The gyrokinetic field invariant and electromagnetic temperature-gradient instabilities in ‘good-curvature’ plasmas

Journal of Plasma Physics Cambridge University Press (CUP) 91:4 (2025) E95

Authors:

PG Ivanov, P Luhadiya, T Adkins, AA Schekochihin

Abstract:

Curvature-driven instabilities are ubiquitous in magnetised fusion plasmas. By analysing the conservation laws of the gyrokinetic system of equations, we demonstrate that the well-known spatial localisation of these instabilities to regions of ‘bad magnetic curvature’ can be explained using the conservation law for a sign-indefinite quadratic quantity that we call the gyrokinetic field invariant. Its evolution equation allows us to define the local effective magnetic curvature whose sign demarcates the regions of ‘good’ and ‘bad’ curvature, which, under some additional simplifying assumptions, can be shown to correspond to the inboard (high-field) and outboard (low-field) sides of a tokamak plasma, respectively. We find that, given some reasonable assumptions, electrostatic curvature-driven modes are always localised to the regions of bad magnetic curvature, regardless of the specific character of the instability. More importantly, we also deduce that any mode that is unstable in the region of good magnetic curvature must be electromagnetic in nature. As a concrete example, we present the magnetic-drift mode, a novel good-curvature electromagnetic instability, and compare its properties with the well-known electron-temperature-gradient instability. Finally, we discuss the relevance of the magnetic drift mode for high- $\beta$ fusion plasmas, and in particular its relationship with microtearing modes.

Measuring the beamlet divergence in NBI systems for fusion: A diagnostic comparison

Fusion Engineering and Design Elsevier 215 (2025) 114951

Authors:

Niek den Harder, Michael Barnes, Andreas Döring, Bernd Heinemann, Bruno Laterza, Isabella Mario, Antonio Pimazzoni, Carlo Poggi, Alessandro La Rosa, Emanuele Sartori, Beatrice Segalini, Gianluigi Serianni, Alastair Shepherd, Christian Wimmer, Dirk Wünderlich, Ursel Fantz

Hydrodynamic simulations of black hole evolution in AGN discs II: inclination damping for partially embedded satellites

(2025)

Authors:

Henry Whitehead, Connar Rowan, Bence Kocsis

Hydrodynamic simulations of black hole evolution in AGN discs I: orbital alignment of highly inclined satellites

(2025)

Authors:

Connar Rowan, Henry Whitehead, Gaia Fabj, Philip Kirkeberg, Martin E Pessah, Bence Kocsis

A higher-order finite-element implementation of the nonlinear Fokker–Planck collision operator for charged particle collisions in a low density plasma

Computer Physics Communications Elsevier 314 (2025) 109675

Abstract:

Collisions between particles in a low density plasma are described by the Fokker–Planck collision operator. In applications, this nonlinear integro-differential operator is often approximated by linearised or ad-hoc model operators due to computational cost and complexity. In this work, we present an implementation of the nonlinear Fokker–Planck collision operator written in terms of Rosenbluth potentials in the Rosenbluth–MacDonald–Judd (RMJ) form. The Rosenbluth potentials may be obtained either by direct integration or by solving partial differential equations (PDEs) similar to Poisson's equation: we optimise for performance and scalability by using sparse matrices to solve the relevant PDEs. We represent the distribution function using a tensor-product continuous-Galerkin finite-element representation and we derive and describe the implementation of the weak form of the collision operator. We present tests demonstrating a successful implementation using an explicit time integrator and we comment on the speed and accuracy of the operator. Finally, we speculate on the potential for applications in the current and next generation of kinetic plasma models.