transport-driven scrape-off layer flows, the role of the...

Transport-driven scrape-off layer flows,the role of the X-point

J.E. Rice, A.E. Hubbard, J.W. Hughes, M. Greenwald, R. Granetz, I. Hutchinson, J. Irby, Y. Lin, B. Lipschultz, E.S. Marmar,K. Marr, D. Mossessian, R. Parker, W. Rowan, N. Smick, J.A. Snipes,

J.L. Terry, S.M. Wolfe, S.J. Wukitch and the Alcator C-Mod Team

AlcatorC-Mod

and connections to the L-H power thresholdin Alcator C-Mod

Presented by B. LaBombard

Key contributors:

Presented at the 32nd EPS Plasma Physics Conference Tarragona, Spain

27 June - 1 July, 2005

AlcatorC-Mod

A leading paradigm for L-H transition physics involves plasma flow shear...yet, a compelling explanation for x-point sensitivity has been elusive

Motivation: The input power required to attain a High confinement mode in a tokamak (H-mode) depends on magnetic topology...

...a factor of ~2 higher power is typically required when Bx—B points away versus toward the active x-point (dating back to ASDEX)

†Nucl. Fusion 44 (2004) 1047, Phys. Plasmas 12 (2005) 056111.

AlcatorC-Mod


Recent results from Alcator C-Mod suggest an (unexpected) explanationfor the topology-dependence of the L-H power threshold†:



transport-driven plasma flows in the scrape-off-layer...

...and the flow boundary conditions that they impose (X-point dependent) on the confined plasma


AlcatorC-Mod

Focus of this talk:

Key experimental observations...


Recent results from Alcator C-Mod suggest an (unexpected) explanationfor the topology-dependence of the L-H power threshold†:



transport-driven plasma flows in the scrape-off-layer...

...and the flow boundary conditions that they impose (X-point dependent) on the confined plasma

transport-driven flows in the SOL connection to magnetic X-point topology, toroidal plasma rotation

...and the L-H threshold


Transport-DrivenScrape-off Layer Flows

Outline of Talk

Flow Boundary Condition on Confined Plasma

Connection to X-point Sensitivity of L-H Threshold

L-H Thresholds & Flowsin other topologies

near-sonic// flows ballooning-like

transportballooning-like transport drive G^


Outline of Talk





G^


=> x-point dependent toroidal rotation of confined plasma

DVf DVfIp

BT

co-currentrotation drive

counter-currentrotation drive

DVf DVfIp

BT



Outline of Talk



...via topology-dependent toroidal plasma rotation


G^


DVf DVfIp

BT



DVf DVfIp

BT




Outline of Talk


=

Lower-limited and lower-null dischargeshave same L-H power thresholds=> SOL flow pattern may play role





Outline of Talk

Outline of Talk





OuterScanningProbeInner

ScanningProbe

VerticalScanningProbe

Diagnostics:

Outline of Talk





Lower Single Null

IpBT

Bx—B

Outline of Talk





Upper Single Null

IpBT

Bx—B

Outline of Talk





Double Null

IpBT

Bx—B

0510 0

0510 0

AlcatorC-Mod

Evidence for Cross-field Transport Asymmetries.... ...Driving Near-Sonic Flows in Inner SOL

0 5 1000.1

1.0

10.0

100.0

0 5 1000.0

0.1

0.2

0.3

0.4

0 5 100-50

-25

0

25

50

0

-25

0

25

Outer SOL

1020

eV

m-3

Distance from Separatrix (mm)

10

100

ElectronPressure

RMS Jsat/<Jsat>

1

10

100

0

0.1

0.2

0.3

510 0

-50

-25

0

25

50

Inner SOL

Parallel FlowVelocity (km/s)

Fluctuations

0510 0

0510 0

AlcatorC-Mod


0 5 1000.1

1.0

10.0

100.0

0 5 1000.0

0.1

0.2

0.3

0.4

0 5 100-50

-25

0

25

50

0

-25

0

25

Outer SOL

1020

eV

m-3


10

100

ElectronPressure

RMS Jsat/<Jsat>

1

10

100

0

0.1

0.2

0.3

510 0

-50

-25

0

25

50

Inner SOL


Inner SOL plasma 'disappears' in Double Null LnT reduced by factor of 4

Fluctuations

0510 0

0510 0

AlcatorC-Mod


Fluctuation levels persistentlylower on Inner SOL

0 5 1000.1

1.0

10.0

100.0

0 5 1000.0

0.1

0.2

0.3

0.4

0 5 100-50

-25

0

25

50

0

-25

0

25

Outer SOL

1020

eV

m-3


10

100

ElectronPressure

RMS Jsat/<Jsat>

1

10

100

0

0.1

0.2

0.3

510 0

-50

-25

0

25

50

Inner SOL


Consistent with low ^ transportin inner SOL


Fluctuations

0510 0

0510 0

AlcatorC-Mod



0 5 1000.1

1.0

10.0

100.0

0 5 1000.0

0.1

0.2

0.3

0.4

0 5 100-50

-25

0

25

50

0

-25

0

25

Outer SOL

1020

eV

m-3


10

100

ElectronPressure

RMS Jsat/<Jsat>

1

10

100

0

0.1

0.2

0.3

510 0

-50

-25

0

25

50

Inner SOL



Near-sonic // flows on Inner SOL


Fluctuations

Always directed from outer to inner SOL in upper and lower-null, but ~stagnant in double-null

0510 0

0510 0

AlcatorC-Mod



Plasma exists on inner SOL because it flows along field lines from outer SOL

0 5 1000.1

1.0

10.0

100.0

0 5 1000.0

0.1

0.2

0.3

0.4

0 5 100-50

-25

0

25

50

0

-25

0

25

Outer SOL

1020

eV

m-3


10

100

ElectronPressure

RMS Jsat/<Jsat>

1

10

100

0

0.1

0.2

0.3

510 0

-50

-25

0

25

50

Inner SOL





Fluctuations


0510 0

0510 0

AlcatorC-Mod



Plasma exists on inner SOL because it flows along field lines from outer SOL

0 5 1000.1

1.0

10.0

100.0

0 5 1000.0

0.1

0.2

0.3

0.4

0 5 100-50

-25

0

25

50

0

-25

0

25

Outer SOL

1020

eV

m-3


10

100

ElectronPressure

RMS Jsat/<Jsat>

1

10

100

0

0.1

0.2

0.3

510 0

-50

-25

0

25

50

Inner SOL


Outer SOL flows weaker, co-current, appear modulated by topology...




Fluctuations


Toroidal Distance (cm)

Ver

tica

l (cm

)

-10 0 10 20-20-6

6

0

Upper Null Discharge

Toroidal Distance (cm)

Ver

tica

l (cm

)-10 0 10 20-20

-6

6

0

Lower Null Discharge

Magnetic Field Line

Direction of Flow

Magnetic Field Line

Direction of Flow

AlcatorC-Mod

CH4 Puff Camera

Plasma flow direction depends on Upper/Lower Null topology,identical to that seen by Inner Scanning Probe

Similar patterns of strong Inner SOL flows are evident in other devices:

Data from C+1 "plumes"at inner midplane location†

C+1 light,515 nm

†D. Jablonski, et al., J. Nucl Mater. 241-243 (1997) 782.

Gas Injection Experiments: Also Reveal Strong Inner SOL Flows, Closely Aligned with Magnetic Field Lines

JET: Mach probe data, inner div. carbon flakes, 13C transport experimentsDIII-D: 13C transport experiments

AlcatorC-Mod

Probe Data can be Mapped onto a "Flux-Tube Coordinate", S,Revealing Transport-Driven Component of Parallel Flow

IpBT0

0.250.75

0.5

1 S

0

0.25

0.5

0.75

1 S

Lower Null Upper Null

Definition of flux-tube coordinate, S

AlcatorC-Mod


r = 4 mm

0

10

20

1020

eV

m-3

Electron Pressure, nTe

0.0 0.5 1.0

0

1

Mac

h#

Mach Number, M//

Normalized distance along field line, S0.25 0.75

Outer SOL Inner SOL

Data from matched Lower-Nulland Upper-Null discharges

0

10

20 nTe(1+ M//2/2)

1020

eV

m-3

IpBT0

0.250.75

0.5

1 S

0

0.25

0.5

0.75

1 S



AlcatorC-Mod


r = 4 mm

0

10

20

1020

eV

m-3


0.0 0.5 1.0

0

1

Mac

h#

Mach Number, M//


Outer SOL Inner SOL


Lower nTe on Inner SOL

0

10

20 nTe(1+ M//2/2)

1020

eV

m-3

IpBT0

0.250.75

0.5

1 S

0

0.25

0.5

0.75

1 S



AlcatorC-Mod


r = 4 mm

0

10

20

1020

eV

m-3


0.0 0.5 1.0

0

1

Mac

h#

Mach Number, M//


Outer SOL Inner SOL



Toroidal rotation, Pfirsch-Schlüter flows, ... ...appear as offsets to average of +

0

10

20 nTe(1+ M//2/2)

1020

eV

m-3

~ transport-driven// flow component

Transport-driven parallel flow from Outer to Inner SOL

IpBT0

0.250.75

0.5

1 S

0

0.25

0.5

0.75

1 S



AlcatorC-Mod


r = 4 mm

0

10

20

1020

eV

m-3


0.0 0.5 1.0

0

1

Mac

h#

Mach Number, M//


Outer SOL Inner SOL




0

10

20 nTe(1+ M//2/2)

1020

eV

m-3

Thermal + flow energy ~constant



=> implies a free-streaming flow response

IpBT0

0.250.75

0.5

1 S

0

0.25

0.5

0.75

1 S



AlcatorC-Mod


r = 4 mm

0

10

20

1020

eV

m-3


0.0 0.5 1.0

0

1

Mac

h#

Mach Number, M//


Outer SOL Inner SOL




0

10

20 nTe(1+ M//2/2)

1020

eV

m-3

Thermal + flow energy ~constant



=> implies a free-streaming flow response

IpBT


Implied transport-driven flow pattern

co-currentV//f G^ G^

counter-current V//f

Outline of Talk





Lower Single Null

Toroidal RotationInferred from Ar17+

X-ray Doppler


ScanningProbe

Outline of Talk





Double Null


X-ray Doppler


ScanningProbe

Outline of Talk





Upper Single Null


X-ray Doppler


ScanningProbe

-15 -10 -5 0 5 10 150

-60

-30

0

30

-15 -10 -5 0 5 10 150-10

0

10

20

-15 -10 -5 0 5 10 150

Distance Between Primary andSecondary Separatrices (mm)

-40

-30

-20

r = 2 mm

r = 1 mm

Inner Probe

Outer Probe

Core Ar17+Doppler

To

roid

al V

elo

city

(km

s-1

)AlcatorC-Mod

X-point Topology Sets Magnitude and Direction of Transport-Driven SOL Flows => Core Plasma Rotation is Affected

-15 -10 -5 0 5 10 150

-60

-30

0

30

-15 -10 -5 0 5 10 150-10

0

10

20

-15 -10 -5 0 5 10 150


-40

-30

-20

r = 2 mm

r = 1 mm

Inner Probe

Outer Probe

Core Ar17+Doppler

To

roid

al V

elo

city

(km

s-1

)AlcatorC-Mod


Toroidal projections of flows nearseparatrix shift toward counter-currentin sequence:lower => double => upper-null

-15 -10 -5 0 5 10 150

-60

-30

0

30

-15 -10 -5 0 5 10 150-10

0

10

20

-15 -10 -5 0 5 10 150


-40

-30

-20

r = 2 mm

r = 1 mm

Inner Probe

Outer Probe

Core Ar17+Doppler

To

roid

al V

elo

city

(km

s-1

)AlcatorC-Mod



Central plasma toroidal rotationcorrespondingly shifts more towardcounter-current direction

-15 -10 -5 0 5 10 150

-60

-30

0

30

-15 -10 -5 0 5 10 150-10

0

10

20

-15 -10 -5 0 5 10 150


-40

-30

-20

r = 2 mm

r = 1 mm

Inner Probe

Outer Probe

Core Ar17+Doppler

To

roid

al V

elo

city

(km

s-1

)AlcatorC-Mod




Toroidal velocity change is largest oninner SOL=> suggests inner SOL flow is responsible for change in rotation of confined plasma

18 km/s

12 km/s

50 km/s

DV

-15 -10 -5 0 5 10 150

-60

-30

0

30

-15 -10 -5 0 5 10 150-10

0

10

20

-15 -10 -5 0 5 10 150


-40

-30

-20

r = 2 mm

r = 1 mm

Inner Probe

Outer Probe

Core Ar17+Doppler

To

roid

al V

elo

city

(km

s-1

)AlcatorC-Mod





18 km/s

12 km/s

50 km/s

DV

~5 mm change in x-point balanceis sufficient to reverse flows=> consistent with scale length of pressure gradients near separatrix

-15 -10 -5 0 5 10 150

-60

-30

0

30

-15 -10 -5 0 5 10 150-10

0

10

20

-15 -10 -5 0 5 10 150


-40

-30

-20

r = 2 mm

r = 1 mm

Inner Probe

Outer Probe

Core Ar17+Doppler

To

roid

al V

elo

city

(km

s-1

)AlcatorC-Mod




\Transport-driven SOL flows impose boundary conditions on confined plasma


18 km/s

12 km/s

50 km/s

DV

~5 mm change in x-point balanceis sufficient to reverse flows=> consistent with scale length of pressure gradients near separatrix

IpBT

V//f V//f

^ transport-driven parallel SOL flows

AlcatorC-Mod

If Transport-Driven SOL Flow/Rotation Paradigm is Correct,Radial Electric Fields in SOL Should Depend on X-point Topology

Ballooning-like transport leads to a helical flowcomponent in the SOL with net volume-averagedtoroidal momentum: co-current for lower null, counter-current for upper null

IpBT

V//f V//f


DVfDVf

IpBT

Influence on plasma rotation

AlcatorC-Mod



Being free to rotate only in the toroidal direction,the confined plasma acquires a correspondingco-current or counter-current rotation increment

IpBT

V//f V//f


DVfDVfEr

DErxBq

IpBT

Influence on plasma rotation

Erweaker

DErxBq

stronger

AlcatorC-Mod



Being free to rotate only in the toroidal direction,the confined plasma acquires a correspondingco-current or counter-current rotation increment

Via momentum coupling across separatrix,a topology-dependent toroidal rotationcomponent, Er/Bq, should appear in the SOL

=> Stronger Er in SOL for lower null=> Weaker Er in SOL for upper null

AlcatorC-Mod

Plasma Potentials Near Separatrix Systematically Increasein the Sequence: Upper, Double, Lower-Null

0 5 1000

20

40

60

0 5 100

30

50

70

0 5 10030

50

70

r (mm)

Inner Probe

Est

imat

ed P

lasm

a P

ote

nti

al (

volt

s)

Vertical Probe

Outer Probe

Double NullLower Null

Upper Null

More positive Er in SOL near separatrix in Lower-Null

Caution: Accuracy of potential profile shape is uncertain!Plasma potential profiles estimated from sheath potential drop

DEr/Bq ~ 8 km/s, ~consistent with measured change in parallel (toroidal) flow in SOL

Outline of Talk





IpBT

DVfEr

DErxBq

strongerDVf

DErxBq

Erweaker

stronger weaker

Bx—B

SOL flows => topology-dependent rotation (and Er) near separatrix

SOL widths same; Rotation fi 0 near wall => Implies toroidal velocity shear (ErxB shear) near separatrix is:

=> Lower L-H power threshold when Bx—B points toward x-point!

L-H transition is thought to involvevelocity shear suppression of plasma turbulence

Novel Hypothesis:

1

2

1020

m-3

-40

0

40

km s

-1

Ar17+ Toroidal Velocity

eV

400

200

0

0123

MW

ICRF Power

Line Averaged Density

Electron Temperature

Max|—pe/ne| from TS100

10

keV

m-1

-0.2 -0.1 0.0 0.1L-H transition time (s)

in region 0.95 < y < 1

ECE:TS:y=0.95

AlcatorC-Mod

L-H Transition Coincides with Plasma Rotation AttainingRoughly the Same Value, Independent of Topology

Input power level to attain L Hdepends on x-point topology

Bx—B

Ohmic+ICRF => no momentum input

1

2

1020

m-3

-40

0

40

km s

-1


eV

400

200

0

0123

MW

ICRF Power




10

keV

m-1



ECE:TS:y=0.95

AlcatorC-Mod



Edge Te and electron pressure gradientsat L H transition also different

Bx—B


1

2

1020

m-3

-40

0

40

km s

-1


eV

400

200

0

0123

MW

ICRF Power




10

keV

m-1



ECE:TS:y=0.95

AlcatorC-Mod




Plasma rotation during ohmic phasestarts out counter-current in USN....

Bx—B


---- SOL flow boundary condition!

1

2

1020

m-3

-40

0

40

km s

-1


eV

400

200

0

0123

MW

ICRF Power




10

keV

m-1



ECE:TS:y=0.95

AlcatorC-Mod




Plasma rotation during ohmic phasestarts out counter-current in USN....

Bx—B

....but ramps toward co-current aspressure gradients build up

similar rotation at the L H transition!


---- SOL flow boundary condition!

1 2 3 4

-40

0

40

0

20 r = 2 mm

1 4

1 2 3 4

0

20

Total Input Power (MW)

r = 2 mm

Lower NullUpper Null

L H(at transition)

AlcatorC-Mod

Two Elements Combine to Affect Toroidal Rotation: SOL Flows (~Topology) + Plasma Pressure (~Power)

Vertical Probe

Outer Probe

Core Ar17+ Doppler

Toroidal Velocities

km s

-1

DV

Transport-driven SOL flows impose acounter-current toroidal rotation offset in Upper Null

1 2 3 4

-40

0

40

0

20 r = 2 mm

1 4

1 2 3 4

0

20


r = 2 mm


L H(at transition)

AlcatorC-Mod


Vertical Probe

Outer Probe

Core Ar17+ Doppler

Toroidal Velocities

km s

-1


Toroidal rotation also depends on~plasma pressure, ramping towardsco-current direction as input powerincreases

Slope:~14 km s-1 MW-1



- Effect extends to separatrix; seen by probes in SOL 1 2 3 4

-40

0

40

0

20 r = 2 mm

1 4

1 2 3 4

0

20


r = 2 mm


L H(at transition)

AlcatorC-Mod


Vertical Probe

Outer Probe

Core Ar17+ Doppler

Toroidal Velocities

km s

-1







-40

0

40

0

20 r = 2 mm

1 4

1 2 3 4

0

20


r = 2 mm


L H(at transition)

AlcatorC-Mod


Vertical Probe

Outer Probe

Core Ar17+ Doppler

Toroidal Velocities

km s

-1

But, a similar relationship between co-current core rotation and plasma pressureis seen during the H-mode phase...

Theories:

- sub-neoclassical transport

- turbulence

B. Coppi, Nucl. Fusion 42, 1 (2002).

A.L. Rogister, et al., Nucl. Fusion 42, 1144 (2002).

K.C. Shaing, Phys. Rev. Lett. 86, 640 (2001)

=> Mechanism for this not resolved

... J. Rice, et al., Nucl. Fusion 41, 277 (2001)

L-H Transition occurs at nominally thesame toroidal rotation speed

=> Upper Null requires more input power




-40

0

40

0

20 r = 2 mm

1 4

1 2 3 4

0

20


r = 2 mm


L H(at transition)

AlcatorC-Mod


Vertical Probe

Outer Probe

Core Ar17+ Doppler

Toroidal Velocities

km s

-1

Outline of Talk





Lower Single Null

IpBT

Bx—B

Outline of Talk





Inner Divertor 'Nose' Grazing

IpBT

Bx—B

Outline of Talk





Inner Divertor 'Nose' Limited

IpBT

Bx—B

Outline of Talk





Inner Wall Limited

IpBT

Bx—B

0

1

2

1020

m-3

0

1

2

MW

3

5

7

a.u

.

Da

0.6 0.8 1.0 1.2 1.4 1.6Time (s)

Line AveragedDensity

ICRF Power0.6 0.8 1.0 1.2 1.4 1.6

0.6 0.8 1.0 1.2 1.4 1.6

AlcatorC-Mod

Lower Limited discharges have L-H power thresholds that are virtually identical to Lower Single Null discharges!

Does not matter if lower X-point is centered in divertor, grazing the wall,or buried inside the vacuum vessel structure

What common element causes L-H threshold to be identical?

0510 00.1

1.0

10.0

100. 0

0510 00.0

0.1

0.2

0.3

0.4

-50

-25

0

25

50

0

10

100

10

00

0.1

0.2

-50

1

10

100

0

0.1

0.2

0.3

-50

-25

0

25

50

Inner SOL

1020

eV

m-3

0510

AlcatorC-Mod

Lower-Limited and Lower X-point discharges have identical SOL flow patterns....perhaps a key commonality

ElectronPressure

Fluctuations


-25

0

25

Outer SOL

0 5 100

0 5 100

10

10

0 5 100

RMS Jsat/<Jsat>


0510 00.1

1.0

10.0

100. 0

0510 00.0

0.1

0.2

0.3

0.4

-50

-25

0

25

50

0

10

100

10

00

0.1

0.2

-50

1

10

100

0

0.1

0.2

0.3

-50

-25

0

25

50

Inner SOL

1020

eV

m-3

0510

AlcatorC-Mod


ElectronPressure

Fluctuations


-25

0

25

Outer SOL

0 5 100

0 5 100

10

10

0 5 100

RMS Jsat/<Jsat>


Pressure profiles near separatrix& fluctuation asymmetries are similar in all topologies

0510 00.1

1.0

10.0

100. 0

0510 00.0

0.1

0.2

0.3

0.4

-50

-25

0

25

50

0

10

100

10

00

0.1

0.2

-50

1

10

100

0

0.1

0.2

0.3

-50

-25

0

25

50

Inner SOL

1020

eV

m-3

0510

AlcatorC-Mod


ElectronPressure

Fluctuations


-25

0

25

Outer SOL

0 5 100

0 5 100

10

10

0 5 100

RMS Jsat/<Jsat>



Lower-Limited, Grazing, Lower X-pointhave co-current inner SOL flows...Upper X-point => counter-current

0510 00.1

1.0

10.0

100. 0

0510 00.0

0.1

0.2

0.3

0.4

-50

-25

0

25

50

0

10

100

10

00

0.1

0.2

-50

1

10

100

0

0.1

0.2

0.3

-50

-25

0

25

50

Inner SOL

1020

eV

m-3

0510

AlcatorC-Mod


ElectronPressure

Fluctuations


-25

0

25

Outer SOL

0 5 100

0 5 100

10

10

0 5 100

RMS Jsat/<Jsat>



Lower-Limited, Grazing, Lower X-pointhave co-current inner SOL flows...Upper X-point => counter-current

IpBT

SOL flow pattern defined by X-points and/or Limiter contact

co-currentV//f G^

Key commonality for L-H sensitivity?

=> same LH threshold

co-currentV//f G^

†Normalized to threshold scaling from Int. H-mode Threshold Database, J. Snipes, et al., PPCF 42, A299 (2000).

†

-20 -10 0 10 200.5

1.0

1.5

2.0

No

rmal

ized

inp

ut

po

wer

Normalized L-H Power Thresholdsfor Various Magnetic Topologies

D, Distance between primary and secondary separatrices or inner wall gap (mm)

AlcatorC-Mod

New Insight: Connection between magnetic topology and L-H threshold may be recast in terms of control of Inner SOL flows

-20 -10 0 10 200.5

1.0

1.5

2.0

No

rmal

ized

inp

ut

po

wer



AlcatorC-Mod


D parameterizes:

Lower x-point or Lower-Limited

Upper x-point or Upper-Limited

(2) Width of Inner SOL available for these flows

(1) Direction of transport-driven SOL flows in Inner SOL (negative = co-current)

-20 -10 0 10 200.5

1.0

1.5

2.0

No

rmal

ized

inp

ut

po

wer



AlcatorC-Mod


D parameterizes:





Lower X-pt, Lower Grazing, Lower LimitedFavorable (co-current) Inner SOL flow => Lowest L-H thresholds

-20 -10 0 10 200.5

1.0

1.5

2.0

No

rmal

ized

inp

ut

po

wer



AlcatorC-Mod


D parameterizes:






Reduced Inner Gap, Double NullInner SOL flow blocked or balanced => Increased L-H thresholds|D| < ~5 mm, corresponds to flow change

-20 -10 0 10 200.5

1.0

1.5

2.0

No

rmal

ized

inp

ut

po

wer



AlcatorC-Mod


D parameterizes:







Upper NullUnfavorable (counter) Inner SOL flow => Highest L-H thresholds

-20 -10 0 10 200.5

1.0

1.5

2.0

No

rmal

ized

inp

ut

po

wer



AlcatorC-Mod


D parameterizes:







Upper NullUnfavorable (counter) Inner SOL flow => Highest L-H thresholds

Inner Wall-Limited - worst of all worlds?no Inner SOL flow, impurity susceptible => Highest L-H thresholds

AlcatorC-ModSummary

A cross field transport-driven plasmacirculation loop is evident in C-Mod

X-points and/or limiter contact points set the // flow direction

SOL flows impose a toroidal rotation boundary condition for confined plasmaX-point/limiter topology and toroidal rotation at boundary are linked!

IpBT

V//f V//f DVfDVf

Bx—B

ErDErxBq

Erweaker

DErxBq

stronger

near-sonic// flows ballooning-like

transportG^

=> Potential explanation for the topology dependence of the L-H power threshold

AlcatorC-ModSummary

L-H threshold studies with different x-point topologies support hypothesisthat SOL flows have a controlling influence

SOL flows impede co-current rotation with upper x-pointCorrespondingly, more input power (which promotes co-rotation) is required

L-H transition is coincident with toroidal rotation achieving similar level,independent of x-point topology

The role of magnetic topology in affecting L-H thresholds can be formulated in terms of the resultant flows in the Inner SOL

L-H Power Thresholds

Co-Current Inner SOL flows

1.0

1.5

2.0

-20 -10 0 10 20D (mm)

Counter-Current Inner SOL flows

No

rmal

ized

Po

wer

Same L-H power threshold

=

transport-driven scrape-off layer flows, the role of the...

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