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Density peaking in gyrokinetic simulations of H-mode plasmas in Alcator C-Mod

Density peaking in gyrokinetic simulations of H-mode plasmas in Alcator C-Mod. D. R. Mikkelsen, M. Bitter, K. Hill, PPPL M. Greenwald, A. Ince-Cushman, J. Rice, MIT J. Candy, R. Waltz, General Atomics ITPA Transport & Confinement Topical Group Workshop Milan, 20-22 October 2008.

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Density peaking in gyrokinetic simulations of H-mode plasmas in Alcator C-Mod

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  1. Density peaking in gyrokinetic simulations of H-mode plasmas in Alcator C-Mod • D. R. Mikkelsen, M. Bitter, K. Hill, PPPL • M. Greenwald, A. Ince-Cushman, J. Rice, MIT • J. Candy, R. Waltz, General Atomics • ITPA Transport & Confinement Topical Group Workshop • Milan, 20-22 October 2008 D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  2. Simulations of H-mode density peaking • Angioni, et al., Phys. Plasmas 12 (2005) 112310 • GS2 quasilinear estimates of particle flux (supercedes GLF23 work). • raising collisionality turns off the pinch, but the transition is too early. • Initial C-Mod simulation of peaked ne(r) H-mode w/TRANSP prediction of Ti(r) [~Te(r)]. • null particle flux requires half the experimental density gradient. • find that higher k modes produce the pinch, then • Angioni and Maslov make new quasilinear est. for higher k modes: see a pinch! • Measured Ti(r) is relaxed in C-Mod peaked density H-modes; low density, so low Qie. • simulate plasmas with low, moderate, and strong peaking. • ~null flux in all simulations with experimental temperature and density profiles. • artificially reduce collisionality with profiles of flattest ne(r) case: get a strong pinch! • null flux is achieved with a density gradient similar to that in peaked density plasmas • Strong peaking if is ITG not too far above threshold; hyper-ITG gives flat density. • Te(r) is far above TEM threshold and contributes strongly to the growth rate. • 'Relaxed' Ti(r) not essential; require Ti(r) near ITG threshold; ITER density peaking? D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  3. Lower Density H-Modes Have Peaked Profiles • We don’t know why these shapes allow operational H-mode at lower target densities • Lower L-H threshold density • ELMs seem to reduce rate of density rise in H-mode • Whatever the cause: • Standard H-mode • ne0/<ne> = 1.1-1.2 • Ln >> a, R/Ln ~ 0, R/LT ~ 6-7 • Low-Density H-mode • ne0/<ne> = 1.5 • R/Ln ~ 3-4, R/LT ~ 6-7 Greenwald APS-07 D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  4. Sawtooth Inversion Radius Difference In Profile Shape Is Notable • Peaking is seen over outer 60% of plasma radius • H-mode profiles evolve quickly; t << a/VWARE • Transport in center of plasma may be affected by sawteeth • These are of large amplitude; dTe/Te ~ 25% • Radii chosen to characterize peaking in ASDEX-U are appropriate for C-Mod profiles Greenwald APS-07 r/a = 0.4 r/a = 0.8 D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  5. LHCD (L-Mode with VS ~ 0; VWare very small) “New Shape” “Standard” Shape Increased Density Peaking At Low Collisionality Observed For ICRF Heated H-Modes In C-Mod Greenwald APS-07 D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  6. Density peaking with experimental profiles • Turbulent heat flux typically ≥ net heating power,but not >> Pheat. • ion/electron heat flux ratio is usually incorrect, though. • See null particle flux at the observed density gradients. • pinch appears first in modes with higher kI, typically for ki > 0.5 • Need to do convergence studies varying max. ki, and radial resolution. • Raise collisionality 3X with peaked density profiles: still see null flux. • higher collisionality does reduce the electron drive and both heat fluxes. • relaxed Ti(r) prevents ITG drive from becoming too strong? • Lower collisionality /3 with flatter density profiles: get strong pinch • need to raise the density gradient to levels similar to the peaked plasmas. • lower collisionality permits stronger electron drive. • Essential condition seems to be significant ratio of electron/ ion drive. • ITER likely near ITG threshold, will electron drive be strong enough? D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  7. Topics to be investigated Experimental profiles are stable in some regions, raising the collisionality broadens or creates stable regions. Simulated particle flux usually has both outward and inward flow regions. need profiles consistent with simulated fluxes; TGYRO can find them? Should examine the implications for ITER now. start with ITER simulations (Budny, et al., Nucl. Fusion 48 (2008) 075005) find conditions in which ITER would have a peaked density profile. are these conditions similar to current experimental regimes? need profiles consistent with simulated fluxes; TGYRO again? D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  8. Extra material D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  9. C-Mod peaked density regime • Operation with JFT-2M shape produced low-density H-mode plasmas. • Recipe:high triangularity, lower>0.75, low , and lower target density. • Pedestal density is especially low, andTped is high, 0.8-0.9 keV • Pedestal is wider, infer larger Dcauses lower ne, higher TeELMs. • Higher density ohmic EDA H-modes with same profile shape are not unusual. Surprise result: Density profile is peaked; in RF-heated H-mode plasmas this shape is never seen at higher densities. ITBs have central density peaking, but flat periphery; this regime has flat central region and a density gradient for r/a > 0.5 D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  10. New Ti profile shape may provide the key • New curved-crystal X-ray spectrometer installed for 2007 campaign. • preliminary data analysis, not fully checked out yet. • Shows Ti profile is much 'flatter' than Te in low density H-mode plasmas. • Te profile is steeper in low density plasmas. • Te-Ti is large, but Qie is < 1 MW. • central Ti is apparently not consistent with neutron rate, but • is the deuterium distribution Maxwellian? At 2nd harmonic resonance. • Measured LTi is much longer than LTe for r/a~0.6 • this reduces ITG drive (eliminates it?) • TEM modes would produce an inward pinch. • New GYRO simulations are based on measured Ti(r). • Simulation particle flux is ~0 for experimental density profiles. • Artificially reducing collisionality produces a pinch in high density cases! • Varied the density gradient, null flux with peaking like low density plasmas. D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  11. Standard H-mode A. Ince-Cushman, et al. Flat density profile ne(0)~3.3x1020 m-3 D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  12. Peaked density H-mode A. Ince-Cushman, et al. ne(0)~2.1x1020 m-3 D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

  13. More Density Peaking with Relaxed Ti(r) D. R. Mikkelsen, ITPA Transport & Confinement Topical Group Workshop, Milan 20-22 October 2008

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