Tony Bell FRS

Abstract:

We present a novel code which solves the Vlasov-Fokker-Planck (VFP) equation in three-dimensional magnetic turbulence using finite difference methods. The approach is distinct from particle tracking codes. The angular component of the velocity space distribution function is represented by a spherical harmonic expansion drawing on an approach pioneered by Bell, et al. (2006 Plasma Phys. Control. Fusion 48 R37) for laser-plasma interaction simulations. This method enables the accurate representation of magnetic fields and of the effect of angular scattering effects on a particle distribution function. The code has been verified against both collisional and quasi-linear turbulent transport theories. It is shown to address successfully the physics of cross-field transport in regimes of magnetic field field perturbation amplitude and collisionality that are difficult to study using other approaches. © 2010 IOP Publishing Ltd.

Abstract:

Supernova remnants (SNR) are the most likely source of galactic cosmic rays (CRs) up to the 'knee' in the spectrum at a few PeV. The theory of diffusive shock acceleration nicely supplies a power law energy distribution with approximately the desired spectral index and with suitably high efficiency. For a SNR blast wave expanding into a typical interstellar magnetic field the predicted maximum CR energy falls short of 1 PeV, but a non-resonant plasma instability allows the CRs themselves to amplify the magnetic field by orders of magnitude to a level capable of accelerating CRs to the knee. © 2009 IOP Publishing Ltd.

Abstract:

Based on an analysis of a specific electron trajectory in counter-propagating beams, Bell and Kirk (2008 Phys. Rev. Lett. 101 200403) recently suggested that laboratory lasers may shortly be able to produce significant numbers of electron-positron pairs. We confirm their results using an improved treatment of non-linear Compton scattering in the laser beams. Implementing an algorithm that integrates classical electron trajectories, we then examine a wide range of laser pulse shapes and polarizations. We find that counter-propagating, linearly polarized beams, with either aligned or crossed orientation, are likely to initiate a pair avalanche at intensities of approximately 10²⁴ W cm^-2 per beam. The same result is found by modelling one of the beams as a wave reflected at the surface of an overdense solid. © 2009 IOP Publishing Ltd.

Abstract:

Prolific electron-positron pair production is possible at laser intensities approaching 10;{24} W cm;{-2} at a wavelength of 1 mum. An analysis of electron trajectories and interactions at the nodes (B=0) of two counterpropagating, circularly polarized laser beams shows that a cascade of gamma rays and pairs develops. The geometry is generalized qualitatively to linear polarization and laser beams incident on a solid target.

Abstract:

Cosmic ray acceleration may occur at a supernova shock expanding into a circumstellar wind. Self-similar solutions for the cosmic ray distribution are derived firstly when diffusion is isotropic and secondly when the wind sustains a magnetic field in the form of a Parker spiral. © 2008 World Scientific Publishing Company.