Third PRC-US Magnetic Fusion Collaboration Workshop 18-19 May 2006

Third PRC-US Magnetic Fusion Collaboration Third PRC-US Magnetic Fusion Collaboration Workshop 18-19 May 2006Workshop 18-19 May 2006

Plans and Results of the Texas

Collaboration with ASIPPLong history of collaboration between

the Fusion Research Center, Texas and the Institute of Plasma Physics, Hefei

Plans for HT-7 and EAST ECE -- Electron Cyclotron Emission radiometer for Te

CXRS -- Charge Exchange Recombination Spectroscopy for Ti and rotation

Expanded divertor

Results of Helimak project

Spatial Coverage of ECE System on HT-7

Schematic of the ECE diagnostic on HT-7

Lo Freq(GHz)

98.5 100.433

102.336

104.299

106.232

108.165

110.098

112.031

Hi Freq(GHz)

112 114 116 118 120 122 124 126

ECE Data with position shift to obtain a

relative calibration.

Te Profile (ECE)

• Relative calibration from shift is position position

• Absolute calibration form Thomson Scattering (central temperature)

ECE Temperature Profile

• Shot 81535:

EAST ECE System

Proposed ECE Antenna for EAST• Diffraction limited spatial

resolution • Integrated hot calibration source

• Possible test of ITER prototype calibration source

CXRS on HT-7 and EASTW. L. Rowan,1 Yuejiang Shi,2 June Huang,2

Huang He1, and B. N. Wan2

1Fusion Research Center, The University of Texas at Austin2Institute of Plasma Physics, Chinese Academy of Sciences

• DNB transferred to ASIPP and brought back into operation through common effort

• CXRS spectrometer and optics installed• Plans

–Develop CXRS analysis codes–Conduct transport experiments on HT-7–Transfer DNB to EAST–Transfer CXRS to EAST

DNB, Component Mix, and

CXRS Viewing Range

€

0.25 ≤ r ≤ −0.07

0.93 ≤ ρ ≤ −0.26

CXRS view range

DNBHT-7

The beam has operated for one campaign with an useful density component mix E:E/2:E/3:E/18 = 10:26:49:15

The CXRS diagnostic is installed for the current campaign and is expected to provide Ti, v over the LFS of the plasma

Divertor Projections

• Although ITER divertor may handle heat loads adequately, the divertor heat loads for the next-step reactor will exceed material limits: This is a show-stopper

• Other divertor configurations including radiating mantle and swept divertor will not scale to ITER or to a reactor

• Need an expanded divertor or other configuration

M. Kotschenreuther, P. M. Valanju, S. M. Mahajan, J. C. Wiley, M. Pekker Sherwood Fusion Theory Conference,

April, 2006

Expanded Divertor for EAST

• A new configuration to reduce the heat load on the divertor plates

• Axisymmetric coils near the divertor plates expand the footprint of the intersection of the field lines with the divertor plates

• Divertor coil currents are comparable to other PF coil currents

• The first test of this idea is proposed for EAST. Use reduced plasma current and pulsed divertor coils as a proof of concept

• A concept could be presented in August at ASIPP

An Experiment for EAST

• Energize coils in blue to yield flux expansion

• To prove the concept, use a set of coils with pulsed current just large enough to observe the expansion effect easily

An Experiment for EAST

• Energize coils in blue to expand the green flux at the divertor plate (in the circle)

Flux Expansion Versus Divertor Coil Current

I = 0 kAexpansion = 2.2



Helimak

Unique concept for a basic plasma experiment

Simple sheared cylindrical slab geometry

Device large compared with all scale lengths

Designed, engineered, and built by ASIPP

Operating successfully at Texas

Helimak

Dimensionless test of drift-wave turbulence

Simple, but physical geometry (curvature)

Open field lines, but long ( up to ~1 km)

Test of flow shear stabilization of turbulence

Dimensionless model of SOL

Helimak

Probe connections

Vacuum Vessel

Toroidal field coils

Vertical field coils

Microwave feed

Magnetron

Amplifiers and A/D

Helimak

Helimak

Helimak Dimensions and ParametersA Sheared Cylindrical Slab

<R> = 1.1 m ∆R = 1 mh = 2 mBT = 0.1 T Bv ≤ 0.01 TPulse ≤ 60 sPlasma source and heating: 6 kW ECH @ 2.45 GHzn ≤ 1011 cm-3 Te ~ 10 eVArgon, Heliumcs = 3 x 104 m/s (Argon) Vdrift = 100 m/s Vdiamagnetic = 103 m/sdrift-wave ~ 1 kHzConnection length: 10 m < L < 1000 m p (parallel loss) > 1 msProbe arrays in end plates provide vertical and full radial profilesIsolated end plates may apply radial electric fields: Vp ≤ ±100 Volts

Helimak

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

Bias plate

0.77

0.82

0.87

0.92

ECH resonance radius

R (m)

Density Profiles for various ECH Resonant Radii

Argon (shot# 402100062: 1 kW, 700 A, 15 ohms) U profile

-12

-7

-2

3

8

0 10 20 30 40 50 60 70 80 90

R - R inner [cm]

Te [eV], Vfl [V], n [10^16 m^-3] electron temperaturefloating potentialdensity

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are needed to see this picture.

Typical Density, Temperature, and Floating Potential Profiles

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0

0.5

1

0.6 0.8 1 1.2 1.4 1.6

Radial Profiles of Fluctuation Amplitude

∆n/n

R

(Various ECHResonant radii)

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0

20

40

60

80

100

120

0 2000 4000

Frequency Spectra

R=1.2Drift waves

in LFSgradient

R=1Coherent modeat density peak

(Hz)

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-5 -3 -1 1 3 5

Density PDF R=1.4 -- Low DensityLow median density with "blobs"

of high densityMaximum of density, coherent modeBimodal PDF typical of

harmonic oscillator

Helimak

0

1

2

3

0 1000 2000 3000 4000

Phase

(Hz)

VPH=400 m/s

kρs~0.5

=1.2R

( - )Vertical Propagation Drift wave

Helimak

Helimak

Major Points

The Helimak provides a good example of a turbulence bifurcation (shear stabilization)

The stabilization is caused by j (not E)

The transition is binary, not gradual -- no intermediate states as threshold approached from either direction

12341234BBhRiRoBRzφProbe

Helimak

Cross-section

Field lines terminate on isolated end plates

Biasing #2 plates with respect to others imposes radial electric field, current

Helimak

Response to Negative BiasProbe n(t) across radial profile

Bias Reduced ∆n Reduced ∆n; increased <n>

Helimak

Response to Positive BiasProbe n(t) across radial profile

Bias Reduced ∆nReduced ∆n; increased <n> Increased <n>

Helimak

Response to Negative BiasProbe n(t) across radial profile

Bias Reduced ∆nIncreased <n>Helium

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Time History of a Bifurcation



Negative Bias Positive Bias

Isat(t)

BiasVoltageCurrent

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Profile Changes at Bifurcation

-50 V

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0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.6 0.8 1 1.2 1.4 1.6R (m)

Profile of ∆n/n Reduction

Positive Bias

Negative Bias

Bias Plate(Various resonance radii)

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0 2000 4000 6000 8000

Γo = 1Γ- = 0.33

Γ+ = 0.53

( )Hz

Frequency-Resolved Particle Transport

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High Field Side

Low Field Side

Phase Velocity

Change with Bias

Larger changes for positive bias Equilibrium flow reversed by positive bias Negative bias adds to equilibrium flow

Helimak

Inferred Velocity Shear Same |∂Vz/∂z| for ± bias ~104 s-1

Equilibrium V from potential profile

∆V with bias from ∆Vphase of turbulence

Rn(r)E x B equilibrium∆V for + Bias∆V for - Bias01100 700 m/s1400 1200 m/s4000 m/s200 m/sVz(-)Vz(+)

Helimak

Velocity Shear vs.

Velocity shear ~104 s-1 comparable with shortest turbulence autocorrelation time

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.0016

Autocorrelation

Low field side c = 0.14 ms

High field side c = 0.7 msDensity

max c = 0.4 ms

Helimak

Drive for Velocity Shear:E x B or j x B?

Plasma floating potentials and Er decrease at bifurcation,

despite large bias

Threshold voltages for positive and negative bias different

Threshold currents for positive and negative bias similar



0

20

40

60

50 1000

5

10

15

Connection Length

Threshold Conditions -- Argon

-V

+V

-I

+I

No Transition

(V) (A)

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0

5

10

15

20

25

0.75 0.8 0.85 0.9 0.95 10

5

10

+V

-V

+I

-I

Threshold Conditions -- Helium

RECH

No Transition

(V)(A)

12341234Rzφ Flow of Bias Current , From plates j||jr across field drives sheared

vz flow

++------



Biasing drives current from+ plates into plasma along field lines, across the field lines, and back out alongfield lines to the - plates.

For typical threshold currents,<jr> ~ 0.1 A/m2

j X B = dp/dt ~ p/p

p = mnVz

For p ~ 1 ms, Vzmax ~ 2 km/sShear, ∂vz/∂r ~ 104 s-1

Helimak

Drive for Velocity Shear:E x B or j x B?

Plasma floating potentials and Er decrease at bifurcation

Threshold voltages for positive and negative bias different

Threshold currents for positive and negative bias similar

Symmetric current flow essential to bifurcation; if one plate isolated to stop current flow, transition absent.

Observations favor j(Shear flow driven by radial shear in j x B)

Helimak

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Normal, Fast Bifurcation

Bias

Isat(t) at various radii

Jump between two steady states

Simultaneous at all radii

No hysteresis; bias directly controls instability

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Isat(t) at various radii

Bias

Slow Sweep Through Threshold Bias always near threshold

Jump between two steady states; sharp threshold, no graded transition

No hysteresis

Helimak

Conclusions

• The Helimak offers a simple, controlled model of shear-flow driven turbulence bifurcations – bifurcation without hysteresis through equilibrium profile

• The shear flows are driven by current ( jxB ) ⇒ momentum transport key

• The bifurcation is a step-function in shearin g rate – no intermediate regimes near threshold

Documents

Third PRC-US Magnetic Fusion Collaboration Workshop 18-19 May 2006