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VERSION:2.0
CALSCALE:GREGORIAN
PRODID:UW-Madison-Physics-Events
BEGIN:VEVENT
SEQUENCE:0
UID:UW-Physics-Event-3779
DTSTART:20151109T180000Z
DTEND:20151109T190000Z
DTSTAMP:20240319T111714Z
LAST-MODIFIED:20151105T180448Z
LOCATION:2241 Physics Bldg
SUMMARY:Dynamics of a reconnection-driven runaway ion tail in a revers
ed field pinch plasma/ Effect of Resonant Magnetic Perturbations on 3D
equilibria in the MST RFP \, Plasma Physics (Physics/ECE/NE 922) Semi
nar\, Jay Anderson/Stefano Munaretto\, UW Madison\, Plasma Physics
DESCRIPTION:Jay Anderson:
\nNon-collisional heating and energizati
on of ions is a powerful process in
\nreversed-field pinch (RFP) p
lasmas and in many astrophysical settings.
\nTearing activity in
the RFP (including linearly and nonlinearly driven modes
\nwhich
span the plasma column) saturates through dynamo-like feedback on
\nthe current density profile\, rapidly releasing magnetic energy and
inducing
\na strong impulsive\, parallel-to-B electric field as p
oloidal magnetic flux is
\nconverted to toroidal flux. The global
reconnection leads to strong ion heating
\nwith a known anisotro
py in temperature (T_perp > T_{||})\, suggestive of a
\nperpendic
ular bulk heating mechanism.
\nIn the subset of strongest reconne
ction events\, multiple mechanisms combine to
\ncreate a most inte
resting ion distribution. Runaway of the reduced-friction naturally h
eated ions generates an asymmetric ion tail with E_{||} >> E_perp. Th
e tail is reinforced by a confinement asymmetry\, where runaway ions a
pproach the limit of classical cross-field transport despite magnetic
stochasticity from the broad spectrum of tearing modes. Confinement i
s lower in other regions of the
\nv_perp/v_{||} plane and reduces
to Rechester-Rosenbluth-like transport experienced by thermal particle
s.
\n
\n
\nStefano Munaretto:
\n The orientation of 3
D\, stellarator-like equilibria in the MST RFP can now be controlled w
ith application of an m = 1 RMP. This has led to greatly improved diag
nosis\, revealing enhancements in both the central electron temperatur
e and density. Coupled to a recent advance in the V3FIT code\, reconst
ructions of the 3D equilibria have also been dramatically improved. Th
e RMP also inhibits the generation of high-energy > 20 keV electrons t
hat is otherwise common with the 3D state. This state occurs when the
normally broad spectrum of core-resonant m = 1 tearing modes condenses
\, with the innermost resonant mode growing to large amplitude\, reach
ing ~ 8% of the axisymmetric field strength. This occurs in plasmas of
sufficiently large Lundquist number ~ IpTe^3/2\, and the duration of
the state is maximized with zero applied Bt (infinite toroidal beta).
As the dominant mode grows\, eddy current in MST's conducting shell sl
ows the mode's rotation. This leads to locking of the 3D structure\, b
ut with an orientation that varies randomly shot to shot\, making diag
nosis difficult. An m = 1 RMP can now be applied with an array of sadd
le coils at the vertical insulated cut in the shell. With an amplitude
br/B ~ 10% and a tailored temporal waveform\, the RMP can force the 3
D structure into any desired orientation relative to MST's diagnostics
. A recent advance in V3FIT allows calculation of the substantial heli
cal image current flowing in MST's shell\, which has in turn allowed s
elf-consistent utilization of both external and internal (Faraday rota
tion) measurements of the magnetic field. The ORBIT code predicts redu
ced stochasticity and improved confinement of high-energy electrons wi
thin the 3D structure. The suppression of these electrons by the m = 1
RMP may reflect a change to the central magnetic topology. The genera
tion of these electrons is unaffected by non-resonant perturbations\,
such as m = 3.
URL:https://www.physics.wisc.edu/events/?id=3779
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