**NJIT
Physics Department Seminar**

**November 30th, Monday **

**Fast Magnetic Reconnection
in Astrophysical & Laboratory Plasmas**

**Prof. Amitava Bhattacharjee**

Dept. of Astrophysical
Sciences & Princeton Plasma Physics Laboratory

Princeton University

(Solar Physics, Host: Kosovichev)

Time: **11:45am-12:45pm with 11:30am tea time**

Room: **ECE202**

**Abstract**:

The problem of fast magnetic reconnection in
high-Lundquist-number plasmas has been an active area of research for several
decades. The main challenge is to explain why reconnection in natural or
laboratory plasmas (including fusion devices) can proceed rapidly from a
relatively quiescent state characterized by high values of the Lundquist
number. Recent work has demonstrated that there is a fundamental shortcoming
in the classical Sweet-Parker theory even within the framework of
resistive magnetohydrodynamics (MHD). When the Lundquist number exceeds a critical value, the
Sweet-Parker layer is unstable to a rapid tearing instability,
hereafter referred to as the plasmoid instability.
Numerical simulations, supported by heuristic scaling arguments, strongly
suggest that within the framework of resistive MHD, the nonlinear reconnection
rate mediated by the plasmoid instability
becomes insensitive to the Lundquist number. Because the plasmoid instability can initiate a cascade to thin current
sheets that are much thinner than the original Sweet-Parker layer, the
so-called Hall current terms in the generalized Ohm’s law become important,
triggering the onset of Hall reconnection, which lead to higher reconnection
rates. Recent results from the largest two-dimensional Hall MHD
simulations to date will be presented that demonstrate the rich dynamics
enabled by the interplay between the plasmoid
instability and the Hall current. In three dimensions, the plasmoids
evolve to form flux ropes that produce turbulence spontaneously, but the
reconnection rate remains robustly at the two-dimensional value. Examples of
applications will be drawn from heliophysical and
magnetically confined laboratory plasmas.