Theoretical astrophysics and plasma physics at RPC

Abstract:

<jats:p>High-beta magnetised plasmas often exhibit anomalously structured temperature profiles, as seen from galaxy cluster observations and recent experiments. It is well known that when such plasmas are collisionless, temperature gradients along the magnetic field can excite whistler waves that efficiently scatter electrons to limit their heat transport. Only recently has it been shown that parallel temperature gradients can excite whistler waves also in collisional plasmas. Here, we develop a Wigner–Moyal theory for the collisional whistler instability starting from Braginskii-like fluid equations in a slab geometry. This formalism is necessary because, for a large region in parameter space, the fastest-growing whistler waves have wavelengths comparable to the background temperature gradients. We find additional damping terms in the expression for the instability growth rate involving inhomogeneous Nernst advection and resistivity. They (i) enable whistler waves to re-arrange the electron temperature profile via growth, propagation and subsequent dissipation, and (ii) allow non-constant temperature profiles to exist stably. For high-beta plasmas, the marginally stable solutions take the form of a temperature staircase along the magnetic field lines. The electron heat flux can also be suppressed by the Ettingshausen effect when the whistler intensity profile is sufficiently peaked and oriented opposite the background temperature gradient. This mechanism allows cold fronts without magnetic draping, might reduce parallel heat losses in inertial fusion experiments and generally demonstrates that whistler waves can regulate transport even in the collisional limit.</jats:p>

Abstract:

Magnetic field generation in ultraintense laser-solid interactions is studied over a range of laser intensities relevant to next-generation laser facilities (a0=50–500) using two-dimensional (2D) particle-in-cell simulations including strong-field quantum electrodynamic effects. It is found that fields O(0.1) MT (1 GG) may be generated by relativistic electrons traveling along the surface of the target. However, a significant fraction of the energy budget is converted to high-energy photons, approximately 38% at a0=500, greatly reducing the available energy for field generation. A model for the evolution of the target-surface fields is created and the scaling of the field strength with a0 is extracted from a set of 2D simulations. The simulated scaling allows for the estimation of field strengths and the model gives insight into the evolution of the fields on the next generation of laser facilities, a necessary component to the proposal of any future magnetized experiment.

Abstract:

Plasma turbulence on disparate spatial and temporal scales plays a key role in defining the level of confinement achievable in tokamaks, with the development of reduced numerical models for cross-scale turbulence effects informed by experimental measurements an essential step. MAST-U is a well-equipped facility having instruments to measure ion and electron scale turbulence at the plasma edge. However, measurement of core electron scale turbulence is challenging, especially in H mode. Using a novel synthetic diagnostic approach, we present simulated measurement specifications of a proposed highly optimised mm-wave based collective scattering instrument for measuring both normal and bi-normal electron scale turbulence in the core and edge of MAST-U. A powerful modelling framework has been developed that combines beam-tracing techniques with gyrokinetic simulations to predict the sensitivity and spectral range of measurement, with a quasi-numerical approach used to analyse the corresponding instrument selectivity functions. For the reconstructed MAST 022769 shot, a maximum measurable normalised bi-normal wavenumber of k⊥ ρe∼ 0.6 was predicted in the core and k⊥ ρe∼ 0.79 near the pedestal, with localisation lengths LFWHM ranging from ∼0.4 m in the core at k⊥ ρe∼ 0.1 to ∼0.08 m at k⊥ ρe> 0.45. Synthetic diagnostic analysis for the 022769 shot using CGYRO gyrokinetic simulation spectra reveal that electron temperature gradient turbulence wavenumbers of peak spectral intensity comfortably fall within the measurable/detectable range of the instrument from the core to the pedestal. The proposed diagnostic opens up opportunities to study new regimes of turbulence and confinement, particularly in association with upcoming non-inductive, microwave based current drive experiments on MAST-U and can provide insight into cross-scale turbulence effects, while having suitability to operate during burning plasma scenarios on future reactors such as Spherical Tokamak for Energy Production.

Abstract:

We present the largest 3D particle-in-cell shearing-box simulations of turbulence driven by the magnetorotational instability, for the first time employing the realistic proton-to-electron mass ratio. We investigate the energy partition between relativistically hot electrons and subrelativistic ions in turbulent accreting plasma, a regime relevant to collisionless, radiatively inefficient accretion flows around supermassive black holes such as those targeted by the Event Horizon Telescope. We provide a simple empirical formula to describe the measured heating ratio between ions and electrons, which can be used for more accurate global modeling of accretion flows with standard fluid approaches such as general-relativistic magnetohydrodynamics.