Plasma flows and critical gradient phenomenanear the last-closed flux surface

B. LaBombard

AlcatorC-Mod

Presented at the 10th ITPA Edge and Pedestal Physics Topical Group Meeting MIT Plasma Science and Fusion Center

April 10-12, 2006

AlcatorC-Mod

Recent C-Mod experiments have revealed important aspects oftransport physics at the SOL interface...

...which may be fundamental to understanding the edge pedestal

AlcatorC-Mod

Recent C-Mod experiments have revealed important aspects oftransport physics at the SOL interface...

Strong (near-sonic) plasma flows just outside the LCFS

Ballooning-like transport drive mechanismConnection to magnetic topology (LSN/USN) Toroidal rotation 'boundary condition' on confined plasma

...which may be fundamental to understanding the edge pedestal

AlcatorC-Mod

Recent C-Mod experiments have revealed important aspects oftransport physics at the SOL interface...

Strong (near-sonic) plasma flows just outside the LCFS

'Critical gradient' behavior of pressure profiles near the LCFS

L-mode: pressure gradients 'clamped' at a value of that depends on collisionality

Ballooning-like transport drive mechanismConnection to magnetic topology (LSN/USN) Toroidal rotation 'boundary condition' on confined plasma

=> edge plasma maps to a 2-D 'phase space' ( , collisionality)=> 'density limit boundary' at high collisionality

aMHD

aMHD

...which may be fundamental to understanding the edge pedestal

AlcatorC-Mod

Recent C-Mod experiments have revealed important aspects oftransport physics at the SOL interface...

Strong (near-sonic) plasma flows just outside the LCFS

'Critical gradient' behavior of pressure profiles near the LCFS

L-mode: pressure gradients 'clamped' at a value of that depends on collisionality

H-mode: scaling of peak pedestal pressure gradients with Ip2 (Hughes)

Ballooning-like transport drive mechanismConnection to magnetic topology (LSN/USN) Toroidal rotation 'boundary condition' on confined plasma

=> edge plasma maps to a 2-D 'phase space' ( , collisionality)=> 'density limit boundary' at high collisionality

aMHD

aMHD

...which may be fundamental to understanding the edge pedestal

AlcatorC-Mod

Recent C-Mod experiments have revealed important aspects oftransport physics at the SOL interface...

Strong (near-sonic) plasma flows just outside the LCFS

'Critical gradient' behavior of pressure profiles near the LCFS

L-mode: pressure gradients 'clamped' at a value of that depends on collisionality

H-mode: scaling of peak pedestal pressure gradients with Ip2 (Hughes)

Ballooning-like transport drive mechanismConnection to magnetic topology (LSN/USN) Toroidal rotation 'boundary condition' on confined plasma

=> edge plasma maps to a 2-D 'phase space' ( , collisionality)=> 'density limit boundary' at high collisionality

Most recent: Potential link between 'critical gradient' and SOL flowsL-mode: attainable value of depends on LSN/USN topology

=> edge flows are correspondingly different

aMHD

aMHD

aMHD

...which may be fundamental to understanding the edge pedestal

AlcatorC-Mod

Recent C-Mod experiments have revealed important aspects oftransport physics at the SOL interface...

Strong (near-sonic) plasma flows just outside the LCFS

'Critical gradient' behavior of pressure profiles near the LCFS

L-mode: pressure gradients 'clamped' at a value of that depends on collisionality

H-mode: scaling of peak pedestal pressure gradients with Ip2 (Hughes)

Ballooning-like transport drive mechanismConnection to magnetic topology (LSN/USN) Toroidal rotation 'boundary condition' on confined plasma

=> edge plasma maps to a 2-D 'phase space' ( , collisionality)=> 'density limit boundary' at high collisionality

L-H threshold power: lower with 'favorable' SOL flows (LSN or lower-limited)

Most recent: Potential link between 'critical gradient' and SOL flowsL-mode: attainable value of depends on LSN/USN topology

=> edge flows are correspondingly different

aMHD

aMHD

aMHD

...which may be fundamental to understanding the edge pedestal

Transport-driven plasma flows in the SOL

JET

JT60-U

JET

C-Mod

Distance to separatrix (mm) Distance to separatrix (mm)

Distance to separatrix (mm)

Distance to separatrix (mm)

Distance to separatrix (mm)

JT60-U

JT60-U

JT60-U

Scrape-off layer flow patterns in a tokamak are complex -Near-sonic flow along field lines occurs far from material surfaces

Representative composite of parallel flow data† from JT60-U, JET, C-Mod

- Strong flows along B (M// ~ 0.5)

- Components which are both dependent and independent of the sign of B

†G. Matthews, J. Nucl. Mater. 137-139 (2005) 1.

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

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)

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

Fluctuations

0510 0

0510 0

AlcatorC-Mod

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

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

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)

Consistent with low ^ transportin inner SOL

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

Fluctuations

0510 0

0510 0

AlcatorC-Mod

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

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

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)

Consistent with low ^ transportin inner SOL

Near-sonic // flows on Inner SOL

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

Fluctuations

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

0510 0

0510 0

AlcatorC-Mod

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

Fluctuation levels persistentlylower on Inner SOL

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

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)

Consistent with low ^ transportin inner SOL

Near-sonic // flows on Inner SOL

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

Fluctuations

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

0510 0

0510 0

AlcatorC-Mod

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

Fluctuation levels persistentlylower on Inner SOL

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

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)

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

Consistent with low ^ transportin inner SOL

Near-sonic // flows on Inner SOL

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

Fluctuations

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

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

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

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

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

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

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

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

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

Central plasma toroidal rotationcorrespondingly shifts more towardcounter-current direction

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

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

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

Central plasma toroidal rotationcorrespondingly shifts more towardcounter-current direction

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

~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

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

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

Central plasma toroidal rotationcorrespondingly shifts more towardcounter-current direction

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

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

~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

Transport-Driven SOL Flows: a mechanism for plasma near the separatrix to 'spin-up' toroidally, depending 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

^ transport-driven parallel SOL flows

DVfDVf

IpBT

Influence on plasma rotation

AlcatorC-Mod

Transport-Driven SOL Flows: a mechanism for plasma near the separatrix to 'spin-up' toroidally, depending 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

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

IpBT

V//f V//f

^ transport-driven parallel SOL flows

DVfDVfEr

DErxBq

IpBT

Influence on plasma rotation

Erweaker

DErxBq

stronger

AlcatorC-Mod

Transport-Driven SOL Flows: a mechanism for plasma near the separatrix to 'spin-up' toroidally, depending 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

Being free to rotate only in the toroidal direction,the confined plasma can acquire 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

Critical gradient phenomena near the separatrix

InverseCollisionalityParameter

AlcatorC-Mod

†[1] Scott, PPCF 39 (1997) 1635.

PoloidalBeta Gradient

Turbulence character & transport level determined primarily by twodimensionless parameters

[3] B. Scott

'Critical Gradient' transport behavior is suggested in first-principles 3-D Electromagnetic Fluid Dift turbulence simulations†

aMHD ~ PB2

q2R —̂ ~1/ 2

qad Rlei( )1 R

L )(1/ 4

[2] Xu, X.Q., et al., Nucl. Fusion 40 (2000) 731. [4] Rogers, Drake, and Zeiler, Phys. Rev. Lett. 81 (1998) 4396.

n

InverseCollisionalityParameter

AlcatorC-Mod

†[1] Scott, PPCF 39 (1997) 1635.

PoloidalBeta Gradient

Turbulence character & transport level determined primarily by twodimensionless parameters

[3] B. Scott

'Critical Gradient' transport behavior is suggested in first-principles 3-D Electromagnetic Fluid Dift turbulence simulations†

aMHD ~ PB2

q2R —̂ ~1/ 2

qad Rlei( )1 R

L )(1/ 4

[2] Xu, X.Q., et al., Nucl. Fusion 40 (2000) 731. [4] Rogers, Drake, and Zeiler, Phys. Rev. Lett. 81 (1998) 4396.

n

Electron Heat Diffusivity [3]

101

102

10-2 10-1 10010-1

100

I

ce

aMHD

InverseCollisionalityParameter

AlcatorC-Mod

†[1] Scott, PPCF 39 (1997) 1635.

PoloidalBeta Gradient

Turbulence character & transport level determined primarily by twodimensionless parameters

[3] B. Scott

'Critical Gradient' transport behavior is suggested in first-principles 3-D Electromagnetic Fluid Dift turbulence simulations†

aMHD ~ PB2

q2R —̂ ~1/ 2

qad Rlei( )1 R

L )(1/ 4

[2] Xu, X.Q., et al., Nucl. Fusion 40 (2000) 731. [4] Rogers, Drake, and Zeiler, Phys. Rev. Lett. 81 (1998) 4396.

n

contours of

constantheat flux

incr

ease

x

100

"critical gradient"

Electron Heat Diffusivity [3]

101

102

10-2 10-1 10010-1

100

I

ce

edge plasma staterestricted to this band

aMHD

InverseCollisionalityParameter

AlcatorC-Mod

†[1] Scott, PPCF 39 (1997) 1635.

PoloidalBeta Gradient

Turbulence character & transport level determined primarily by twodimensionless parameters

[3] B. Scott

'Critical Gradient' transport behavior is suggested in first-principles 3-D Electromagnetic Fluid Dift turbulence simulations†

transport depends on location in (aMHD, ad) 'phase-space'

aMHD ~ PB2

q2R —̂ ~1/ 2

qad Rlei( )1 R

L )(1/ 4

Transport

Incre

asingIn

acce

ssib

le

ad

aMHD

[2] Xu, X.Q., et al., Nucl. Fusion 40 (2000) 731. [4] Rogers, Drake, and Zeiler, Phys. Rev. Lett. 81 (1998) 4396.

'Phase Space' of EMFDT [4]

n

<= increasing collisionality

contours of

constantheat flux

incr

ease

x

100

"critical gradient"

Electron Heat Diffusivity [3]

101

102

10-2 10-1 10010-1

100

I

ce

edge plasma staterestricted to this band

aMHD

AlcatorC-Mod

Results from 2000 campaign:†

Plasma states near separatrix are indeed found to occupy a well-defined region in the phase space of EMFDT

†Nuclear Fusion 45 (2005) 1658.

Lower single-nullForward Ip, BT

Low-power Ohmic L-mode dischargesDensity: 0.14 < n/nG < 0.53

Discharges with differentmachine parameters: BT, Ip, ne

0.4 0.6 0.8 1.02

3

4

5

6 q95 = 6.5

q95 = 5

q95 = 3.5

Ip (MA)

BT (

tesl

a)

AlcatorC-Mod

Results from 2000 campaign:†

Plasma states near separatrix are indeed found to occupy a well-defined region in the phase space of EMFDT

~ 1q

lei

RÊ

Ë Á

ˆ

¯ ˜

1/ 2 RLn

Ê

Ë Á

ˆ

¯ ˜

1/ 4

ad

aMHD

~ nTe

B2q2RLpe In

acce

ssib

le

~ —pe

Ip2

†Nuclear Fusion 45 (2005) 1658.

Lower single-nullForward Ip, BT

Low-power Ohmic L-mode dischargesDensity: 0.14 < n/nG < 0.53

Discharges with differentmachine parameters: BT, Ip, ne ...occupy in a similar band in space

0.80.5

1.0IP (MA)

aMHD ad,

A region of high at high density is inaccessible, owing to an explosivegrowth of cross-field transport

<== increasing ne

aMHD

0.4 0.6 0.8 1.02

3

4

5

6 q95 = 6.5

q95 = 5

q95 = 3.5

Ip (MA)

BT (

tesl

a)

0.2 0.4 0.6 0.80

0.5

1.0

1.5

0

aMHDad2 = 0.15

ELM-freeEDAL-mode

Inac

cess

ible

~ 1q

lei

RÊ

Ë Á

ˆ

¯ ˜

1/ 2 RLn

Ê

Ë Á

ˆ

¯ ˜

1/ 4

ad

aMHD

~ nTe

B2q2RLpe

~ —pe

Ip2

<== increasing ne

0.2 0.4 0.6 0.80

0.5

1.0

1.5

AlcatorC-Mod

Results from 2000 campaign:†

Plasma states near separatrix are indeed found to occupy a well-defined region in the phase space of EMFDT

†Nuclear Fusion 45 (2005) 1658.

Lower single-nullForward Ip, BT

Low-power Ohmic L-mode dischargesDensity: 0.14 < n/nG < 0.53

Discharges with differentmachine parameters: BT, Ip, ne ...occupy in a similar band in spaceaMHD ad,

0.4 0.6 0.8 1.02

3

4

5

6 q95 = 6.5

q95 = 5

q95 = 3.5

Ip (MA)

BT (

tesl

a)

Ohmic H-modes evolve from L-modesat the low collisionality boundary,increasing in aMHD

0

0.4 0.6 0.8 1.02

3

4

5

6 q95 = 6.5

q95 = 5

q95 = 3.5

Ip (MA)

BT (

tesl

a)AlcatorC-Mod

Pressure gradients near the separatrix appear to clamp at similar values of

0

1

1

-1 0 1 2 3 4 5 6

-1 0 1 2 3 4 5 6

-1 0 1 4 5 6

10

-1 0 6

1020

eV

m-3

mm

-1

0 2 4 60.1

1.0

3

30

10

3

30

aMHD

54

6BT (T)

0.80.5

1.0IP (MA)

aMHD when normalized collisionality is held fixed

Distance from into SOL (mm)

ad

Look at pressure profile data from dischargeswith ~ 0.35, 2 mm from separatrix

— nTe^

— nTe^

0.4 0.6 0.8 1.02

3

4

5

6 q95 = 6.5

q95 = 5

q95 = 3.5

Ip (MA)

BT (

tesl

a)AlcatorC-Mod

Pressure gradients near the separatrix appear to clamp at similar values of

0

1

1

-1 0 1 2 3 4 5 6

-1 0 1 2 3 4 5 6

-1 0 1 4 5 6

10

-1 0 6

1020

eV

m-3

mm

-1

0 2 4 60.1

1.0

3

30

10

3

30

aMHD

54

6BT (T)

0.80.5

1.0IP (MA)

aMHD when normalized collisionality is held fixed

Distance from into SOL (mm)

ad

Look at pressure profile data from dischargeswith ~ 0.35, 2 mm from separatrix

— nTe^

— nTe^

Ip Scan:Pressure gradients scale roughly as Ip => similar

2

aMHD

0.4 0.6 0.8 1.02

3

4

5

6 q95 = 6.5

q95 = 5

q95 = 3.5

Ip (MA)

BT (

tesl

a)AlcatorC-Mod

Pressure gradients near the separatrix appear to clamp at similar values of

0

1

1

-1 0 1 2 3 4 5 6

-1 0 1 2 3 4 5 6

-1 0 1 4 5 6

10

-1 0 6

1020

eV

m-3

mm

-1

0 2 4 60.1

1.0

3

30

10

3

30

aMHD

54

6BT (T)

0.80.5

1.0IP (MA)

aMHD when normalized collisionality is held fixed

Distance from into SOL (mm)

ad

Look at pressure profile data from dischargeswith ~ 0.35, 2 mm from separatrix

No sensitivity to toroidal field

— nTe^

— nTe^

Ip Scan:Pressure gradients scale roughly as Ip => similar

2

aMHDBT Scan:

=> Pressure gradient near separatrix set by a 'critical poloidal beta gradient'

Coupling between flows and critical gradient?

AlcatorC-Mod

New experiments (2005 & 2006)

Extended range of Ip, BT

Density scans: 0.1 < n/nG < 0.5 with lower currents (0.4 MA)and fields (4, 3.2=>2.7 tesla)

Improved scanning probe diagnostics

0.4 0.6 0.8 1.02

3

4

5

6 q95 = 6.5

q95 = 5

q95 = 3.5

Ip (MA)

BT (

tesl

a)

LSNUSN

Is there any evidence that edge plasma flows affect the 'critical gradient' ( ) seen near the separatrix?aMHD

AlcatorC-Mod

New experiments (2005 & 2006)

Extended range of Ip, BT Lower vs upper-null topologies

Density scans: 0.1 < n/nG < 0.5 with lower currents (0.4 MA)and fields (4, 3.2=>2.7 tesla)

Improved scanning probe diagnostics

0.4 0.6 0.8 1.02

3

4

5

6 q95 = 6.5

q95 = 5

q95 = 3.5

Ip (MA)

BT (

tesl

a)

LSNUSN

Is there any evidence that edge plasma flows affect the 'critical gradient' ( ) seen near the separatrix?aMHD

AlcatorC-Mod

New experiments (2005 & 2006)

co-currentrotation drive

counter-currentrotation drive

Extended range of Ip, BT Lower vs upper-null topologies

Density scans: 0.1 < n/nG < 0.5 with lower currents (0.4 MA)and fields (4, 3.2=>2.7 tesla)

Improved scanning probe diagnostics

SOL flows change dramaticallywith X-point location

0.4 0.6 0.8 1.02

3

4

5

6 q95 = 6.5

q95 = 5

q95 = 3.5

Ip (MA)

BT (

tesl

a)

LSNUSN

Is there any evidence that edge plasma flows affect the 'critical gradient' ( ) seen near the separatrix?aMHD

AlcatorC-Mod

New experiments (2005 & 2006)

co-currentrotation drive

counter-currentrotation drive

Extended range of Ip, BT Lower vs upper-null topologies

What is influence on SOL 'phase-space'?

Density scans: 0.1 < n/nG < 0.5 with lower currents (0.4 MA)and fields (4, 3.2=>2.7 tesla)

=> Run matched discharges with upper and lower null

Improved scanning probe diagnostics

SOL flows change dramaticallywith X-point location

0.4 0.6 0.8 1.02

3

4

5

6 q95 = 6.5

q95 = 5

q95 = 3.5

Ip (MA)

BT (

tesl

a)

LSNUSN

Is there any evidence that edge plasma flows affect the 'critical gradient' ( ) seen near the separatrix?aMHD

New Results (2005 & 2006) - AlcatorC-ModPressure gradients near sep. consistently scale as Ip

... but value depends on lower / upper X-point topology

2

0.1 0.2 0.3 0.4 0.5 0.61/ 2

q Rlei( )1 ~

RLnÊ

Ë Á

ˆ

¯ ˜

1/ 4

ad

0

1

2

3

0.4 MA

0.8 MA

1.1 MA

0

1

2

3

4

0.4 MA

0.8 MA

1.1 MA

— nTe^

— nTe^

1021

eV

m-3

mm

-1

1/ 2

q Rlei( )1

0.1 0.2 0.3 0.4 0.5 0.60.0

0.2

0.4

0.6

0.8r = 1 mm

New Results (2005 & 2006) - AlcatorC-Mod

aMHD

Pressure gradients near sep. consistently scale as Ip

... but value depends on lower / upper X-point topology

2

Edge plasma states again align in EMFDT phase-space, but in two bands

Lower null achieves higher valuesof compared to upper null athigh collisionality

aMHD

0.1 0.2 0.3 0.4 0.5 0.61/ 2

q Rlei( )1 ~

RLnÊ

Ë Á

ˆ

¯ ˜

1/ 4

ad

0

1

2

3

0.4 MA

0.8 MA

1.1 MA

0

1

2

3

4

0.4 MA

0.8 MA

1.1 MA

— nTe^

— nTe^

1021

eV

m-3

mm

-1

AlcatorC-Mod

Plasma flows in the SOL are dramatically different inLower vs Upper null topologies

... perhaps affecting the attainable values of aMHD

- Plasma flows from low to high-field side(ballooning-like transport drive)

r = 2 mm

High-field side SOL

-1.0

-0.5

0.0

0.5

1.0

Parallel FlowMach Number

1/ 2

q Rlei( )1

0.1 0.2 0.3 0.4 0.5 0.6

AlcatorC-Mod

Plasma flows in the SOL are dramatically different inLower vs Upper null topologies

... perhaps affecting the attainable values of aMHD

- Plasma flows from low to high-field side(ballooning-like transport drive)

- Low-field side flows near sep. are affected (~toroidal rotation)

r = 2 mm

High-field side SOL Low-field side SOLr = 1 mm Parallel Flow

Mach Number

-1.0

-0.5

0.0

0.5

1.0

-0.2

0.0

0.2

0.4

1/ 2

q Rlei( )1

0.1 0.2 0.3 0.4 0.5 0.6

Parallel FlowMach Number

1/ 2

q Rlei( )1

0.1 0.2 0.3 0.4 0.5 0.6

AlcatorC-Mod

Plasma flows in the SOL are dramatically different inLower vs Upper null topologies

... perhaps affecting the attainable values of

- Highest is achieved when flow is positive (co-current) on low-field side

=> favors lower null topology

(Note: lower null also has lowest L-H threshold power)

aMHD

aMHD

- Plasma flows from low to high-field side(ballooning-like transport drive)

- Low-field side flows near sep. are affected (~toroidal rotation)

1/ 2

q Rlei( )1

aMHD

r = 2 mm

High-field side SOL Low-field side SOLr = 1 mm

r = 1 mm

Parallel FlowMach Number

-1.0

-0.5

0.0

0.5

1.0

-0.2

0.0

0.2

0.4

0.1 0.2 0.3 0.4 0.5 0.60.0

0.2

0.4

0.6

0.8

Parallel FlowMach Number

1/ 2

q Rlei( )1

0.1 0.2 0.3 0.4 0.5 0.6

AlcatorC-ModSummary

Accessible L-mode edge states map to a ( , ) 'phase space'aMHDMapping is invariant of machine parameters for fixed magnetic topology: 0.4 < Ip < 1 MA, 2.7 < BT < 6T, 0.1 < ne/nG < 0.5

when equilibrium plasma flows near the separatrix are differentLower null topology leads to higher than Upper nullaMHD

Co-current plasma flows in the SOL are associated with higher aMHD

=> Flow is another phase space parameter ( , , ,...)aMHD ad M

ad

Strong 'transport-driven' plasma flows exist just outside the LCFS

Plasma near the separatix exhibits a 'critical gradient' ( ) behavioraMHD

Broadly consistent with behavior in EMFDT simulations

Ballooning-like transport drive, x-point (and limiter) dependent flow pattern,a flow boundary condition for the confined plasma

Key plasma phenomena in edge/pedestal region