Ohmic and RF Heated ITBs in Alcator C-Mod

William L. Rowan, Igor O. BespamyatnovFusion Research Center, The University of Texas at Austin

C.L. Fiore, A. Dominguez, A.E. Hubbard, A. Ince-Cushman,M.J. Greenwald, L. Lin, E.S. Marmar, M. Reinke, J.E. Rice,

K. ZhurovichMIT Plasma Science and Fusion Center

Session NP8: Poster Session V, Abstract: NP8.00072 9:30am - 12:30pm, Wednesday, November 14, 2007 Rosen Centre Hotel - Grand Ballroom

49th Annual Meeting of the Division of Plasma Physicsof the American Physical Society

November 12–16 2007; Orlando, FL

Ohmic and RF Heated ITBsin Alcator C-Mod

Internal transport barrier (ITB) plasmas can arise as a result of RFheating or spontaneously in Ohmic Alcator C-Mod plasmas. Theoperational prescription for the ITB includes formation of an EDA H-mode. In Ohmic ITBs, the toroidal magnetic field is ramped down andthen increased. Like ITBs generated with off-axis ICRF heating, OhmicITBs have peaked pressure profiles which can be suppressed by on-axisICRF heating. Recent work on onset conditions for the ICRF generatedITB (K. Zhurovich, et al., Nuclear Fusion 47, 1220 (2007))demonstrates that the broadening of the ion temperature profile due tooff-axis ICRF reduces the ion temperature gradient and suppresses theITG-instability-driven particle flux. The object of this study is to addadditional impurity transport information to the ITB description andto examine the characteristics of ITBs to find whether this model foronset is supported.

Summary

Two ITB modes in Alcator C-Mod– Ohmic, no auxiliary heating– Auxiliary heated: off-axis ICRF– Common features previously identified

» EDA H-Mode precursor required for both» Peaked pressure profiles evolve in both» Central ICRF heating suppresses peaking

New results are for ITBs with off-axis ICRF– New boron impurity profiles are reported and used to study

impurity transport at the onset of the ITB» Inward advection is enhanced over diffusion during the

ITB» Impurity transport approaches neoclassical

– Clear profile Ti increased but cannot confidently interpret aprofile broadening

– ITB gradient for nz is inboard of gradient for Ti

ITB in C-Mod

Two ITB scenarios– Same precursor-plasma target: EDA H-mode– Ohmic: requires a BT ramp– ICRF: Off axis deposition

Very similar characteristics– Form from an H-mode– Strong pressure profile peaking– Peaking can be clamped by deposition of central ICRF power

Results from either will apply to the other There are advantages of studying one in preference to the other

– ICRF deposition complicates the transport analysis– ICRF deposition has more operational similarities to ITBs on

other devices

Ohmic ITB

H-ModeTransition

ITB

TeTS

neTS

BT

ne peaking

ne_L(4)

Hα

NoteBT Ramp

Auxiliary Heated ITBs

Off-axis ICRF deposition ITB forms from EDA H-mode

plasma ITB is characterized by

peaking of the electron densityprofile. The change of theelectron temperature is lessdramatic

Similar ITB can be formed inan Ohmic plasma

H-Mode

ITB

nepeaking

Tepeaking

Off-axis ICRF and Improved Confinement

Off-axis ICRF deposition leads to ITBformation

For the results shown here, depositionwas on the high field side.0.55 < R (m) < 0.6(vertical dashed lines)

Deposition can be on the low field side. Improved impurity transport is

observed between the two vertical lineson the low field side

Improved impurity transport on fluxsurfaces in-board of surfaces on whichthe heating is resonant.

Ti gradient just out-board of thegradient

ICRF Resonance Improved boron confinement

Compare ne and Te for ITBs

ITB characterized by peaking in the pressure profile for ρ < 0.6 Density peaking is dominant contributor Peaking can be clamped by deposition of central RF

(Fiore, et al., POP)

H-modeITB

H-mode

ITB

Ohmic ITB Auxiliary Heated ITB

Ion TemperatureITB foot ICRF Resonance Range

ITBImpurityGradientRegion

Ion Temperature

Ti profile changes when the plasma transitions from H-mode to ITB The Ti gradient is just outside the ICRF deposition and clearly

outside the ITB foot as defined by the electron density profile. In-board of the gradient, Ti during ITB is larger than Ti in H-Mode The Ti increase is well developed at the location of the impurity

gradient enhancements

Impurity Transport in an ITB

The current work extends earlier C-Mod impurity transportresearch (J. W. Rice, et al., Nuclear Fusion 42 ,510 (2002)) with localmeasurements of lighter ions.

New measurements of fully stripped B+5 profiles with spatial andtemporal resolution are reported here.

In the earlier work on C-Mod, chord-averaged measurements ofpuffed argon ions were reproduced in numerical simulations ofimpurity transport to characterize the transport.

This measurement is local. There is just one boron ion present atthe measurement locations. The analysis is therefore moretransparent.

Differences between light and heavy impurity transport have beenobserved in other experiments. (ref JET)

Measured B+5 Density

The boron density profile steepens during the ITB The deposition location for the ICRF is just outside the foot of the

gradient (see the configuration) The inset shows the MIST analysis for a similar discharge. In

measurement and simulation, B+5 is the only boron charge state forR < 0.87 m. B+5 density for R < 0.87 is the entire boron density.

H-mode

ITB

B+4

B+5 MIST1070831028

ICRF Resonance

ITBImpurityGradientRegion ITB Foot

Mag

netic

axi

s

H-modeITB

1070831028

Measured Electron Density

The boron density profile steepens during the ITB The heating location for the ICRF is predominately just outside the

foot of the ne gradient (see the configuration)

ICRF Resonance

Impurity transport"v/D" analysis

The transport equation is

For quasi-linear or neoclassicaldescriptions of impuritytransport transport

Dynamic equilibrium For fully-stripped boron (B+5)

R < 0.85 m That region is "source free" so

that

Inward convection is increasedover diffusion in ITB

zj

zj

zj

St

n=!•"+

#

#

zj

zjz

j nr

nD !+

"

"#=$

0=!

!

t

nzj

0=zjS

D

v

r

n

n

zj

zj

=!

!1

H-mode

ITB

v/D

v/D

v/D

Is B+5 neoclassical?

Is B+5 neoclassical?-or-Can it be shown to obey neoclassical descriptions?

The diffusion coefficient is The convection coefficient is purely neoclassical Limit collisions of the impurity to collisions with the main plasma

ion, D+ in this case TD=Tz

!

D = Dneo

+Dan

!

v = vneo

!

vneo

= Dneo

ZI

ZD

1

nD

dnD

dr"H

1

TD

dTD

dr

#

$ %

&

' ( , 0.2 ) H ) 0.5

!

1

nZ

dnZ

dr=1

nD

dnD

dr

ZI

ZD

Dneo

Dneo

+ Dan

1"H#D( )

!

"D

=

1

TD

dTD

dr

1

nD

dnD

dr

#1

Is the transport neoclassical?

B+5 is neoclassical over a narrow range in-board of the ITB at thefoot

For this, use Dan << Dneo

!

1

nZ

dnZ

dr

1

nD

dnD

dr

=ZI

ZD

1"H( ) # 2.5 " 4.5

ITB1070831028

1070831027

1070831026

( )HZI

!1

Comparison withPrevious C-Mod Results

J.E. Rice, et al., Nuclear Fusion, 42, 510 (2002)

D

v

S.J. Wukitch, et al., Physics of Plasmas, 9, 2149 (2002)

present work

Comparison with JET Results

R.Dux, et al., Nucl. Fus., 44, 260 (2004)

D

v

present work

Summary

Confinement of light impurities is improved during ITB– Inward convection is enhanced relative to diffusion, or

equivalently, diffusion is reduced relative to inward convection.– Transport for boron is neoclassical over a limited range of the

plasma radius.– Further confirmation of ITB particle transport similarities

between C-Mod and other devices. Ti profiles change during the transition from H-mode to ITB.

– Ti increases on flux surfaces connected to the ICRF resonanceregion.

– Ti increases out-board of the ITB foot (defined using ne(R)) andof the region where the light impurity gradient steepens

Additional experiments planned for the next campaign.