Theoretical astrophysics and plasma physics at RPC

Abstract:

Efficient control of turbulent heat transport is crucial for magnetic confinement fusion reactors. This work discusses the complex interplay between density gradients and microinstabilities, shedding light on their impact on turbulent heat transport in different fusion devices. In particular, the influence of density gradients on turbulent heat transport is investigated through an extensive inter-machine study, including various stellarators such as W7-X, LHD, TJ-II and NCSX, along with the Asdex Upgrade tokamak (AUG) and the tokamak geometry of the Cyclone Base Case (CBC). Linear and nonlinear simulations are performed employing the δf-gyrokinetic code stella across a wide range of parameters to explore the effects of density gradients, temperature gradients, and kinetic electrons. A strong reduction in ion heat flux with increasing density gradients is found in NCSX and W7-X due to the stabilization of temperature-gradient-driven modes without significantly destabilizing density-gradient-driven modes. In contrast, the tokamaks exhibit an increase in ion heat flux with density gradients. Notably, the behavior of ion heat fluxes in stellarators does not align with that of linear growth rates, if only the fastest-growing mode is taken into account. Additionally, this study provides physical insights into the microinstabilities, emphasizing the dominance of trapped-electron-modes (TEMs) in CBC, AUG, TJ-II, LHD and NCSX, while both the TEM and the passing-particle-driven universal instability contribute significantly in W7-X.

Abstract:

We present numerical simulations used to interpret laser-driven plasma experiments at the GSI Helmholtz Centre for Heavy Ion Research. The mechanisms by which non-thermal particles are accelerated, in astrophysical environments e.g., the solar wind, supernova remnants, and gamma ray bursts, is a topic of intense study. When shocks are present the primary acceleration mechanism is believed to be first-order Fermi, which accelerates particles as they cross a shock. Second-order Fermi acceleration can also contribute, utilizing magnetic mirrors for particle energization. Despite this mechanism being less efficient, the ubiquity of magnetized turbulence in the universe necessitates its consideration. Another acceleration mechanism is the lower-hybrid drift instability, arising from gradients of both density and magnetic field, which produce lower-hybrid waves with an electric field which energizes particles as they cross these waves. With the combination of high-powered laser systems and particle accelerators it is possible to study the mechanisms behind cosmic-ray acceleration in the laboratory. In this work, we combine experimental results and high-fidelity threedimensional simulations to estimate the efficiency of ion acceleration in a weakly magnetized interaction region. We validate the FLASH MHD code with experimental results and use OSIRIS particle-in-cell (PIC) code to verify the initial formation of the interaction region, showing good agreement between codes and experimental results. We find that the plasma conditions in the experiment are conducive to the lower-hybrid drift instability, yielding an increase in energy ∆E of ∼ 264 keV for 242 MeV calcium ions.

Abstract:

While dynamical tides only become relevant during the last couple of orbits for circular inspirals, orbital eccentricity can increase their impact during earlier phases of the inspiral by exciting tidal oscillations at each close encounter. We investigate the effect of dynamical tides on the orbital evolution of eccentric neutron star binaries using post-Newtonian numerical simulations and construct an analytic stochastic model that reproduces the numerical results. Our study reveals a strong dependence of dynamical tides on the pericenter distance, with the fractional energy transferred to dynamical tides over that dissipated in gravitational waves (GWs) exceeding ∼1% at separations rp≲50 km for large eccentricities. We demonstrate that the effect of dynamical tides on orbital evolution can manifest as a phase shift in the GW signal. We show that the signal-to-noise ratio of the GW phase shift can reach the detectability threshold of 8 with a single advanced Laser Interferometer Gravitational Wave Observatory detector at design sensitivity for eccentric neutron star binaries at a distance of 40 Mpc. This requires a pericenter distance of rp0≲68 km (rp0≲76 km) at binary formation with eccentricity close to 1 for a reasonable tidal deformability and f-mode frequency of 500 and 1.73 kHz (700 and 1.61 kHz), respectively. The observation of the phase shift will enable measuring the f-mode frequency of neutron stars independently from their tidal deformability, providing significant insights into neutron star seismology and the properties of the equation of state. We also explore the potential of distinguishing between equal-radius and twin-star binaries, which could provide an opportunity to reveal strong first-order phase transitions in the nuclear equation of state.