Triggering the L-H Transition Lothar Schmitz 1 for B. Grierson 2 , - - PowerPoint PPT Presentation

▶
triggering the l h transition
SMART_READER_LITE
LIVE PREVIEW

Triggering the L-H Transition Lothar Schmitz 1 for B. Grierson 2 , - - PowerPoint PPT Presentation

Oct 22, 2023 401 likes •825 views

First Direct Evidence of Turbulence-Driven Main Ion Flow Triggering the L-H Transition Lothar Schmitz 1 for B. Grierson 2 , L. Zeng 1 , T.L. Rhodes 1 , G.R. McKee 3 , Z. Yan 3 , J.A. Boedo 4 , C. Chrystal 4 , G.R. Tynan 4 , P. Gohil 5 , R.

slide-1
SLIDE 1

1

  • L. Schmitz/IAEA2014

1 1 1

Lothar Schmitz1

for

  • B. Grierson2, L. Zeng1, T.L. Rhodes1, G.R. McKee3,
  • Z. Yan3, J.A. Boedo4, C. Chrystal4, G.R. Tynan4,
  • P. Gohil5, R. Groebner5, P.H Diamond4,6, G. Wang1

W.A. Peebles1, E.J. Doyle1, K.H. Burrell5, C.C. Petty5,

1University of California Los Angeles, Los Angeles, CA, USA 2Princeton Plasma Physics Laboratory, Princeton, NJ, USA 3University of Wisconsin-Madison, Madison, WI, USA 4University of California San Diego, La Jolla, CA, USA 5General Atomics, San Diego 6NFRI, and World Class Institute, Daejeon, Korea

25th IAEA Fusion Energy Conference

  • St. Petersburg, Russia

October 13-18, 2014

1

First Direct Evidence of Turbulence-Driven Main Ion Flow Triggering the L-H Transition

slide-2
SLIDE 2

2

  • L. Schmitz/IAEA2014
  • Investigate L-H transitions at marginal

heating power:

  • expanded transition timescale
  • can exhibit limit cycle oscillations (LCO)
  • Er, E⤬B shear periodically modulated;

edge turbulence periodically quenched:

  • LCO can reveal the detailed turbulence-

flow interaction and trigger physics

2

Predicting the L-H Transition Power Threshold in ITER Requires a Physics-based L-H Transition Model

slide-3
SLIDE 3

3

  • L. Schmitz/IAEA2014

3

  • New: Evidence that turbulence-driven ion flow

triggers the L-Mode – LCO transition

  • Causality: Turbulence-driven flow quenches turbulence initially;

pressure gradient-driven flow locks in H-mode confinement

  • New: A modified predator-prey model captures essential

LCO physics

  • New: L-mode seed flow shear at L-mode – LCO transition

has a density dependence similar to the L-H power threshold

Outline / Summary

slide-4
SLIDE 4

4

  • L. Schmitz/IAEA2014

Backscattering off density fluctuations with

ks = ki + + , ,

=

= -2ki

Several Effects localize back- scattering to the cut-off layer

E⤬B velocity from Doppler shift:

Doppler = vturb

vturb: Turbulence advection Here, vph << vExB

vExB~ Doppler/2ki

Doppler Backscattering (DBS) Measures Local Density Fluctuation Level and Turbulence Advection Velocity

4

X-mode cutoff-layer = c,x

ki ks ,ki

Launch angle

slide-5
SLIDE 5

5

  • L. Schmitz/IAEA2014

Density fluctuations and E×B velocity measured by DBS with high spatial/ temporal resolution Radial mapping using density profiles from fast Profile Reflectometry (25 s) Main ion poloidal/toroidal flow via CER measurements E×B flow shearing rate calculated from neighboring DBS channels:

Time Evolution and Radial LCO Structure via Multi-channel Doppler Backscattering and Main Ion CER

DBS/Main Ion CER probing locations

LCFS

Poloidal/Tor. CER Chords 5

slide-6
SLIDE 6

6

  • L. Schmitz/IAEA2014

6 3

Outline

Evidence of Turbulence-driven Ion Flow; Meso-scale Dipolar Flow Structure

slide-7
SLIDE 7

Time Evolution and Radial Mapping of LCO Structure via Multi- channel Doppler Backscattering

7

  • L. Schmitz/IAEA2014

Time Evolution and Radial LCO Structure via Multi-channel Doppler Backscattering

Schmitz et al, PRL 108, 2012 Outer Shear Layer Inner Shear Layer

7

  • L-Mode: Weak ExB shear layer

turbulence peaks at/outside the separatrix

  • LCO phase: Periodic ExB flow

and turbulence suppression (starting at separatrix)

  • H-mode: Wider and deeper

shear layer; turbulence suppression maintained across the edge

slide-8
SLIDE 8

8

  • L. Schmitz/IAEA2014

Time Evolution and Radial LCO Structure via Multi-channel Doppler Backscattering

Schmitz et al, PRL 108, 2012 Outer Shear Layer Inner Shear Layer

8

  • L-Mode: Weak ExB shear layer

turbulence peaks at/outside the separatrix

  • LCO phase: Periodic ExB flow

and turbulence suppression (starting at separatrix)

  • H-mode: Wider and deeper

shear layer; turbulence suppression maintained across the edge

slide-9
SLIDE 9

9

  • L. Schmitz/IAEA2014

How is the LCO Triggered? Obtain Turbulence-Driven Ion Flow from the Radial Ion Force Balance

E×B velocity measured via DBS v×B term evaluated from radial momentum balance (subtracting term)

from DBS from profile reflectometer/CER

9

Ñpi

slide-10
SLIDE 10

10

  • L. Schmitz/IAEA2014

10

How is the LCO Triggered? Evidence for Turbulence-Driven vixB Flow in the Ion Diamagnetic Direction

Positive transient in vixB (ion diamagnetic direction) inside the LCFS at the initial turbulence quench Turbulence suppressed within ~ 100

from DBS from profile reflectometer/CER Radial ion momentum balance:

slide-11
SLIDE 11

11

  • L. Schmitz/IAEA2014

11

Meso-scale Shear Triggers Initial Turbulence Quench

Peak negative ExB flow does not coincide with time of maximum shear (across outer shear layer) Local meso-scale ExB shear reversal initiates first turbulence quench:

slide-12
SLIDE 12

12

  • L. Schmitz/IAEA2014

12

ExB Shear Across Outer Layer Increases Periodically Preceding Turbulence Suppression

Peak negative ExB flow does not coincide with time of maximum shear (across outer shear layer) Local meso-scale ExB shear reversal initiates first turbulence quench: ExB Shear across outer layer increases; quenches turbulence periodically during successive LCO cycles

slide-13
SLIDE 13
  • L. Schmitz/IAEA2014

12

Turbulence Drives Main Ion Poloidal Flow

  • Main ion flow (measured

via main ion CER) lags ñ

  • Phase delay of (~90º)

is qualitatively consistent with ion flow acceleration via Reynolds stress :

  • BES velocimetry confirms

(positive) Reynolds stress gradient in outer layer

He Plasma: Cross-Correlation

  • f ñ and

 tD~0.15 ms

vqvr

Measured early in the LCO (t0+1.5 ms)

¶ vq ¶t = - ¶ vqvr ¶r

  • m vq
slide-14
SLIDE 14

14

  • L. Schmitz/IAEA2014

Poloidal Flow is the Main Contribution to the vExB Oscillation Early in the LCO

Phase-lock analysis: Triangular CER waveforms due to limited CER time resolution

is the dominant

contribution to vExB early in the LCO

Outer Shear Layer t0+4ms

14

= 0.27 ms

IDD

slide-15
SLIDE 15

15

  • L. Schmitz/IAEA2014

BES Shows Formation of Large Scale Eddies and Eddy Tilting/ Break-up in High Shear Regions

  • Large eddies grow

at expense of smaller eddies

  • Break-up/turbulence

reduction after large eddies tilt

  • E×B flow reversal near

LCFS: IDD turbulence- driven flow at LCFS; EDD turbulence-driven flow further inboard

15

6 4 2

LCFS

Inner Outer

Z (CM)

slide-16
SLIDE 16

16

  • L. Schmitz/IAEA2014

Outline

16

Causality of shear flow generation

slide-17
SLIDE 17

17

  • L. Schmitz/IAEA2014

Final Transition to H-mode is due to Increasing Pressure-Gradient Driven Shear; Modulation/Increase of ∇n (∇pi)

17

  • ∇n is used as proxy for ∇pi

as Ln < 0.3LTi

  • Density gradient only changes

significantly well into the LCO

  • Gradual increase and periodic

modulation of ∇n during LCO

  • Increasing ∇p slows down LCO

frequency (increasing shear inhibits turbulence recovery)

slide-18
SLIDE 18

18

  • L. Schmitz/IAEA2014

18

  • Expanded time scale:

∇n (∇p) increase after each fluctuation quench

Final Transition to H-mode is due to Increasing Pressure-Gradient Driven Shear; Modulation/Increase of ∇n,∇pi

slide-19
SLIDE 19

19

  • L. Schmitz/IAEA2014

Early in the LCO, ∇pi lags : E×B Shear is not caused by the pressure gradient Later in the LCO, ∇pi leads : Pressure-gradient driven shear is dominant

Causality of Shear Flow Generation: Turbulence-Driven Flow Shear Dominates Early in the LCO

19

Correlation delay Between and ∇pi

∇pi leads ∇pi lags

slide-20
SLIDE 20

20

  • L. Schmitz/IAEA2014

Outline

20

A modified Predator-prey Model Captures Essential LCO Physics

slide-21
SLIDE 21

21

  • Total ExB flow includes pressure-gradient-driven equilibrium flow
  • Pressure gradient is modulated via the periodic change in

turbulence level and transport: two interacting feedback cycles

Gradient Drive

Damping ( ii)

ZF Inhibition

Two Coupled Feedback Cycles: Synergy of Turbulence-Driven Flow and Pressure-Gradient-Driven Flow

21

Pressure Gradient (Predator II)

slide-22
SLIDE 22

22

  • L. Schmitz/IAEA2014

Predator-Prey Model Reproduces Important Experimental LCO Features and Scalings

Total ExB flow (includes , vDia, and turbulence- driven flow): (Er,ñ) phasing shifts from 90º closer to 0º as diamagnetic shear becomes dominant

*based on Miki, Diamond, PoP 2012

0-D Predator-Prey Modeling results*, including:

  • neoclassical

poloidal ion velocity (no toroidal flow)

  • shearing by

turbulence-driven and ∇p driven E⤬B flow

  • pressure profile

evolution (radial transport)

22

Predator-Prey Model Predicts LCO with Opposing Turbulence-Driven and ∇p-Driven (vDia) Flow

slide-23
SLIDE 23

23

  • L. Schmitz/IAEA2014

Predator-Prey Model Reproduces Important Experimental LCO Features and Scalings

Total ExB flow (includes , vDia, and turbulence- driven flow): (Er,ñ) phasing shifts from 90º closer to 0º as diamagnetic shear becomes dominant

*based on Miki, Diamond, PoP 2012

0-D Predator-Prey Modeling results*, including:

  • neoclassical

poloidal ion velocity (no toroidal flow)

  • shearing by

turbulence-driven and ∇p driven E⤬B flow

  • pressure profile

evolution (radial transport)

23

Predator-Prey Model Predicts LCO with Opposing Turbulence-Driven and ∇p-Driven (vDia) Flow

slide-24
SLIDE 24

24

  • L. Schmitz/IAEA2014

24

Predator-prey Model Qualitatively Reproduces the Measured Phase Shift between ñ and vExB

Early LCO (t0+ 1.5ms): Experiment: ~70-90º model: ~50-70º Late LCO (tH-1.5ms): Experiment: ~20-30º model: ~10-20º

Quantitative differences due to variations of Zonal- and mean turbulence-driven ion flow

slide-25
SLIDE 25

25

  • L. Schmitz/IAEA2014

25

Outline

E×B and v×B seed flow shear at the L-mode-LCO Transition

slide-26
SLIDE 26

26

  • L. Schmitz/IAEA2014

Importance of Seed Flow Shear: L-Mode E×B and v×B Flow Shear (and Pth) Increase at Low and High Density

  • Reynolds work PRe depends on

Reynolds stress and seed shear flow:

  • Total E⤬B shearing rate and

v⤬B shear show a minimum at intermediate density (similar to Pth)

  • L-mode diamagnetic seed

flow shearing rate does not reflect the Pth density dependence

  • 26
slide-27
SLIDE 27

27

  • L. Schmitz/IAEA2014
  • Strong evidence that turbulence-driven ion flow triggers LCO;

evidence of dipolar meso-scale flow structure

  • Causality of shear flow generation: Pressure-gradient-driven shear

increases only well after the initial fluctuation quench, and locks in the final transition to H-mode

  • 0-D /1-D predator-prey models captures synergy of turbulence-driven

and pressure-gradient driven flow and reproduces essential experimental LCO properties

  • Connection to power threshold: Both total E×B shear and v×B velocity

shear increase at very low and at high plasma density (qualitatively similar to Pth scaling)

Conclusions/Physical Picture

27

slide-28
SLIDE 28

28 28

  • L. Schmitz/IAEA2014

Positive Flow Transients in Outer Shear Layer Suppress ñ

  • E×B Shearing rates

peak in the outer shear layer where turbulence level is high

  • Positive flow transients

suppress turbulence

Outer Outer Inner Inner

LCFS

LCFS

t0

28

vExB ñ/n

slide-29
SLIDE 29

29

  • L. Schmitz/IAEA2014

Negative Flow Transients Occur after Turbulence Suppression

  • Negative E×B transients

reflect turbulent-driven flow early in the LCO

  • Pressure-gradient-

driven flow only changes significantly well into the LCO

Outer Outer Inner Inner

LCFS

LCFS

t0

29

vExB ñ/n

slide-30
SLIDE 30

30

  • L. Schmitz/IAEA2014

30 24

  • L. Schmitz/EU-US TTF 2014

24

0-D Predator-Prey modeling results*, including:

  • neoclassical

poloidal ion velocity (no toroidal flow)

  • shearing by

turbulence-driven and mean flows

  • pressure profile

evolution

  • radial transport

Turbulence-drivenZonal flow vZF lags density fluctuation level ñ by 90º Equilibrium flow is out of phase (180º) with ñ (both consistent with observed limit cycle phasing)

Predator-Prey Model Predicts LCO with Opposing Turbulence-Driven and ∇p-driven (vDia) Flow

slide-31
SLIDE 31

31

  • L. Schmitz/IAEA2014

31 24

  • L. Schmitz/EU-US TTF 2014

0-D Predator-Prey modeling results*, including:

  • neoclassical

poloidal ion velocity (no toroidal flow)

  • shearing by

turbulence-driven and mean flows

  • pressure profile

evolution

  • radial transport

Turbulence-driven flow vZF lags ñ by 90º (qualitatively consistent with experiment Poloidal Ion Flow lags ñ by 10-30º consistent with observed limit cycle phasing)

Predator-Prey Model Predicts LCO with Opposing Turbulence-Driven and p-Driven (vDia) Flow

slide-32
SLIDE 32

32

  • L. Schmitz/IAEA2014

24

  • L. Schmitz/EU-US TTF 2014

0-D Predator-Prey modeling results*, including:

  • neoclassical

poloidal ion velocity (no toroidal flow)

  • shearing by

turbulence-driven and mean flows

  • pressure profile

evolution

  • radial transport

Turbulence-driven flow vZF lags density fluctuation level ñ by 90º (consistent with observed limit cycle phasing)

Predator-Prey Model Predicts LCO with Opposing Turbulence-Driven and p-Driven (vDia) Flow

32

slide-33
SLIDE 33

33 33

  • L. Schmitz/IAEA2014
  • Positive transients

in inner shear layer delayed;

  • Consistent with radial

inward propagation of LCO E×B flow*

  • Mesoscale radial

structure:  i < LExB < Lp

Outer Outer Inner Inner

LCFS

LCFS

t0

*L. Schmitz et al., PRL 2012

33

vExB ñ/n Flow Layer Propagates Radially Inwards

slide-34
SLIDE 34

34

  • L. Schmitz/IAEA2014

34

Limit Cycle Directions (ñ,vExB Phase Relation) are Consistent with Meso-scale Turbulence-Driven Flow

Opposite Limit cycle directions are

  • bserved in outer/inner shear layer

ñ,vExB phase relationship is consistent with observed radial E×B flow propagation

*L. Schmitz et al.,

CCW CW

Limit Cycle-Outer Shear Layer Limit Cycle-Inner Shear Layer

Cross Correlation of ñ and vE×B

H-mode H-mode L-Mode H-mode

slide-35
SLIDE 35

35

  • L. Schmitz/IAEA2014

35

Predator-Prey Model Predicts LCO with Opposing Turbulence-Driven and ∇p-Driven (vDia) Flow

slide-36
SLIDE 36

36

  • L. Schmitz/IAEA2014

36 10 10

Evidence of Turbulence-Driven Poloidal Ion Flow from Main Ion CER and DBS

Main ion flow v lags the density fluctuation level ñ E×B velocity approximately in phase with v: Driven Poloidal ion flow is main contribution to vE×B

He Plasma: Cross-Correlation

  • f ñ and v

Poloidal flow acceleration via turbulence-generated Reynolds stress : vqvr

Measured early in the LCO

slide-37
SLIDE 37
  • L. Schmitz/IAEA2014

11 11

How Does the LCO Start? Compelling Evidence for Turbulence- Driven Ion Flow from Main Ion CER and DBS

Poloidal main ion flow v (blue, green) lags the density fluctuation level ñ The E×B flow is in phase with vθ (expected if the Er modulation results from vθ) Less clear correlation of ñ with toroidal velocity vφ in the early LCO

R=Rs-0.8 cm

He Plasma: Cross-Correlation

  • f ñ and vE×B with vθ
slide-38
SLIDE 38

38

  • L. Schmitz/IAEA2014

Toroidal Flow Modulation is Out of Phase with ExB Velocity in Outer Shear Layer

Toroidal velocity is positive (co-current); increases locally towards LCFS (orbit-loss effect?) Shown is the electric field component due to v Weak toroidal velocity modulation

  • bserved in Inner

Shear Layer

Outer Shear Layer Inner Shear Layer t0+4ms

38

slide-39
SLIDE 39

39

  • L. Schmitz/IAEA2014

39 29

  • L. Schmitz/TTF2014

30

Miki-Diamond Model* (1-D, coupled with radial transport model)

*Miki and Diamond, PoP 2012)

Mean Shear Flow Mean poloidal flow (Reynolds stress + neoclassical flow)

Predator-Prey Equations (1-D), Pressure Gradient (1-D) + Transport Model*,

Turbulence Evolution Turbulence-driven shear flow energy Pressure gradient evolution

slide-40
SLIDE 40
  • L. Schmitz/IAEA2014

2 2

Motivation

  • The presently used empirical L-H power threshold scaling

does not reflect important parameters, or the observed non-monotonic dependency of Pth on density:

  • Predicting the L-H transition power threshold in ITER requires

a physics-based L-H transition model:

  • Link trigger physics/microscopic flow/turbulence dynamics

to the macroscopic power threshold scaling

  • Extract critical seed shear flow/ critical turbulence-driven

shear flow and determine their role in the Pth scaling

(2008 multi-machine scaling)

Pth(MW) =0.049BΦ0.8ne0.72S0.94

slide-41
SLIDE 41

41

  • L. Schmitz/IAEA2014

14 14

Meso-Scale Dipole Structure of Turbulence-Driven Flow: Alternating Transients in Outer / Inner Shear Layer

Outer Shear Layer Inner Shear Layer E×B Shearing rates peak in the

  • uter shear layer

(pos. flow: magenta arrows) where turbulence level is high Radial profile consistent with radial inward propagation of LCO E×B flow*

*L. Schmitz et al., PRL 2012

41