Volume 21, Issue 7, July 2014

The cosmic magnetic fields in regions of low plasma pressure and large currents, such as in interstellar space and gaseous nebulae, are forcefree in the sense that the Lorentz force vanishes. The threedimensional ArnoldBeltramiChildress (ABC) field is an example of a forcefree, helical magnetic field. In fluid dynamics, ABC flows are steady state solutions of the Euler equation. The ABC magnetic field lines exhibit a complex and varied structure that is a mix of regular and chaotic trajectories in phase space. The characteristic features of field line trajectories are illustrated through the phase space distribution of finitedistance and asymptoticdistance Lyapunov exponents. In regions of chaotic trajectories, an ensembleaveraged variance of the distance between field lines reveals anomalous diffusion—in fact, superdiffusion—of the field lines. The motion of charged particles in the forcefree ABC magnetic fields is different from the flow of passive scalars in ABC flows. The particles do not necessarily follow the field lines and display a variety of dynamical behavior depending on their energy, and their initial pitchangle. There is an overlap, in space, of the regions in which the field lines and the particle orbits are chaotic. The time evolution of an ensemble of particles, in such regions, can be divided into three categories. For short times, the motion of the particles is essentially ballistic; the ensembleaveraged, mean square displacement is approximately proportional to t ^{2}, where t is the time of evolution. The intermediate time region is defined by a decay of the velocity autocorrelation function—this being a measure of the time after which the collective dynamics is independent of the initial conditions. For longer times, the particles undergo superdiffusion—the mean square displacement is proportional to t^{α} , where α > 1, and is weakly dependent on the energy of the particles. These superdiffusive characteristics, both of magnetic field lines and of particles moving in these fields, strongly suggest that theories of transport in threedimensional chaotic magnetic fields need a shift from the usual paradigm of quasilinear diffusion.
 LETTERS


Possible energy gain for a plasmalinerdriven magnetoinertial fusion concept
View Description Hide DescriptionA onedimensional parameter study of a MagnetoInertial Fusion (MIF) concept indicates that significant gain may be achievable. This concept uses a dynamically formed plasma shell with inwardly directed momentum to drive a magnetized fuel to ignition, which in turn partially burns an intermediate layer of unmagnetized fuel. The concept is referred to as Plasma Jet MIF or PJMIF. The results of an adaptive mesh refinement Eulerian code (Crestone) are compared to those of a Lagrangian code (LASNEX). These are the first published results using the Crestone and LASNEX codes on the PJMIF concept.

LH transition dynamics in fluid turbulence simulations with neoclassical force balance
View Description Hide DescriptionSpontaneous transport barrier generation at the edge of a magnetically confined plasma is reproduced in fluxdriven threedimensional fluid simulations of electrostatic turbulence. Here, the role on the radial electric field of collisional friction between trapped and passing particles is shown to be the key ingredient. Especially, accounting for the selfconsistent and precise dependence of the friction term on the actual plasma temperature allows for the triggering of a transport barrier, provided that the input power exceeds some threshold. In addition, the barrier is found to experience quasiperiodic relaxation events, reminiscent of edge localised modes. These results put forward a possible key player, namely, neoclassical physics via radial force balance, for the low to highconfinement regime transition observed in most of controlled fusion devices.
