Edge Probe Measurements of the MHD Dynamo and Anomalus Ion Heating in MST

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Edge Probe Measurements of the MHD Dynamo and
Anomalus Ion Heating in MST M. Miller, D. Craig,
D. Ennis, G. Fiksel, S. Gangadhara, A.
Kuritsyn University of Wisconsin - Madison
Center forMagnetic Self-Organization in
Laboratory and Astrophysical Plasmas
CMSO meetingAugust 2 - 4, 2006, Chicago, IL
Sawtooth crashes
Madison Symmetric Torus
Probes measure MHD Dynamo
How MHD dynamo depends on the phase difference

• Sawtooth crashes happen quickly, 100 ?s.
• During a sawtooth crash, magnetic fluctuations
  increase. It is thought that magnetic energy is
  transferred to kinetic energy.
• Preceded by a rapid increase in core resonant
  magnetic modes (m 1, n 6-9)
• Followed by increases in edge resonant modes (m
  0, n 1-4).

• The parallel component of flux surface averaged
  Ohms law is
• Coherent action of fluctuating magnetic fields
  and plasma flows create MHD dynamo.
• The optical probe and a fast magnetic probe were
  both used to measure the dynamo in the edge. The
  two probes were separated by 15º along the
  equilibrium magnetic field line (poloidal at the
  edge).

MST is a reversed field pinch plasma confinement
device. The toroidal component of the magnetic
field reverses direction near the edge and is on
orderthe same magnitude as the poloidal
field.Major radius R 1.5 mMinor radius a
0.52 mElectron density ne ? 1.0 ? 1013 cm-3
The dynamo term depends on the magnitude of the
fluctuating parameters along with their coherence
(?) and phase difference (?)
MHD Dynamo
Hall Dynamo
Core Electron temperature Te ? 200 eV - 1.2 keV
Phase profile of Velocity and Magnetic Field
Introduction

• Periodic reconnection events (Sawtooth crashes)
  occur in MST plasmas that redistribute plasma
  current, flow, and energy.
• Anomalous ion heating was previously measured in
  the core (IDS I, CHERS). See S. Gangadhara
  poster this session.
• The MHD dynamo that contributes to the current
  drive was also previously measured in the core
  (IDS I, CHERS). See D. Ennis presentation this
  meeting
• This presentation reports on the measurements of
  anomalous ion heating and MHD dynamo at the edge.

The MHD dynamo changes sign across the reversal
surface because the phase of the fluctuating Bt
changes phase while the phase of fluctuating vr
remains constant.

• Only one component of the MHD dynamo is measured
• Shown is a sawtooth ensemble of
• Note that changes sign across the
  reversal surface.

Ion temperature at the crash

• In MST and other lab plasmas, ion temperature is
  observed to be higher than can be explained by
  collisions with the heated electrons. An
  anomalously high ion temperature is also observed
  in astrophysical plasmas, such as the solar
  corona.
• Sawtooth ensembles average over many similar
  events.
• Averaging over many realizations approximates a
  flux surface average (e.g. lt Ti gt).

Why does the MHD dynamo change sign across the
reversal surface?
Optical Probe

• Passive light collection of the collisionally
  excited He II line.
• Localized measurement. View dump provides a
  spatial resolution of about 3 cm.
• Based on an older IDS probe that was coupled to
  the IDS-I spectrometer.
• The new IDS-II spectrometer has higher throughput
  and spectral resolution.

To answer that question, the probe signals were
correlated with the toroidal magnetic array. In
the toroidal array there are 64 coil locations.
This array is used to resolve the toroidal (n)
spectrum of the fluctuating magnetic signal In
MST, the n 1 - 4 modes are associated with m
0 and n 6 - 8 with m 1.
Shown is a sawtooth ensemble of the ion
temperature. The equilibrium temperature gets
higher at deeper insertion depths. During a
sawtooth crash, ions have an observed temperature
rise of 40 in 200?s.
Correlated Velocity Fluctuations
Future work

• -0.50 ? -0.40 ms -0.05 ? 0.05
  ms 0.40 ? 0.50 ms
• Velocity fluctuations are correlated with the
  magnetic modes.
• Velocity fluctuations are seen to increase for
  all modes during the sawtooth crash
• The n 1 mode dominates
• Noise floor at 60 m/s

• Finish radial scan with the IDS probe and
  magnetic probe.
• Can infer electric field by magnetic measurements
• Use the IDS probe and a Lagmuir probe to measure
  particle flux.
• Measure ne ni, lt v, negt
• Look at other lines, e.g. Boron III line
• Mach Probe development
• Measure other components of MHD dynamo
• Look for flow shear at the reversal surface
  during crash. (possible mechanism for ion
  heating)
• Use old IDS probe with IDS II spectrometer.

Equilibrium radial velocity at the crash
At the sawtooth crash, a blue shift of the HeII
line is measured. The velocity is not absolutely
calibrated yet. If one assumes that the
equilibrium flow is small, the shift of the line
towards smaller wavelengths indicates an outward
transport of particles due to the relaxation
event.

• Thermal motions broaden the line, ion temperature
  is calculated from the width.
• Ion flows will shift the centroid of the line.