118523771
RESONANCE BROADENING AND HEATING OF CHARGED PARTICLES IN MAGNETOHYDRODYNAMIC TURBULENCE
The heating, acceleration, and pitch-angle scattering of charged particles by magnetohydrodynamic (MHD) turbulence are important in a wide range of astrophysical environments, including the solar wind, accreting black holes, and galaxy clusters. We simulate the interaction of high-gyrofrequency test particles with fully dynamical simulations of subsonic MHD turbulence, focusing on the parameter regime with β ~ 1, where β is the ratio of gas to magnetic pressure. We use the simulation results to calibrate analytical expressions for test particle velocity-space diffusion coefficients and provide simple fits that can be used in other work. The test particle velocity diffusion in our simulations is due to a combination of two processes: interactions between particles and magnetic compressions in the turbulence (as in linear transit-time damping; TTD) and what we refer to as Fermi Type-B (FTB) interactions, in which charged particles moving on field lines may be thought of as beads sliding along moving wires. We show that test particle heating rates are consistent with a TTD resonance that is broadened according to a decorrelation prescription that is Gaussian in time (but inconsistent with Lorentzian broadening due to an exponential decorrelation function, a prescription widely used in the literature). TTD dominates the heating for vs vA (e.g., electrons), where vs is the thermal speed of species s and vA is the Alfven speed, while FTB dominates for vs vA (e.g., minor ions). Proton heating rates for β ~ 1 are comparable to the turbulent cascade rate. Finally, we show that velocity diffusion of collisionless, large gyrofrequency particles due to large-scale MHD turbulence does not produce a power-law distribution function.
55286238
Production of ultra-cold neutrons using Doppler-shifted Bragg scattering and an intense pulsed neutron spallation source
We present an analytic and a computer generated simulation of the production of ultra-cold neutrons (UCN) using Bragg scattering from a moving crystal to Doppler-shift higher velocity neutrons into the UCN region. The calculation was carried out with a view toward its application at the Intense Pulsed Neutron Source (IPNS) now under construction at Argonne National Laboratory. This method for the production of UCN appears well matched to a pulsed source, and we show that the UCN can be stored in a neutron bottle at the peak flux which can potentially be much higher than at the present high flux reactors. The predicted density of stored UCN indicates that a highly precise measurement of the neutron electric dipole moment (EDM) will be possible within the next few years.
45238244
Quantum Monte Carlo calculations of light nuclei using chiral potentials.
We present the first Green's function Monte Carlo calculations of light nuclei with nuclear interactions derived from chiral effective field theory up to next-to-next-to-leading order. Up to this order, the interactions can be constructed in a local form and are therefore amenable to quantum Monte Carlo calculations. We demonstrate a systematic improvement with each order for the binding energies of A=3 and A=4 systems. We also carry out the first few-body tests to study perturbative expansions of chiral potentials at different orders, finding that higher-order corrections are more perturbative for softer interactions. Our results confirm the necessity of a three-body force for correct reproduction of experimental binding energies and radii, and pave the way for studying few- and many-nucleon systems using quantum Monte Carlo methods with chiral interactions.