# Publications

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

Journal of Plasma Physics (2021)

JM Garcia-Regaña, M Barnes, I Calvo, FI Parra, JA Alcuson, R Davies, A Gonzalez-Jerez, A Mollen, E Sanchez, JL Velasco, A Zocco

Copyright © The Author(s), 2021. Published by Cambridge University Press. A study of turbulent impurity transport by means of quasilinear and nonlinear gyrokinetic simulations is presented for Wendelstein 7-X (W7-X). The calculations have been carried out with the recently developed gyrokinetic code stella. Different impurity species are considered in the presence of various types of background instabilities: ion temperature gradient (ITG), trapped electron mode (TEM) and electron temperature gradient (ETG) modes for the quasilinear part of the work; ITG and TEM for the nonlinear results. While the quasilinear approach allows one to draw qualitative conclusions about the sign or relative importance of the various contributions to the flux, the nonlinear simulations quantitatively determine the size of the turbulent flux and check the extent to which the quasilinear conclusions hold. Although the bulk of the nonlinear simulations are performed at trace impurity concentration, nonlinear simulations are also carried out at realistic effective charge values, in order to know to what degree the conclusions based on the simulations performed for trace impurities can be extrapolated to realistic impurity concentrations. The presented results conclude that the turbulent radial impurity transport in W7-X is mainly dominated by ordinary diffusion, which is close to that measured during the recent W7-X experimental campaigns. It is also confirmed that thermodiffusion adds a weak inward flux contribution and that, in the absence of impurity temperature and density gradients, ITG- and TEM-driven turbulence push the impurities inwards and outwards, respectively.

## Eccentric Black Hole Mergers in Active Galactic Nuclei

ASTROPHYSICAL JOURNAL LETTERS 907 (2021) ARTN L20

H Tagawa, B Kocsis, Z Haiman, I Bartos, K Omukai, J Samsing

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

Nuclear Fusion IOP Publishing 60 (2020) 126045

FI Parra, CM Roach, C Giroud, WD Dorland, DR Hatch, M Barnes, J Hillesheim, N Aiba, J Ball, P Ivanov

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

MR HARDMAN, M BARNES, CM Roach

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

MW Kunz, J Squire, AA Schekochihin, E Quataert

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

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

MC Sormani, J Magorrian, F Nogueras-Lara, N Neumayer, R Schönrich, RS Klessen, A Mastrobuono-Battisti

&lt;jats:title&gt;Abstract&lt;/jats:title&gt; &lt;jats:p&gt;The nuclear stellar disc (NSD) is a flattened stellar structure that dominates the gravitational potential of the Milky Way at Galactocentric radii 30 &#x2272; R &#x2272; 300&#x2009;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&#xA0;al. (2002). Our fiducial model has a mass contained within spherical radius r = 100&#x2009;pc of $M(r&amp;amp;lt;100\, {\rm pc}) = 3.9 \pm 1 \times 10^8 \, \rm M_\odot$ 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 &#x3C3;z/&#x3C3;R &amp;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 &#x3C3;z/&#x3C3;R &amp;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&#x2009;pc of our Galaxy.&lt;/jats:p&gt;

## Elasticity of tangled magnetic fields

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

DN Hosking, A Schekochihin, S Balbus

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

B Deme, B-M Hoang, S Naoz, B Kocsis

## Fluctuation dynamo in a weakly collisional plasma

JOURNAL OF PLASMA PHYSICS 86 (2020) ARTN 905860503

DA St-Onge, MW Kunz, J Squire, AA Schekochihin

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

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

A Rasskazov, G Fragione, B Kocsis

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

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

H Tagawa, Z Haiman, I Bartos, B Kocsis

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

ASTROPHYSICAL JOURNAL American Astronomical Society 898 (2020) ARTN 25

H Tagawa, Z Haiman, B Kocsis

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

ASTROPHYSICAL JOURNAL American Astronomical Society 896 (2020) ARTN 138

Y Yang, I Bartos, Z Haiman, B Kocsis, S Marka, H Tagawa

## Trapped orbits and solar-neighbourhood kinematics

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

J Binney

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

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

J Binney

## 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)

G Fragione, BD Metzger, R Perna, NWC Leigh, B Kocsis

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.

## Intermediate-mass Black Holes' Effects on Compact Object Binaries

ASTROPHYSICAL JOURNAL American Astronomical Society 892 (2020) ARTN 130

B Deme, Y Meiron, B Kocsis

Although their existence is not yet confirmed observationally, intermediate mass black holes (IMBHs) may play a key role in the dynamics of galactic nuclei. In this paper, we neglect the effect of the nuclear star cluster itself and investigate only how a small reservoir of IMBHs influences the secular dynamics of stellar-mass black hole binaries, using N-body simulations. We show that our simplifications are valid and that the IMBHs significantly enhance binary evaporation by pushing the binaries into the Hill-unstable region of parameter space, where they are separated by the SMBH's tidal field. For binaries in the S-cluster region of the Milky Way, IMBHs drive the binaries to merge in up to 1-6% of cases, assuming five IMBHs within 5 pc of mass 10,000 solar masses each. Observations of binaries in the Galactic center may strongly constrain the population of IMBHs therein.

## Transport of high-energy charged particles through spatially-intermittent turbulent magnetic fields

Astrophysical Journal American Astronomical Society 892 (2020) 114

LE Chen, AFA Bott, P Tzeferacos, A Rigby, A Bell, R Bingham, C Graziani, J Katz, R Petrasso, G Gregori, F Miniati

Identifying the sources of the highest energy cosmic rays requires understanding how they are deflected by the stochastic, spatially intermittent intergalactic magnetic field. Here we report measurements of energetic charged-particle propagation through a laser-produced magnetized plasma with these properties. We characterize the diffusive transport of the particles experimentally. The results show that the transport is diffusive and that, for the regime of interest for the highest-energy cosmic rays, the diffusion coefficient is unaffected by the spatial intermittency of the magnetic field.

## Effective spin distribution of black hole mergers in triples

Monthly Notices of the Royal Astronomical Society 493 (2020) 3920-3931

G Fragione, B Kocsis

&#xA9; 2020 The Author(s). Many astrophysical scenarios have been proposed to explain the several black hole (BH) and neutron star binary mergers observed via gravitational waves (GWs) by the LIGO-Virgo collaboration. Contributions from various channels can be statistically disentangled by mass, spin, eccentricity, and redshift distributions of merging binaries. In this paper,we investigate the signatures of BH-BH binary mergers induced by a third companion through the Lidov-Kozai mechanism in triple systems. We adopt different prescriptions for the supernovae natal kicks and consider different progenitor metallicities and initial orbital parameters. We show that the typical eccentricity in the LIGO band is 0.01-0.1 and that the merger rate is in the range 0.008-9Gpc-3 yr-1, depending on the natal kick prescriptions and progenitor metallicity. Furthermore, we find that the typical distribution of effective projected spin is peaked at Xeff ~ 0 with significant tails. We show that the triple scenario could reproduce the distribution of Xeff. We find that the triple channel may be strongly constrained by the misalignment angle between the binary component spins in future detections with spin precession.

## Making a Supermassive Star by Stellar Bombardment

ASTROPHYSICAL JOURNAL American Astronomical Society 892 (2020) ARTN 36

H Tagawa, Z Haiman, B Kocsis

Approximately two hundred supermassive black holes (SMBHs) have been discovered within the first $\sim$Gyr after the Big Bang. One pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs). A possible obstacle to this scenario is that the collapsing gas fragments and forms a cluster of main-sequence stars. Here we raise the possibility that stellar collisions may be sufficiently frequent and energetic to inhibit the contraction of the massive protostar, avoiding strong UV radiation driven outflows, and allowing it to continue growing into an SMS. We investigate this scenario with semianalytic models incorporating star formation, gas accretion, dynamical friction from stars and gas, stellar collisions, and gas ejection. We find that when the collapsing gas fragments at a density of $\lesssim 3\times 10^{10}\,\mathrm{cm^{-3}}$, the central protostar contracts due to infrequent stellar mergers, and in turn photoevaporates the remaining collapsing gas, resulting in the formation of a $\lesssim 10^4~{\rm M_\odot}$ object. On the other hand, when the collapsing gas fragments at higher densities (expected for a metal-poor cloud with $Z\lesssim10^{-5}\,{\rm Z_\odot}$ with suppressed ${\rm H_2}$ abundance) the central protostar avoids contraction and keeps growing via frequent stellar mergers, reaching masses as high as $\sim 10^5-10^6\,{\rm M_\odot}$. We conclude that frequent stellar mergers represent a possible pathway to form massive BHs in the early universe.