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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. 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

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  1. 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

  2. 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

  3. 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

  4. 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

  5. ICRF Only NBI + ICRF More Density Peaking With NBI Heating (Fueling)

  6. C-Mod Data Fit Reasonably By Scaling Derived for JET/AUG ne(0.2)/<ne> = 1.347 – 0.117log(nEFF) + 1.331GNBI – 4.03bT

  7. This Isn’t The Whole Story:q95 Dependence Observed on C-Mod C-Mod ICRF Data

  8. At Same CollisionalityMore Peaking Seen As q95 Is Increased C-Mod ICRF Data

  9. At Same CollisionalityMore Peaking Seen As q95 Is Increased C-Mod ICRF Data

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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.

  15. 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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