Transcript Experimental Measurements of Non

Experimental Measurements of NonMHD Dynamo Effects
Summarized by
S.C. Prager
many contributors:
J. Anderson
D. Brower
D. Craig
D. Den Hartog
plus most of MST group
W. Ding
G. Fiksel
H. Ji
J. Sarff
<E>|| + dynamo = <j>||
Dynamo
MHD
v˜  B˜
theory status
QL, NL comp
Hall
˜j  B˜
QL

Diamagnetic
B˜ p˜ 
very little

Kinetic

B˜ r p˜ ||
RR,
constraints from
Landau resonances
Three Effects from Two-Fluid Theory
j  B pe
E v B

 j
ne
ne
parallel mean-field component,
E ||  v˜  B˜ || 

MHD
Linear part,

||
ne
 j
||
Hall
˜j
E˜  B p˜ e  B
v˜ 


2
2
ne
B
B
Using in top eqn,

˜j  B˜
E || 
E˜  B˜ 
B
2

(  p˜ e ) B˜ 
B
2
 j
Pressureless diamagnetic
MHD
||

The “kinetic dynamo”
radial transport of parallel current along stochastic magnetic
field
for electrons, the flux of parallel momentum
or
B
r  ||  rˆ  ||  rˆ 
B
r 
p˜ || e B˜ r
B
˜ B˜

|| r
B
Experimental Observations
To my knowledge,
Only in RFP
(some MHD dynamo measurements in
SPHEX spheromak)
Dynamo Measurements
• MHD: some detailed measurements
• Hall: some detailed measurements
• Diamagnetic: one measurement
• Kinetic: no measurements
All measurements are ongoing;
Diagnostics being expanded
E  j
in experiment
2.0
E ||
1.5

E||
1.0
V/m
0.5
neo J||
j
||
(Zeff = 2)
0.0
-0.5
0.0
0.2
0.4
/a
0.6
0.8
1.0
radius
additional current drive mechanism (dynamo)
Dynamo occurs in bursts
Toroidal
Magnetic
Flux
(Wb)
MST
time (ms)
MHD Dynamo
(exists, but not the whole story)
Early measurement,
passive Doppler spectroscopy
(1995)
MHD dynamo in edge
E˜ B˜
v˜  B˜
By Langmuir and magnetic probes
By spectroscopic and magnetic probes
1995
Consistent with MHD dynamo
Spectroscopic Probe
Fiksel,
Den Hartog
yields local flow velocity
Measurement of MHD dynamo
Volts
m
 v˜  B˜ 
0
-10
r/a = 0.9
 j   E
r/a = 0.9
-20
Volts
m

0
-10
-20
-0.5

r/a
=
0.8
r/a = 0.8
time (ms)
0
time (ms)
0.5
MHD dynamo dominant at some radii, not everywhere
Hall Dynamo
• Edge: magnetic probes
• Core: laser Faraday rotation
Fluctuation Power Spectra
B
probes at
edge
Shen et al
1993
j
Frequency (kHz)
Correlation between j and B
phase
coherence
Frequency (kHz)
Result
at r/a ~ 0.9,
Hall term accounts for less than 25%
of the total current
Hall term is significant at r/a = 0.8
Fiksel, Almagri
ongoing
V/m
Hall dynamo (V/m)
 ˜j  B˜ 
ne
5
0
-5
-10
-15
-20
-25
-1
-0.5
0
0.5
1
Time from crash (ms)
time (ms)
1.5
2
Core measurements of Hall Dynamo
W. Ding, D. Brower, B. Deng UCLA 2003
E vector of injected laser rotates by angle 
~
 nBdl
Wave phase-shifted by angle 


~
 ndl
Can infer
B˜ , ˜j
3-Wave Polarimeter-Interferometer
System
Faraday
rotation/interferometer system
MST
R0 = 1.50 m
a = 0.52 m
Ip = 400 kA
ne ~ 1019 m-3
B0 ~ 4 kG
Faraday rotation angle: detects mean B, fluctuating B,
sawteeth
m=1 activity
Faraday Rotation [deg.]
6
x=-17 cm
4
-9
2
-2
6
0
13
-2
(a)
-4
1.0
0.5
0.0
24.0
21
external magnetic coil
24.5
25.0
Time [ms]
25.5
26.0
Current fluctuations increase during dynamo event
˜jT
(kA/m2)
Hall dynamo large near resonant surface
Diamagnetic dynamo: significant at
some conditions
Kinetic dynamo
•No direct measurement
•In ZT-40, fast electrons detected in edge
may imply transport from core
Summary
• Evidence for importance of MHD and Hall
dynamos
• Some indications that diamagnetic dynamo may be
important
• Would not be surprised if kinetic dynamo was
important.
Evidence for stochastic transport of particles,
energy - why not electron momentum?
• Goal: understand what mechanisms are
important, under what conditions
• Possibly dynamo always relaxes plasma
toward preferred MHD state - specific
mechanisms vary with conditions
• Experimental dynamo program is active, with
new diagnostics.