Matthew R. Stoneking

Research Interests

Team Electron 2006

(left to right: Joan Marler, Bao Ha, Sarah Curry, Julian Hector, and Matt Stoneking)

Plasma Physics

Plasma physics is the study of hot, ionized gases, sometimes referred to as the fourth state of matter. A plasma, being a collection of charged particles interacting with each other over long distances via electric and magnetic forces, exhibits a wide array of complex behavior including waves and instabilities. Plasmas occur abundantly in space (stars, the solar wind and the interstellar medium) and in the earth's ionosphere (evidenced by, for example, auroral activity).

One of the potential applications of plasma physics is the production of electric power from nuclear fusion. To release significant power from fusion reactions requires that the fuel be 1) hot, 2) dense, and 3) well contained. The most promising method for containing (or confining) a hot plasma is magnetic confinement whereby a strong magnetic field insulates the plasma from the walls of the container. Perfect insulation is not generally achieved because turbulent fluctuations arise in the plasma that transport plasma particles and energy out of the magnetic field and into the wall of the container. A major emphasis of my graduate and post-doctoral research was dedicated to quantifying the turbulent transport due to magnetic turbulence, to understanding the physics governing the turbulence and attempting to reduce the turbulence, i.e. plugging the leaks in the magnetic bottle. Since coming to Lawrence University in 1997, my research has been of a more basic and fundamental nature…

Toroidal Pure Electron Plasmas

Non-neutral plasmas are collections of charged particles of a single sign of charge. The most commonly studied non-neutral plasmas are pure electron plasmas and (positive) ion plasmas, although other exotic examples such as positron (i.e. anti-electron) plasmas or negative ion plasmas are also possible. Non-neutral plasmas exhibit many of the same phenomena as traditional, more commonly studied plasmas that contain nearly equal quantities of positive and negative charge. Collective (or many-body) effects such as the screening out of electric potential perturbations, the propagation of particle density waves, and the growth of unstable perturbations are characteristic features of both traditional and non-neutral plasmas. However, since non-neutral plasmas are charged, they possess a self-electric field or “space-charge electric field.” The space charge electric field of the non-neutral plasma introduces behavior that is peculiar to such systems. In my experiment, I exploit one such peculiar feature of non-neutral plasmas, namely plasma flow or rotation, to confine it.

The Lawrence Non-neutral Torus (LNT) experiment and its successor, the LNT II continue a sparse but historically extended line of experiments in which electron plasmas are confined in a purely toroidal magnetic field. In 2003-2004 we demonstrated that electron plasmas could be trapped for more than 10 milliseconds in a purely toroidal magnetic field. This was the first unambiguous demonstration that the self-electric field (space-charge electric field) of the plasma and its associated plasma flow can act to like the poloidal magnetic field of a tokamak to provide and effective rotational transform. Without this effect, the plasma would drift out of the trap on the timescale of about 100 microseconds. Confinement was ultimately limited by either diffusion associated with collisions with neutral gas atoms or transport associated with asymmetries in the trap.

Construction of the successor experiment, LNT II, was recently completed. LNT II operates at lower neutral pressure, higher magnetic field and with improved trap symmetry. The improved symmetry is achieved by construction of a conducting (gold-plated aluminum) toroidal shell that surrounds the plasma and is segmented to permit experimental control and diagnosis (image charge signals). The first experiments in the new device achieved confinement times longer than one second, consistent with the most optimistic design projections.

Papers from the Lawrence Non-neutral Plasma Physics Group:

J.P. Marler and M.R. Stoneking, Confinement Time Exceeding One Second for a Toroidal Electron Plasma, to be published in Phys. Rev. Lett. (2008).

J.P. Marler and M.R. Stoneking, Non-neutral Plasma Confinement in Toroidal Geometry, Journal of Physics: Conference Series 71, 012003 (2007). Workshop on Nonequilibrium Processes in Plasma Physics and Studies of the Environment.

J.P. Marler and M.R. Stoneking, A Kilogauss-scale, High-vacuum Toroidal Electron Plasma Experiment, in Non-neutral Plasma Physics VI, AIP Conf. Proc. 862, edited by M. Drewsen, U. Uggerhoj, and H. Knudsen, (American Institute of Physics, New York, 2006), p. 71. Workshop on Non-neutral Plasmas.

M. R. Stoneking, M. A. Growdon, M.L. Milne, and R. T. Peterson, Poloidal ExB drift used as an effective rotational transform to achieve long confinement times in a toroidal electron plasma, Phys. Rev. Lett. 92, 095003 (2004).

M.R. Stoneking, M.A. Growdon, M.L. Milne, and R.T. Peterson, Millisecond Confinement and Observation of the m=1Diocotron Mode in a Toroidal Electron Plasma, in Non-neutral Plasma Physics V, AIP Conf. Proc. Vol. 692, edited by M. Schauer, T. Mitchell, and R. Nebel, 2003.

M.R. Stoneking, P.W. Fontana, R.L. Sampson, and D.J. Thuecks, Electron plasmas in a "partial" torus, Phys. Plasmas 9, (2002).

M.R. Stoneking, P.W. Fontana, R.L. Sampson, and D.J. Thuecks, Electron Plasma Confinement in a Partially Toroidal Trap, in Non-neutral Plasma Physics IV, AIP Conf.Proc. Vol. 606, edited by F. Anderegg, L. Schweikhard and C.F. Driscoll, 2001.