Evidence forElectromagnetic Fluid Drift Turbulence

Controlling the Edge Plasma Statein Alcator C-Mod

B. LaBombard, J.W. Hughes, D. Mossessian, M. Greenwald, J.L. Terry,

Alcator C-Mod Team

AlcatorC-Mod

Contributed talk CO1.002Presented at the 45th Annual Meeting of the APS Division of Plasma Physics

October 27 - 31, 2003 Albuquerque, N.M.

AlcatorC-Mod

Motivation: Quantitative, Physics-Based Description of Edge/SOL Transport is Still Lacking.....

....yet, impact on reactor operation (edge pedestal, main-chamber recycling, density limit physics,...) is apparent

Progress is being made on two fronts:

DALFTI, NLET, BOUT, PARTURB, ....- 3-D plasma-fluid turbulence simulation codes

- Detailed diagnosis of profiles/turbulence in experiments

Probes, BES and Gas-Puff Turbulence Imaging, ...

Questions:To what degree do the turbulence-simulations capture experimentally observed plasma response?Do they contain the essential physics?

Focus of this talk: Dimensionless 'phase-space' occupied by the plasma edge and its relationship to turbulence theory/simulation

Collisionality

AlcatorC-Mod

3-D Turbulence Theory/Simulations† Identify DimensionlessParameters that Determine Turbulence & Transport State

Primary Dimensionless Parameters:

RDZ [2]:

Scott [1]: C ~qR( )2

L^lei

Physics:

†[1] Scott, PPCF 39 (1997) 1635[2] Rogers, Drake, and Zeiler, Phys. Rev. Lett. 81 (1998) 4396

Beta Gradient

ˆ b ~ nTeB2

qRL^

Ê

Ë Á

ˆ

¯ ˜

2

aMHD ~n(Ti + Te )

B2q2RL^

~lei( )

1/ 2

q(RL )1/ 4ad

^

Result:Turbulence character & transport leveldetermined primarily by location in (b, C) or (aMHD, ad) 'phase-space'ˆ

= f ( ˆ b ,C,...) g(aMHD,ad ,...)Transport or =

Example from DALFTI [3]:

[3] Scott, to be published in Phys. Plasmas

=> strong dependence on aMHD for aMHD > 0.2

Electron Heat Diffusivity

101

102

10-2 10-1 10010-1

100I

ce

aM

ElectroMagnetic parallel inductance, finite B, parallel resistivity,Fluid Drift non-linear drift-wave, curvature,3-D Turbulence "ballooning-like" asymmetries, x-point effects

~

linear drift-resistiveballooning normalization

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In Actual Experiments, Gradients are Not Controlled But Constrained to Satisfy Particle & Power Balance

lei q

lines of ~constant heat flux

Heat fluxes set by input power, particle fluxes set by fueling

incr

ease

x

100

Look at "edge plasma state" in discharges with different and

- EMFD dimensionless parameters are setting transport levels

=> edge plasma state constrained to a characteristic curve in (b, C)- or (aMHD, ad)-space ˆ

Test:

"critical gradient"

If:

Then:- Edge gradients ~(b or aMHD) may appear as a function of the "other" key dimensionless parameters ~(C or ad)

ˆ

- Transport is a strong function of local gradient

Electron Heat Diffusivity

101

102

10-2 10-1 10010-1

100I

ce

aMAnd:

AlcatorC-Mod

Tests for EMFD transport physics in the SOL should focus on region near separatrix (the Near SOL)

r=3 mm

Gaussians

skewness=0.5kurtosis=0.5

-11

10

100

Da Light: Signal - Mean

skewness=1.0kurtosis=1.1

r=9 mm

0 1 2 3

1

10

100

Da-Light Probability Distributions

NearSOL

FarSOL

250 5 10 15 20

1.0

0.2(10

20 m

-3)

Distance into SOL, r (mm)

Limiter Shadow

0.0 1.5

0.0 0.5 1.0 1.5Time (milliseconds)012

0.0 1.5

I sat

/<I s

at>

0.0 1.501

2

0.0 0.5 1.0 1.5Time (ms)

Density

Far SOL: flattened time-averaged gradients, intermittent 'bursty/blobby' transport

Near SOL: steep time-averaged gradients, less 'bursty'

Two-zone SOL profiles:

NearSOL

FarSOL

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Experiment: Collect Edge Profile Data at Different ne and q in Otherwise Identical Discharges

Lower single-nullForward & reversed Ip, BT

Low-power Ohmic dischargesL- and H-mode

Goal: Examine "edge plasma state" with respect to EMFD parameters

Density: .14 < n/nG < .53Plasma current: 0.5 < Ip < 1.0 MA Toroidal field: 4 < BT < 6 tesla

0.4 0.6 0.8 1.0Ip (MA)

2

3

4

5

6

7

BT

(te

sla)

LElm-FreeEDAL:Rev BT, Ip

BT, Ip, q Parameter Space

qy95 = 6.5

qy95 = 5

qy95 = 3.5

In particular, vary and lei q

-10 -5 0 5 10 15

1

10

100

-10 -5 0 5 10 15

1

10

100

-10 -5 0 5 10 15 1

10

100

NL=0.66 x1020 m-2

-10 -5 0 5 10 15 1

10NL=0.83

-10 -5 0 5 10 151

10

NL=1.01

- "Transport barrier" near separatrix is present in both L- and H-mode

Ohmic L-mode Profiles

Elm-Free

EDA

Edge Thomson Scanning-Probe

AlcatorC-ModExperiment: Examine "Edge Plasma State", 2 mm into SOL

LnTe

LnTe

LnTe(mm)

(mm)

(mm)

Ohmic H-mode Profiles

- Cross-checks between Edge Thomson & Scanning Probe look goodGradient scale-lengths similar; similar trends with discharge

r (mm)r (mm)

=> Location where electron pressure-gradient scale-lengths exhibit minima in all discharges

Notes:

1.0 1.5 2.01

10

0.4 0.6 0.8 1.01

10

0.1 0.2 0.3 0.4 0.51

10

Data taken at r = 2.0 mm

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Result: Gradient Scale-Lengths Near Separatrix Correlate with Collisionality, Scaled According to EMFD Parameters!

leiR

Ê

Ë Á

ˆ

¯ ˜

1/ 2

leiqR

Ê

Ë Á

ˆ

¯ ˜

1/ 2

1q

leiR

Ê

Ë Á

ˆ

¯ ˜

1/ 2

~ 1C1/ 2

RL^

Ê

Ë Á

ˆ

¯ ˜

1/ 2

5.03.5

6.5

qy95

(mm)

LnTe

LnTe

LnTe

- LnTe and Ln map to a simple function of C or ad over the full parameter range!

L-mode, normal BT, Ip 0.5 < Ip < 1.0 MA 4 < BT < 6 tesla .14 < n/nG < .53 10

1

10

3

3

30

Ln(mm)

LnTe

5.03.5

6.5

qy95

~ 1q

leiR

Ê

Ë Á

ˆ

¯ ˜

1/ 2 RL^

Ê

Ë Á

ˆ

¯ ˜

1/ 4

ad

0 0.2 0.4 0.6 0.8

0.80.51.0

IP (MA)

(mm)

Note: No fitted-parameters!ad computed from directlyfrom data.

Discharge Conditions:

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- Electron pressure gradients near the separatrix increase as Ip , yielding similar values of aMHD or b for the same value of ad

~ 1q

leiR

Ê

Ë Á

ˆ

¯ ˜

1/ 2 RL^

Ê

Ë Á

ˆ

¯ ˜

1/ 4

ad

Result: Pressure-Gradients Near Sep. Scale with aMHD (or b) => Edge plasma state clusters around curve in (aMHD,ad) space!

0.0 0.2 0.4 0.6 0.80

40

80

120

0.0 0.2 0.4 0.6 0.80.0

0.4

0.8

1.2

0.80.51.0

IP (MA)

5.03.5

6.5

qy95

~ RL^

aMHD

aMHD

ˆ b

Edge Plasma Phase-Space Suggested by 3-D Turbulence Simulation (Ref. [2])

~ nTeB2

q2RL^

Data taken at r = 2.0 mm

ˆ

[2] Rogers, Drake, and Zeiler, Phys. Rev. Lett. 81 (1998) 4396

Tran

spor

t

Incr

easin

g

- similar behavior suggested by some turbulence simulations....

2

ˆ

Inac

cess

ible

increasing plasma density

Inac

cess

ible

- A sharp boundary in (aMHD, ad)-space for ad < 0.3 is implied by the data, defining a region of inaccessible states

~ —nTeIp

2

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Result: Direction of Magnetic Field Influences Mapping of Edge Plasma State in EMFD-Space

Larger gradient scale-lengths in edge (~50%)

=> Indicates additional parameter(s) "control" transport-gradient relationships

=> Suggests link between "additional control parameter(s)" and L-H threshold

ad

aMHD

(mm)

LnTe

1

10

0 0.2 0.4 0.6 0.80

0.4

3

0.0 0.2 0.4 0.6 0.8MID_Alpha_Diamag ()

0.8

1.2 Reversed BT, Ip

Normal BT, Ip

Note: No Ohmic H-mode observed with reversed field

Differences in Reversed BT, Ip Discharges:

Smaller values of (x ~1/2) aMHD

Similarities in Reversed BT, Ip Discharges:Core density, power input, particle & power fluxes across SOL

=> same transport fluxes but gradients are weaker

Data taken at r = 2.0 mm

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Summary: Strong Evidence for Electromagnetic Fluid Drift Turbulence Controlling Edge Plasma State

Edge plasma state clusters around curve in (b, C)- or (aMHD, ad) -space => consistent with transport being a strong function of EMFD parameters, as seen in 3-D turbulence simulations

- Reversed-field data map to different characteristic curve in EMFD-space=> evidence for additional parameter(s) controlling flux-gradient relationships => possible link to L-H threshold conditions

but, need to:

- Plasma profiles near separatrix examined in ohmic discharges with range of densities, currents, fields

- resolve additional control parameter(s) (theory and experiment)

- resolve quantitative differences in flux-gradient relationships (theory vs. exp.)

- connect with transport phenomena in Far SOL ('blob' propagation zone)

ˆ

Conclusion:- endorsement for EMFD description of edge turbulence and transport