ENERGY TRANSPORT IN NEUTRAL BEAM HEATED DIII D DISCHARGES WITH NEGATIVE MAGNETIC SHEAR Presented by D.P. SCHISSEL for the DIII D Team* Presented to 38th APS/DPP Meeting NOVEMBER 11 15, 1996 Denver, Colorado
IN COLLABORATION WITH C.M. GREENFIELD, J.C. DeBOO, L.L. LAO, E.A. LAZARUS, 1 G.A. NAVRATIL, B.W. RICE,3 G.M. STAEBLER, B.W. STALLARD,3 E.J. STRAIT, H.E. ST. JOHN, M.E. AUSTIN, K.H. BURRELL, T.A. CASPER,3 D.R. BAKER, V.S. CHAN, E.J. DOYLE,5 J.R. FERRON, C.B. FOREST, P. GOHIL, R.J. GROEBNER, W.W. HEIDBRINK,6 R.-M. HONG, A.W. HOWALD, C.-L. HSIEH, A.W. HYATT, G.L. JACKSON, J. KIM, C.J. LASNIER,3 A.W. LEONARD, J. LOHR, R.J. LA HAYE, R. MAINGI,7 R.L. MILLER, M. MURAKAMI, 1 T.H. OSBORNE, C.C. PETTY, C.L. RETTIG, T.L. RHODES,5 S. SABBAGH, T.C. SCOVILLE, R.T. SNIDER, R.D. STAMBAUGH, R.E. STOCKDALE, P.L. TAYLOR, T.S. TAYLOR, D.M. THOMAS, M.R. WADE,1 R.E. WATLZ, R.D. WOOD,3 and D.G. WHYTE8 1Oak Ridge National Laboratory Columbia University 3Lawrence Livermore National Laboratory University of Maryland 5University of California, Los Angeles 6University of California, Irvine 7Oak Ridge Associated Universities 8INRS Energie et Materiaux 76-96 8Q.1/APS 96
KEY POINTS Plasmas with weak or negative central magnetic shear have exhibited reduced particle, heat, and momentum transport Local suppression of plasma turbulence has been correlated with transport reduction E B shear decorrelation of turbulence is the leading candidate for explaining the reduced transport 76-96 8Q.1/APS 96
NEGATIVE CENTRAL SHEAR PLASMAS SHOW A DRAMATIC IMPROVEMENT OVER DISCHARGES WITH A MONOTONIC q PROFILE Negative Central Shear (NCS) meaning a plasma with either weak or negative shear NCS with an L Mode edge NCS combined with L mode edge conditions Strong T i and Ω φ peaking inside NCS region NCS with an H Mode edge NCS combined with ELM free H mode edge conditions Broader profiles 15 1 5 H mode NCS H mode Edge....6.8 1. Τ i (kev) 3 1 Ω ϕ (1 5 r/s)....6.8 1. 1 8 6 NCS L mode Edge q....6.8 1.
Er (kv/m) In High-Performance DIII-D Plasmas, the Radial Electric Field Er Must be Included in Analysis of MSE Data E r is dominated by v B in DIII-D E r fields of kv/m become comparable to the v beam B pol motional field 15 1 5 E v B E tot E p -5....6.8 1. Bz (T). MSE Bz data. neglecting Er Mag. Probe -. MSE Bz data -.8 with measured Er 1. 1.6 1.8....6 R (m) 8 JR /R 6 15 1 5 q q=1-5....6.8 1. (A/cm)
PLASMA POSITION ALLOWS NCS L AND H MODE EDGE CONTROL Double -null divertor with δ~.8 Early NBI during I p ramp Low target ne, high Te Freeze J(r) in core resulting in reversed or flat q profile L mode and H mode edge control accomplished by null bias Internal barrier forms with higher power NBI H factors (τ relative to ITER89 P) of up to have been achieved Rapid increase in T i and Ω φ Modest increase in T e and n e resulting in larger T i / T e 3 1..5. q = q() - q min I p (MA) P b (MW) 87935 H D α 1 n e (1 19 m 3 ). 5..8 T i (kev) R (m) 1.78 1 1.91 1.98.6 5 1 15 5 3 Time (ms) 1
L MODE EDGE PLASMA HAS REDUCED ION DIFFUSIVITY INSIDE NCS AREA AFTER FORMATION OF TRANSPORT BARRIER Ion transport about times smaller inside NCS region (<.5) with more power factor of 1 reduction has been observed at constant power Electron transport does not change within calculated uncertainties 5% reduction has been observed in other discharges 1 χ i Early Formation (5 MW) Extended Barrier (1 MW) 1 χ e Early Formation (5 MW) ExtendedBarrier (1 MW) 8731 1 1 Improved χ i Region m /s 1 ion neo CH m /s 1.1...6.8 1.1...6.8 1
NCS L MODE EDGE DISCHARGE HAS LOWER CORE FLUCTUATIONS Reduced core fluctuations (FIR) correlated with reduced ion transport Scattered Power proportional to n ~ e Fluctuation reductions consistent with reduced turbulence e.g. Ion Temperature Gradient (ITG or η i ) modes Core Plasma Edge Plasma Core Scattered Power (AU) 1 3 1 L-mode NCS L mode Edge 1 5 5 1 15 Frequency (khz)
FLUCTUATIONS DECREASE IN TIME DURING NCS DISCHARGE NCS L mode Edge 8993 Core density fluctuations (FIR) reduce as T i increases with NCS established T i (kev) 15 1 5 R (m) 1.77 1.895 1.97.36.88.189 n e ~ n e (AU) 5 1 15 Time (ms)
ADDITION OF H MODE EDGE TO THE NCS L MODE REDUCES ION DIFFUSIVITY OVER ENTIRE PLASMA L Mode edge discharge has reduced χ i inside weak central shear area H Mode edge discharge has reduced χ i over entire cross section Approaches Chang Hinton neoclassical at all radii Neutral beam power balanced by ion-electron exchange and dw i /dt Electron diffusivity remains relatively unchanged m /s 1. 1..1 χ i L Edge (3 ms) H Edge (5 ms) ion neo CH.1....6.8 1. 87935 m /s 1. 1. χ e L mode Edge H mode Edge.1.1....6.8 1. MW 15 1 5 NCS H mode Edge Ion Power Balance Beam dw/d t cond + conv 5 i e exchange 1....6.8 1.
NCS H MODE EDGE DISCHARGE HAS LOW CORE/EDGE FLUCTUATIONS Reduced edge fluctuations (FIR) in NCS H mode edge case correlated with reduced ion transport over entire plasma cross-section Approximatately neo-classical ion transport in NCS H mode edge 5. Core Plasma Edge Plasma Core Scattered Power (AU). 3.. 1. NCS L mode Edge NCS H mode Edge. 1 5 5 1 15 Frequency (khz)
NCS H MODE EDGE DISCHARGE HAS LOW CORE/EDGE FLUCTUATIONS Reduced fluctuations (FIR) in NCS plasmas correlated with regions of reduced ion transport k θ = cm -1 Color Enhanced Contour Plot of FIR Scattered Power Spectra Evolution intensity map 87953 1 Frequency (khz) -1 L-mode Edge H-mode Edge 18 19 1 3 Time (ms) } } } core edge core
BASIC FEATURES OF SHEAR STABILIZATION MODEL Negative or weak magnetic shear allows stabilization of high n MHD modes (e.g., ideal ballooning modes) q > 1 everywhere stabilizes sawteeth Lack of these instabilities plus application of additional heating allows pressure and rotation gradients to build, thus increasing radial electric field =( ) + 1 r i i i i i E Z en P v θ B φ v φ B θ Local transport bifurcation can occur based on sheared E B flow decorrelation of turbulence [Hinton & Staebler, Phys. Fluids B5, 181 (1993)] 76-96 8Q.1/APS 96
E B FLOW SHEAR AND TURBULENCE Effect of E B flow shear can be quantified by comparing the change in flow shear to turbulence growth rates Change in E B flow shear determined by Doppler shift shear rate [Hahm & Burrell, Phys. Plasmas, 168 (1995)] ( ω E B = RB θ) B ψ E r RB θ In previous work turbulent transport is completely suppressed when ω E B > γ max based on 3-D non linear ITG simulations [Waltz et al., Phys. Plasmas 1, 9 (199)] γ max is the maximum growth rate without E B shear In this paper maximum linear growth rate γ max is calculated considering both the ITG and dissipative trapped electron modes Calculated from 3-D ballooning mode gyrokinetic stability code in the electrostratic limit [Kotchenreuther et al., Bull. Am. Phys. Soc. 37, 13 (199)] 76-96 8Q.1/APS 96
GYROKINETIC LINEAR STABILITY CALCULATIONS Written by Mike Kotschenreuther, Institute for Fusion Studies, University of Texas at Austin. Bull. Am. Phys. Soc. 37 (199) 13 Initial value code in ballooning coordinates (finds the most unstable mode, ion or electron branch, at the specified wavenumber) Full gyro averaged kinetic equations for electron, ions and one impurity species. (Impurity response not used in our calculations but Z eff and dilution retained) Lorentz collision operator used to correctly treat collisional boundary layers at the trapped boundary Full electromagnetic response implemented (only electrostatic perturbations used in our calculations). Correct ideal MHD ballooning mode threshold requires extension to real flux surface geometry Model shifted circle (s,α) magnetic equilibrium used with profile gradients taken along the low field midplane to approximate effect of elongation Wavenumber optimized to find maximum growth rate for comparison with E B shear rate 76-96 8Q.1/APS 96
ExB FLOW SHEAR IS A LEADING CANDIDATE TO EXPLAIN STABILIZATION OF MICROTURBULENCE L Mode edge NCS plasma In the early low power phase γ max is greater than ω ExB over most of the plasma In the high power phase ω ExB > γ max in the region of reduced transport (1 5 / sec) Low Power (5 MW) γ max 3 ω ExB 1....6.8 1. Improved χ i Region (1 5 / sec) 3 1 High Power (1 MW) ω ExB γ max 8731....6.8 1.
THE REGION OF STABILITY IN THE CORE RESULTS FROM SEVERAL FACTORS REDUCING THE GROWTH RATE Negative Magnetic Shear and Shafranov Shift are Stabilizing T i > T e and Thermal Ion Dilution by Fast Ions are Stabilizing 3 Without negative shear and Shafranov Shift (shear =.5) 3 Without T i > T e and n i < n e (T i = T e and n i = n e ) 8731 (1 5 / sec) γ max (1 5 / sec) 1 1 γ max ω ExB ω ExB...6.8 1...6.8 1 ω ExB and γ max calculated from experimental profiles during low power phase
THE REGION OF STABILITY IN THE CORE RESULTS FROM SEVERAL FACTORS REDUCING THE GROWTH RATE Peaked density destabilizing for pure trapped electron modes and stabilizing for pure ηi modes Mixed net effect when the two drives are combined Weak and or negative magnetic shear (s<) and Shafranov shift (α > ) Generic feature of ballooning modes: peak γ max at s =.5 and α = Ti > Te Larger ion gyro oribit relative to electrons which decouples ions from electrons (FLR) Thermal ion dilution by fast ions (ni < ne) ñ e = ñ Z with fast ion fluctuations neglected yields ñe = n i i i n e n e ñ i n i 76-96 8Q.1/APS 96
TRANSPORT SIMULATIONS WITH ExB SHEAR REPRODUCES TEMPERATURE DATA IN DIII D NCS L MODE DISCHARGE Predicted temperature profiles well simulated when ExB shear included Predictive transport model GLF3 (Waltz, 1996 IAEA) includes ExB shear in ions via χanom = Σ k (γ max ω ExB ) γ k γ d k (γ k + ω k ) 1 T i (kev) 6 8731 T e (kev) 7 With ExB No ExB...6.8 1 3 With ExB No ExB...6.8 1
GREATER NEGATIVE SHEAR IN L MODE EDGE REDUCES TRANSPORT χ i reduced with larger q; within uncertainty χ e remains the same γ max and ω ExB comparison does not explain the reduced ion transport No calculated instability (.1 < k θ s < 1) at =. Yet electron transport not observed to be electron neoclassical 6 q case 1 case....6.8 1. m /s 1 1. 1..1 χ i 873 case 1 case case 1 ion neo CH.1....6.8 1. 1 5 /s 3 1 ω ExB....6.8 1. 8731 case γ max
GREATER NEGATIVE SHEAR DOES NOT CHANGE TRANSPORT IN NCS H MODE EDGE PLASMA χ i and χ e do not change with larger q γ max smaller than ω ExB at all radii consistent with neoclassical ion transport over entire plasma cross section combining NCS L mode core transport with H mode edge transport q 6 case case 1...6.8 1 m /s 1 1.1 χ i 87953 case 1 case 1 case ion neo CH.1...6.8 1 (1 5 / sec) 87937 case 8 case 6 ω ExB γ max...6.8 1
SUMMARY NCS plasmas provide a robust and reliable enhanced confinement regime with both an L mode and H mode edge τ E up to times ITER89 P NCS with L mode edge Peaked toroidal rotation, ion temperature and plasma density profiles consistent with an internal transport barrier χ i reduced to ion-neoclassical inside the transport barrier Larger negative shear lowers ion transport NCS with H mode edge Broad plasma profiles consistent with improved transport over the entire plasma cross-section χ i reduced to ion-neoclassical over the entire plasma Larger negative shear does not alter transport Lower transport is accompanied by reduced plasma fluctuations Primary candidate for microturbulence stabilization is sheared E B flow NCS is necessary but not sufficient for enhanced confinement Necessary for ballooning nd stability Sheared E B flow must be large enough to overcome turbulence growth rates 76-96 8Q.1/APS 96