Internal Transport Barriers in Alcator C-ModCatherine Fiore

MIT Plasma Science and Fusion CenterAPS-DPP

November 14, 2007Orlando, Fl

With Contributions from:W. L. Rowan†, A. Dominguez, A. E. Hubbard , A. Ince-Cushman, M. J.

Greenwald, L. Lin, E.S. Marmar , M. Reinke, J. E. Rice, K. Zhurovich£, MIT-PSFC, †FRC-UTA, £Goldman Sachs

Supported by US DoE

C-ModAlcator

What’s New:• Gyrokinetic stability study at ITB onset• Improved ion temperature profile

measurements• Localization of fluctuation measurements• Ohmic H-mode ITBs

Introduction: ITBs in Alcator C-Mod

Internal Transport Barriers (ITBs) in C-Mod arise from steady H-mode plasmas lasting 2 or more energy confinement times when the central power input is low: they are seen in both Ohmic and Off-axis heated ICRF plasmas

Features of C-Mod ITBs

t=0.8 st=0.9 s H-mode

t=1.1 st=1.0 s

Transitionto ITB

t=1.3 st=1.2 s

ITBProfiles peak

Major Radius (m)0.65 0.70 0.75 0.80 0.85 0.90

2

4

6

8

10

12

n s

qrt(Z

) (1

0

/m

)e

eff

203

ITb foot position

Divertor

Molytiles

Alcator C-Mod

RFantenna

fci min

C-Mod plasmas are a unique platform for ITB study:– No particle or momentum input– Monotonic q profiles– Collisionally coupled ions and electrons with Ti ≈Te

Reduction in particle and thermal transport in the barrier region and core allows the Ware pinch to dominate the transport. This results in strongly peaked pressure and density profiles. Ion thermal transport is reduced to neoclassical

Control of particle and impurity accumulation is achieved through application of central ICRF heating: TEM stability plays a role.Ernst, IAEA 2004, 2006

Features of C-Mod ITBs

Increasing magnetic field moves ICRF resonance off-axis on low field side. ITBs form when Bt > 6.2 T.

R/LTe decreases in the regionnear the ITB foot at the time of

onset.

The region of stability to ITG modes widens with increasing magnetic field.

K. Zhurovich, et al, NP8.00073K. Zhurovich et al 2007 Nucl. Fusion 47 1220-1231

ITBR/L

Te

5.4 5.6 5.8 6.0 6.46.2Toroidal Field (T)

12

0

4

8 R=0.80 m(barrier foot)

ITBR/L

Te

5.4 5.6 5.8 6.0 6.46.2Toroidal Field (T)

0

4

8 R=0.78 m(near barrier foot,inside)

ITG growth rate profiles

0.000.050.100.150.200.25

0.02

0.15

0.28

0.42

0.55

0.68

0.82

rho

Mra

d/s 5.6 T

5.8 T6.0 T6.2 T6.3 T

Upgraded high resolution x-ray spectrometer measurement of Ti profile shows barrier in temperature during ITB

Excellent agreement is found between Ti from HIREX, central Ti from neutrons, and Te from Thomson Scattering.

•A temperature barrier was found previously with sawtooth heat pulse measurements and inferred from pressure profile increase•Ion temperature profile from HIREX confirms Ti transport barrier.See: Ince-Cushman, P03.00014, this session

Hill, NP8.00080Bitter, NP8.00084

0.70 0.75 0.80 0.85Major radius (m)

0.0

0.5

1.0

1.5

2.0

Ti,T

e (k

ev)

Ti, HIREX

Te, Thomson ScatteringTi, Neutron Inversion

ITB footregion

t=0.71 s, pre ITB

t=0.85, transition to ITB

t=1.09, ITB

Off-Axis ICRF ITB

0.4 0.6 0.8 1.0 1.2 1.4 1.6t (s)

0.0

0.5

1.0

1.5

2.0

T ,T

(k

ev)

Ti, HIREX

Te, Thomson ScatteringTi, Neutron Inversion

Off-axis ICRF ITB

ei

Normalized temperature gradients (R/LTi) are obtained from high resolution x-ray spectrometer ion

temperature profiles

R/LTi increases from the center, with a drop at larger radius after ITB is established.

R/LTi in outer channels increases with time as ITB grows in the barrier region.

0.0 0.2 0.4 0.6 0.8r/a

0

5

10

15

R/L T

i

t=0.71 H-mode

t=0.85 s, transition to ITB

t=1.09 s. ITB

0.6 0.8 1.0 1.2time (s)

0

5

10

15

R/L

Ti

Normalized T Gradient fromHigh Resolution X-ray Spectrometer

r/a=0.39

r/a=0.58

H-mode

ITB

i

Fluctuations arising during ITB density peaking have been identified as TEM mode

PCI was configured to preferentially view half of the plasma

The fluctuations in 20-60 kHz propagate in the electron diamagnetic direction

pci Fluctuations

20

40

60

80

100

Freq

(kH

z)

0.00.10.20.30.4

0.9 1.0 1.1 1.2 1.3 1.40.9 1.0 1.1 1.2 1.3 1.4

20

40

60

80

100

1.5

2.0

2.5

2.0

time (s)

density peaking

0.9 1.0 1.1 1.2 1.3 1.40.9 1.0 1.1 1.2 1.3 1.4

ITB

B

PCI:TEM(GS2):QC

TEM-like

-1

shot:1070831021shot:1070831021shot:1070831021t:[1.04,1.10]st:[1.04,1.10]sf:[20,60]kHzf:[20,60]kHz

directione

Aut

o po

wer

[a.u

.]

Wavenumber [cm ]

See Lin, NP8.00076

Density peaking in an ITB begins shortly after an EDA H-mode develops for both Ohmic and Off-Axis ICRF ITBs

Electron Density, Shot 1070628019

0.8 0.9 1.0 1.1 1.2 1.30.70

0.75

0.80

0.85

0.90

Rm

id, m

0 2•1020 4•1020 6•1020

t (s)

Electron Density, Shot 1070831024

0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.75

0.80

0.85

0.90

Rm

id, m

0 1•1020

2•1020

3•1020

t (s)

0.65 0.70 0.75 0.80 0.85 0.90Major Radius (m)

1

2

3

4

0

Den

sity

(10

/m

)

203

Off-Axis ICRF Heated ITB

t=1.29 s ITB

t=0.79 s L-mode

t=0.89 s H-mode

0.65 0.70 0.75 0.80 0.85 0.90Major Radius (m)

Ohmic H-Mode ITB

1

2

3

4

5

0

Den

sity

(10

/m

)

203

t=1.14 st=1.04 s

} ITB

t=0.79 s L-mode

t=0.89 s H-mode

Density contour plot shows that the peaking time and intensity are similar

Thermal transport characteristics are similar for off-axis RF induced and Ohmic H-mode ITBs

m

/sec

2

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.6 0.8 1.0 1.2 1.4

χeff

t (s)

L-H transition

ITB

Off-axis ICRF ITB

r/a0.6

0.2

m

/sec

2

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.6 0.8 1.0 1.2 1.4

χeff

t (s)

Ohmic

L-H transition

ITB

Back transition

r/a

0.60.2

R/LT appears to have different trend in Ohmic H-mode ITB than in off-axis ICRF ITB

0.8 1.0 1.2 1.4

10

5

0time (s)

R/L

Ti

Off-axis ICRF ITB, Transp

0.6

15

0.6 0.8 1.0 1.2 1.4

108

64

2

0

time (s)

R/L

TiOhmic H-mode ITB, Transp

0.8 1.0 1.2 1.4

5

0

time (s)

R/L

Te

Off-axis ICRF ITB, Transp15

0.6

10

ITB

ITB

ITB

H-mode

H-mode

H-mode

0.20.4

0.6

r/a

0.8 1.0 1.2 1.4time (s)

R/L

Te

Ohmic H-mode ITB, Transp

0.6

10

5

0

15ITB

H-mode

0.20.4

0.6

r/a

0.20.4

0.6

r/a

0.2

0.40.6r/a

ConclusionsNon-linear gyrokinetic modeling shows that broadening of the temperature profile with off-axis ICRF injection increases the size of the core-stable region of the plasma, reducing the outgoing particle flux, allowing Ware pinch to dominate particle transport.

Improved temperature profiles establish that a thermal barrier exists at or near the particle barrier seen in the density profile. These profiles will improve data for stability analysis.

Density fluctuations that arise and strengthen during ITB growthpropagate in the electron direction and are consistent with TEM characteristics.

ITBs arising in Ohmic H-mode plasmas appear identical to those forming in off-axis ICRF heated cases, except for trends in R/LT. Stability analysis is needed.