2 05 6 J Overview of HSX Results and Future Directions 0€¦ · 51st APS-DPP Annual Meeting, 2-6...
Transcript of 2 05 6 J Overview of HSX Results and Future Directions 0€¦ · 51st APS-DPP Annual Meeting, 2-6...
51st APS-DPP Annual Meeting, 2-6 November 2009, Atlanta GA
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JPfirsh-Schlüter Poloidal Index
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• Emphasis now is investigating anomalous and
neoclassical electron transport by heating
electrons to low collisionality regime at B = 1.0 T.
• ECRH at 100 kW show Te ~ 2.5 keV highly
peaked in core => Evidence of internal transport
barrier (CERC) in a quasisymmetric stellarator.
• E x B suppression of turbulence needed to
explain peaked Te profile.
• CHERS measurements of Er show agreement
with ion root calculation outside plasma core.
toroidal
polo
ida
l
Overview of HSX Results and Future DirectionsD.T. Anderson, F.S.B. Anderson, A. Briesemeister, C. Clark, C. Deng, K. M. Likin,
J. Lore, J. C. Schmitt, J.N. Talmadge, G. Weir, R. Wilcox, K. Zhai HSX Plasma Laboratory, Univ. of Wisconsin, Madison, USA
Summary
Field line travels once around
toroidally 3 periods in |B| ι ~ 3
Energetic Particle Instability• Collaborative effort with Brower (UCLA), Spong (ORNL) & Breizman (Texas)
• Energetic electrons produced by 2nd harmonic ECRH at 0.5T produce
coherent, global fluctuations in range 20 – 120 kHz.
• Mode frequency has weak dependence on transform making it unlikely that it
is Alfvenic mode. Stellgap (Spong) calculation including coupling of Alfvenic
to sound waves; this coupling best explains experimental results.
See paper by C Deng, D.L. Brower et al., PRL 103, 025003 (2009).
0 0.5 10
0.5
1
1.5
2
2.5
3
r/a
Te (
ke
V)
QHS
Mirror
Internal Transport Barrier
• Te (0) ~ 2.5 keV steep Te gradient at plasma core is evidence of CERC for QHS
configuration.
• Quasi-linear Weiland model simulates transport due to Trapped Electron Mode.
• 2-D model assumes single class of trapped electrons.
• Validated by 3D GS2 code.
• Electric field profile modeled with diffusion equation large Te gradient in location
of ExB shear layer
• Inside plasma core, anomalous χe ~ 10 times experimental value.
• Shearing rate greater than maximum linear growth rate inside r/a ~ 0.3
• ExB shear suppresses turbulence: diffusivity scaled by quench rule:
D D * max (1-αEγE/γmax ,0); γE = shearing rate; γmax = maximum growth rate
• Without shear suppression (αE = 0), Te at core is underestimated.
• αE = 0.3 gives good agreement with temperature at core.
• Density threshold (~ 5 x 1012 cm-3 ) for transport barrier consistent with having ion
root throughout entire plasma.
Invited talk by Lore (if you didn’t see it – you missed it!)
W. Guttenfelder, J. Lore, et. al,
Phys. Rev. Lett 101, 215002
(2008).
CHERS MeasurementsNeutral
Beam View 1
View 2
• 30 keV neutral H beam for charge
exchange; two 0.75m spectrometers
measure 539 nm C+5 line
• 10 mostly ‘toroidal’ (view 1 above) and
10 mostly ‘poloidal’ (view 2)
Plasma Currents
New Directions
Second ECRH gyrotron operational by end of year; additional 400 kW available
with steerable mirror for off-axis heating. Also, power modulation for transient
diffusivity.
• Comparison of impurity transport on TJ-II and HSX. Laser blow-off experiments of
light (B) and heavy (Al) impurities for ECRH discharges in TJ-II (Zurro) beginning
November, 2009. Determination of convective and diffusive transport, as well as
density and power scans. Also, initial calculations of impurity transport with PENTA
code. Goal is to determine ‘temperature screening’ as function of symmetry-
breaking and the role of Er on impurity transport.
•Collaboration with TJ-II (Hidalgo) on magnetic geometry effects on turbulence and
zonal flows. Theory predicts reduced zonal flow damping with quasisymmetry. We
have begun looking at effect of electrode biasing on long-range correlations. First
results show improved particle confinement. Large increase in flow shear at edge,
consistent with flow in symmetry direction.
•Collaboration with ORNL (Diem and Rasmussen) on ICRF experiments in HSX.
First experiments will concentrate on antenna design and coupling with 5 kW
source. Higher power transmitter (100 kW / 5-30 MHz) is also available.
• Experiments will investigate effect of magnetic geometry on ion distribution.
Collaboration with Kyoto University (Murakami) on 5-D GNET calculation.
Acquisition of CX analyzer to compare distribution function to model predictions.
HSX Parameters
<R> 1.2 m
<a> 0.12 m
i 1.051.12
B0 0.5 -1.0 T
ECRH<100 kW
28 GHz
0 0.2 0.4 0.6 0.8 110
-3
10-2
10-1
r/a
e
ff
Mirror
QHS
Conventional
Stellarators
r/a ~ 2/3HSX is a quasi-helically symmetric
stellarator (QHS) with almost no
toroidal curvature and a high effective
transform: ιeff ~ 3. This yields small
banana widths, low plasma currents,
low neoclassical transport.
Auxiliary coils degrades quasisymmetry
and increases effective ripple, viscous
damping and neoclassical transport for
comparison to QHS mode. This is
called the Mirror mode.
Summary of Results
• Large (15-20 km/s) parallel flow is due to
quasisymmetry; often assumed to be zero for
conventional stellarators.
• First observation of helical Pfirsch-Schluter current,
as expected for device with no toroidal curvature.
Signals from pick-up coils in agreement with V3FIT
calculation of field due to evolving bootstrap current.
• Electrode biasing increases flow shear in direction
of quasisymmetry.
• At B=0.5 T, coherent mode due to energetic
electrons observed; consistent with acoustic mode.New Directions & Collaborations
Second gyrotron comes on-line in December; Impurity transport (TJ-II); Magnetic geometry effects
on turbulence and zonal flows(TJ-II, IPP, PPPL, Warwick);Ion heating (ORNL);
Radial electric field in
reasonable agreement with ion
root at edge, large uncertainty
at location of electron root.
Large parallel flow (15-20 km/s)
observed for QHS configuration,
often neglected in standard
neoclassical calculation.
See poster by Briesemeister
• First results of Zeff profile based on plasma bremsstrahlung
measured by poloidal array of CHERS system. Intensity calibrated
with integrating sphere. Reconstruction yields preliminary profile.
ICRF
Zonal Flows and Electrode Biasing
Impurity Transport
Double ECRH Power
0.8 0.81 0.82 0.83 0.84 0.85 0.86-300
-200
-100
0
100
200
300
sec
Am
ps
BCW
BCCW
• Toroidal current in HSX is due to bootstrap current.
• Current evolves during discharge Use Strand-Houlberg model.
• Steady-state bootstrap current is calculated by PENTA code. Total
integrated current depends on electron ( -250 A) or ion root (-400 A).
• Ultimate goal is to use V3FIT+ code for equilibrium reconstruction
At present, we use code in forward direction to calculate field due to
plasma currents
• 16 3-axis coils measure field at two toroidal locations
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1x 10
5
JB
S A
/ m
2
Jbs e-root
Jbs i-root
Multiple ambipolar
solutions
• Early in time (5-7ms),
signals dominated by Pfirsch-
Schluter current (dotted line).
• Bootstrap current (dashed
line) has minimal influence.
• At later times (see above)
(15-45 ms) coils track
increase in bootstrap current
• The helical Pfirsch-Schluter current in
HSX has been experimentally
demonstrated there is little toroidal
curvature in HSX.
• Both the PS and bootstrap current are
reduced in magnitude compared to a
tokamak because of the high effective
transform.
• Bootstrap current verified to flow in
opposite direction to current in tokamak
decreases rotational transform.
See poster by Schmitt
0 0.2 0.4 0.6 0.8 1
10-1
100
101
r/a
e (
m2/s
)
Weiland
EXP
Neoclassical
0 0.2 0.4 0.6 0.8 1
0
200
400
600
r/a
Er (V
/cm
)
Ion root
Electron root
DE = 0.3 m2/s
0 0.2 0.4 0.6 0.8 10
0.5
1
1.5
2
2.5
3
r/a
Te (
ke
V)
Weiland + ExB
shear (αE = 0.28)
Weiland w/o shear
(αE = 0)
Experiment
0 0.2 0.4 0.6 0.8 10
5
10
15
20
r/a
lin, E
(1
05 s
-1)
lin
E
Shearing and
growth rates
0 0.2 0.4 0.6 0.8 1
0
100
200
300
400
500
r/a
Er (
V/c
m)
ChERS
∇Te
∇TI
∇Ti
Temperature
Screening
Accumulation
+S.P. Hirshman, et. al., Phys Plasma, 11, 595
(2004). Thanks to J. Hanson & S. Knowlton for
assistance.
PENTA calculation of C+6 transport in a tokamak,
showing temperature screening in banana and PS
regime, accumulation in plateau.
CHERS
measurement of
flow shear during
electrode bias
Left: Antenna design
(S. Diem, ORNL).
Right: 5 keV banana
orbit in HSX (top) and
equivalent tokamak.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
0.5
1
1.5
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3.5
4
r/a
Zef
f
0 0.2 0.4 0.6 0.8 1-5
0
5
10
15
20
25
30Boxport View Measured Velocity with 450V bias
r/a
Velo
cit
y (
km
/s)
Bias applied
Biased (450V)
Unbiased
0 0.2 0.4 0.6 0.8 1-20
0
20
40
60
r/a
U|| (
km
/s)
NC Electron root
NC Ion root
ChERS