Internal transport barriers occur in C-Mod plasmas with off-axis ICRF heating and also in Ohmic H-mode plasmas These ITBs are marked by highly peaked density and pressure profiles as they rely on a reduction of particle and thermal flux in the barrier region which then allows the neoclassical pinch to peak the central density without reducing the central temperature. Enhancement of several core diagnostics has resulted in increased understanding of C-Mod ITBs. Ion temperature profile measurements have been obtained using an innovative design for x-ray crystal spectrometry and

clearly show a barrier forming in the ion temperature profile. The phase contrast imaging (PCI) provides limited localization of the ITB related fluctuations that increase in strength as the central density increases. Simulation of triggering conditions,

integrated simulations with fluctuation measurements, parametric studies, and transport implications of fully ionized boron impurity profiles in the plasma are under study. A summary of these results will be presented.

Work supported by US-DoE DE-FC02-99ER54512

A bs trac t

Introduction to C-Mod ITBs Internal Transport Barriers (ITBs) in Alcator C-Mod have strongly peakedpressure and density profiles

�

Transport Studies in Alcator C-Mod ITB PlasmasC. L. Fiore, P. T. Bonoli, D. Ernst, A. Ince-Cushman, L. Lin, E. S. Marmar, M. Porkolab, J. E. Rice, S. J. Wukitch, MIT-PSFC, I Bespamyatnov, P. Phillips, W. Rowan, FRC-UTA

Summary and Future Work:

Internal Transport Barriers (ITBs) in C-Mod arise in 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 ICRF heated plasmas.

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

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 density and pressure profiles. The ion thermal transport is reduced to the neoclassical level.

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

~~i e

The ITB rises gradually after the H-modeis established in off-axis heatedICRF. Both n sqrt (Z ), where 1 < Z <2, and electron pressure from Thomson scattering show peaking

e effeff

1040310023

t=1.26 s, ITBt=0.89 s, ITBt=0.66 s, H-modet=0.49 s, L-mode

0.0 0.2 0.4 0.6 0.8 1.0

Ele

ctro

n P

ress

ure

(1

0 P

asca

ls)

0.5

1.0

1.5

2.0

0.0

5

r/a

ITB foot

0.8 < t< 0.9 s H-mode

1.2 < t<1.3 s ITB Profiles peak

0

4

8

12

n s

qrt(Z

) (

10

/m

)20

3

840

MW

/m3ICRF Power

eff

ITB foot

1.0 < t<1.1 s Transition to ITB

e

Off-axis RF power distribution is peakedat or outside of the ITB foot position.

Experimental ParametersToroidal field: 2.8 <Bt <6.4 T (Bt max=8.1 T)

Plasma current 0.4 < Ip <1.3 MA (Ip max=2 MA)

Major radius~0.67 m

Minor radius ~0.22 m

q profiles are monotonic

ICRF power:

3 MW @ 80 MHz and

2 MW@70 MHz or

2 MW @50 MHz

Diver tor

M olytiles

A lcator C -M od

R Fantenna

fci min

Alcator C-Mod experiments are conducted without internalparticle and momentum sources

Tightly coupled ions and electrons with T Ti e~~

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 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

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

Ti

from TRANSP calculation

R/L

Ti

Normalized temperature gradients (R/L ) show sametrend with time in Ohmic H-mode ITBs as that seen

R/LTi from measurement decreases with time after plasma enters H-mode, rises with ITB onset and as ITB grows in the barrier region.

0.6 0.8 1.0 1.2 1.4 1.6time (s)

0

2

4

6

8

1070628014

r/a=0.38r/a=0.29r/a=0.20

Normalized Ti gradient from x-ray spectrometer

H-Modeonset

ITB

H-Mode and ITB end

1070831028

0.6 0.8 1.0 1.2time (s)

0

5

10

15

R/L

Ti

r/a=0.16r/a=0.39r/a=0.49r/a=0.58

HIREX Data

r/a= 0.2r/a= 0.4r/a= 0.5r/a= 0.6

Transp

t=0.7st=0.8st=0.9st=1.0st=1.1s

0.0 0.2 0.4 0.6 0.8r/a

0

5

10

15

R/L

Ti

t=0.71 st=0.75 st=0.85 st=0.99 st=1.09 s

Transp

HIREX Data

R/LTi measured by HIREX can be compared with that inferred from TRANSP

R/LTi from TRANSP

increases from the center for all times; drop outside of core is not seen

R/LTi from TRANSP is comparable for core channels; shows discrepency at larger radii.

PCI sees fluctuations arising during ITB density peaking also in Ohmic H-mode ITBs

PCI spectrogram 1080313021

time (s) Hmode; ITB starts 1.07 s,continues untilend of shot

Freq

uenc

y, K

hz

PCI spectrogram 1080313032

time (s) H-Mode,No ITB forms

Freq

uenc

y, K

hz

0.6 0.8 1.0 1.2 1.4

2 .02 .42 .83 .2

1.161.201.24

2030405060

2468

time (s)

J /Jbs tot (%) r/a=0.4

Density Peaking

Jbs A/cm2

q

r/a=0.4

r/a=0.4

1080514027Off-axis heated ITBwith central ICRF power added

Off-axis RF, 2 MWCentral RF, 1 MW

H-modeH-mode +ITB

0.3 0.4 0.5 0.6 0.7

2 .22 .42 .62 .8

1.401.451.50

30405060

48

12 J /Jbs tot (%) r/a=0.3

Density Peaking

Jbs A/cm r/a=0.32

q r/a=0.3

1080516026Off-axis heated ITB

Off-axis RF, 2 MW

time (s)

H-modeH-mode +ITB

0.6 0.8 1.0 1.2 1.4 1.6

2 .02 .53 .0

1.31.41.5

20304050

468

10

time (s)

J /Jbs tot (%) r/a=0.4

Density Peaking

Jbs A/cm2

q

r/a=0.4

r/a=0.4

1080430034Ohmic ITB

H-mode

H-mode +ITB

i,eη decrease before observable density peaking in ITB; decrease to 1 during ITB

R/LTi profile shows decrease near barrier as ITB develops (TRANSP Calculation)

0.0 0.2 0.4 0.6 0.8r/a

0

5

10

15

20

R/L

Ti

t=0.30st=0.35st=0.45s, H-mode startt=0.50st=0.55st=0.60st=0.65s

L-mode

ITB

}

}

0.0 0.2 0.4 0.6 0.8r/a

0

5

10

15

20

R/L

Ti

t=0.60st=0.80st=0.90st=1.02st=1.10s

L-modeH-mode

ITB}Transition to ITB

0.0 0.2 0.4 0.6 0.8r/a

0

5

10

15

20

R/L

Ti

t=0.90st=1.00st=1.10st=1.15st=1.20st=1.26s

1080430034Ohmic ITB

L-modeH-mode

ITB}

Density peaking indicates ITB development

0.0 0.2 0.4 0.6 0.8 1.0r/a

0

1

2

3

dens

ity x

10

/m

20

3

t=0.90st=1.00s

L-modeH-mode

t=1.10st=1.20st=1.26s

ITB}

0.0 0.2 0.4 0.6 0.8 1.0r/a

0

1

2

3

4

5

dens

ity x

10

/m

20

3

t=0.70st=0.80s

L-modeH-mode

t=0.90st=1.00st=1.12s

ITB}Off-axis ICRF ITB 1080514027

0.0 0.2 0.4 0.6 0.8 1.0r/a

0

0.5

1.0

1.5

2.0

2.5

dens

ity x

10

/m

20

3

t=0.30st=0.45s

L-modeH-mode

t=0.50st=0.55st=0.64s

ITB}Off-axis ICRF ITB 1080516026

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.

PCI Measurements (Lin, JO3.00010; Porkolab, PP6.00081)

10+1

10-3

[a.u.]

On-axis ICRF

off-axis ICRF

H-Mode

starts

ITB

ITBforms

Central RF

Central RF added

Shot: 1080516005Channel: 17

10-7

3.0

1.5

0.00.6 0.8 1.0 1.2 1.4 1.6

Time [sec]

ICR

F P

ower

[MW

] Fr

eque

ncy

[kH

z]

10

100

200

300

400

500Fluctuations above 300 kHz prior to ITB formation are corelocalized and are consistent with ITG turbulence found in Gyro simulations; Slowing of plasma rotation as the ITB forms reduces the EXB Doppler shift resulting in overlap of the core and edge PCI measurements, so that the location of the broadband fluctiontions cannot be resolved.

ITBs develop readily in both off-axis ICRF heated H-mode plasmasand in Ohmic H-mode plasmas; Profile and transport characteristicsare quite similar.

Flattening of the temperature profiles as the ICRF resonance moves offaxis has been observed. GS2 simulation indicates that this causes an increase in the size of the ITG stable region of the plasma.

Expanded capability in fluctuation diagnostics is now available for direct studyof turbulence associated with ITBs. Gyro simulations show that highfrequency turbulence measured before the onset of the ITB are consistentwith ITG instability. At this time the turbulence measurement associated with the ITB density peaking cannot be well localized and definitively identified.

Fully stripped boron ion, B is the only boron ion present for R < 0.85 m. Impurity +5transport analysis using a fully stripped ion is much simpler that for other species.Numerical simulation (MIST) demonstrates v/D is inward. During the ITB, the boron profile peaks but is still far from neoclassical.

Calculated q profile appears to narrow before ITB onset in typical ITBs of all types. Profile and transport characteristics are also consistent between different ITBs. Furtherstudy of these changes along with stability analysis will be done.

Spontaneous Ohmic ITB

H-Mode

Transp 43531080430034

0.6 0.8 1.0 1.2 1.4time (s) at r/a=0.4

1

2

3

4

5

6 = d log (T )/d log (n )j jj

0.40H-mode

ITB Transp 43241080516026

0.2 0.3 0.4 0.5 0.6 0.7 0.8time (s) at r/a=0.4

0

1

2

3

4

5

6 = d log (T )/d log (n )j jj

ie

H-mode

ITB

η η

ηη

ie

ηη

Transp 42421080514027

0.6 0.8 1.0 1.2 1.4time (s) at r/a=0.4

1

2

3

4

5

6

0.40

= d log (T )/d log (n )j jj

H-modeITB

η

ie

ηη

0.65 0.70 0.75 0.80 0.85 0.90Major Radius (m)

0

2

4

6

8

10

Ele

ctro

n D

ensi

ty (1

0 20

/

m3

)

H-mode

∆t = 0.29 s∆t = 0.13 s

∆t = 0.43 s

Strong densitypeaking also occursin Ohmic EDAH-mode plasmas

Upgraded high resolution x-ray (HIREX) 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.

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

• +5

•

••

Boron Profiles

B5+ density3.02.5

2.01.5

1.0

0.50.0B

d

ensi

ty x

10

/m

5+

1

8

3

0.70 0.75 0.80 0.85 0.90major radius (m)

3.0

2.5

2.0

1.5

1.0

0.5

Impu

rity

den

sity

x 1

0

/m

18

3

0.0 0.2 0.4 0.6 0.8 1.0ρ

0.0

10

0

-10

-20

20

V/D

(m )-1

Boron Profiles(Bespamyatov, Rowan PP6.00090)

+5

Typical ITB, induced withoff-axis ICRF heating, 4.5 T,80 Mhz. Density peakingbegins shortly after H-mode onset.

Spontaneous Ohmic H-modeITB, no ICRF. Again, densitypeaking begins soon after H-mode starts.

Off-axis ICRF induced earlyin a low current plasma, 70 MhzICRF in 5.4 T plasma. Noteatypical density peaking in L-mode and at the L-H transition

q (TRANSP calculation) increases outside of thesawtooth radius in the barrier region prior to theobservable density peaking in ITB plasmas

Typical ITB, induced withoff-axis ICRF heating, 4.5 T,80 Mhz. Central ICRF, 70 Mhzwas added after the ITB formedbut failed to halt the density riseand ultimately triggered a backtransition to L-mode

Spontaneous Ohmic H-modeITB, no ICRF. Similar trendsin q and bootstrap current areseen.

Off-axis ICRF induced earlyin a low current plasma, 70 MhzICRF in 5.4 T plasma.

q profile (TRANSP calculation) narrows as ITB develops

0.0 0.1 0.2 0.3 0.4r/a

0.8

1.0

1.2

1.4

1.6

q

1080430034Ohmic ITB

t=1.10st=1.20st=1.26s

ITB}t=0.90st=1.00s

L-modeH-mode

0.0 0.1 0.2 0.3 0.4r/a

0.81.0

1.2

1.4

1.6

1.82.0

q

t=0.50st=0.55st=0.64s

ITB}t=0.30st=0.45s

L-modeH-mode

Off-axis ICRF ITB 1080516026

0.0 0.1 0.2 0.3 0.4r/a

0.9

1.0

1.1

1.2

1.3

q

t=0.90st=1.00st=1.12s

ITB}t=0.70st=0.80s

L-modeH-mode

Off-axis ICRF ITB 1080514027

Typical ITB, induced withoff-axis ICRF heating, 4.5 T,80 Mhz.

Spontaneous Ohmic H-modeITB, no ICRF.

Off-axis ICRF induced earlyin a low current plasma, 70 MhzICRF in 5.4 T plasma.

Typical ITB, induced withoff-axis ICRF heating, 4.5 T,80 Mhz. η increasesat H-mode onset, falls towards1 as ITB forms and density peaks

Spontaneous Ohmic H-modeITB, no ICRF. η increasesat H-mode onset, falls towards1 as ITB forms and density peaks

Off-axis ICRF induced earlyin a low current plasma, 70 MhzICRF in 5.4 T plasma. η increaseis not so dramatic at H-mode onsetbecause density is relatively peakedthroughout. It still falls towards1 as ITB forms and density peaks further.

i,ei,e

Typical ITB, induced withoff-axis ICRF heating, 4.5 T,80 Mhz. .

Spontaneous Ohmic H-modeITB, no ICRF.

Off-axis ICRF induced earlyin a low current plasma, 70 MhzICRF in 5.4 T plasma.

Fully stripped boron ion B is the only boron ion present for R < 0.85 m.Impurity transport analysis using a fully stripped ion is much simpler thanthat for other species.The shaded area defines the region for analysis of the impurity profiles. R=0.81defines the foot of the the ITB as indicated in the density profiles. Analysis is notdone inside of R=0.74.Numerical simulation (MIST) demonstrates that v/D is inward.During the ITB the boron profile peaks but is still far from neoclassical. Thedeuterium main ion is in the banana regime and the impurity is in the plateau regime.

0.0 0.2 0.4 0.6 0.8

r/a at t=0.64

density x 10 /m

q

J /Jbs tot (%)

JbsA/cm 2

1080516026Off-axis ICRF ITB

20 3

2

4

48

12

70503010

2.01.61.2

0.0 0.2 0.4 0.6 0.8

density x 10 /m

1.52.53.5

103050

26

10

r/a at t=1.26 s

2.5

1.02.0 20 3

q

J /Jbs tot (%)1080430034Ohmic ITB

JbsA/cm2

0.0 0.2 0.4 0.6 0.8

density x 10 /m 20 3

q

J /Jbs tot (%)

JbsA/cm2

r/a at t=1.12 s

1080514027Off-axis ICRF ITB

1.01.52.02.5

20406026

2.5

3.5

10

Bootstrap fraction at ITB peak time reaches 12% in barrier region

Typical ITB, induced withoff-axis ICRF heating, 4.5 T,80 Mhz.

Spontaneous Ohmic H-modeITB, no ICRF.

Off-axis ICRF induced earlyin a low current plasma, 70 MhzICRF in 5.4 T plasma.

1080430034Ohmic ITB Off-axis ICRF ITB 1080516026Off-axis ICRF ITB 1080514027

Off-axis ICRF ITB 1080516026Off-axis ICRF ITB 1080514027

1080430034Ohmic ITB