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Experimental Observation of High-k Turbulence Evolution across L-H Transition in NSTX
NSTX-U Supported by
Culham Sci CtrYork U
Chubu UFukui U
Hiroshima UHyogo UKyoto U
Kyushu UKyushu Tokai U
NIFSNiigata UU Tokyo
JAEAInst for Nucl Res, Kiev
Ioffe InstTRINITI
Chonbuk Natl UNFRI
KAISTPOSTECH
Seoul Natl UASIPP
CIEMATFOM Inst DIFFER
ENEA, FrascatiCEA, Cadarache
IPP, JülichIPP, Garching
ASCR, Czech Rep
Coll of Wm & MaryColumbia UCompXGeneral AtomicsFIUINLJohns Hopkins ULANLLLNLLodestarMITLehigh UNova PhotonicsOld DominionORNLPPPLPrinceton UPurdue USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU IllinoisU MarylandU RochesterU TennesseeU TulsaU WashingtonU WisconsinX Science LLC
Y. Ren1
R.E. Bell1, Linming Shao2, D.R. Smith3, S. Zweben1, W. Guttenfelder1, S.M. Kaye1, B.P. LeBlanc1, E.
Mazzucato1, K.C. Lee4, C.W. Domier5, H. Yuh6 and the NSTX Team
1. PPPL 2. ASIPP 3. UW-Madison 4. NFRI 5. UC-Davis 6. Nova Photonics
2014 U.S. Transport Task Force Workshop, San Antonio, Texas, April 22-25, 2014
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Highlights
• First detailed measurements of high-k (electron-scale) turbulence across L-H transition in NSTX– L-H transition at current flattop – High-k turbulence quasi-stationary before L-H transition, intermittent
after L-H transition and significantly suppressed ~15 ms after L-H transition
– Suppression of high-k turbulence at lower wavenumbers, i.e. k┴ρs ≤ 9-10• Low-k turbulence measured by BES and GPI at different radii but
similar temporal behavior with high-k measurement – Showing suppression of low-k turbulence into H-mode at r/a>0.8
• Linear stability analysis using GS2 code showing some consistency with turbulence measurements– Decreased ETG growth rate into H-mode
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Turbulence Diagnostics in the Experiment Cover from Low-k to High-k
Probe beam
Scattered light
Spherical mirror
kp
ks
Probe beam
θs
ki
D.R. Smith, PhD thesis, 2009
High-k turbulence measurement
r/a Measured quantity
Spatial & temporal resolution
280 GHz microwave scatteringsystem
~0.7-0.8 (core-edge transition region)
Density fluctuation
3 ≤ k┴ρs ≤ 12 R ~ ±2 cm f~ 5 MHz
Low-k turbulence measurementBES >0.8 Density
fluctuationR~ 1cm f~ 1 MHz
GPI >0.8 GPI D emission
R~ 1 cmf~ 500 kHz
Turbulence diagnostic configuration for the experiment
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L-H Transition is Triggered by NBI Power Step-up during Current Flattop
Shot=139442 BT =5.5 kG
L-H transition
From TRANSP
• L-H Transition at current flattop reduces measurement complications
• Better high-k measurement due to favorable Doppler frequency shift
• Less MHD activity
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L-H Transition is Triggered by NBI Power Step-up during Current Flattop
Shot=139442 BT =5.5 kG
L-H transition
From TRANSP
• L-H Transition at current flattop reduces measurement complications
• Better high-k measurement due to favorable Doppler frequency shift
• Less MHD activity
Time range of interest
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D Dithering is Observed after the L-H Transition at t≈350 ms
• Spikes in divertor D after the L-H transition show the dithering feature of the L-H transition
• Low-f MHD activity is relatively benign after the L-H transition
LCFS at R=146 cm
L-H transition
D dithering
High-k measurement region (r/a~0.7-0.8)
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Edge Transport Barrier (ETB) is Established after the Dithering Phase
High-k measurement region (r/a~0.7-0.8)
• Clear H-mode density ear appears at t=365 ms, showing the establishment of ETB– The density ear due to edge accumulation of carbon impurity
LCFS at R=146 cm
L-H transition
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Edge Transport Barrier (ETB) is Established after the Dithering Phase
• H-mode pedestal continuously builds up after the dithering phase– Electron density and temperature increase
• High-k scattering system measures turbulence k spectrum at r/a~0.7-0.8 and ETB is at r/a>0.9
LCFS at R=146 cm
High-k measurement region (r/a~0.7-0.8)
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Clear Suppression of High-k Turbulence into L-H Transition from the High-k Scattering System
L-H transition
• Off-center peak denotes the scattered signal
• f=0 peak is from stray radiation
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Four Phases in the High-k Turbulence Evolution can be Identified
L-H transition
• Quasi-stationary turbulence before the L-H transition
• Off-center peak denotes the scattered signal
• f=0 peak is from stray radiation
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• Quasi-stationary turbulence before the L-H transition• Dithering in high-k spectral power observed after L-H transition
Four Phases in the High-k Turbulence Evolution can be Identified
L-H transition
Dithering Dithering
• Off-center peak denotes the scattered signal
• f=0 peak is from stray radiation
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• Quasi-stationary turbulence before the L-H transition• Dithering in high-k spectral power observed after L-H transition• Significant suppression of high-k turbulence in 365<t<380 ms
Four Phases in the High-k Turbulence Evolution can be Identified
L-H transition
• Off-center peak denotes the scattered signal
• f=0 peak is from stray radiation
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• Quasi-stationary turbulence before the L-H transition• Dithering in high-k spectral power observed after L-H transition• Significant suppression of high-k turbulence in 365<t<380 ms• Some increase in turbulence at t>380 ms
Four Phases in the High-k Turbulence Evolution can be Identified
L-H transition
• Off-center peak denotes the scattered signal
• f=0 peak is from stray radiation
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Broad Band High-k Turbulence is Seen across the L-H Transition
Electron direction
• Frequency spectra are shown for an exact Thomson time point: t=365 ms
• Turbulence propagates in the electron diamagnetic direction
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High-k Turbulence Changes in Amplitude and Frequency across the L-H Transition
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Significant Drop of Spectral Power in Lower Wavenumbers is Observed in the High-k Spectrum • Small variation before the L-H transition: quasi-stationary• Significant drop, i.e. a factor of about 7, in the peak spectral
power at t=365 ms after the L-H transition– Even smaller at 382 ms
Before L-H transition
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Significant Drop of Spectral Power in Lower Wavenumbers is Observed in the High-k Spectrum • Small variation before the L-H transition: quasi-stationary• Significant drop, i.e. a factor of about 7, in the peak spectral
power at t=365 ms after the L-H transition– Even smaller at 382 ms
• The drop in spectral power only occurs at k┴ρs <9-10 Before L-H transition
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Similar Observations in Spectral Power Drop in Lower Wavenumbers in Different NSTX Scenarios
After L-H transition
Before L-H transition
Increase of ExB shear
Ren et al., NF 2013
NBI-heated L-mode
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Similar Observations in Spectral Power Drop in Lower Wavenumbers in Different NSTX Scenarios
Increase of ExB shear
Increase of density gradient
Ren et al., NF 2013
Ren et al., PRL 2011
NBI-heated L-mode
NBI-heated H-mode
After L-H transition
Before L-H transition
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Similar Observations in Spectral Power Drop in Lower Wavenumbers in Different NSTX Scenarios
Increase of ExB shear
Increase of density gradient
RF heating turned-off
Ren et al., NF 2013
Ren et al., PRL 2011
NBI-heated L-mode
NBI-heated H-mode
RF-heated L-mode
After L-H transition
Before L-H transition
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Similar Observations in Spectral Power Drop in Lower Wavenumbers in Different NSTX Scenarios
• The drop in spectral power only occurs at k┴ρs <9-10
Ren et al., NF 2013
Ren et al., PRL 2011
NBI-heated L-mode
NBI-heated H-mode
RF-heated L-mode
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• Overall turbulence power decreases into H-mode
Large Intermittency (Dithering) in High-k Turbulence from t=350 to 365 ms
• Off-center peak denotes the scattered signal
• f=0 peak is from stray radiation
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• Overall turbulence power decreases into H-mode• Periods of minimum turbulence appear intermittently (~1-1.5 ms)
Large Intermittency (Dithering) in High-k Turbulence from t=350 to 365 ms
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• Overall turbulence power decreases into H-mode• Periods of minimum turbulence appear intermittently (~1-1.5 ms)
– Fast decrease and rise of turbulence power in 0.5-1 ms
Large Intermittency (Dithering) in High-k Turbulence from t=350 to 365 ms
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• High-k turbulence intermittency is on the same time scale as the dithering of divertor D
• A definite correlation is not yet established
Large Intermittency (Dithering) in High-k Turbulence from t=350 to 365 ms is Similar to Divertor D
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• High-k channel 3 measuring k┴ρs ~6-7 is compared with BES measurement at R=142 cm (top of the H-mode pedestal)
Low-k Turbulence Measured by BES Shows Similar Temporal Behavior as High-k Turbulence
A BES channel at R=142 cm
High-k channel 3k┴ρs ~6-7R=135.5-140
High-k measurement region
BES channel location
L-H transition
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Low-k Turbulence Measured by BES Shows Similar Temporal Behavior as High-k Turbulence
A BES channel at 142 cm
High-k channel 3k┴ρs ~6-7
• High-k channel 3 measuring k┴ρs ~6-7 is compared with BES measurement at R=142 cm (top of the H-mode pedestal)
L-H transition • Quasi-stationary turbulence before the L-H transition– from BES %9.2~
nn
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Low-k Turbulence Measured by BES Shows Similar Temporal Behavior as High-k Turbulence
A BES channel at 142 cm
High-k channel 3k┴ρs ~6-7
L-H transition • Quasi-stationary turbulence before the L-H transition– from BES
• Reduced turbulence into H-mode− from BES
%9.2~
nn
%94.0~
nn
• High-k channel 3 measuring k┴ρs ~6-7 is compared with BES measurement at R=142 cm (top of the H-mode pedestal)
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Low-k Turbulence Measured by BES Shows Similar Temporal Behavior as High-k Turbulence
A BES channel at 142 cm
High-k channel 3k┴ρs ~6-7
• Quasi-stationary turbulence before the L-H transition– from BES
• Reduced turbulence into H-mode− from BES
• Intermittent turbulence right after the L-H transition−t ~ 350-365 ms−Similar temporal
intermittency in low-k and high-k
%9.2~
nn
%94.0~
nn
• High-k channel 3 measuring k┴ρs ~6-7 is compared with BES measurement at R=142 cm (top of the H-mode pedestal)
L-H transition
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Low-k Turbulence Measured by BES Shows Similar Temporal Behavior as High-k Turbulence
A BES channel at 142 cm
High-k channel 3k┴ρs ~6-7
• Quasi-stationary turbulence before the L-H transition– from BES
• Reduced turbulence into H-mode− from BES
• Intermittent turbulence right after the L-H transition−t ~ 350-365 ms−Similar temporal
intermittency in low-k and high-k
%9.2~
nn
%94.0~
nn
• High-k channel 3 measuring k┴ρs ~6-7 is compared with BES measurement at R=142 cm (top of the H-mode pedestal)
L-H transition
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GPI Measurements also Show Similar Overall Temporal Behavior as the High-k Turbulence
GPI emission at R=144 cm
L-H transition
• is the RMS value of the GPI emission fluctuation over every 0.13 ms
• is the mean of the GPI emission over every 0.13 ms
• Last closed flux surface at R~146 cm
• High-k measurement region at R~135.5 -140 cm
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• Quasi-stationary turbulence before the L-H transition
GPI Measurements also Show Similar Overall Temporal Behavior as the High-k Turbulence
GPI emission at R=144 cm
L-H transition
• is the RMS value of the GPI emission fluctuation over every 0.13 ms
• is the mean of the GPI emission over every 0.13 ms
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• Intermittent and decreasing turbulence right after the L-H transition
GPI Measurements also Show Similar Overall Temporal Behavior as the High-k Turbulence
GPI emission at R=144 cm
L-H transition
• is the RMS value of the GPI emission fluctuation over every 0.13 ms
• is the mean of the GPI emission over every 0.13 ms
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• Suppressed turbulence in 365 ms<t<380 ms
GPI Measurements also Show Similar Overall Temporal Behavior as the High-k Turbulence
GPI emission at R=144 cm
L-H transition
• is the RMS value of the GPI emission fluctuation over every 0.13 ms
• is the mean of the GPI emission over every 0.13 ms
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GPI Measurements are more Intermittent than the High-k Turbulence
• is the RMS value of the GPI emission fluctuation over every 0.13 ms
• is the mean of the GPI emission over every 0.13 ms
GPI emission at R=144 cm
L-H transition
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• ~30% decrease in normalized inverse ETG scale length
• ~30% variation in normalized inverse ITG scale length
• Significant decrease in normalized inverse density gradient scale length
• Significant increase in Te, Ti and ne (~45-60%)
Equilibrium Profiles Changes can be Significant across the L-H Transition
Quantities averaged in the high-k measurement region
The time of L-H transition
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Linear Stability Analysis Shows that ITG and ETG are both Unstable
• ETG linear growth rates decrease into H-mode– ETG mode real frequency increases
• ITG growth rates vary much less significantly
• Stability Analysis was performed with the GS2 code (Kotschenreuther et al., 1995) with Miller local equilibrium
ITG
ETG
Before L-H transitionR=138 cm
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ETG Stability across the L-H Transition is Consistent with the Measured High-k Turbulence Variation
• ETG linear growth rates decrease into H-mode• The measured high-k turbulence also decreases into H-mode• The observed intermittency requires nonlinear processes
• Stability Analysis was performed with the GS2 code (Kotschenreuther et al., 1995) with Miller local equilibrium
ITG
ETG
R=138 cm
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ETG Close to Marginal Stability in the High-k Measurement Region Further into H-mode
• The decrease in ETG linear growth rates is due to the decrease of ETG and increase of critical ETG
• Stability Analysis was performed with the GS2 code (Kotschenreuther et al., 1995) with Miller local equilibrium
ITG
ETG
R=138 cm
The time of L-H transition
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Summary• First detailed measurement of high-k (electron-scale) turbulence
across L-H transition in NSTX– Quasi-stationary high-k turbulence before L-H transition, intermittent after
L-H transition and significantly suppressed ~15 ms after L-H transition– Suppression of high-k turbulence at lower wavenumbers, i.e. k┴ρs ≤ 9-10,
similar observations also in different NSTX scenarios• Low-k turbulence measured by BES and GPI at different radii but
similar temporal behavior as high-k turbulence observed– Intermittency observed after L-H transition, similar to high-k turbulence– Showing suppression of low-k turbulence into H-mode at r/a>0.8
• Linear stability analysis using GS2 code showing some consistency with turbulence measurements– Decreased ETG growth rate into H-mode – Nonlinear processes needed for explaining the observed intermittency
Acknowledgement: Work supported by DoE
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Backup Slides
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Broad Band High-k Turbulence is Seen across the L-H Transition
Electron direction
• Frequency spectra are shown for an exact Thomson time point: t=382 ms
• Real frequency distinguishable from the Doppler frequency shift for channel 3
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• Frequency spectra are shown for an exact Thomson time point: t=348 ms
• Mean real frequency much smaller than Doppler frequency shift
Broad Band High-k Turbulence is Seen across the L-H Transition
Electron direction
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Ion-scale Modes are more Suppressed into H-mode
• is used to assess ExB shear effect on ion-scale modes
• increases in the high-k measurement region into H-mode
• Stability Analysis was performed with the GS2 code (Kotschenreuther et al., 1995) with Miller local equilibrium
ITG
ETG
R=138 cm
Before L-H
After L-H transition