High speed images of edge plasmas in NSTX

IEA WorkshopEdge Transport in Fusion Plasmas

September 11-13, 2006Kraków, Poland

GPI outer midplane – shot 118152 – 208.762 ms to 208.837 ms

2cm2cm

R. J. MaquedaNova Photonics Inc., USA

in collaboration with

R. Maingi, T. Munsat, J. R. Myra, D. P. Stotler, A. E. White, S.J. Zweben and the NSTX Research Team

2

Outline

• Introduction: National Spherical Torus Experiment (NSTX) and diagnostics

• Edge turbulence: Gas Puff Imaging (GPI) images

• Edge Localized Modes (ELMs)

• Summary

3

Typical NSTX parametersGeneralR ~ 0.85 m a ~ 0.7 mBaxis = 4.5 kG

Ip = 0.7-1.2 MA

PNBI < 7 MW

Te(0) ~ 1 keV

ne(0) ~ 2.5 x 1013 cm-3

<ne> ~ 2 x 1013 cm-3

Outer edge (Rmid = 1.46 m)ne ~ 5 x 1012 cm-3

Te ~ 13 eV ~ 10-3

Ln ~ 2 cm

s ~ 0.2 cmei ~ 6 x 106 s-1

Lc ~ 5 m (connection length to divertor)ei/Lc ~ 0.05

q = (BT/Bpol)(R/A) ~ 2

LRBM ~ 1 cm

(shot 108332 at 0.18s)

Center stack(18.5 cm radius)Carbon tiles

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Time (ms)

Co

ntr

as

t e

nh

an

ce

d-5

0 t

o 2

00

sc

ale

Div

ert

or

D (

a.u

.)

Phantom 7 cameraFrame rate:

≤68000 frames/sat 128 x 128 pixels

≤120000 frames/sat 64 x 64 pixels

Minimum frame exposure: 2 s

Digitization: 12-bit

Full discharge coverage (2 GB of on-board memory)

Complex filament structure and dynamicsGood performance, long pulse at 1 MA

Clip: no filter2 s exposures

7 ms at 100000 frame/splayback at 150 s/s

Ra

w i

ma

ge

s0

to

40

0 s

ca

le

1 MA - 4.0 MW NBI - Double null

Click on image above to play movie clip. (Caution: 29 MB file)

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GPI Diagnostic

• Camera used to view visible emission from edge just above midplane.

• Gas puff is injected to increase image contrast and brightness. Gas puff does not perturb local (nor global) plasma.

• Emission filtered for D (He) light from

gas puff: I none f(ne,Te) ( ne

Te)

with 0.5 < , < 2

• D (He) emission only seen in range

~ 5 eV < Te < 50 eV

• View aligned along B field line to see 2-D structure B. Typical edge phenomena has a long parallel wavelength, filament structure.

• For more details: “Gas puff imaging of edge turbulence”, R.J. Maqueda et al., Rev. Sci. Instrum. 74(3), p. 2020, 2003.

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GPI: L-H Transition

Transition takes place at ~192.1 ms

L-mode

Separatrix

Antenna limiter

shadow

23 cm radial

23 cm poloidal

Spontaneous transition into quiescent H-

mode

“Blobs”

Ohmic H-mode

~8 s between frames

0.65 ms mosaicD2 puffD filter

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GPI: “Quiescent” H-modeSeparatrix

Antenna limiter

shadow

23 cm radial

23 cm poloidal

Ohmic H-mode

L-H transition takes place at

~192.1 ms

~8 s between frames

0.65 ms mosaicD2 puffD filter

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GPI: Active H-mode

H-mode edge with “blobs” ...micro-ELMs?

Separatrix Antenna limiter shadow 23 cm radial

23 cm poloidal

“Blobs”Active

Active

Quiet

Quiet

4.5 MW NBI

~8 s between frames

0.65 ms mosaicD2 puffD filter

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Quiescent vs. active H-modes

Gas puff imaging

D2 puffsFOV: 23cm x 23 cm

Quiescent900 kAOhmic

Lower single null

Active1 MA

4.7 MW NBILower single null

po

loid

al

R

D2 puff

Clip: D filter3 s exposures

5 ms at 120000 frame/splayback at 125 s/s

Click on image above to play movie clip.(Caution: 23 MB file)

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0 1.0 2.0 3.0 4.0 5.0

115513 Ohmic LSN 900 kA

116107 2-4 MW NBILSN 700 kA

118152 4-7 MW NBIDN 1-1.2 MA

119286 2 MW NBILSN 800 kA .

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0 100.0 200.0 300.0 400.0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0 2.0 4.0 6.0 8.0 10.0

Turbulence/blob activity during H-mode• The characteristics of the H-mode turbulence and blobs present a

continuum from a turbulence level just above that measurable (a “quiescent” H-mode) to that approaching L-mode level (an “active” H-mode), at least for brief periods of time.

• The level of activity correlates well with the pedestal ne or Pe.

Blo

b a

cti

vit

y (

a.u

.)

Pedestal ne (1013 cm-3) Pedestal Te (eV) Pedestal Pe (kPa)

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232.291 ms

232.366 msShot 118152

Radial

Pol

oida

l

75

s

Blob history

1. Blob birth (frames 28156-28158)

2. Detached blob: polarization and ExB drift (frames 28159-28163)

3. Blob dissipation (frames 28164-28165)

...at the same time the blob “trail” gives rise to a secondary blob that flows poloidally

CAUTION: Inflection points in the blob trajectory can be seen at all radial positions

Secondary blob

2cm

2cm

SeparatrixAntenna limiter

shadow

8.3 s

12

SOL flows

2cm2cm

208.853 ms

Separatrix

Antenna limiter shadow

SOL flows (“wind”) visible

Shot 118152

208.738 ms

Blob “shread” upward

2cm2cm

309.898 ms

310.014 msShot 116107

~8 s between frames

D2 puff/D filter

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Ra

w i

ma

ge

s0

to

30

0 s

ca

leC

on

tra

st

en

ha

nc

ed

-50

to

20

0 s

ca

leD

ive

rto

r D

(

a.u

.)

Time (ms)

Small ELM (“Type V”) filament: propagating ionization front

800 kA6.5 MW NBI

Lower single nullType V ELMs

23

cm

po

loid

al

23 cm radial

Clip: D filter3 s exposures

5 ms at 120000 frame/splayback at 125 s/s

1193

18 @

0.6

6841

7 s

separatrix limitershadow

Type V ELM filament ribbon

Tangential edge imaging

NO gas puff

• Crossfield (poloidal) width: ~12 cm

• Crossfield (radial) width: ~3-4 cm

• Plasma within filament similar to that on pedestal.

Type V ELM: W/W < 1%

Click on image to play movie clip.(Caution: 23 MB file)

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Filament coincident in time with divertor signature

Time (ms)312.0 312.5 313.0 313.5

1136

65

InterferometerK.C. Lee (UC-Davis)

Mirnov arrayJ. Menard (PPPL)

E. Fredrickson (PPPL)

123

7

USXR arrays

Filament also carries current

~400 A

Ip Lin

e av

erag

e d

ensi

ty (

1019

m-3)

USXR

Divertor light

I US

XR (

a.u

.)I v

is (

a.u

.)T

oro

idal

an

gle

(d

eg.)

300

200

100

0

Chord #1

Chord #2

Chord #3

Chord #7

Midplane chord

BII

0

100

200

300

NSTX plan view (midplane)

K. Tritz (JHU)

R. Maingi (ORNL)

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Small ELM (“Type V”) filament: no detachment

• Toroidal velocity: ~8 km/s (~0.9 kHz at R~1.45 m) ...counter IP and plasma rotation

• Radial velocity: ≤0.2 km/s

• Current: ~400 A (~100 kA/m2) ...co-IP

• Lifetime: 0.5 to 1 ms• Filament coincident in time with divertor signature• Plasma within filament similar to that on pedestal

• Filament detachment not observed• “Soft” ELM crash due to enhanced transport on perturbed flux

surfaces

Blob characteristics during H-mode different from small Type V ELMs: magnetic signature, characteristic sizes, propagation, detachment

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After large ELMs edge similar to L-mode edge

Gas puff imaging

Field of view23 cm x 23 cm

L-mode800 kA

2 MW NBILower single null

H-mode1 MA

4.7 MW NBILower single null

Clip: D filter3 s exposures

5 ms at 120000 frame/splayback at 125 s/s

po

loid

alR

D2 puff

ELM at ~221.1 ms

Click on image above to play movie clip.(Caution: 23 MB file)

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Summary• Edge of toroidally confined plasma (like NSTX) show a

complex filament structure and dynamics: blobs and ELMs.

• Fast-frame imaging is a very useful tool to study these phenomena. Gas Puff Imaging (GPI) enhances the usefulness of fast imaging for edge turbulence studies.

• While “blob” (and turbulent) activity is much reduced in H-mode compared to L-mode, H-modes present a continuum from “quiescent” to “active” edges.

• H-mode blob activity increases with edge pedestal density (and pressure).

• Long-lived “Type V” ELM filaments have very different characteristics and dynamics than blob filaments. Type V ELM crash associated with enhanced transport during filament lifetime.

• Large ELMs revert edge turbulence characteristics to L-mode like.

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Blob, GPI and ELM structure related NSTX papers...and references within

• “High-speed imaging of edge turbulence in NSTX”, S. J. Zweben et al., Nucl. Fusion 44 (2004) 134.

• “Three-dimensional neutral transport simulations of gas puff imaging experiments”, D. P. Stotler et al., Contrib. Plasma Phys. 44, 294 (2004).

• “Structure and motion of edge turbulence in the National Spherical Torus Experiment and Alcator C-Mod”, S. J. Zweben et al., Phys. Plasmas 13, 056114 (2006).

• “Bispectral analysis of low- to high-confinement mode transitions in the National Spherical Torus Experiment”, A. E. White et al., Phys. Plasmas 13, 072301 (2006).

• “Characterization of small, Type V ELMs in the National Spherical Torus Experiment”, R. Maingi et al., accepted Phys. Plasmas (2006).

• “Blob birth and transport in the tokamak edge plasma: analysis of imaging data”, J. R. Myra et al., accepted Phys. Plasmas (2006).

• “Structure of MARFEs and ELMs in NSTX”, R. J. Maqueda et al., submitted J. Nucl. Mater. (2006).

• “Derivation of time depedent 2-D velocity field maps for plasma turbulence studies”, T. Munsat and S. J. Zweben, submitted Rev. Sci. Instrum. (2006).