Energy Transfer; Gamma Rays; Solar Atmosphere; Solar X-Rays; Solar Physics; ACCELERATION OF PARTICLES; SUN: FLARES; SUN: X-RAYS; GAMMA RAYS; SUN: PARTICLE EMISSION; SUN: RADIO RADIATION full text sources ADS |

Relativistic detuning limit on energy in electron cyclotron resonance x-ray source

In this paper, a detailed theoretical analysis of electron acceleration in a TE111 cylindrical resonant cavity used as a cyclotron resonance accelerator is presented. The relativistic detuning limit on energy is found to depend critically on the externally applied dc magnetic field. The optimum value of the magnetic field, for a given operating frequency, electric field, and the cavity dimensions, is calculated. The cavity dimensions are optimized to get maximum peak-to-peak electric field so that higher electron energy can be obtained. It is also shown that most of the electrons, depending on the time of emission, end up in two accelerating bunches.

Structure optimization of linear resonance accelerators by sensitivity functions

An algorithm is proposed for optimizing linear accelerator structure parameters using sensitivity functions and practical stability methods. This approach takes into account the actual perturbations of particle trajectories as a function of the computational parameters, analyzes the distribution of system parameter tolerances, and performs optimization with allowance for these factors. Numerical results for specific optimization problems of beam dynamics at the outlet are reported.

Subject index

This chapter discusses the neutron sources. All neutron sources are based on nuclear reactions that free neutrons bound in nuclei. These are categorized according to the mechanisms by which this is done: fission chain reactions, fusion reactions, e--bremsstrahlung-induced photoneutron and photofission reactions, charged-particle nuclear reactions, and spallation. The last three together termed as accelerator-based reactions. Muon-catalyzed fusion and the neutron focus represent other categories of neutron-producing reactions. Electron-bremsstrahlung sources produce evaporation neutrons with an average energy of a few mega-electron-volts, whether due to photofission or photonuclear reaction. Since the neutron production mechanisms involve first producing bremsstrahlung photons, which then interact to produce neutrons, these sources produce very large gamma-ray fluxes as well, which sometimes represent a troublesome background. Neutron shielding is like that for a reactor. Spallation sources also produce predominantly evaporation neutrons, which boil off in the process of “cooling off” of highly excited nuclei. The types of accelerators that provide the highest intensities are resonant accelerators, which all have inherent pulse structure. In cyclotrons, synchrotrons, and storage rings, bunches of particles circulate at frequencies of a few megahertz.

Test particle motion in the cyclotron resonance regime

Test particles moving in the field of an electromagnetic wave propagating in a background magnetic field can gain significant energy when the wave parameters and particle energy are such that the cyclotron resonance condition is satisfied. Central to the acceleration process and long time scale periodic behavior is the coherent accumulation over many cyclotron orbits of a small change in energy during each orbit, a result of the circularly polarized component of the wave electric field. Also important is the small change in the relative wave phase during each orbit resulting from relativistic variations of the cyclotron frequency and wave-induced streaming along the background magnetic field. The physical mechanisms underlying cyclotron resonance acceleration are explored using a set of heuristic mapping equations (the PMAP) describing changes in the particle momentum and relative wave phase. More accurate (but less transparent) descriptions of the particle motion are pursued in the context of orbit-averaged Hamiltonian theory. A discrete set of mapping equations for the slowly varying canonical action and angle are derived (the QMAP) but are found to generate inaccurate solutions in certain regions of phase space when the resonance number l is such that {↓l}↓=1 and the particles are initially cold. These difficulties are avoided by constructing a continuous time orbit-averaged Hamiltonian and solving the resultant canonical equations of motion. Assuming the momentum is small relative to mc (where m is the particle mass and c is the speed of light), details of the distribution of particle trajectories in the action-angle phase space for {↓l}↓=1 and {↓l}↓=2 are presented and criteria for the existence of orbits oscillatory in angle are derived.

Subthreshold stochastic diffusion with application to selective acceleration of He-3 in solar flares

view Abstract Citations (22) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Subthreshold Stochastic Diffusion with Application to Selective Acceleration of 3He in Solar Flares Riyopoulos, Spilios Abstract A mechanism of resonant acceleration driven by waves below the threshold for global chaos is discussed. It affects only resonant ions and proceeds through a stochastic network in phase space (Arnold web) characterizing this particular type of wave-particle interaction. A theory for selective acceleration of He-3 and He-3-like ions through resonant subthreshold diffusion is proposed. It relies on the coincidence of two natural resonances, the two-ion (He-4, H) hybrid plasma resonance with the ion cyclotron resonance of He-3. This occurs over a wide range of plasma parameters for relative abundances He-4/H about equal to 0.25. It is argued that existing low-frequency fluctuations or high-frequency noise tends to amplify the selective diffusion of He-3. Publication: The Astrophysical Journal Pub Date: November 1991 DOI: 10.1086/170682 Bibcode: 1991ApJ...381..578R Keywords: Diffusion; Particle Acceleration; Solar Flares; Helium Plasma; Hydrogen Plasma; Plasma Resonance; Stochastic Processes; Wave-Particle Interactions; Solar Physics; DIFFUSION; PARTICLE ACCELERATION; SUN: FLARES full text sources ADS |

Resonant acceleration of alpha particles by ion cyclotron waves in the solar wind

Preferential acceleration of alpha particles interacting with left‐hand polarized ion cyclotron waves is studied. It is shown that a small positive drift velocity between alpha particles and protons can lead to alpha particle velocities well in excess of the proton bulk velocity. During the acceleration process, which is assumed to take place at heliocentric distances less than 0.3 AU, the alpha particle drift velocity should exceed the proton bulk velocity, and then the gap which exists around the alpha particle gyrofrequency should disappear. It is also shown that for proton thermal anisotropies of the order of those observed in fast solar wind, the waves either grow or are not damped excessively, so that the waves can exist and might thus lead to the observed differential speeds. However, the way in which the alpha particles exceed the proton velocity remains unexplained.

Ion acceleration in a current sheet with magnetic islands

This paper reports on results of an experimental study of proton acceleration processes in a neutral current sheet. An effect of resonance acceleration during the interaction of particles with a macroscopic potential jump in the sheet (V ρ × B - acceleration) was detected. An inhomogeneity of the acceleration at the O- and X-points was observed. Results of laboratory experiments are compared with measurements in the geomagnetic tail.

High‐resolution sounding rocket observations of large‐amplitude Alfvén waves

Shear Alfvén waves with amplitudes >100 mV/m were observed on two recent sounding rocket flights: one flown from Poker Flat, Alaska, to an altitude of 1040 km in the evening auroral zone, and another flown from Sondre Stromfjord, Greenland, to an altitude of 770 km in the morning cusp region. The largest waveforms are best described as a series of step functions, rather than as broadband noise or as single frequency waves. Complete two‐dimensional E and B measurements at 4‐ms time resolution were made, showing a downward propagation direction and implying insignificant reflection from the ionosphere at frequencies greater than 1 Hz. Intense, field‐aligned, low‐energy electron fluxes accompany the waves. Acceleration of these electrons by the Alfvén waves, either through the resonant acceleration mechanism described by Temerin et al. (1986), or through the nonresonant mechanism described by Goertz and Boswell (1979), is shown to be feasible. The waves in at least one case have a sufficiently large ponderomotive potential to generate the observed density fluctuations of order 1.

Ion acceleration in a quasi-neutral current sheet

This paper reports on results derived from a study of proton acceleration processes in a neutral sheet with a small transverse component of the magnetic field on a theta-pinch device. Effects of ion reflection, resonance acceleration during an interaction of particles with a macroscopic potential jump in the sheet ( V p × B—acceleration), preacceleration in the plasma-turbulence region, and particle fluxes along a neutral line were revealed. It is found that the acceleration is inhomogeneous at the O- and X-points and is anisotropic with respect to the direction of current in the sheet. Results of laboratory experiments are compared with measurements in the geomagnetic tail, and the usefulness of such a comparison is pointed out.

Particle Acceleration Processes in the Solar Corona

Theoretical ideas on particle acceleration associated with solar flares are reviewed. A historical outline is used to introduce the various acceleration mechanisms. These are stochastic acceleration in its various forms, diffusive acceleration at shock fronts, shock drift acceleration, resonant acceleration, acceleration during magnetic reconnection and acceleration by parallel electric fields in double layers or electrostatic shocks. Particular emphasis is placed on so-called first phase acceleration of electrons in solar flares, which is conventionally attributed to bulk energisation of electrons (Ramaty et al. 1980). There is no widely accepted theory for bulk energisation, which may be regarded as an enhanced form of heating. Ideas on bulk energisation are discussed critically. It is argued that the dissipation cannot be due to classical resistivity and involves anomalous resistivity or hyperresistivity, e.g., in multiple double layers. The dissipation must occur in very many localised regions. Bulk energisation due to magnetic reconnection is discussed briefly. A model for bulk energisation due to the continual formation and decay of weak double layers is outlined

Plasma Heating by a Magnetosonic Shock Wave through Resonant Ion Acceleration

Resonant wave-particle interactions in magnetosonic shock waves are studied by theory and simulation. The number of ions trapped by a perpendicular laminar shock in a finite beta plasma is evaluated, and the amount of shock heating due to resonant ion acceleration is analytically obtained in terms of the Alfven Mach number and upstream plasma parameters. Some effects of trapped ions on shock waves are also discussed. In addition, it is shown that a laminar oblique shock can reflect some electrons by a magnetic mirror effect, in spite of a large positive potential in the shock region. These theoretical predictions are confirmed by a fully electromagnetic particle simulation with full ion and electron dynamics in one spatial dimension.

Solar flares and particle acceleration.

The theory and observations of solar flares are reviewed with emphasis on particle acceleration. Hard X-ray and γ-ray measurements of solar flares have revealed that the acceleration of particles to relativistic energies can occur promptly (within-1s). This can be explained by the theory of resonant particle acceleration in a large-amplitude magnetosonic wave. Some models of magnetic reconnection and loop-loop interactions are also discussed which are believed to play an essential role in initiating the flare and/or releasing the magnetic energy.

Particle Reflection and Plasma Heating by a Laminar Magnetosonic Shock Wave

The fraction of ions trapped by a laminar magnetosonic shock wave is evaluated theoretically. The heating rate due to the resonant ion acceleration is then obtained interms of the Mach number and upstream plasma parameters. In addition, it is shown that, in spite of the large positive potential in the shock region, a laminar shock can reflect some electrons, as well as ions, because of the magnetic mirror effect.

Acceleration of energetic ions by a nearly perpendicular interplanetary shock

The theory of resonant ion acceleration which has been recently developed is compared quantitatively with the acceleration of energetic ions by a quasiperpendicular shock observed by Voyager 2 at a distance of ∼ 1.9 AU from the Sun. The theory explains many important features of the experimental results.

Theory for resonant ion acceleration by nonlinear magnetosonic fast and slow waves in finite beta plasmas

A Korteweg–de Vries equation that is applicable to both the nonlinear magnetosonic fast and slow waves is derived from a two-fluid model with finite ion and electron pressures. As in the cold plasma theory, the fast wave has a critical angle θc. For propagation angles greater than θc (quasiperpendicular propagation), the fast wave has a positive soliton, whereas for angles smaller than θc, it has a negative soliton. Finite β effects decrease the value of θc. The slow wave has a positive soliton for all angles of propagation. The magnitude of resonant ion acceleration (the vp×B acceleration) by the nonlinear fast and slow waves is evaluated. In the fast wave, the electron pressure makes the acceleration stronger for all propagation angles. The decrease in θc resulting from finite β effects results in broadening of the region of strong acceleration. It is also found that fairly strong ion acceleration can occur in the nonlinear slow wave in high β plasmas. The possibility of unlimited acceleration of ions by quasiperpendicular magnetosonic fast waves is discussed.

Resonant ion acceleration by oblique magnetosonic shock wave in a collisionless plasma

A magnetosonic shock wave propagating obliquely to a magnetic field is studied by theory and simulation, with particular attention to the resonant ion acceleration (the vp×B acceleration) by the shock. Theoretical analysis based on a two-fluid model shows that, in the laminar shock, the electric field strength in the direction normal to the wave is about (mi/me)1/2 times larger for the quasiperpendicular shock than that for the quasiparallel shock, which is a reflection of the fact that the width of the quasiperpendicular shock is much smaller than that of the quasiparallel shock. Time evolution of a totally self-consistent magnetosonic shock wave is studied by using a 2 1/2 -dimensional fully relativistic, fully electromagnetic particle simulation with full ion and electron dynamics. Even the low Mach number shock wave can significantly accelerate some ions by the vp×B acceleration. The resonant ion acceleration occurs more strongly in the quasiperpendicular shock, because the magnitude of this acceleration is proportional to the electric field strength.