#
59th Annual Meeting of the APS Division of Plasma Physics

## Volume 62, Number 12

##
Monday–Friday, October 23–27, 2017;
Milwaukee, Wisconsin

### Session PT2: Tutorial: Experiments and models of MHD jets and their relevance to astrophysics and solar physics

2:00 PM–3:00 PM,
Wednesday, October 25, 2017

Room: 102ABC

Chair: Mike Brown, Swarthmore College

Abstract ID: BAPS.2017.DPP.PT2.1

### Abstract: PT2.00001 : Experiments and models of MHD jets and their relevance to astrophysics and solar physics*

2:00 PM–3:00 PM

Preview Abstract

####
Author:

Paul Bellan

(Caltech)

MHD-driven flows exist in both space and lab plasmas because the MHD\
force-balance equation $\mathbf{J\times B}-\nabla P=0$ can only be satisfied
in situations having an unusual degree of symmetry. In the normal situation
where such symmetry does not exist, an arbitrary magnetic field $\mathbf{B}$
and its associated current $\mathbf{J}=\mu _{0}^{-1}\nabla \times \mathbf{B}$
provide a magnetic force $\mathbf{F=J\times B}$ having the character of a
torque, i.e., $\nabla \times $ $\mathbf{F}\neq 0$. Because $\nabla \times
\nabla P=0$ is a mathematical identity, no pressure gradient can balance
this torque so a flow is driven.
Additionally, since ideal MHD\ has magnetic flux frozen into the \textit{%
frame} of the moving plasma, the flow convects frozen-in magnetic flux. If
the flow slows and piles up, both the plasma and the frozen-in magnetic flux
will be compressed. This magnetic flux compression amplifies both the
frozen-in $\mathbf{B}$ and its associated $\mathbf{J}$. Slowing down thus
increases certain components of $\mathbf{F}$, in particular the\ pinch force
associated with the electric current in the flow direction. This increased
pinching causes the flow to self-collimate if the leading edge of the flow
moves slower than the trailing part so there is compression in the flow
frame. The result\ is that the flow self-collimates and forms a narrow jet.
Self-collimating jets with embedded electric current and helical magnetic
field are analogous to the straight cylindrical approximation of a tokamak,
but now with the length\ of the cylinder continuously increasing and the
radius depending on axial position. The flows are directed from axial
regions having small radius to axial regions having large radius. The flow
velocity is proportional to the axial electric current and is a significant
fraction of the Alfv\'{e}n velocity. Examples of these MHD-driven flows are
astrophysical jets, certain solar coronal situations, and the initial plasma
produced by the coaxial magnetized plasma guns used for making spheromaks.
The above picture has been developed from laboratory measurements, analytic
models, and numerical simulations. Upon attaining a critical length,
laboratory jets develop a complex but resolvable sequence of instabilities
which is effectively a cascade from the large-scale MHD regime to the
small-scale two-fluid and kinetic regimes. This cascade involves kinking,
Rayleigh-Taylor instabilities, magnetic reconnection, whistler waves, ion
and electron heating, and generation of hard X-rays.
An extended model shows how clumps of particles in a weakly ionized
accretion disk move like a metaparticle having its charge to mass ratio
reduced from that of an ion by the fractional ionization. These weakly
charged metaparticles follow an inward spiral trajectory that is neither a
cyclotron nor a\ Kepler orbit and accumulate at small radius where they
produce a disk-plane radial EMF that drives astrophysical jets.

*Supported by DOE, NSF, and AFOSR

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2017.DPP.PT2.1