Compressible Convection in Stars

My work with the MUSIC code centers on understanding the effect of compressibility on stellar convection, e.g. convective overshooting. MUSIC is designed as a hydrodynamic extension of a 1D stellar evolution code. It therefore uses a realistic equation of state and realistic opacities for stars. The initial profiles of density and temperature are taken directly from stellar evolution data, and so these are also as realistic as possible. MUSIC a fully compressible, time-implicit, implicit large-eddy simulation, hydrodynamic code for exploring convection in the interiors of stars. MUSIC uses spherical geometry.

Papers:

J. Pratt, I. Baraffe, T. Goffrey, C. Geroux, M. Viallet, D. Folini, T. Constantino, M.V. Popov, R. Walder. Spherical-shell boundaries for two-dimensional compressible convection in a star. Astronomy and Astrophysics, 2016. arXiv:1606.07200 [astro-ph.SR]

T. Goffrey, J. Pratt, M. Viallet, I. Baraffe, M. V. Popov, R. Walder, D. Folini, C. Geroux, T. Constantino. Benchmarking the Multi-dimensional Stellar Implicit Code MUSIC. Astronomy and Astrophysics, 2016.

C. Geroux, I. Baraffe, M. Viallet, T. Goffrey, J. Pratt, T. Constantino, R. Walder, D. Folini, and M. V. Popov. Multi-dimensional structure of accreting young stars. accepted by Astronomy and Astrophysics, February 2016.

M. Viallet, T. Goffrey, I. Baraffe, D. Folini, C. Geroux, M. V. Popov, J. Pratt, and R.Walder. A Jacobian-free Newton-Krylov method for time-implicit multidimensional hydrodynamics. Astronomy and Astrophysics, February 2016.

J. Pratt, M. Viallet, I. Baraffe, C. Geroux, T. Goffrey, M.V. Popov, D. Folini, R.Walder Multi-dimensional models of the interior of stars. Proceedings of the 22nd UK Conference of the Association for Computational Mechanics in Engineering, 2-4 April 2014, University of Exeter, Exeter, UK.

MHD instabilities in tokamaks: magnetic reconnection and tearing modes

My work with the JOREK code centers on understanding magnetic reconnection in tokamaks, and how the growing magnetic islands that are the most dangerous in a fusion machine can be limited or suppressed with electron cyclotron current drive. JOREK is a compressible MHD code in a realistic tokamak geometry. JOREK has been developed over the last decade at CEA Cadarache, ITER, IPP Garching, and by a large international group of developers.

Papers:

J. Pratt, G.T.A Huismans, E. Westerhof. Early evolution of electron cyclotron driven current during suppression of tearing modes in a circular tokamak. Physics of Plasmas, 2016. arXiv:1609.08850 [physics.plasm-ph]

J.W. Haverkort, H.J. de Blank, G.T.A. Huysmans, J. Pratt, B. Koren. Implementation of the full viscoresistive magnetohydrodynamic equations in a nonlinear finite element code. Journal of Computational Physics, Volume 316, Pages 281-302, 2016.

E. Westerhof, H.J. de Blank, J. Pratt. New insights into the generalized Rutherford equation for nonlinear neoclassical tearing mode growth from 2D reduced MHD simulations. Volume 56, Number 3, Nuclear Fusion, February 2016. Eurofusion preprint

E. Westerhof and J. Pratt. "Closure of the single fluid magnetohydrodynamic equations in presence of electron cyclotron current drive." accepted by Physics of Plasmas, Sept 2014. arXiv:1409.5613 [physics.plasm-ph].

E. Westerhof, J. Pratt. "Expression of electron cyclotron current drive in plasma fluid models." Proceedings of the 40th EPS Conference on Plasma Physics. Espoo, Finland, July 1st -- 5th 2013. (pdf download)

Turbulence, MHD turbulence, and Boussinesq convection in astrophysical plasmas

My work with the MHDT code centers on understanding properties of diffusion/dispersion, transport, the formation of large-scale structures, and the influence of anisotropy on the flow. The MHDT code is a pseudo-spectral direct numerical simulation of MHD turbulence. MHD turbulence can be driven by Boussinesq convection or by various means of isotropic random forcing. Lagrangian tracer particles have been implemented to allow the examination of Lagrangian statistics.

Papers:

J. Pratt, A. Busse, W.-C. Müller, S.C. Chapman, N.W. Watkins Extreme-value statistics from Lagrangian convex hull analysis I. Validation for homogeneous turbulent Boussinesq convection and MHD convection. submitted to New Journal of Physics, 2016. arXiv:1605.05983v3 [physics.flu-dyn]

J. Pratt, A. Busse, W.-C. Müller. A Lagrangian Perspective on Anisotropy in Turbulent Flows. Forschung im HLRN-Verbund 2015 Project Book of the Northern German Computing Alliance).

J. Pratt, A. Busse, W.-C. Müller. "Fluctuation dynamo amplified by intermittent shear-bursts in convectively-driven magnetohydrodynamic turbulence." A&A 557 A76 (2013). arXiv:1304.0190v4 [astro-ph.SR]

R. Moll, J. Pietarila Graham, J. Pratt, R.H. Cameron, W.-C. Müller, and M. Schüssler. Universality of the Small-Scale Dynamo Mechanism. The Astrophysical Journal, Volume 736 Number 1 p36. July 2011. arXiv:1105.0546 [astro-ph.SR]

MHD instabilities in Fusion Plasmas (Tandem Mirrors): flute modes and trapped particle modes

Mirror magnetic fields interact with MHD instabilities, waves, and energetic particles in interesting ways. Mirror magnetic fields are of great interest in the solar wind and many other astrophysical settings. A tandem mirror machine is an idea for a fusion machine that uses mirror magentic fields to confine a hot plasma. My PHD thesis determined the limits of stability for MHD instabilities and trapped particle instabilities in several different designs for tandem mirror machines. One of these machine designs was the GAMMA-10, a giant fusion experiment built in Tsukuba, Japan. Another was the kinetically stabilized tandem mirror, a new design for a fusion machine developed at Livermore.

Papers:

J. Pratt, H. L. Berk, W. Horton. "Drift-Wave Eigenmodes and Spectral Gaps in Tandem Mirrors." Fusion Science and Technology, v55,2T,p25, 2009.

H. L. Berk and J. Pratt. Trapped Particle Stability for the Kinetic Stabilizer. Nuclear Fusion, 51 (2011) 083025.

J. Pratt, W. Horton. "Global Energy Confinement scaling predictions for the kinetically stabilized tandem mirror." Physics of Plasmas. 13 (4), 2006.

Fluid Closures and MHD

This was my undergraduate thesis work, undertaken partly during my summers as an undergraduate research student at Los Alamos National Laboratory. I proved that the EDQNM fluid closure is realizable for the MHD equations. I independently developed a small pseudo-spectral code that solved the MHD equations to numerically show the reasonable results of this closure.

Papers:

Turner, Leaf and Jane Pratt. "Eddy-Damped Quasinormal Markovian Closure: A Closure for Magnetohydrodynamic Turbulence? J. Phys. A: Math. Gen. 35 (2002) 781-793.