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

slide2

Particle Transport and Density Profiles

  • 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
slide3

Lower Density H-Modes Show Modest Profile Peaking

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

slide4

ITER

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

slide5

ICRF Only

NBI + ICRF

More Density Peaking With NBI Heating (Fueling)

slide6

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

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

slide10

Magnetic Axis

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

slide11

Residuals from JET/AUG scaling vs q95

  • No NBI data
  • Apparent linear scaling with q95

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

ICRF Data

slide12

New Scaling Including q95 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
slide13

q95 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

slide14

Initial Simulation Work Begun

  • 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.
slide15

Summary

  • 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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