Colloquium Schedule

Spring 2018

The Physics of Collisionless Shock Waves

Lynn Wilson, Goddard Spaceflight Center
Fri., Apr. 13, 2018, 4:30 PM in Science Center 101

Collisionless shock waves are an ubiquitous phenomena throughout the universe including solar-generated interplanetary shocks, planetary bow shocks, cometary bow shocks, and astrophysical sources, e.g., supernova.  In traditional thermodynamics of collisional media, a shock wave is the result of a nonlinearly steepening wave balancing steepening with energy dissipation through binary particle collisions. Shock waves generated in an ionized gas – called a plasma – not mediated by collisions posed an interesting challenge when they were first hypothesized.  That is, how does such a phenomena irreversibly transform bulk flow kinetic energy into other forms like random kinetic energy (i.e., heat)?  The problem is further complicated by the fact that the plasma is considered a non-equilibrium kinetic gas, not a fluid, which poses challenges for entropy generation and/or irreversibility.  I will discuss our current understanding of the physics of collisionless shocks from an observational point of view and conclude with some unanswered questions.

Plasma Science - From Laboratory Fusion to Astrophysical Plasmas

Fatima Ebrahimi, Princeton Plasma Physics Laboratory and Princeton University
Fri., Apr. 20, 2018, 12:30 PM in Science Center 199

Our universe is immersed in magnetized plasma, electrically conducting ionized gas.  Some of the most fundamental and long-standing astrophysical problems, such as the magnetization of the universe, collimation of astrophysical jets, the accretion process and transport in astrophysical disks (surrounding e.g. black holes) and their coronas can only be explored through plasma physics. Our sun as a natural laboratory for plasma physics provides inspiring as well as challenging problems, including its dynamo cycles, heating, and the replication of its core reaction, fusion energy, on earth in a lab. There is an abundance of observational/experimental data emerging from natural phenomena of space and astrophysical plasmas, as well as laboratory plasma experiments, for plasma physicists to explore. I will review some of these topics, in particular magnetic reconnection, the rearrangement of the magnetic field topology of plasmas, which energizes many processes in nature and has been shown to also be critical in the nonlinear dynamics of many processes in toroidal fusion plasmas. Using global simulations, I will demonstrate the instrumental role of magnetic reconnection, which enables an innovative technique for producing current in fusion plasmas.