Supported by. The dependence of H-mode energy confinement and transport on collisionality in NSTX

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NSTX-U Supported by The dependence of H-mode energy confinement and transport on collisionality in NSTX Coll of Wm & Mary Columbia U CompX General Atomics FIU INL Johns Hopkins U LANL LLNL Lodestar MIT Lehigh U Nova Photonics ORNL PPPL Princeton U Purdue U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Illinois U Maryland U Rochester U Tennessee U Tulsa U Washington U Wisconsin X Science LLC Stanley M. Kaye S. Gerhardt, W. Guttenfelder, R. Maingi, R. Bell, A. Diallo, B. LeBlanc, M. Podesta and the NSTX Research Team 24 th IAEA, FEC! San Diego, CA! 8-13 October 212! Culham Sci Ctr York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAEA Inst for Nucl Res, Kiev Ioffe Inst TRINITI Chonbuk Natl U NFRI KAIST POSTECH Seoul Natl U ASIPP CIEMAT FOM Inst DIFFER ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep

H-mode confinement scales differently in two wall conditioning scenarios used in NSTX NSTX has used HeGDC+boronization as well as lithium evaporation for wall conditioning Strong B T, weak I p scaling with HeGDC+B! H 98y,2 scaling trends with Li evaporation! 6 4 2 HeGDC+B! I p.37.6.8 1. 1.2 6 4 2 B T 1.1.3.4.5.6 Li remains outside the! main plasma! (Podesta EX/P3-2)! 6 4 2 Li evap! I p.79.6.8 1. 1.2 I p (MA) 6 4 2 B T -.15.3.4.5.6 B T (T) Kaye (27), Gerhardt (211)! NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 2!

Can the difference in dimensional parameter scalings be reconciled? We find that: Discharges using lithium evaporation generally have lower collisionality Collisionality unifies the scalings: Strong increase of normalized confinement time with decreasing ν Favorable implications for ST-based Fusion Nuclear Science Facility (FNSF) Collisionality decreases primarily due to broadening of the electron temperature profile The reasons for the strong scaling with collisionality will be explored in this talk Global scaling Profile and transport changes (in both e - and i + ) with collisionality Results from linear gyrokinetic calculations NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 3!

Two methods were used to change collisionality in NSTX NBI-heated H-mode discharges Results will be reported from both: Vary I p, B T at constant I p /B T (no Li evap + fixed Li evap): Nu scan No Li evap: Type V (small) ELMs that have minimal impact on confinement Li evap: no ELMs q, β vary strongly: constrain dataset to limited q and β ranges for analysis Vary amount of between-shots Li evaporation (fixed I p & B T ): Li scan Little Li evap: Type I ELMS: choose analysis times to be inter-elm Large Li evap: no ELMs I p, B T, q, <β>, κ,. constant for all discharges For both scans, choose analysis times When P rad /P heat < 2% During steady periods NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 4!

A strong dependence of global confinement on between-shot Li deposition and collisionality is prominent in the Li Scan Strong increase in total thermal and electron confinement Factor of five decrease in collisionality Strong and favorable dependence of τ E with decreasing collisionality Implications for ST-FNSF (over one order of magnitude lower ν ε ) 1 # E (ms) # Ee (ms) (a).5.4 (b)!" i!" e x=.7.4.3!* e -.67±.14.3.2.2.1.1 1 2 4 6 8 1 Between-Shot Li Deposition (mg) -2 2 4 6 8 1 Between-Shot Li Deposition (mg)...5.1.15.2!* e (x=.5) Maingi et al. PRL (211), EX/11-2! x=[φ/φ a ] 1/2! NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 5!

Not all dimensionless variables are fixed across the range of ν ρ (=ρ s /a, a constant) changes across range of collisionality! Primarily due to T e profile broadening! 1..8 2.5 x 1 7 2. 1.5! e " (x=.7).1.6 1.5 1. Decreasing! e ".4 1..5.2 " s c s.5 @x=.7.2.4.6!* e (x=.7).2.4.6.8 1. [#/# a ] 1/2.75 Need to normalize confinement trends by ρ variation! NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 6!

Dependence on ν even stronger when ρ variations are taken into account Express confinement scaling in terms of dimensionless parameters Ωτ E = Bτ E = ρ α f(ν, β, T e /T i, κ, q,.) where α = -2 for Bohm and α = -3 for gyrobohm scaling NSTX HeGDC+B discharges found to be consistent with gyrobohm (Kaye, 26) For the Li scan, B, q, <β>, κ, a constant for all discharges Normalize τ E further by ρ α : test both Bohm and gyrobohm!.5 Bohm normalization!.5 gyrobohm normalization!.4.4.3.2! * e -1.5±.18.3.2!* e -1.91±.22.1.1...5.1.15.2!* e (x=.5)...5.1.15.2!* e (x=.5) NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 7!

Strong dependence of normalized confinement on ν also in Nu scan Constrain data to q a/2 = 2-2.5 and <β T > = 8.5-12.5%.4.3 no Li Li.4.3 Bohm normalization! no Li Li.5.4 gyrobohm normalization! no Li Li.2! * e -.79±.1.2! * e -1.6±.12.3.2! * e -1.21±.16.1 q x=.5 =2-2.5 <" T,th >=8.5-12.5%...5.1.15.2.25!* e (x=.5).1 q x=.5 =2-2.5 <" T,th >=8.5-12.5%...5.1.15.2.25!* e (x=.5).1 q x=.5 =2-2.5 <" T,th >=8.5-12.5%...5.1.15.2.25!* e (x=.5) ITER98y,2 ν scaling weak! NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 8!

n e and Z eff variations do not control the variation of ν Would expect a linear dependence between parameter pairs if they were controlling factors (ν ~ n e Z eff ).8 Li scan Nu scan.8 Li scan Nu scan.6.6.4.4.2.2 2 4 6 8 n e (1 19 m -3 ) @ x=.7 1 2 3 4 5 6 Z eff (x=.7) NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 9!

The variation in T e and T e profile broadness is the fundamental reason ν (and ρ ) varies ν ~ 1/T e 2.8.6 Li scan Nu scan.8.6.4.4.2.2.4.6.8 1. T e (kev) @ x=.7.2 Li scan Nu scan 1.4 2 2.6 T e ()/<T e > Broader! NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 1!

T e broadening reflects a strong reduction in electron transport with decreasing collisionality in the outer region of the plasma This can be seen in both χ e and χ e /χ GB, where χ GB ~ ρ s2 c s /a 1 (a) 1 (b) " e # (x=.7).1 1 1 1 Decreasing " # e Nu scan.1.4.6.8 1. [!/! a ] 1/2.1 Nu scan.1.4.6 Decreasing " # e.8 1. [!/! a ] 1/2.75 Curves color coded relative to value over full range of collisionality! NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 11!

There is a general increase of anomalous ion transport in outer regions with decreasing collisionality The dependences are more complicated Overall increase in χ i /χ i,neo with decreasing collisionality, but there is large scatter even at similar ν e Neoclassical (NCLASS) ion transport at lowest collisionality! (factor of ~2 uncertainty in χ i /χ i,neo )! Ion transport also correlated with rotation shear 1 Li scan Nu scan x=.6 1 Li scan Nu scan x=.6 1 1.1.1.2.3.4.5! e ".1-3 -2-1 Now look at microstability properties of plasmas at high- and low-k NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 12! R M s (= R2!" c s )

High-k ETG becomes more stable for lower collisionality discharges Comparison of experimental R/L Te to analytic ETG critical gradient (Jenko et al., 21) indicates reduction of ETG drive as collisionality decreases Consistent with reduction in electron transport 1 Nu scan 1 1 " e # (x=.7).1 Linear gyrokinetic indicated ETG completely stabilized for low collisionality discharges! Stability due to reduced T e gradient!!(guttenfelder TH/6-1)!.1.1.2.4.6.8 1. [!/! a ] 1/2 Decreasing " e #.75 Reduction of high-k turbulence (k r ρ θ ~ 5 3) at lower collisionality in pedestal region!!(canik 211, EX/P7-16)! NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 13!

Low-k modes show more complicated dependence Linear GYRO calcs indicate microtearing growth dominates low-k spectrum at high collisionality.5 Nu Scan! High $ e * x=.6.4.3 µtearing.2.1 # ExB.2.4.6.8 1 k! " s NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 14!

Low-k modes show more complicated dependence Linear GYRO calcs indicate microtearing growth dominates low-k spectrum at high collisionality At low collisionality, microtearing becomes weaker Consistent with reduction in electron transport going from high to low collisionality.5.4.3 Nu Scan! High $ e * Low $ e * x=.6 µtearing.2.1 # ExB.2.4.6.8 1 k! " s NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 15!

Low-k modes show more complicated dependence Linear GYRO calcs indicate microtearing growth dominates low-k spectrum at high collisionality At low collisionality, microtearing becomes weaker Consistent with reduction in electron transport going from high to low collisionality.5.4.3 Nu Scan! High $ e * Low $ e * x=.6 Low-k hybrid mode (TEM/KBM) predicted to exist at low collisionality Consistent with increase in ion transport Can provide some electron transport.2.1 µtearing TEM/KBM # ExB Mode growth rates near γ EXB at low collisionality Non-linear calculations underway to assess effect on predicted transport levels Li scan shows similar result.2.4.6.8 1 k! " s Guttenfelder TH/6-1! (next talk)! NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 16!

Summary and Conclusions Collisionality is the unifying parameter in understanding confinement trends in NSTX plasmas Normalized confinement shows a strong and favorable dependence with decreasing collisionality Trend is even stronger when Bohm or gyrobohm variation of ρ * is taken into account Improved confinement is governed primarily by reduction in electron transport in outer region Broader T e profiles with decreasing ν e * ETG, microtearing more stable going from high to low ν e * Ions, however, become more anomalous going from high to low collisionality Hybrid TEM/KBM mode unstable at low ν e * Need to assess respective roles of ν e * and rotation shear Will be able to explore these trends at even lower collisionality (5x) with more control of the rotation profile on NSTX-U NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 17!

Sign Up for Copies NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 18!

Two methods were used to change collisionality in NSTX H-mode discharges Results will be reported from both: Vary I p, B T at constant I p /B T (fixed Li + no Li): Nu scan Vary amount of between-shots Li evaporation (fixed I p & B T ): Li scan 8.45 T HeGDC+B wall conditioning! n e (1 19 m -3 ) 3 8.45 T Lithium wall conditioning! n e (1 19 m -3 ) 3 1 W total (kjoules) D! (arb. units) 1 W total (kjoules) D! (arb. units).5 P rad /P heat 1 I p (MA)..2.4.6.8 1. Time (s) Type V (small) ELMs)!.5 P rad /P heat >2%! 1 I p (MA)..2.4.6.8 1. 1.2 Time (s) Type I No ELMs (Maingi EX/11-2)! Choose toi when P rad /P heat <.2! NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 19!

4 2 Nu Scan! a/l ne a/l nd a/l nimp 15 1 5 Li Scan -2-5 -4-6.2.4.6.8! * e (x=.7) -1-15 a/l ne a/l nd a/l nimp -2.2.4.6! * e (x=.7) NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 2!

Nu Scan! a/l Te a/l Ti -2 Li Scan! -2-4 -4-6 -6.2.4.6.8! * e (x=.7) -8 a/l Te a/l Ti -1.2.4.6! * e (x=.7) NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 21!

.6.4 Nu Scan!.8.6 Li Scan! W e,ped /W e,tot W i,ped /W i,tot W t,ped /W t,tot.4.2 W e,ped /W e,tot W i,ped /W i,tot.2 W t,ped /W t,tot.2.4.6.8.2.4.6! * e (x=.7)! * e (x=.7) NSTX-U! FEC 212 Effects of collisionality, S. Kaye (1/11/212)! 22!

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