Theoretical astrophysics and plasma physics at RPC

Abstract:

Here we present details of a mixed implicit-explicit numerical scheme for the solution of the gyrokinetic-Poisson system of equations in the local limit. This scheme has been implemented in a new code called $\texttt{stella}$, which is capable of evolving electrostatic fluctuations with full kinetic electron effects and an arbitrary number of ion species in general magnetic geometry. We demonstrate the advantages of this mixed approach over a fully explicit treatment and provide linear and nonlinear benchmark comparisons for both axisymmetric and non-axisymmetric magnetic equilibria.

Abstract:

© 2018 IOP Publishing Ltd. Differential rotation is induced in tokamak plasmas when an underlying symmetry of the governing gyrokinetic-Maxwell system of equations is broken. One such symmetry-breaking mechanism is considered here: the turbulent acceleration of particles along the mean magnetic field. This effect, often referred to as the 'parallel nonlinearity', has been implemented in the δf gyrokinetic code stella and used to study the dependence of turbulent momentum transport on the plasma size and on the strength of the turbulence drive. For JET-like parameters with a wide range of driving temperature gradients, the momentum transport induced by the inclusion of turbulent acceleration is similar to or smaller than the ratio of the ion Larmor radius to the plasma minor radius. This low level of momentum transport is explained by demonstrating an additional symmetry that prohibits momentum transport when the turbulence is driven far above marginal stability.

Abstract:

In a recent paper, we determined the measurement accuracy of physical parameters for eccentric, precessing, non-spinning, inspiraling, stellar-mass black hole - black hole (BH-BH) binaries for the upcoming second-generation LIGO/VIRGO/KAGRA detector network at design sensitivity using the Fisher matrix method. Here we extend that study to a wide range of binary masses including neutron star - neutron star (NS-NS), NS-BH, and BH-BH binaries with BH masses up to $110 \, M_{\odot}$. The measurement error of eccentricity $e_{10 \,\rm Hz}$ at a gravitational-wave (GW) frequency of $10 \, {\rm Hz}$ is in the range $(10^{-4}-10^{-3}) \times (D_L/ 100\,\rm Mpc)$ for NS-NS, NS-BH, and BH-BH binaries at a luminosity distance of $D_L$ if $e_{10 \,\rm Hz} \gtrsim 0.1 $. For events with masses and distances similar to the detected 10 GW transients, we show that nonzero orbital eccentricities may be detected if $0.081 \lesssim e_{10 \,\rm Hz}$. Consequently, the LIGO/VIRGO/KAGRA detector network at design sensitivity will have the capability to distinguish between eccentric waveforms and circular waveforms. In comparison to circular inspirals, we find that the chirp mass measurement precision can improve by up to a factor of $\sim 20$ and $\sim 50-100$ for NS-NS and NS-BH binaries with BH companion masses $\lesssim 40 \, M_{\odot}$, respectively. The identification of eccentric sources may give information on their astrophysical origin; it would indicate merging binaries in triple or higher multiplicity systems or dynamically formed binaries in dense stellar systems such as globular clusters or galactic nuclei.