Diffusion of charged particles in strong magnetic fields

23.01.2015

How a magnetic field affects the trajectory of a charged particle is one of the first questions which arise in the study of plasmas: The trajectory is bent around the magnetic field lines, resulting in a gyrating motion perpendicular to the magnetic field. Along the magnetic field, the particle can move freely and the particle's total motion is a helical path. The diffusion of an ensemble of only weakly interacting particles is, therefore, unaltered along the magnetic field lines. Across the magnetic field, the diffusion is reduced proportionally to 1/B in the limit of strong fields, the so-called Bohm diffusion. If, however, the particles are strongly interacting, for example because they are very cold or very highly charged, this well-accepted picture has to be changed (see Figure): While the diffusion across the magnetic field shows Bohm diffusion as in weakly coupled plasmas, now also the migration of particles along the field lines is slowed down - an effect of the strong correlations between the particles. The diffusion coefficient falls as 1/Bα, where α is 1 for very strongly correlated systems and between 0 and 1 for less strongly correlated systems. This is illustrated in the figure where β denotes the strength of the magnetic field (ratio of cyclotron frequency and plasma frequency) and Γ, the coupling strength (ratio of interaction energy to kinetic energy). Using computing resources at the HLRN, the research group of Prof. Michael Bonitz has discovered this effect and investigated it in detail in large scale first principle computer simulations: http://http://link.aps.org/doi/10.1103/PhysRevLett.107.135003