Tony Bell FRS

Abstract:

Miniati and Bell (2011) proposed a mechanism for the generation of magnetic seeds that is based the finite resistivity of the low temperature IGM in the high redshift universe. In this model, cosmic-ray protons generated by the first generation of galaxies, escape into the intergalactic medium carrying an electric current that induces return currents, $j_t$, and associated electric fields, $\vec E=\eta\vec j_t$ there. Because the resistivity, $\eta$, depends on the IGM temperature, which is highly inhomogeneous due to adiabatic contraction and shocks produced by structure formation, a non-vanishing curl of the electric field exists which sustains the growth of magnetic field. In this contribution we have developed an approximate numerical model for this process by implementing the source terms of the resistive mechanism in the cosmological code CHARM. Our numerical estimates substantiate the earlier analysis in Miniati and Bell (2011) which found magnetic seeds between 10$^{-18}$ and 10$^{-16}$ Gauss throughout cosmic space at redshift z~6, consistent with conservative estimates of magnetic fields in voids at z~0 from recent gamma-ray experiments.

Abstract:

Diffusive shock acceleration is the prime candidate for efficient acceleration of cosmic rays. Galactic cosmic rays are believed to originate predominantly from this process in supernova remnant shock waves. Confinement of the cosmic rays in the shock region is key in making the mechanism effective. It has been known that on small scales (smaller than the typical gyroradius) high-amplitude non-resonant instabilities arise due to cosmic ray streaming ahead of the shock. For the efficiency of scattering of the highest energy cosmic rays it is of interest to determine the type of instabilities that act on longer length scales, i.e. larger than the cosmic ray gyroradius. We will present the results of our analysis of an instability that acts in this regime and will discuss its driving mechanism and typical growth times.

Abstract:

Inertial fusion energy (IFE) offers one possible route to commercial energy generation. In the proposed 'shock ignition' route to fusion, the target is compressed at a relatively low temperature and then ignited using high intensity laser irradiation which drives a strong converging shock into the centre of the fuel. With a series of idealized calculations we analyse the electron transport of energy into the target, which produces the pressure responsible for driving the shock. We show that transport in shock ignition lies near the boundary between ablative and heat front regimes. Moreover, simulations indicate that non-local effects are significant in the heat front regime and might lead to increased efficiency by driving the shock more effectively and reducing heat losses to the plasma corona. © 2011 IOP Publishing Ltd.

Abstract:

Relativistic charged particles (CRs for cosmic rays) produced by supernova explosion of the first generation of massive stars that are responsible for the reionization of the universe escape into the intergalactic medium, carrying an electric current. Charge imbalance and induction give rise to a return current, → jt , carried by the cold thermal plasma which tends to cancel the CR current. The electric field, →E = n→ jt , required to draw the collisional return current opposes the outflow of low-energy CRs and ohmically heats the cold plasma. Owing to inhomogeneities in the resistivity, ν (T ), caused by a structure in the temperature, T, of the intergalactic plasma, the electric field possesses a rotational component which sustains Faraday's induction. It is found that a magnetic field is robustly generated throughout intergalactic space at a rate of 10-17 to 10-16GGyr-1, until the temperature of the intergalactic medium is raised by cosmic reionization. The magnetic field may seed the subsequent growth of magnetic fields in the ntergalactic environment. The role of CR-driven instabilities is discussed, and nonlinear effects are briefly considered.

Abstract:

OSHUN is a parallel relativistic 2D3P Vlasov-Fokker-Planck code, developed primarily to study electron transport and instabilities pertaining to laser-produced-including laser-fusion-plasmas. It incorporates a spherical harmonic expansion of the electron distribution function, where the number of terms is an input parameter that determines the angular resolution in momentum-space. The algorithm employs the full 3D electromagnetic fields and a rigorous linearized Fokker-Planck collision operator. The numerical scheme conserves energy and number density. This enables simulations for plasmas with temperatures from MeV down to a few eV and densities from less than critical to more than solid. Kinetic phenomena as well as electron transport physics can be recovered accurately and efficiently. © 2011 Elsevier Inc.