# Publications

## Continuous-in-time approach to flow shear in a linearly implicit local gyrokinetic code

Journal of Plasma Physics Cambridge University Press (CUP) **87** (2021) 905870230

<jats:p>A new algorithm for toroidal flow shear in a linearly implicit, local <jats:inline-formula> <jats:alternatives> <jats:tex-math>$\delta f$</jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0022377821000453_inline2.png" /> </jats:alternatives> </jats:inline-formula> gyrokinetic code is described. Unlike the current approach followed by a number of codes, it treats flow shear continuously in time. In the linear gyrokinetic equation, time-dependences arising from the presence of flow shear are decomposed in such a way that they can be treated explicitly in time with no stringent constraint on the time step. Flow shear related time dependences in the nonlinear term are taken into account exactly, and time dependences in the quasineutrality equation are interpolated. Test cases validating the continuous-in-time implementation in the code GS2 are presented. Lastly, nonlinear gyrokinetic simulations of a JET discharge illustrate the differences observed in turbulent transport compared with the usual, discrete-in-time approach. The continuous-in-time approach is shown, in some cases, to produce fluxes that converge to a different value than with the discrete approach. The new approach can also lead to substantial computational savings by requiring radially narrower boxes. At fixed box size, the continuous implementation is only modestly slower than the previous, discrete approach.</jats:p>

## Mass-gap Mergers in Active Galactic Nuclei

ASTROPHYSICAL JOURNAL **908** (2021) ARTN 194

## Turbulent impurity transport simulations in Wendelstein 7-X plasmas

JOURNAL OF PLASMA PHYSICS **87** (2021) ARTN 855870103

## Eccentric Black Hole Mergers in Active Galactic Nuclei

ASTROPHYSICAL JOURNAL LETTERS **907** (2021) ARTN L20

## Toroidal and slab ETG instability dominance in the linear spectrum of JET-ILW pedestals

Nuclear Fusion IOP Publishing **60** (2020) 126045

Local linear gyrokinetic simulations show that electron temperature gradient (ETG) instabilities are the fastest growing modes for $k_y \rho_i \gtrsim 0.1$ in the steep gradient region for a JET pedestal discharge (92174) where the electron temperature gradient is steeper than the ion temperature gradient. Here, $k_y$ is the wavenumber in the direction perpendicular to both the magnetic field and the radial direction, and $\rho_i$ is the ion gyroradius. At $k_y \rho_i \gtrsim 1$, the fastest growing mode is often a novel type of toroidal ETG instability. This toroidal ETG mode is driven at scales as large as $k_y \rho_i \sim (\rho_i/\rho_e) L_{Te} / R_0 \sim 1$ and at a sufficiently large radial wavenumber that electron finite Larmor radius effects become important; that is, $K_x \rho_e \sim 1$, where $K_x$ is the effective radial wavenumber. Here, $\rho_e$ is the electron gyroradius, $R_0$ is the major radius of the last closed flux surface, and $1/L_{Te}$ is an inverse length proportional to the logarithmic gradient of the equilibrium electron temperature. The fastest growing toroidal ETG modes are often driven far away from the outboard midplane. In this equilibrium, ion temperature gradient instability is subdominant at all scales and kinetic ballooning modes are shown to be suppressed by $\mathbf{ E} \times \mathbf{ B} $ shear. ETG modes are very resilient to $\mathbf{ E} \times \mathbf{ B}$ shear. Heuristic quasilinear arguments suggest that the novel toroidal ETG instability is important for transport.

## Stabilisation of short-wavelength instabilities by parallel-to-the-field shear in long-wavelength E × B flows

Journal of Plasma Physics Cambridge University Press (CUP) **86** (2020) 905860601

Magnetised plasma turbulence can have a multiscale character: instabilities driven by mean temperature gradients drive turbulence at the disparate scales of the ion and the electron gyroradii. Simulations of multiscale turbulence, using equations valid in the limit of infinite scale separation, reveal novel cross-scale interaction mechanisms in these plasmas. In the case that both long-wavelength (ion-gyroradius-scale) and shortwavelength (electron-gyroradius-scale) linear instabilities are driven far from marginal stability, we show that the short-wavelength instabilities are suppressed by interactions with long-wavelength turbulence. Two novel effects contributed to the suppression: parallel-to-the-field-line shearing by the long-wavelength E x B flows, and the modification of the background density gradient by the piece of the long-wavelength electron adiabatic response with parallel-to-the-field-line variation. In contrast, simulations of multiscale turbulence where instabilities at both scales are driven near marginal stability demonstrate that when the long-wavelength turbulence is sufficiently collisional and zonally dominated the effect of cross-scale interaction can be parameterised solely in terms of the local modifications to the mean density and temperature gradients. We discuss physical arguments that qualitatively explain how a change in equilibrium drive leads to the observed transition in the impact of the cross-scale interactions.

## Self-sustaining sound in collisionless, high-beta plasma

JOURNAL OF PLASMA PHYSICS **86** (2020) ARTN 905860603

## Jeans modelling of the Milky Way’s nuclear stellar disc

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2020)

<jats:title>Abstract</jats:title> <jats:p>The nuclear stellar disc (NSD) is a flattened stellar structure that dominates the gravitational potential of the Milky Way at Galactocentric radii 30 ≲ R ≲ 300 pc. In this paper, we construct axisymmetric Jeans dynamical models of the NSD based on previous photometric studies and we fit them to line-of-sight kinematic data of APOGEE and SiO maser stars. We find that (i) the NSD mass is lower but consistent with the mass independently determined from photometry by Launhardt et al. (2002). Our fiducial model has a mass contained within spherical radius r = 100 pc of $M(r&amp;lt;100\, {\rm pc}) = 3.9 \pm 1 \times 10^8 \, \rm M_\odot$ and a total mass of $M_{\rm NSD} = 6.9 \pm 2 \times 10^8 \, \rm M_\odot$. (ii) The NSD might be the first example of a vertically biased disc, i.e. with ratio between the vertical and radial velocity dispersion σz/σR &amp;gt; 1. Observations and theoretical models of the star-forming molecular gas in the central molecular zone suggest that large vertical oscillations may be already imprinted at stellar birth. However, the finding σz/σR &amp;gt; 1 depends on a drop in the velocity dispersion in the innermost few tens of parsecs, on our assumption that the NSD is axisymmetric, and that the available (extinction corrected) stellar samples broadly trace the underlying light and mass distributions, all of which need to be established by future observations and/or modelling. (iii) We provide the most accurate rotation curve to date for the innermost 500 pc of our Galaxy.</jats:p>

## Elasticity of tangled magnetic fields

Journal of Plasma Physics Cambridge University Press **86** (2020) 905860511

The fundamental difference between incompressible ideal magnetohydrodynamics and the dynamics of a non-conducting fluid is that magnetic fields exert a tension force that opposes their bending; magnetic fields behave like elastic strings threading the fluid. It is natural, therefore, to expect that a magnetic field tangled at small length scales should resist a large-scale shear in an elastic way, much as a ball of tangled elastic strings responds elastically to an impulse. Furthermore, a tangled field should support the propagation of ‘magnetoelastic waves’, the isotropic analogue of Alfvén waves on a straight magnetic field. Here, we study magnetoelasticity in the idealised context of an equilibrium tangled field configuration. In contrast to previous treatments, we explicitly account for intermittency of the Maxwell stress, and show that this intermittency necessarily decreases the frequency of magnetoelastic waves in a stable field configuration. We develop a mean-field formalism to describe magnetoelastic behaviour, retaining leading-order corrections due to the coupling of large- and small-scale motions, and solve the initial-value problem for viscous fluids subjected to a large-scale shear, showing that the development of small-scale motions results in anomalous viscous damping of large-scale waves. Finally, we test these analytic predictions using numerical simulations of standing waves on tangled, linear force-free magnetic-field equilibria.

## Detecting Kozai–Lidov Imprints on the Gravitational Waves of Intermediate-mass Black Holes in Galactic Nuclei

The Astrophysical Journal American Astronomical Society **901** (2020) 125-125

## Fluctuation dynamo in a weakly collisional plasma

JOURNAL OF PLASMA PHYSICS **86** (2020) ARTN 905860503

## Binary Intermediate-mass Black Hole Mergers in Globular Clusters

The Astrophysical Journal American Astronomical Society **899** (2020) 149-149

## Spin Evolution of Stellar-mass Black Hole Binaries in Active Galactic Nuclei

The Astrophysical Journal American Astronomical Society **899** (2020) 26-26

## The Sixth Data Release of the Radial Velocity Experiment (Rave). II. Stellar Atmospheric Parameters, Chemical Abundances, and Distances

ASTRONOMICAL JOURNAL **160** (2020) ARTN 83

## The Sixth Data Release of the Radial Velocity Experiment (RAVE). I. Survey Description, Spectra, and Radial Velocities

ASTRONOMICAL JOURNAL **160** (2020) ARTN 82

## Formation and Evolution of Compact-object Binaries in AGN Disks

ASTROPHYSICAL JOURNAL American Astronomical Society **898** (2020) ARTN 25

## Cosmic Evolution of Stellar-mass Black Hole Merger Rate in Active Galactic Nuclei

ASTROPHYSICAL JOURNAL American Astronomical Society **896** (2020) ARTN 138

## Trapped orbits and solar-neighbourhood kinematics

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY **495** (2020) 895-904

## Angle-action variables for orbits trapped at a Lindblad resonance

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY **495** (2020) 886-894

## Electromagnetic transients and gravitational waves from white dwarf disruptions by stellar black holes in triple systems

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2020)

Mergers of binaries comprised of compact objects can give rise to explosive transient events, heralding the birth of exotic objects which cannot be formed through single star evolution. Using a large number of direct N-body simulations, we explore the possibility that a white dwarf (WD) is dynamically driven to tidal disruption by a stellar-mass black hole (BH) as a consequence of the joint effects of gravitational wave (GW) emission and Lidov-Kozai oscillations imposed by the tidal field of a outer tertiary companion orbiting the inner BH-WD binary. We explore the sensitivity of our results to the distributions of natal kick velocities imparted to the BH and WD upon formation, adiabatic mass loss, semi-major axes and eccentricities of the triples, and stellar mass ratios. We find rates of WD-TDEs in the range $1.2\times 10^{-3}-1.4$ Gpc$^{-3}$ yr$^{-1}$ for $z\leq 0.1$, rarer than stellar TDEs in triples by a factor of $\sim 3$--$30$. The uncertainty in the TDE rates may be greatly reduced in the future using gravitational wave (GW) observations of Galactic binaries and triples with LISA. WD-TDEs may give rise to high energy X-ray or gamma-ray transients of duration similar to long gamma-ray bursts but lacking the signatures of a core-collapse supernova, while being accompanied by a supernova-like optical transient which lasts for only days. WD--BH and WD--NS binaries will also emit GWs in the LISA band before the TDE. The discovery and identification of triple-induced WD-TDE events by future time domain surveys and/or GWs could enable the study of the demographics of BHs in nearby galaxies.