51st APS-DPP Annual Meeting, 2-6 November 2009, Atlanta GA

18 20 22 24 26 28 30 32

0

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Ga

uss

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-2

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Ga

uss

Tim

eBθ

Φ≈ 1/6 FP

1716

32

181

2

Φ≈1/2 FP

JPfirsh-Schlüter Poloidal Index

9

5

25

26

Bθ

Br

13

2 4 6 8 10 12 14 16

-1-0.5

00.5

1

Gauss

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-1

-0.50

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1

Gauss

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-1-0.5

0

0.51

Gauss

18 20 22 24 26 28 30 32

-1-0.5

00.5

1

Gauss

Br

Bθ

• 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

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10-1

100

101

r/a

e (

m2/s

)

Weiland

EXP

Neoclassical

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0

200

400

600

r/a

Er (V

/cm

)

Ion root

Electron root

DE = 0.3 m2/s

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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

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5

10

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20

r/a

lin, E

(1

05 s

-1)

lin

E

Shearing and

growth rates

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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

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r/a

Zef

f

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5

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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

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20

40

60

r/a

U|| (

km

/s)

NC Electron root

NC Ion root

ChERS