Supported by Office of Science NSTX H. Yuh (Nova Photonics) and the NSTX Group, PPPL Presented by S. Kaye 4 th T&C ITPA Meeting Culham Lab, UK March 2010 ITB Formation and Suppression of Turbulent Transport in NSTX Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAEA Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST POSTECH ASIPP ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec College W&M Colorado Sch Mines Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, Both Ion and Electron ITBs Develop in NSTX L-mode Plasmas Electron and ion ITB locations –Relation to q, q’, ExB shear, …… Suppression of electron-scale turbulence associated with electron ITBs –ID of electron-scale turbulence as ETG –Development of supercritical T e gradients Comparison to linear gyrokinetic results indicate need for non-linear calculations

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, Internal Transport Barriers Develop When Magnetic Shear is Negative MSE constrained q, current profiles

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, Reversed Shear Plasmas Can Have Simultaneous Electron/Ion/Momentum Transport Barriers Peaked core gradients in electron and ion temperatures, and toroidal velocity Electron density gradient does not show much change with ITB q min s min R/L Te, max R/L Ti, max R/L v , max tanh or spline fitted Profiles from over 80 ITB discharges with over 500 timeslices

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, Locations of Ion and Electron ITBs Do NOT Coincide Ion ITB location closely correlated to that of peak ExB shear Occurs just outside Occurrence Fraction

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, Location of Electron ITB Coincides with that of Minimum Magnetic Shear Locations of electron or ion ITBs do NOT coincide with either q min or rational q Occurrence Fraction

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, Electron Transport Significantly Reduced During e - ITBs What is responsible for improvement? Can it be suppression of ETG turbulence?

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, Microwave Scattering Diagnostic Measures n e Fluctuations at Electron-Scale Wavenumbers k r  e ≤ 0.6 can be measured Multiple detection channels can measure fluctuations at multiple k-values simultaneously Localized scattering volume with radial resolution of  2.5 cm

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, High-k Fluctuations are Reduced Inside the e - ITB Reduced high-k fluctuation amplitude seen with strong reversed shear Weak negative shear shows higher amplitude high-k fluctuations even at lower  T e Modes can be bursty in nature (~100  s) – will see later (Ignore central peaks – due to reflections) Doppler shifted scattered power

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, High-k Fluctuations Consistent with Characteristics of ETG Modes Rest frequency of mode determined by subtraction of Doppler shift due to toroidal rotation Measured rest frequency of ~-630 kHz agrees with result from linear GS2 calculation for fastest growing eigenmode (-590 kHz) Rest frequency

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, Measured Growth Rates of High-k Modes Agrees with Linear GYRO Results for Fastest Growing Mode Even bursty high-k modes can suppress e - ITBs Large linear growth rates (up to 400 kHz) Expt. (shear = -0.5)

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, Reversed Magnetic Shear Suppresses ETG Mode Growth Rates Even at “Supercritical”  T e Intermittant, short duration bursts of ETG observed during RS phase –Average ETG mode amplitude low,  T e well above ETG critical (linear) Series of large amplitude, closely spaced ETG bursts collapses T e profile –Magnetic shear is zero/positive during this time T e profile at with small magnetic shear can be heated only to ETG critical gradient level –ETG mode amplitude grows to a moderate continuous level –ETGs capable of transporting a large amount of e - heat flux GYRO critical R/L Te GS2 critical R/L Te

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, ETG Modes Are Predicted to be Unstable at the High  T e Achieved Linear GYRO calculations indicate ETG is unstable at the large R/L Te accessible with strong reversed magnetic shear –Modes not suppressed by s<0 –Calcs also suggest possibility of another mode at higher R/L Te Non-linear calculations needed to determine effect of reversed magnetic shear on mode amplitudes

NSTX 4 th T&C ITPA Mtg - CulhamMarch 22-25, Summary e - ITBs  s min (negative); ion ITBs  max ExB shear Reduced  T e associated with high levels of high-k turbulence –High-k bursts limit maximum  T e –High-k turbulence reduced inside regions of negative magnetic shear (e - ITB) Electron transport significantly reduced in this region High-k turbulence characteristics consistent with those predicted for ETG modes –Suppression of these ETG modes by negative magnetic shear relaxes electron temperature profile stiffness ETG modes predicted to be highly linearly unstable at the high  T e s achieved –Need to understand non-linear impact of negative magnetic shear on ETG mode amplitudes/structure