Particle transport and density peaking at low collisionality on alcator c mod
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Particle Transport And Density Peaking At Low Collisionality On Alcator C-Mod. 49 th Annual Meeting of APS - DPP Orlando, 11/14/2007 M. Greenwald, J.W. Hughes, D. Mikkelsen, J. Terry, Alcator Group C. Angioni, H. Weisen. Particle Transport and Density Profiles.

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Particle transport and density peaking at low collisionality on alcator c mod
Particle Transport And Density Peaking At Low Collisionality On Alcator C-Mod

49th Annual Meeting of APS - DPP

Orlando, 11/14/2007

M. Greenwald, J.W. Hughes, D. Mikkelsen, J. Terry, Alcator Group

C. Angioni, H. Weisen


Particle Transport and Density Profiles On Alcator C-Mod

  • We want to be able to predict density profile

    • Better fusion performance with moderate density peaking

    • Effects on stability, divertor operation etc.

  • Results from ASDEX (Angioni et al., PRL 2003), JET (H. Weisen, et al., NF 2005) show increase in density peaking at low n* for H-mode plasmas.

    • Central fueling (NBI) can play an important role as well.

  • Scales to ITER (with weak fueling): ne(0)/<ne> ~ 1.4-1.5

  • In this talk, we’ll also look at an additional effect: the role of safety factor (or magnetic shear) in the particle transport


Lower Density H-Modes Show Modest Profile Peaking On Alcator C-Mod

BT = 5.2 T

IP = 1.0 MA

PIN = 3.9 MW

H98 = 1.05

BT = 5.4 T

IP = 0.9 MA

PIN = 2.3 MW

H98 = 1.3


ITER On Alcator C-Mod

ITER

ICRF Only

ICRF Only

C-Mod Data Helps Break Covariance Between nEFF and n/nGMakes Extrapolation To ITER More Certain.

nEFF= nei/wD≡ 0.1RZEFF<ne>/<Te>2


ICRF Only On Alcator C-Mod

NBI + ICRF

More Density Peaking With NBI Heating (Fueling)


C-Mod Data Fit Reasonably By Scaling Derived for JET/AUG On Alcator C-Mod

ne(0.2)/<ne> = 1.347 – 0.117log(nEFF) + 1.331GNBI – 4.03bT


This Isn’t The Whole Story: On Alcator C-Modq95 Dependence Observed on C-Mod

C-Mod ICRF Data


At Same Collisionality On Alcator C-ModMore Peaking Seen As q95 Is Increased

C-Mod ICRF Data


At Same Collisionality On Alcator C-ModMore Peaking Seen As q95 Is Increased

C-Mod ICRF Data


Magnetic Axis On Alcator C-Mod

Separatrix

Main Difference In Density Profiles Is Extent Of Peaked Region

R/Ln ~ 3.1

R/Ln ~ 2.2

Time to establish peaking << a/VWare


Residuals from JET/AUG scaling vs q On Alcator C-Mod95

  • No NBI data

  • Apparent linear scaling with q95

ne(0.2)/<ne> = 1.347 – 0.117log(nEFF) + 1.331GNBI – 4.03bT

ICRF Data


New Scaling Including q On Alcator C-Mod95 Dependence

  • Weak linear q dependence added to previous scaling

  • Best fit to C-Mod data

  • (JET and AUG not included)

  • ne(0.2)/<ne> = 1.1 – 0.117log(nEFF) + 1.331GNBI – 4.03bT+0.45q95


q On Alcator C-Mod95 Dependence Not As Evident In JET & AUG Data

  • When important dependences are factored out, q95 (or li) have small statistical significance for JET/AUG scaling. (Angioni, Weisen, et al, NF 2007).

  • We haven’t closed the loop with this new C-Mod data.

ICRF + NBI Data


Initial Simulation Work Begun On Alcator C-Mod

  • GYRO simulations have been performed using profile data from C-Mod shots (see Mikkelsen poster NP8.00071)

  • Density profile is adjusted to achieve zero particle flux

    • With R/LTi > R/LTe (as measured), null flux is seen at the observed density gradient

    • (Raising gradient by 20% produces outward flux)

  • Physics seems to be reduction of ITG instability-drive, enabling TEM-driven pinch with kqri > 0.5

  • In this analysis, collisionality dependence is through relaxation of LTi from reduced ion-electron coupling – raising nEFF by factor of 2 in simulations did not remove pinch.

  • Differences/similarities with AUG and JET work need to be explored.


Summary On Alcator C-Mod

  • Collisionality is the leading dependency for density profile peaking in H-modes.

    • C-Mod data helps break covariance between nEFF and n/nG(Greenwald et al., Nucl. Fus. L26, 2007)

  • In C-Mod, secondary effect found: weak q scaling

    • Commonly seen in L-Mode (see Baker DIII-D, Weisen TCV)

    • This dependence is not as evident in JET and AUG data

  • Peaking/particle transport is beginning to be understood

    • Initial simulations suggest that interplay of ITG and TEM turbulence is the key

    • More simulations/analysis required

    • Experiments to look for changes in fluctuations are planned


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