Theoretical astrophysics and plasma physics at RPC

Abstract:

Low collisionality stellarator plasmas usually display a large negative radial electric field that has been expected to cause accumulation of impurities due to their high charge number. In this paper, two combined effects that can potentially modify this scenario are discussed. First, it is shown that, in low collisionality plasmas, the kinetic contribution of the electrons to the radial electric field can make it negative but small, bringing the plasma close to impurity temperature screening (i.e., to a situation in which the ion temperature gradient is the main drive of impurity transport and causes outward flux); in plasmas of very low collisionality, such as those of the large helical device displaying impurity hole (Ida et al (The LHD Experimental Group) 2009 Phys. Plasmas 16 056111; Yoshinuma et al (The LHD Experimental Group) 2009 Nucl. Fusion 49 062002), screening may actually occur. Second, the component of the electric field that is tangent to the flux surface (in other words, the variation of the electrostatic potential on the flux surface), although smaller than the radial component, has recently been suggested to be an additional relevant drive for radial impurity transport. Here, it is explained that, especially when the radial electric field is small, the tangential magnetic drift has to be kept in order to correctly compute the tangential electric field, that can be larger than previously expected. This can have a strong impact on impurity transport, as we illustrate by means of simulations using the newly developed code kinetic orbit-averaging-solver for stellarators, although it is not enough to explain by itself the behavior of the fluxes in situations like the impurity hole.

Abstract:

Nuclear star clusters around massive black holes are expected to be abundant in stellar mass black holes and black hole binaries. These binaries form a hierarchical triple system with the massive black hole at the center. Gravitational perturbations from the massive black hole can cause high eccentricity excitation. During this process, the eccentricity may approach unity, and the pericenter distance may become sufficiently small that gravitational wave emission drives the binary to merge. In this paper, we consider a simple proof of concept and explore the effect of the eccentric Kozai-Lidov mechanism for unequal mass binaries. We perform a set of Monte Carlo simulations on BH-BH binaries in galactic nuclei with quadrupole and octupole-level secular perturbations, general relativistic precession, and gravitational wave emission. For a nominal number of steady-state BH-BH binaries, our model gives a total merger rate $\sim 1 - 3$$Gpc^{-3} yr^{-1}$, depending on the assumed density profile. Thus, our model potentially competes with other dynamical mechanisms, such as the dynamical formations and mergers of BH binaries in globular clusters or dense nuclear clusters without a massive black hole. We provide predictions for the distributions of these LIGO sources in galactic nuclei.