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 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 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

C-Mod Data Helps Break Covariance Between nEFF and n/nG Makes Extrapolation To ITER More Certain. ICRF Only ITER ICRF Only ITER nEFF= nei/wD ≡ 0.1RZEFF<ne>/<Te>2

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

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

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

At Same Collisionality More Peaking Seen As q95 Is Increased C-Mod ICRF Data

At Same Collisionality More Peaking Seen As q95 Is Increased C-Mod ICRF Data

Main Difference In Density Profiles Is Extent Of Peaked Region Magnetic Axis Separatrix R/Ln ~ 3.1 R/Ln ~ 2.2 Time to establish peaking << a/VWare

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

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

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

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.

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