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By Michael Brown
The APS Topical Group on Plasma Astrophysics (GPAP) was formed in 1999 to provide an intellectual home for plasma physicists who have an interest in astrophysical phenomena. At present, GPAP has 400 members from broad backgrounds and includes scientists at both universities and national labs. The topical group serves as a bridge between the APS Division of Astrophysics (DAP) and the Division of Plasma Physics (DPP), which organizes the annual meeting for GPAP.
GPAP members are engaged in many exciting research areas, particularly laboratory astrophysics. Laboratory astrophysicists try to uncover fundamental plasma physics processes that might be at play in astrophysical settings. They ask scientific questions such as: “What happens when plasma waves collide?” “What happens when you stir plasma?” and “What happens when you compress plasma?”
To answer these questions, three laboratory astrophysics experiments have been designed by GPAP members and provide good examples of current work. First is an experiment performed at UCLA by Greg Howes from the University of Iowa on what gives rise to plasma turbulence. Next is an experiment by Cami Collins in the lab of Cary Forest at the University of Wisconsin that gives insight into what happens to plasma on galactic scales. Last is a recent experiment by Manjit Kaur in my lab at Swarthmore in which we probe the equation of state in a magnetized plasma.
A hallmark of turbulence in electrically conducting or magnetohydrodynamic (MHD) fluids is the spectral transfer of energy in both spatial and temporal frequencies, from large and slow scales to small and fast ones. The typical picture of turbulent dynamics is that energy is introduced into the system at large spatial scales (i.e. low spatial frequency k) by either stirring or interaction with boundaries. The fundamental nonlinear process by which large-scale structures bifurcate in plasmas is due to the interaction of two counter-propagating Alfvén waves .
An Alfvén wave is one of the three normal modes of oscillation in MHD, along with fast and slow magnetosonic waves. MHD turbulence theory predicts that the collision of two Alfvén waves transfers energy nonlinearly to a third wave that has both higher spatial and temporal frequencies. Howes and his team refer to this interaction as the fundamental building block of astrophysical plasma turbulence.
In experiments carried out at UCLA, Howes and coworkers launched counter-propagating Alfvén waves in the magnetized plasma column at the Large Plasma Device (LAPD). After mapping out the resultant fields, the researchers discovered the signature of a nonlinear daughter Alfvén wave with the correct properties, just as predicted.
Astrophysical plasmas at galactic scales (e.g., accretion disks formed as black holes draw matter from companion stars) are stirred and sheared by differential rotation. These objects are typically flow-dominated in the sense that the kinetic energy density far exceeds the magnetic field energy density. By contrast, most laboratory plasma experiments are magnetically dominated, since a strong magnetic field is necessary to confine hot plasma, and typically lab plasmas are nearly at rest. In order to study fundamental processes of flow-dominated plasmas, Collins and the Forest group at the University of Wisconsin have developed a technique to stir unmagnetized plasma in the lab. For her work on this project, Collins won the 2015 Marshall N. Rosenbluth Outstanding Doctoral Thesis Award.
This technique is based on using J×B torques at the edge of the device to stir unmagnetized plasma . A magnetic field is supplied by azimuthal rings of permanent magnets of alternating polarity, forming a cusp magnetic geometry. Interspersed between the magnet poles are alternating anodes and hot cathodes to draw current. The resulting localized J×B force generates a torque on the plasma near the wall.
These ideas have been applied to a device called the Big Red Ball (BRB) at the University of Wisconsin-Madison, which is newly funded by the U.S. Department of Energy as a national user facility. The BRB is a flexible, 3 meters in diameter spherical plasma machine capable of generating arbitrary flow patterns. Some experiments proposed on BRB include driving a large-scale dynamo and studying collisionless magnetic reconnection.
Little is known about what happens in astrophysics when hot, magnetized plasma is compressed and expanded. The equation of state (EOS) of an ideal gas relates pressure, volume, and temperature of the gas, but charged particles in plasma need not obey an ideal gas EOS, particularly if there is a strong magnetic field. Motion of charged particles parallel and perpendicular to the background magnetic field need not be coupled. Indeed, magnetized plasmas are often described with two different temperatures, Tl/ and T⊥. To complicate matters, there are adiabatic invariants, one associated with Tl/ and the other with T⊥ (the magnetic moment), that are separately conserved. For astrophysical events such as the solar wind, the EOS is unknown, if one exists at all.
My plasma physics group recently published a paper detailing the study of thermodynamics of compressed magnetized plasmas, referred to by the authors as magnetothermodynamics . The paper reports experiments in which a parcel of magnetized, fully relaxed, non-axisymmetric plasma is generated in the lab and compressed against a conducting cylinder that is closed at one end. The plasma parameters such as temperature, density, magnetic field, and volume are measured during compression, and a PV diagram is constructed to identify instances of associated ion heating during these compression events. The MHD ideal-gas-like EOS is inconsistent with their observations, but an EOS related to the adiabatic invariants is consistent.
New things are in store for GPAP: leadership has recently turned over, and in the coming months, we will resurrect the group’s newsletter. Stay tuned for announcements regarding student travel grants — we offer five $500 grants. We encourage any APS member interested in plasma astrophysics to join GPAP by visiting the Join an APS Unit page. For more on GPAP go to their website.
The author is professor of physics at Swarthmore College and vice chair of GPAP.
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