Plasma and Fluids
Research in the physics of plasmas and fluids is carried out by Professor Barrett Rogers and Emeritus Professor David Montgomery, and includes studies of nonlinear magnetohydrodynamics and turbulence in plasmas and fluids, investigations of magnetic reconnection, the physics of fusion devices, analytical dynamics, computational physics, plasma simulation, and plasma theory. The department also has a large vacuum calibration and test system for quantifying the response of particle detectors flown on auroral sounding rockets to the space environment, including a plasma source. Further details are described under Professor Kristina Lynch's rocket lab.
Through the efforts of an an international fusion research program, dramatic progress has been made in recent years toward the goal of confining a fusion-grade plasma in the laboratory using intense magnetic fields. The performance of these fusion devices is typically limited by a host of instabilities, which produce turbulence and degrade the confinement of particles and thermal energy. Research on this topic at Dartmouth (Rogers and Montgomery) is focused on understanding the physics of these instabilities as well as the turbulence and transport that they produce. This work relies heavily on both analytic methods as well as state-of-the-art massively parallel numerical simulations.
Professor Barrett Rogers's research is focussed on theoretical and computational plasma physics and addresses topics such as basic plasma physics, magnetic reconnection, plasma turbulence, particle acceleration, instabilities, magnetic fusion, and plasma astrophysics of the present and early universe.
Space and laboratory plasmas such as the sun, the magnetopshere, and laboratory fusion experiments often store large quantities of energy in embedded magnetic fields. Magnetic reconnection is a ubiquitous process that can convert some fraction of this energy, often explosively, into high speed flows and thermal energy. Reconnection is the fundamental process underlying cataclysmic phenomena such as coronal mass ejections, magnetospheric substorms, and ``sawtooth-crash'' reconnection in tokamak fusion reactors. The rapidity of such events is often so extreme that it has been an ongoing challenge to explain theoretically. Reconnection also generates intense electric fields that can accelerate particles to very high energies.
Turbulence is another ubiquitous feature of plasma systems. It can arise, for example, from non-uniformities in plasma flows, or be driven by instabilities arising from non-uniformities in the plasma density or temperatures. In magnetic fusion experiments, small-scale instabilities generate turbulence at small scales, which in turn transport plasma and heat from the core of fusion reactors to the walls. The convective losses generated in this way are among the greatest difficuties facing the design of a feasible fusion device.
Professor Montgomery's recent activities have included studies of:(1) the effect of an externally-imposed dc magnetic field's partial or total suppression of turbulent magnetohydrodynamic (MHD) dynamo action (see, e.g., D.C. Montgomery et al, Phys. Plasmas 9, 1221 (2002) and 6, 2727 (1999)); (2) the mass flows which necessarily arise in steady-state toroidal confinement devices for magnetic fusion plasmas (see, e.g., L.P. Kamp and D.C. Montgomery, Phys. Plasmas 10, 157 (2003)). The two figures below are taken from a 2004 Kamp/Montgomery paper, and show field lines (magnetic and mechanical streamlines) inside a toroid that is being Ohmically driven when viscous and resistive boundary conditions are being imposed. Both toroidal and poloidal flows necessarily result (see Kamp and Montgomery, J. Plasma Phys, 70, 113 (2004)) even in the laminar steady state; (3) together with P.D. Minninni (Buenos Aires) and L. Turner (Cornell), three-dimensional, spectral-method, magnetohydrodynamic computations of dynamos inside spherical shells have been carried out, with particular reference to the role that magnetic and mechanical helicity might play in the generation of planetary magnetic fields (the figure at the top of this page shows some computer graphics displaying typical evolving computed energy densities and field lines for mechanical and magnetic driven helical flows in the interior of the sphere (Mininni et al, New Journal of Physics 9, 303 (2007)); (4) maximum entropy predictions for the evolution of two-dimensional turbulent vortices both in a Malmberg-Penning electron trap and in numerical solutions of the neutral fluid dynamical equations have been in progress with an experimental group from the University of Delaware (e.g., see D.J. Rodgers et al, Phys. Rev. Letters 102, 244501 (2009)), and also in the completely unbounded case where the Oseen vortex has been shown to be the relaxed long-time state of a compact vorticity distribution with circulation in the absence of boundaries (D.C. Montgomery and W.H. Matthaeus, Phys. Fluids 23, 075104 (2011).
D. J. Rodgers,1 S. Servidio, W. H. Matthaeus,1 D. C. Montgomery, T. B. Mitchell,* and T. Aziz, "Hydrodynamic Relaxation of an Electron Plasma to a Near-Maximum Entropy State" Phys. Rev. PRL 102, 244501 (2009).
D. J. Rodgers,1 S. Servidio, W. H. Matthaeus,1 D. C. Montgomery, T. B. Mitchell,* and T. Aziz "Nonlinear Magnetohydrodynamics by Galerkin-method Computation" Phys. Rev. PRL 102, 244501 (2009).
P.D. Mininni, D.C. Montgomery, and L.Turner "Hydrodynamic and magnetohydrodynamic computations inside a rotating sphere" New J. Phys. 9, 303(25 pages), 2007.
D.C. Montgomery and W.H. Matthaeus, "Oseen vortex as a maximum entropy state in a two dimensional fluid," Phys. Fluids 23, 075104 (2011)