Microtearing instabilities and electron transport in NSTX By K.L. Wong, S. Kaye, D.R. Mikkelsen, J.A. Krommes, K. Hill, R. Bell, B. Le Blanc, 34 th EPS Conference on Plasma Physics Warsaw, Poland, July 2-6, 2007 NSTX

Outline ________________________________________ Introduction Properties of microtearing modes Calculation of unstable microtearing modes for NSTX experimental data Theoretical electron thermal conductivity due to microtearing instabilities in NSTX Comparison with TRANSP analysis Reversed magnetic shear plasma Summary NSTX

Introduction ____________________________________ _ Anomalous electron transport is an old subject, almost as old as magnetic fusion research itself While ETG turbulence is a possible candidate for the explanation in tokamaks, here for NSTX, we investigate the stochastic magnetic field effects from microtearing instabilities 1,2 1. M.H. Redi et al., EPS (2003) 2. D.J. Applegate et al.,Plasma Phys.(2004) NSTX

Why is it important for NSTX ? ___________________________________________ Microtearing modes are thought to be important only at the edge of conventional tokamaks like D-III 1 and C-MOD 2 They are stable in the interior of a tokamak where T e is high enough such that ei <  *e They can be the most unstable mode in the interior of NSTX 3, and should saturate at high amplitude due to the low magnetic field 4 1.N. Ohyabu et al., Phys. Rev. Lett. 58, 120 (1987) 2.J. Kesner & S. Migliuolo, Nucl. Fusion 39, 163 (1999) 3.M.H. Redi et al., EPS(2003) 4.J.F. Drake et al., Phys. Rev. Lett. 44, 994 (1980) NSTX

Properties of microtearing modes ____________________________________ _ High-m tearing modes (k || =0) Driven by T e gradient -  ’ is actually negative at high m (stabilizing) Perturbed electric field  E r has odd parity, elongated along B - different from the even parity for ITG modes Propagate in the electron drift direction (GS2:  <0) - opposite to the ITG mode Perturbed magnetic field  B r has even parity and creates magnetic islands at q=m/n In slab geometry, instability requires (a)  e =dlnT e /dlnn e >0.3 (b) Electron Coulomb collision rate e >  *e NSTX

Microtearing modes in toroidal geometry __________________________________________________ In toroidal geometry, trapped particle effects come into play: (1) L. Chen (1977) - collisional detrapping is destabilizing - Krook e (2) Catto (1981) - Lorentz e This driving term has to compete with stabilizing effects like: (1) Field line bending effect (  ’ ≈ -2k  < 0) (2) Collisional effect of passing electrons (3) Ion orbit effects - stabilizing if dT i /dr < dT e /dr - Cowley (1986) When all stabilizing terms are retained, the trapped electron term is too feeble to drive microtearing instabilities in the interior of a realistic conventional tokamak - J. W. Connor (1990) NSTX

The GS2 gyrokinetic stability code _______________________________________ Initial-value algorithm Flux tube geometry (ballooning limit) Calculate the growth rate and mode structure of the most unstable (fastest growing) eigenmode in a preset range of wavenumber Get input parameters directly from TRANSP output file Ref: M. Kotschenreuther et al., Comp. Phys. Comm. 88, 128 (1995) W. Dorland et al., Phys. Rev. Lett. 85, 5579 (2000) NSTX

Plasma equilibrium for NSTX # at 0.9 s ____________________________________ _ NSTX

Mirnov coil signal spectrum __________________________________________

Plasma Equilibrium ____________________________________________________

Results from GS2 code ____________________________________ _ Microtearing modes can be the most unstable mode in the region r/a = In the region where T e has steep gradient (r/a= ), a broad spectrum of microtearing modes are unstable k   s is in the range of , comparable to ITG modes NSTX

Cautionary Remark _______________________________________ The GS2 code only finds the most unstable mode Microtearing modes may still be unstable at other values of k  s, but they do not appear in the GS2 output because some other modes (usually the ITG) are more unstable ITG and microtearing modes have opposite k , which is reflected in the opposite signs of their real frequency NSTX

Benchmark to analytic results __________________________________________ Plasma parameters taken from TRANSP#116313A11, 0.9 s (NBI, H-mode)  e > 0.3 in the region 0.35 <  < 0.75 Choose  = 0.5 where: n e =6.5e13 cm -3, T e =650 ev, L Te =42 cm, L ne =78 cm, B=5 kG, T i =800 ev, e >  *e is satisfied when microtearing modes are unstable according to the GS2 code NSTX

Instability depends on ei __________________________________________________

Nonlinear saturation ____________________________________ _ While the GEM code is still being developed, we rely on exiting nonlinear theory. The nonlinear term causes an energy flow from short (unstable) to long wavelength modes which are stable. Growth and damping rates balance at  B/B ≈  e /L T, and low magnetic field implies large  e and high  B/B. Ref: J. F. Drake et al., Phys. Rev. Lett. 44, 994 (1980) NSTX

Overlapping resistive layers _________________________________________ Resistive layers overlap when m > m o = q(2q’  s ) -1/2 or k>k o For shot A11 at 0.9 s :  s =(2T e /m i ) 1/2 /  ci, B=5kG, a=65cm,  =r/a  q T i T e  s q’=dq/dr m o k o =m o /r _____________________________________________________________________________________ ev 780ev 1.1cm 0.06cm cm * D.A. D’Ippolito et al., Phys. Fluids 23, 771 (1980) NSTX

Global stochastic magnetic field ______________________________________________ When adjacent island chains are separated by KAM surfaces(as electron transport barriers), stochastic field lines are localized near the separatrix When adjacent island chains overlap, global stochastic magnetic field is expected ( usually the KAM surfaces are destroyed before islands overlap - the 2/3 rule ) One can estimate the island width from (  B r ) mn and find that adjacent islands overlap Substantial electron transport should ensue when either adjacent resistive layers or island chains overlap - see D’Ippolito et al.,Phys.Fluids(1980) NSTX

Electron thermal conductivity in stochastic magnetic field ____________________________________ _ When islands overlap, magnetic field lines become stochastic with diffusion coeff D M, the electron thermal conductivity in the collisional regime ( mfp << L c ) can be estimated by :  e = D M v e ( mfp /L c ) where D M ≈ R|  B| 2 /B 2 Ref: A.B. Rechester & Rosenbluth, Phys. Rev. Lett. 40, 38 (1978) T.H. Stix, Nucl. Fusion 18, 353 (1978) B.B. Kadomtsev & O.P. Pogutse (IAEA,Innsbruck-1978) NSTX

Field line correlation length _______________________________________ A rigorous theory 1 on plasma transport in stochastic magnetic fields is extremely complex. The precise formula for the field line correlation length L c is unknown We use L c =qR = field line connection length 2,3 instead of the Kolmogorov-Lyapunov length For NSTX plasmas, the electrons are weakly collisional: 1 < L c / mfp < 10 1.J.A. Krommes et al., J. Plasma Phys. 30, 11 (1983) 2.J.A. Krommes, private communication 3.B.B. Kadomtsev et al., IAEA (1978) NSTX

Comparison between  e exp and  e theo ________________________________________________ Drake’s theory assumes L n >>L T which is not strictly valid in the experiment In the region where L n >L T (  = ), the agreement between  e exp and  e theo is not bad (within a factor 3) In the region  ≤0.4, L n < L T ≥80cm, the driving term from dT e /dr is too weak, and  e may be determined by some other mechanism Get much better agreement (within 40%) in the entire range (  = ) if we replace L T by L where L -1 = L T -1 + L n -1 NSTX

 e due to saturated microtearing modes _________________________________________________ Put  B/B=  e /L T, get  e = (  e /L T ) 2 Rv e (mfp/L c )= (  e /L) 2 v e 2 /( ei q) Use parameters from #116313A11 at 0.9s, L c = qR  Z eff T e n e  e (cm) L n L T (cm) ei (s -1 ) q  e exp  e theo (cm 2 /s) _________________________________________________________________ ev 7.2e e e e5 0.71e

NSTX

Higher T e (0) when microtearing modes are stabilized by reversed magnetic shear _______________________________________________ Growth rate at  =0.3, t=0.32s # normal shear # reversed shear NSTX

Final Remark _________________________________________ NSTX has the flexibility to operate in many regimes. What I show here is just one type of discharge that allows us to study the effect of microtearing modes. There are several ways to improve electron confinement in ST: 1. Raise B T - HHFW heating efficiency improves 2. Raise T e so that e <  *e - can get T e (0) ~ 4 keV by HHFW heating 3. Reversed magnetic shear - High m modes stabilized and T e (0) becomes significantly higher with same NBI power NSTX

Summary ________________________________________ _ Quantitative analysis of anomalous electron transport in NSTX is carried out - use no fudge factor In the entire region of strong T e gradient,  e observed in experiment is in reasonable agreement with nonlinear microtearing mode theory (within 2X), better agreement with grad(n e ) included. This is not surprising because of the low toroidal magnetic field. The high mode amplitude causes global stochastic magnetic field. Microtearing modes are stable in reversed shear plasmas where T e (0) is substantially higher. Existing nonlinear theory can be improved by electromagnetic gyrokinetic simulation (GEM code). NSTX