Acceleration Of Alpha Particles Research Articles

DISSIPATION OF PARALLEL AND OBLIQUE ALFVÉN-CYCLOTRON WAVES—IMPLICATIONS FOR HEATING OF ALPHA PARTICLES IN THE SOLAR WIND

We perform 2.5D hybrid simulations with massless fluid electrons and kinetic particle-in-cell ions to study the temporal evolution of ion temperatures, temperature anisotropies, and velocity distribution functions in relation to the dissipation and turbulent evolution of a broadband spectrum of parallel and obliquely propagating Alfven-cyclotron waves. The purpose of this paper is to study the relative role of parallel versus oblique Alfven-cyclotron waves in the observed heating and acceleration of alpha particles in the fast solar wind. We consider collisionless homogeneous multi-species plasma, consisting of isothermal electrons, isotropic protons, and a minor component of drifting α particles in a finite-β fast stream near the Earth. The kinetic ions are modeled by initially isotropic Maxwellian velocity distribution functions, which develop nonthermal features and temperature anisotropies when a broadband spectrum of low-frequency nonresonant, ω ≤ 0.34 Ωp, Alfven-cyclotron waves is imposed at the beginning of the simulations. The initial plasma parameter values, such as ion density, temperatures, and relative drift speeds, are supplied by fast solar wind observations made by the Wind spacecraft at 1 AU. The imposed broadband wave spectra are left-hand polarized and resemble Wind measurements of Alfvenic turbulence in the solar wind. The imposed magnetic field fluctuations for all cases are within the inertial range of the solar wind turbulence and have a Kraichnan-type spectral slope α = −3/2. We vary the propagation angle from θ = 0° to θ = 30° and θ = 60°, and find that the heating of alpha particles is most efficient for the highly oblique waves propagating at 60°, whereas the protons exhibit perpendicular cooling at all propagation angles.

Interactions of Alfvén-Cyclotron Waves with Ions in the Solar Wind

The results of a study of linear and nonlinear properties of Alfven-cyclotron waves propagating along a background magnetic field in collisionless plasmas are presented. Kinetic theory and one-dimensional hybrid simulations are used to identify the complete family of growing and damped wave modes and the connection with the spontaneous fluctuations in plasmas. The self-consistent reshaping of initially cold ion velocity distributions, the simultaneous generation of beam protons, and the preferential heating and acceleration of alpha particles are shown to occur by the same mechanism: ion trapping and induced pitch-angle scattering by parametrically driven ion acoustic waves.

On the acceleration of alpha particles in the fast solar wind

Recently, Gomberoff and Elgueta (1991) showed that in a plasma composed of anisotropic protons and alpha particles drifting along an external magnetic field with a small velocity relative to the protons, strong left‐hand polarized electromagnetic ion cyclotron waves can be generated. These waves can accelerate the alpha particles to velocities well in excess of the proton bulk velocity. Here we assume a more realistic model of the solar wind by considering a double‐humped proton distribution. It is shown that the secondary proton beam has no important effects on the ion cyclotron waves for beam densities of the order of those observed in fast solar wind conditions. The fact that the alpha proton drift velocity is modulated by the Alfvén velocity remains unexplained.

Time decay of electric fields probed by charged reaction products in six-beam symmetrical implosion experiments

Alpha-particle spectra have been measured with a time-of-flight detector in laser fusion experiments on the symmetrical illumination, six-beam ZETA laser system (Nd-doped phosphate glass, λ = 1.054 μm). For a series of fast imploding targets, upward mean energy shifts of alpha particles were observed. These energy shifts were attributed to a time-varying electric field which charged reaction products experience as they pass through the corona of the imploding microballoon. A one-dimensional, two-temperature hydrodynamics code was used to simulate both the expanding corona and the imploding glass. The corona simulation gave rise to an electric field with a decay time of the order of the laser pulse width. The acceleration of alpha particles by this field satisfactorily accounted for their observed alpha-energy shifts.

Acceleration of alpha particles by the space radiation effects laboratory synchro-cyclotron

A modification of the SREL synchro-cyclotron has been completed, allowing acceleration of alpha particles ( 2 4He ++) as well as protons. The main parts of the modification are in the rf system and the ion source. No changes in the main magnet, extraction channel or beam transport system have been required. The energy of extracted protons from the SREL synchrocyclotron is 588 MeV and the corresponding energy of alpha particles is 707 MeV. At the time of the initial testing, the beam intensity was found to be over 0.2 μA (6 × 10 11 alphas per second) near the extraction radius, and 0.3 μA at the radius of 28″ (86 MeV). The modified cyclotron operates reliably in both the proton and alpha-particle modes, with a period of about eight hours required for the changeover between modes.