Correlation Between Plasma Rotation and Electron Temperature Gradient Scale Length in LOC/SOC Transition at Alcator C-Mod

Transcription

1 Correlation Between Plasma Rotation and Electron Temperature Gradient Scale Length in LOC/SOC Transition at Alcator C-Mod Saeid Houshmandyar 1 W. L. Rowan, 1 P. E. Phillips, 1 M. J. Greenwald, 2 J. W. Hughes, 2 J. E. Rice, 2 S. M. Wolfe, 2 T. Golfinopoulos 2 1 Institute for Fusion Studies, The University of Texas at Austin 2 Plasma Science and Fusion Center, Massachusetts Institute of Technology 03/30/16 S. Houshmandyar, TTF

2 Motivation Why study LOC and SOC plasmas? Ø Intrinsic rotation reversal has been observed in the transition from linear ohmic confinement (LOC) to saturated ohmic confinement (SOC). 1,2,3 Ø Early ITER operations will be with low external momentum input, so understanding the intrinsic rotation is important. Ø It is commonly accepted that LOC/SOC transition from dominance of electron-scale turbulence to turbulence dominated by ITG. 1 Parker et al, Nucl. Fus. 25, 1127 (1985). 2 Greenwald et al, Fus. Sci & Tech 51, 266 (2007). 3 Rice et al, PRL 107, (2011). 4 Rice et al, Nucl. Fus. 53, (2013). 03/30/16 S. Houshmandyar, TTF

3 Motivation Recent study at AUG on LOC/SOC transition Ø Intrinsic rotation reversal has been observed in the transition from linear ohmic confinement (LOC) to saturated ohmic confinement (SOC). 1,2,3 Ø Early ITER operations will be with low external momentum input, so understanding the intrinsic rotation is important. Ø It is commonly accepted that LOC/SOC transition from dominance of electron-scale turbulence to turbulence dominated by ITG. Ø A recent study at AUG suggests that the intrinsic rotation behavior is rather determined by local plasma parameters, regardless of the heating method or the confinement regime.* * McDermott et al, Nucl. Fusion 54, (2014). 03/30/16 S. Houshmandyar, TTF

4 Theoretical Context v The toroidal momentum flux (Γ φ ) in radial direction can be written as three terms*: Γ ϕ = χ ϕ u ʹ+V ϕ u + Γ RS v The magnitude of the off-diagonal terms, Γ RS, are proportional to logarithmic gradients (R/L Te, R/L Ti, ). v In the absence of momentum injection, (Γ φ = 0, intrinsic rotation) and no convection, Γ RS = - χ φ u. Thus a relation between u and R/L Te should exist. This relation characterizes the correlation between the rotation and the scale length. * u (= RΩ/v thi = V rot /v thi ) is the Mach number, u (= -R 2 Ω/v thi = -R dv rot /dr/v thi ) is the normalized rotation gradient, v thi = (2T i /m i ) 1/2, and V φ is the convection (momentum pinch). 03/30/16 S. Houshmandyar, TTF

5 ECE diagnostic can measure L Te directly Ø Radiometer-based ECE on Alcator C-Mod (FRCECE) q 32 channels. q Collects 2 nd harmonic ECE radiation ( GHz) n e m -3. q s temporal, 7mm (radial) and ~3cm (poloidal) resolution. Ø B T -jog technique allows direct L Te profile measurements. 03/30/16 S. Houshmandyar, TTF

6 ECE diagnostic can measure L Te directly Ø B T -jog technique allows direct L Te profile measurements. q For the B T jog technique, a 1.5% change in the toroidal magnetic field shifts the viewing volume of the ECE by ~1 cm, and the ratio of the average of the signal to the change in the signal during its ramp-up (or ramp-down) yields L Te. L Te = T e T e = ΔR ECE(t) ΔECE(t) during ramp up, where ΔR( t) = R B ΔB ( t ) 03/30/16 S. Houshmandyar, TTF

7 ECE diagnostic can measure L Te directly T e profile averaged over the 170 ms of B T ramp-down L Te profile measured from B T -jog technique 03/30/16 S. Houshmandyar, TTF

8 ECE diagnostic can measure L Te directly Additionally, the technique identifies any flattening in T e profiles (i.e., magnetic islands), which can be overlooked in common fitting procedures. 03/30/16 S. Houshmandyar, TTF

9 Main Results Ø Study of intrinsic rotation correlation to L Te. Intrinsic rotation behavior shows strong correlation with temperature scale lengths, however its behavior is eventually found to be dependent on the confinement regime. Ø Localized low frequency modes, correlated with LOC and SOC plasmas. ~3 khz in LOC plasmas (consistent with core rotation frequency), at a steep temperature gradient location. ~6 khz in SOC plasmas (consistent with core rotation frequency), at a steep temperature gradient location. 03/30/16 S. Houshmandyar, TTF

10 Hydrogen plasmas. B T = 5.4T n e = ( m -3 ) [or n e.l = ( m -2 )] I P = 0.8 MA 1 T e / T i 3 Alcator C-Mod Experimental Setup 03/30/16 S. Houshmandyar, TTF

11 u dependence on L Te within core L Te measured through B T -jog. L Ti calculated from T i profiles (HirexSr). 03/30/16 S. Houshmandyar, TTF

12 u dependence on L Te within core The u vs. L T plots are fitted linearly. The slope of these plots are chosen as correlation criterion between rotation and the scale lengths. 03/30/16 S. Houshmandyar, TTF

13 u dependence on L Te within core The u vs. L T plots are fitted linearly. The slope of these plots are chosen as correlation criterion between rotation and the scale lengths. These slopes are then plotted versus core electron density. The correlation (slopes) follow the same trend as the energy confinement time (τ E ) would follow for LOC and SOC plasmas 03/30/16 S. Houshmandyar, TTF

14 Low Frequency Mode observed in the ECE spectrum in LOC plasma SOC LOC Exists in R /30/16 S. Houshmandyar, TTF

15 Low Frequency Modes observed in the ECE spectrum in LOC and SOC plasmas: they have the same core rotation frequency SOC LOC Exists in R Exists in R 0.78 Core rotation frequency 03/30/16 S. Houshmandyar, TTF 2016 Core rotation frequency 15

16 Low Frequency Modes observed in the ECE spectrum in LOC and SOC plasmas: localized to a steep temperature gradient radius LOC SOC Steepest temperature gradient Steepest temperature gradient 03/30/16 S. Houshmandyar, TTF

17 Conclusions Ø Intrinsic rotation behavior shows strong correlation on temperature scale lengths, however its behavior is eventually found to be dependent on confinement regime. Ø A very localized low frequency mode (~3 khz) is observed in LOC plasmas, consistent with the core intrinsic rotation frequency. The mode disappears above LOC/SOC critical density. Also, the mode disappears at the steepest gradient. Ø A very localized low frequency mode (~6 khz) is observed in SOC plasmas, consistent with the core intrinsic rotation frequency. The mode disappears at the steepest gradient. 03/30/16 S. Houshmandyar, TTF