Publications by Alexander Schekochihin

Suppressed effective viscosity in the bulk intergalactic plasma

NATURE ASTRONOMY 3 (2019) 832-837

I Zhuravleva, E Churazov, AA Schekochihin, SW Allen, A Vikhlinin, N Werner

Supersonic plasma turbulence in the laboratory.

Nature communications 10 (2019) 1758-

TG White, MT Oliver, P Mabey, M Kühn-Kauffeldt, AFA Bott, LNK Döhl, AR Bell, R Bingham, R Clarke, J Foster, G Giacinti, P Graham, R Heathcote, M Koenig, Y Kuramitsu, DQ Lamb, J Meinecke, T Michel, F Miniati, M Notley, B Reville, D Ryu, S Sarkar, Y Sakawa, MP Selwood, J Squire, RHH Scott, P Tzeferacos, N Woolsey, AA Schekochihin, G Gregori

The properties of supersonic, compressible plasma turbulence determine the behavior of many terrestrial and astrophysical systems. In the interstellar medium and molecular clouds, compressible turbulence plays a vital role in star formation and the evolution of our galaxy. Observations of the density and velocity power spectra in the Orion B and Perseus molecular clouds show large deviations from those predicted for incompressible turbulence. Hydrodynamic simulations attribute this to the high Mach number in the interstellar medium (ISM), although the exact details of this dependence are not well understood. Here we investigate experimentally the statistical behavior of boundary-free supersonic turbulence created by the collision of two laser-driven high-velocity turbulent plasma jets. The Mach number dependence of the slopes of the density and velocity power spectra agree with astrophysical observations, and supports the notion that the turbulence transitions from being Kolmogorov-like at low Mach number to being more Burgers-like at higher Mach numbers.

Overview of new MAST physics in anticipation of first results from MAST Upgrade

NUCLEAR FUSION 59 (2019) ARTN 112011

JR Harrison, RJ Akers, SY Allan, JS Allcock, JO Allen, L Appel, M Barnes, N Ben Ayedl, W Boeglin, C Bowman, J Bradley, P Browning, P Bryant, M Carr, M Cecconello, CD Challis, S Chapman, IT Chapman, GJ Colyer, S Conroy, NJ Conway, M Cox, G Cunningham, RO Dendy, W Dorland, BD Dudson, L Easy, SD Elmore, T Farley, X Feng, AR Field, A Fil, GM Fishpool, M Fitzgerald, K Flesch, MFJ Fox, H Frerichs, S Gadgil, D Gahle, L Garzotti, Y-C Ghim, S Gibson, KJ Gibson, S Hall, C Ham, N Heiberg, SS Henderson, E Highcock, B Hnat, J Howard, J Huang, SWA Irvine, AS Jacobsen, O Jones, I Katramados, D Keeling, A Kirk, I Klimek, L Kogan, J Leland, B Lipschultz, B Lloyd, J Lovell, B Madsen, O Marshall, R Martin, G McArdle, K McClements, B McMillan, A Meakins, HF Meyer, F Militello, J Milnes, S Mordijck, AW Morris, D Moulton, D Muir, K Mukhi, S Murphy-Sugrue, O Myatra, G Naylor, P Naylor, SL Newton, T O'Gorman, J Omotani, MG O'Mullane, S Orchard, SJP Pamela, L Pangione, F Parra, RV Perez, L Piron, M Price, ML Reinke, F Riva, CM Roach, D Robb, D Ryan, S Saarelma, M Salewski, S Scannell, AA Schekochihin, O Schmitz, S Sharapov, R Sharples, SA Silburn, SF Smith, A Sperduti, R Stephen, NT Thomas-Davies, AJ Thornton, M Turnyanskiy, M Valovic, F Van Wyk, RGL Vann, NR Walkden, I Waters, HR Wilson, MAST-U Team, EM Team

Field reconstruction from proton radiography of intense laser driven magnetic reconnection

PHYSICS OF PLASMAS 26 (2019) ARTN 083109

CAJ Palmer, PT Campbell, Y Ma, L Antonelli, AFA Bott, G Gregori, J Halliday, Y Katzir, P Kordell, K Krushelnick, SV Lebedev, E Montgomery, M Notley, DC Carroll, CP Ridgers, AA Schekochihin, MJV Streeter, AGR Thomas, ER Tubman, N Woolsey, L Willingale

Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma.

Nature communications 9 (2018) 591-

P Tzeferacos, A Rigby, AFA Bott, AR Bell, R Bingham, A Casner, F Cattaneo, EM Churazov, J Emig, F Fiuza, CB Forest, J Foster, C Graziani, J Katz, M Koenig, C-K Li, J Meinecke, R Petrasso, H-S Park, BA Remington, JS Ross, D Ryu, D Ryutov, TG White, B Reville, F Miniati, AA Schekochihin, DQ Lamb, DH Froula, G Gregori

Magnetic fields are ubiquitous in the Universe. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter. The standard theoretical model for the origin of these strong magnetic fields is through the amplification of tiny seed fields via turbulent dynamo to the level consistent with current observations. However, experimental demonstration of the turbulent dynamo mechanism has remained elusive, since it requires plasma conditions that are extremely hard to re-create in terrestrial laboratories. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is indeed capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. These results support the notion that turbulent dynamo is a viable mechanism responsible for the observed present-day magnetization.

Polarization of Sunyaev-Zel'dovich signal due to electron pressure anisotropy in galaxy clusters


I Khabibullin, S Komarov, E Churazov, A Schekochihin

Generation of Internal Waves by Buoyant Bubbles in Galaxy Clusters and Heating of Intracluster Medium

Monthly Notices of the Royal Astronomical Society Blackwell Publishing Inc. (2018)

C Zhang, E Churazov, AA Schekochihin

Buoyant bubbles of relativistic plasma in cluster cores plausibly play a key role in conveying the energy from a supermassive black hole to the intracluster medium (ICM) - the process known as radio-mode AGN feedback. Energy conservation guarantees that a bubble loses most of its energy to the ICM after crossing several pressure scale heights. However, actual processes responsible for transferring the energy to the ICM are still being debated. One attractive possibility is the excitation of internal waves, which are trapped in the cluster's core and eventually dissipate. Here we show that a sufficient condition for efficient excitation of these waves in stratified cluster atmospheres is flattening of the bubbles in the radial direction. In our numerical simulations, we model the bubbles phenomenologically as rigid bodies buoyantly rising in the stratified cluster atmosphere. We find that the terminal velocities of the flattened bubbles are small enough so that the Froude number ${\rm Fr}\lesssim 1$. The effects of stratification make the dominant contribution to the total drag force balancing the buoyancy force. In particular, clear signs of internal waves are seen in the simulations. These waves propagate horizontally and downwards from the rising bubble, spreading their energy over large volumes of the ICM. If our findings are scaled to the conditions of the Perseus cluster, the expected terminal velocity is $\sim100-200{\,\rm km\,s^{-1}}$ near the cluster cores, which is in broad agreement with direct measurements by the Hitomi satellite.

Self-inhibiting thermal conduction in a high-beta, whistler-unstable plasma


S Komarov, AA Schekochihin, E Churazov, A Spitkovsky

Proton imaging of stochastic magnetic fields


AFA Bott, C Graziani, P Tzeferacos, TG White, DQ Lamb, G Gregori, AA Schekochihin

Disruption of sheet-like structures in Alfvenic turbulence by magnetic reconnection


A Mallet, AA Schekochihin, BDG Chandran

Kinetic Simulations of the Interruption of Large-Amplitude Shear-Alfvén Waves in a High-β Plasma.

Physical review letters 119 (2017) 155101-155101

J Squire, MW Kunz, E Quataert, AA Schekochihin

Using two-dimensional hybrid-kinetic simulations, we explore the nonlinear "interruption" of standing and traveling shear-Alfvén waves in collisionless plasmas. Interruption involves a self-generated pressure anisotropy removing the restoring force of a linearly polarized Alfvénic perturbation, and occurs for wave amplitudes δB_{⊥}/B_{0}≳β^{-1/2} (where β is the ratio of thermal to magnetic pressure). We use highly elongated domains to obtain maximal scale separation between the wave and the ion gyroscale. For standing waves above the amplitude limit, we find that the large-scale magnetic field of the wave decays rapidly. The dynamics are strongly affected by the excitation of oblique firehose modes, which transition into long-lived parallel fluctuations at the ion gyroscale and cause significant particle scattering. Traveling waves are damped more slowly, but are also influenced by small-scale parallel fluctuations created by the decay of firehose modes. Our results demonstrate that collisionless plasmas cannot support linearly polarized Alfvén waves above δB_{⊥}/B_{0}∼β^{-1/2}. They also provide a vivid illustration of two key aspects of low-collisionality plasma dynamics: (i) the importance of velocity-space instabilities in regulating plasma dynamics at high β, and (ii) how nonlinear collisionless processes can transfer mechanical energy directly from the largest scales into thermal energy and microscale fluctuations, without the need for a scale-by-scale turbulent cascade.

Disruption of Alfvenic turbulence by magnetic reconnection in a collisionless plasma


A Mallet, AA Schekochihin, BDG Chandran

Experimental determination of the correlation properties of plasma turbulence using 2D BES systems


MFJ Fox, AR Field, F van Wyk, Y-C Ghim, AA Schekochihin, MAST Team

Magneto-optic probe measurements in low density-supersonic jets


M Oliver, T White, P Maybe, M Kuehn-Kauffeldt, L Dohl, R Bingham, R Clarke, P Graham, R Heathcote, M Koenig, Y Kuramitsu, DQ Lamb, J Meinecke, T Michel, F Miniati, M Notley, B Reville, S Sarkar, Y Sakawa, AA Schekochihin, P Tzeferacos, N Woolsey, H-S Park, G Gregori

Overview of recent physics results from MAST

Nuclear Fusion 57 (2017)

A Kirk, J Adamek, RJ Akers, S Allan, L Appel, F Arese Lucini, M Barnes, T Barrett, N Ben Ayed, W Boeglin, J Bradley, PK Browning, J Brunner, P Cahyna, S Cardnell, M Carr, F Casson, M Cecconello, C Challis, IT Chapman, S Chapman, J Chorley, S Conroy, N Conway, WA Cooper, M Cox, N Crocker, B Crowley, G Cunningham, A Danilov, D Darrow, R Dendy, D Dickinson, W Dorland, B Dudson, D Dunai, L Easy, S Elmore, M Evans, T Farley, N Fedorczak, A Field, G Fishpool, I Fitzgerald, M Fox, S Freethy, L Garzotti, YC Ghim, K Gi, K Gibson, M Gorelenkova, W Gracias, C Gurl, W Guttenfelder, C Ham, J Harrison, D Harting, E Havlickova, N Hawkes, T Hender, S Henderson, E Highcock, J Hillesheim, B Hnat, J Horacek, J Howard, D Howell, B Huang, K Imada, M Inomoto, R Imazawa, O Jones, K Kadowaki, S Kaye, D Keeling, I Klimek, M Kocan, L Kogan, M Komm, W Lai, J Leddy, H Leggate, J Hollocombe, B Lipschultz, S Lisgo, YQ Liu, B Lloyd, B Lomanowski, V Lukin, I Lupelli, G Maddison, J Madsen, J Mailloux, R Martin, G McArdle, K McClements, B McMillan, A Meakins, H Meyer, C Michael

© 2017 Culham Centre for Fusion Energy. New results from MAST are presented that focus on validating models in order to extrapolate to future devices. Measurements during start-up experiments have shown how the bulk ion temperature rise scales with the square of the reconnecting field. During the current ramp-up, models are not able to correctly predict the current diffusion. Experiments have been performed looking at edge and core turbulence. At the edge, detailed studies have revealed how filament characteristics are responsible for determining the near and far scrape off layer density profiles. In the core the intrinsic rotation and electron scale turbulence have been measured. The role that the fast ion gradient has on redistributing fast ions through fishbone modes has led to a redesign of the neutral beam injector on MAST Upgrade. In H-mode the turbulence at the pedestal top has been shown to be consistent with being due to electron temperature gradient modes. A reconnection process appears to occur during edge localized modes (ELMs) and the number of filaments released determines the power profile at the divertor. Resonant magnetic perturbations can mitigate ELMs provided the edge peeling response is maximised and the core kink response minimised. The mitigation of intrinsic error fields with toroidal mode number n > 1 has been shown to be important for plasma performance.

Numerical modeling of laser-driven experiments aiming to demonstrate magnetic field amplification via turbulent dynamo

PHYSICS OF PLASMAS 24 (2017) ARTN 041404

P Tzeferacos, A Rigby, A Bott, AR Bell, R Bingham, A Casner, F Cattaneo, EM Churazov, J Emig, N Flocke, F Fiuza, CB Forest, J Foster, C Graziani, J Katz, M Koenig, C-K Li, J Meinecke, R Petrasso, H-S Park, BA Remington, JS Ross, D Ryu, D Ryutov, K Weide, TG White, B Reville, F Miniati, AA Schekochihin, DH Froula, G Gregori, DQ Lamb

Ion-scale turbulence in MAST: anomalous transport, subcritical transitions, and comparison to BES measurements


F van Wyk, EG Highcock, AR Field, CM Roach, AA Schekochihin, FI Parra, W Dorland

Amplitude limits and nonlinear damping of shear-Alfven waves in high-beta low-collisionality plasmas


J Squire, AA Schekochihin, E Quataert

Collisionality scaling of the electron heat flux in ETG turbulence


GJ Colyer, AA Schekochihin, FI Parra, CM Roach, MA Barnes, Y-C Ghim, W Dorland

A statistical model of three-dimensional anisotropy and intermittency in strong Alfvenic turbulence


A Mallet, AA Schekochihin