### Collisionless relaxation of a Lynden-Bell plasma

Journal of Plasma Physics Cambridge University Press (CUP) 88:5 (2022) 925880501

#### Authors:

RJ Ewart, A Brown, T Adkins, AA Schekochihin

#### Abstract:

Plasmas whose Coulomb-collision rates are very small may relax on shorter timescales to non-Maxwellian quasi-equilibria, which, nevertheless, have a universal form, with dependence on initial conditions retained only via an infinite set of Casimir invariants enforcing phase-volume conservation. These are distributions derived by Lynden-Bell (Mon. Not. R. Astron. Soc., vol. 136, 1967, p. 101) via a statistical-mechanical entropy-maximisation procedure, assuming perfect mixing of phase-space elements. To show that these equilibria are reached dynamically, one must derive an effective ‘collisionless collision integral’ for which they are fixed points – unique and inevitable provided the integral has an appropriate H-theorem. We describe how such collision integrals are derived and what assumptions are required for them to have a closed form, how to prove the H-theorems for them, and why, for a system carrying sufficiently large electric-fluctuation energy, collisionless relaxation should be fast. It is suggested that collisionless dynamics may favour maximising entropy locally in phase space before converging to global maximum-entropy states. Relaxation due to interspecies interaction is examined, leading, inter alia, to spontaneous transient generation of electron currents. The formalism also allows efficient recovery of ‘true’ collision integrals for both classical and quantum plasmas.

### Electromagnetic instabilities and plasma turbulence driven by electron-temperature gradient

JOURNAL OF PLASMA PHYSICS 88:4 (2022) ARTN 905880410

#### Authors:

T Adkins, AA Schekochihin, PG Ivanov, CM Roach

### New stability parameter to describe low-$β$ electromagnetic microinstabilities driven by passing electrons in axisymmetric toroidal geometry

ArXiv 2208.10615 (2022)

#### Authors:

MR Hardman, FI Parra, BS Patel, CM Roach, J Ruiz Ruiz, M Barnes, D Dickinson, W Dorland, JF Parisi, D St-Onge, H Wilson

### Astrophysical gravitational-wave echoes from galactic nuclei

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 515:3 (2022) 3299-3318

#### Authors:

Laszlo Gondan, Bence Kocsis

### Gyrokinetic electrostatic turbulence close to marginality in the Wendelstein 7-X stellarator

Physical Review E American Physical Society 106 (2022) L013202

#### Authors:

Alessandro Zocco, Linda Podavini, José Manuel Garcìa-Regaña, Michael Barnes, Felix I Parra, A Mishchenko, Per Helander

#### Abstract:

The transition from strong (fluidlike) to nearly marginal (Floquet-type) regimes of ion-temperature-gradient (ITG) driven turbulence is studied in the stellarator Wendelstein 7-X by means of numerical simulations. Close to marginality, extended (along magnetic field lines) linearly unstable modes are dominant, even in the presence of kinetic electrons, and provide a drive that results in finite turbulent transport. A total suppression of turbulence above the linear stability threshold of the ITG modes, commonly present in tokamaks and known as the “Dimits shift,” is not observed. We show that this is mostly due to the peculiar radial structure of marginal turbulence, which is more localized than in the fluid case and therefore less likely to be stabilized by shearing flows.