EURATOM/UKAEA Fusion Association, Culham Science Centre Abingdon, Oxfordshire, OX14 3DB, UK This...

EURATOM/UKAEA Fusion Association,Culham Science Centre Abingdon, Oxfordshire, OX14 3DB, UK

This work was funded jointly by the United Kingdom Engineering and

Physical Sciences Research Council and by EURATOM.

“Hidden” Variables in the L/H transitiona discussion

“Hidden” Variables in the L/H transitiona discussion

Hendrik Meyer6th October 2009

Hendrik Meyer6th October 2009

Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 2

ContentsContents

What is a “hidden” variable? – Try to define what we don’t know!What is a “hidden” variable? – Try to define what we don’t know!

Wall conditions and impuritiesWall conditions and impurities

Divertor geometry, plasma shape, recycling and fuellingDivertor geometry, plasma shape, recycling and fuelling

3D effects – stellarators and RMPs (MHD, ripple)3D effects – stellarators and RMPs (MHD, ripple)

Plasma flows and the radial electric fieldPlasma flows and the radial electric field

DiscussionDiscussion


What is a “hidden” variable? – Try to define what we don’t know!What is a “hidden” variable? – Try to define what we don’t know!

Concept of the “hidden” variable arose from the scatter of PLH with respect to the power threshold scaling and the variability of PLH in single devices.

Concept of the “hidden” variable arose from the scatter of PLH with respect to the power threshold scaling and the variability of PLH in single devices.

There are three reasons for scatter in the database:1. Scatter due to variables effecting the transition, but are not captured in the scaling.

2. Scatter under the “same” conditions due to effects that are not measured.

3. Scatter due to a statistical nature of the transition.

There are three reasons for scatter in the database:1. Scatter due to variables effecting the transition, but are not captured in the scaling.

2. Scatter under the “same” conditions due to effects that are not measured.

3. Scatter due to a statistical nature of the transition.

9.07.020

8.0005.0 SnBPLH 9.07.0

208.0

005.0 SnBPLH Conventional:

)(2

072.07.0

9.07.020

7.0

, AFZ

SnBP eff

outnewLH

)(

2072.0

7.0

9.07.020

7.0

, AFZ

SnBP eff

outnewLH

More comprehensive: Also IpAlso Ip

impuritiesimpurities

Trapped particlesTrapped particles

The scaling captures global parameters, but the transition is governed by local variables – Scatter not due to “hidden” variables!

The scaling captures global parameters, but the transition is governed by local variables – Scatter not due to “hidden” variables!

Some effects are reproducible, but we don’t understand them – Still “hidden” variables? Some effects are reproducible, but we don’t understand them – Still “hidden” variables?


To see what’s hidden we need to identify what is seen!To see what’s hidden we need to identify what is seen!

“There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we now know we don't know. But there

are also unknown unknowns. These are things we do not know we don't know.”

Donald H. Rumsfeld (Secretary of State, USA 2001– 2006)

“There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we now know we don't know. But there

are also unknown unknowns. These are things we do not know we don't know.”

Donald H. Rumsfeld (Secretary of State, USA 2001– 2006)


The known knowns – there are not that manyThe known knowns – there are not that many

There is evidence for a critical Te(B) or Te(B)

− Often hard to distinguish experimentally.

There is evidence for a critical Te(B) or Te(B)

− Often hard to distinguish experimentally.

The threshold power indicates that thermodynamic processes are important: PLH/ (n S) B (approx.)

The threshold power indicates that thermodynamic processes are important: PLH/ (n S) B (approx.)

This may be a critical Ti(B) or Ti(B) This may be a critical Ti(B) or Ti(B)

DIII-D

R.Groebner, PoP 8 (2001) 2272R.Groebner, PoP 8 (2001) 2272

At L/H transition

JET

Y.Andrew, PPCF 46 (2004) 337Y.Andrew, PPCF 46 (2004) 337

AUG/JET: Transition at similar *, *, , but different absolute T No atomic processes.

AUG/JET: Transition at similar *, *, , but different absolute T No atomic processes.

AUG/CMOD: Transition not at similar *, *, atomic processes?

AUG/CMOD: Transition not at similar *, *, atomic processes?


More known knowns – but maybe less well known?More known knowns – but maybe less well known?

H-mode can be initiated by a sheared EB flow Er changes before n− Sufficient condition, but is it

necessary? see later.

H-mode can be initiated by a sheared EB flow Er changes before n− Sufficient condition, but is it

necessary? see later.DIII-D

K.H. Burrell et.al. PPCF 34 (1992) p.1859 K.H. Burrell et.al. PPCF 34 (1992) p.1859

RB

E

B

RB rBEturb

2

RB

E

B

RB rBEturb

2

Interval of measurement

DIII-D

R. Weynants et.al. PPCF 40 (1998) p.635R. Weynants et.al. PPCF 40 (1998) p.635


The (well and less well) known unknowns – Why “hidden”?The (well and less well) known unknowns – Why “hidden”?

The H-mode threshold is much higher if the ion B-drift is away from the X-point. The H-mode threshold is much higher if the ion B-drift is away from the X-point.

The H-mode threshold in limiter discharges is higher than in divertor discharges – or is it? The H-mode threshold in limiter discharges is higher than in divertor discharges – or is it?

The divertor is important for H-mode access. The divertor is important for H-mode access.

The H-mode can be initiated by changes to the fuelling− Shutting off gas fuelling may initiate H-mode.− Pellet injection can initiate H-mode.

The H-mode can be initiated by changes to the fuelling− Shutting off gas fuelling may initiate H-mode.− Pellet injection can initiate H-mode.

Boronisation and other wall conditioning seem to improve H-mode access. Boronisation and other wall conditioning seem to improve H-mode access.


Wall conditions and impuritiesWall conditions and impurities


Standard shot on ASDEX Upgrade is a good indicator for wall conditionsStandard shot on ASDEX Upgrade is a good indicator for wall conditions

Threshold reduces over time as wall and vacuum conditions improve.

Threshold reduces over time as wall and vacuum conditions improve.

B and Si coatings have no statistical significant effect.

B and Si coatings have no statistical significant effect.

Does not explain the scatter in the database.

Does not explain the scatter in the database.

Change of divertor reduces PLH by 20%

Change of divertor reduces PLH by 20%

F. Ryter et.al. PPCF 44 (2002) p.A407 F. Ryter et.al. PPCF 44 (2002) p.A407

AUG

On smaller devices boronisation usually beneficial – quantify?− MAST, COMPASS,

CMOD?

On smaller devices boronisation usually beneficial – quantify?− MAST, COMPASS,

CMOD?


Low H-mode threshold with tungsten wall in ASDEX UpgradeLow H-mode threshold with tungsten wall in ASDEX Upgrade

Trend for PLH/Pscal to decrease with time: effect of tungsten? – Why? Trend for PLH/Pscal to decrease with time: effect of tungsten? – Why?

0

0.5

1

1.5

0

50

100

1200

0

1400

0

1600

0

1800

0

2000

0

2200

0

2400

0

Pth

/Psc

al0

8T

ungsten PF

C coverage in %

shot

40 boronizations

last

bo

ron

iza

tion

Se

pt

20

09

Scatter only partly correlated with vessel vents and boronisation− Other factors: uncertainty due to PNBI modulation for example.

Scatter only partly correlated with vessel vents and boronisation− Other factors: uncertainty due to PNBI modulation for example.

Different standard shotdue to generator failure

AUG

F. RyterF. Ryter


Li evaporation reduces PLH on NSTXLi evaporation reduces PLH on NSTX

Without Li evaporation: PLH ~ 2.7 MW NBI

− Pheat/ne~0.9 10-19 MW m3

Without Li evaporation: PLH ~ 2.7 MW NBI

− Pheat/ne~0.9 10-19 MW m3

NSTX

With Li evaporation: PLH~ 1.4 MW NBI

− Pheat/ne~0.6 10-19 MW m3

With Li evaporation: PLH~ 1.4 MW NBI

− Pheat/ne~0.6 10-19 MW m3

PLH ne from HHFW expts. PLH ne from HHFW expts.

Normalize PLH by ne due to density differences between plasmas with and without Li evaporation.

Normalize PLH by ne due to density differences between plasmas with and without Li evaporation.

Lithium evaporation produced plasmas with long ELM-free phases.− PNBi from 2 to 6 MW, 200 mg Li between shots.

Lithium evaporation produced plasmas with long ELM-free phases.− PNBi from 2 to 6 MW, 200 mg Li between shots.

S. Kaye, R. Maingi et.al. 12th H-mode

workshop 2009

S. Kaye, R. Maingi et.al. 12th H-mode

workshop 2009


Divertor geometry, plasma shape, recycling and fuelling.

Divertor geometry, plasma shape, recycling and fuelling.


JET

The divertor geometry is importantThe divertor geometry is important

Lower X-point gives lower PLH on JET. Lower X-point gives lower PLH on JET.

JET

Y Andrew et al. PPCF 46 (2004) p.A87Y Andrew et al. PPCF 46 (2004) p.A87

Strike points onStrike points onhorizontal plateshorizontal plates

Strike points onStrike points onvertical platesvertical plates

Effect only visible if strike points are on horizontal targets.

Effect only visible if strike points are on horizontal targets.

Difficult to distinguish effect of recycling from other changes.

Difficult to distinguish effect of recycling from other changes.

Old studies: Lowest PLH when strike point sits on the septum! [Horton, PPCF, 42 (2000) A37]

Old studies: Lowest PLH when strike point sits on the septum! [Horton, PPCF, 42 (2000) A37]

JET

JET


DIII-D also sees lower PLH with lower X-point heightDIII-D also sees lower PLH with lower X-point height

Same effect for He and H with NBI and ECH. – recycling, flows?

Same effect for He and H with NBI and ECH. – recycling, flows? DIII-D

P. Gohil et.al. NF 49 (2009) acceptedP. Gohil et.al. NF 49 (2009) accepted


A shorter outer divertor leg facilitates H-modeA shorter outer divertor leg facilitates H-mode

Repetitive L-mode phases induced by change of the connection length on MAST.− 2 ms < t < 5 ms − Lc > 2 m

Repetitive L-mode phases induced by change of the connection length on MAST.− 2 ms < t < 5 ms − Lc > 2 m

H. Meyer et.al. PPCF 50 (2008) p.53H. Meyer et.al. PPCF 50 (2008) p.53

jjsatsat

. . Loss of bootstrap current pulls leg inwards Loss of bootstrap current pulls leg inwards

Shortening of leg H-mode. Shortening of leg H-mode.

SOLPS calculations indicate a change of the parallel SOL flow with connection length

SOLPS calculations indicate a change of the parallel SOL flow with connection length

MAST

AUG

Does Lc change as well with DIV IIb on AUG?

Does Lc change as well with DIV IIb on AUG?


Also on NSTX indications for higher PLH with long divertor legsAlso on NSTX indications for higher PLH with long divertor legs

SN discharge with low fails to achieve H-mode with PNBI < 6 MW on NSTX

− Mode locking occurs before LH transition.

SN discharge with low fails to achieve H-mode with PNBI < 6 MW on NSTX

− Mode locking occurs before LH transition.

R. Maingi, H Meyer NSTX Research Forum (2007)R. Maingi, H Meyer NSTX Research Forum (2007)

123644 @ 0.28s124657 @ 0.22s

NSTX (H-mode OK) NSTX (no H-mode)

Similar DN discharge has H-mode transition with PNBI < 2 MW.

Similar DN discharge has H-mode transition with PNBI < 2 MW.

Change to lower plasma shape (mostly) allows H-mode access in SN with PNBI < 2 MW as well

Change to lower plasma shape (mostly) allows H-mode access in SN with PNBI < 2 MW as well

NSTX

Effect of X-point height is also observed on Alcator C-MOD

Effect of X-point height is also observed on Alcator C-MOD


t=0.22 sec

Lowest PLH~ 1.1 MW with = 0.4 Lowest PLH~ 1.1 MW with = 0.4

Discharges with higher values have similar PLH~ 2 MW. Discharges with higher values have similar PLH~ 2 MW.

= 0.40 = 0.55 = 0.70

= 0.4

NSTX

R.Maingi et.al. 12th H-mode workshop 2009R.Maingi et.al. 12th H-mode workshop 2009

Lowest divertor leg length does not give lowest PLH – Lc? Lowest divertor leg length does not give lowest PLH – Lc?


Change in ZX () by 6cm improves H-mode on MAST Change in ZX () by 6cm improves H-mode on MAST

At similar PNBI dithering H-mode becomes sustained at higher . – PLH could be 30%.

At similar PNBI dithering H-mode becomes sustained at higher . – PLH could be 30%.

MAST

MAST

MAST

H. Meyer et.al. 12th H-mode workshop 2009H. Meyer et.al. 12th H-mode workshop 2009


The fuelling location can influence the L/H transitionThe fuelling location can influence the L/H transition

Improved H-mode access is observed with high field side (HFS) fuelling on COMPASS-D, MAST and NSTX.

Improved H-mode access is observed with high field side (HFS) fuelling on COMPASS-D, MAST and NSTX.

M. Valovic et.al. PPCF 44 (2002) A175M. Valovic et.al. PPCF 44 (2002) A175

A.R. Field et.al. PPCF 46 (2004) 981A.R. Field et.al. PPCF 46 (2004) 981

MAST

COMPASS-D

Measurements and theory suggest a change of perpendicular flow with gas puff location change in Er

Measurements and theory suggest a change of perpendicular flow with gas puff location change in Er

Lower density limit with HFS fuelling on COMPASS-D Lower density limit with HFS fuelling on COMPASS-D

COMPASS-D


Pellets can initiate H-mode far away from usual H-mode operating regimePellets can initiate H-mode far away from usual H-mode operating regime

Density wave from pellet can trigger H-mode. Density wave from pellet can trigger H-mode.

M. Valovic et.al. J. of Phys.:

Conf. Ser. 123 (2008) 012039

M. Valovic et.al. J. of Phys.:

Conf. Ser. 123 (2008) 012039

P. Gohil et.al PRL 86 (2001) 644P. Gohil et.al PRL 86 (2001) 644

H-mode access far away from usual threshold conditions on DIII-D .

H-mode access far away from usual threshold conditions on DIII-D .

MAST

DIII-D

DIII-D


Plasma flows and the radial electric fieldPlasma flows and the radial electric field


In DN configuration PLH is reducedIn DN configuration PLH is reduced

H. Meyer et.al. NF 46 (2006) 64H. Meyer et.al. NF 46 (2006) 64

H. Meyer et.al. PPCF 47 (2005) 843H. Meyer et.al. PPCF 47 (2005) 843

AUGMAST


Different L-mode Er in U-SN, C-DN, and L-SNDifferent L-mode Er in U-SN, C-DN, and L-SN

In L-mode: No change of Te, ne, and Ti

between DN, LSN, and USN. Inboard SOL decay length shorter

in DN. Most negative Er with rsep 0 mm .

In L-mode: No change of Te, ne, and Ti

between DN, LSN, and USN. Inboard SOL decay length shorter

in DN. Most negative Er with rsep 0 mm .

L-mode

MASTMAST

AUG

L-mode

EErr

Lowest EB shear in USN in AUG, but not in MAST.

Lowest EB shear in USN in AUG, but not in MAST.

New measurements on MAST indicate DN field not always lower than L-SN field.

New measurements on MAST indicate DN field not always lower than L-SN field.

H. Meyer et.al. NF 46 (2006) 64H. Meyer et.al. NF 46 (2006) 64

H. Meyer et.al. PPCF 47 (2005) 843H. Meyer et.al. PPCF 47 (2005) 843

Er ¼ -1 kV/m between DN and LSN on both devices.

Er ¼ -1 kV/m between DN and LSN on both devices.

SOLPS: Changes due to changes in SOL flow.

SOLPS: Changes due to changes in SOL flow.


L/H transition at same SOL flowL/H transition at same SOL flow

B.LaBombard et.al. NF 44 (2004) p.1047B.LaBombard et.al. NF 44 (2004) p.1047

CMOD

CMOD


Lower PLH if SOL flow augments negative Er Lower PLH if SOL flow augments negative Er

SOL flow changes sign between USN and LSN independent of ion B-drift direction.− Same flow direction contributes positively or

negatively to Er depending on the Ip direction.

SOL flow changes sign between USN and LSN independent of ion B-drift direction.− Same flow direction contributes positively or

negatively to Er depending on the Ip direction.

PLH factor 2-3 higher with unfavourable ion B-drift direction.

PLH factor 2-3 higher with unfavourable ion B-drift direction.

Results are consistent with USN/LSN comparison with respect to a critical Er

Results are consistent with USN/LSN comparison with respect to a critical Er

A. Hubbard PoP 14 (2007) 056109A. Hubbard PoP 14 (2007) 056109

CMOD

CMOD


No change of Er during power scan towards LH transition on MASTNo change of Er during power scan towards LH transition on MAST

“L-mode” Er shows considerable shear “L-mode” Er shows considerable shear

Measurement period

MAST

MASTH. Meyer et.al. 12th H-mode workshop 2009H. Meyer et.al. 12th H-mode workshop 2009

Measurements can’t resolve dithers Measurements can’t resolve dithers


Neither Te nor Er changes significantly with Neither Te nor Er changes significantly with

High has marginally deeper Er well, but no change in Er in L-mode.

High has marginally deeper Er well, but no change in Er in L-mode.

MAST

MAST

Measurements of Te at R-Rsep= -4cm don’t support critical Te

Measurements of Te at R-Rsep= -4cm don’t support critical Te

H. Meyer et.al. 12th H-mode workshop 2009H. Meyer et.al. 12th H-mode workshop 2009


On COMPASS-D no change of v prior to the H-modeOn COMPASS-D no change of v prior to the H-mode

Poloidal velocity is a marker for Er

−Measurement on He+ ions.

Poloidal velocity is a marker for Er

−Measurement on He+ ions.

No change within 0.1ms before the transition.

No change within 0.1ms before the transition.

How fast can the velocity change?−ion-ion collision time kin.−Bounce time for trapped particles NEO.

−In COMPASS-D kin NEO 36 s.

How fast can the velocity change?−ion-ion collision time kin.−Bounce time for trapped particles NEO.

−In COMPASS-D kin NEO 36 s.

How fast do we need to measure? How fast do we need to measure?

Bootstrapping into the H-mode.−Initiated by plasma potential

fluctuations?

Bootstrapping into the H-mode.−Initiated by plasma potential

fluctuations?


As net torque input is reduced the geodaesic acoustic mode (GAM) amplitude decreases

As net torque input is reduced the geodaesic acoustic mode (GAM) amplitude decreases

GAM vanishes and ZF grows as L/H transition is approachedGAM vanishes and ZF grows as L/H transition is approached

DIII-DDIII-D

G.R. McKee NF 49 (2009) 115016G.R. McKee NF 49 (2009) 115016

r/a = 0.9

co-injection phase

balanced injection

GAM is replaced by low frequency zonal flow. GAM is replaced by low frequency zonal flow.

Shear in ZF may be the trigger of the H-mode Shear in ZF may be the trigger of the H-mode GAM

ZF


No change in Er observed prior to L/H transition on TJ-IINo change in Er observed prior to L/H transition on TJ-II

Fast measurements using Doppler reflectometry

Fast measurements using Doppler reflectometry

T. Estrada et.al NF 50 (2009) in printT. Estrada et.al NF 50 (2009) in print

Reduction in ne fluctuations precedes increase of Er

Reduction in ne fluctuations precedes increase of Er

Reduction of ne fluctuations simultaneous with increase of low frequency fluctuations of sheared flow

Reduction of ne fluctuations simultaneous with increase of low frequency fluctuations of sheared flow

TJ-II

TJ-II


3D effects – stellarators and RMPs (MHD, ripple)3D effects – stellarators and RMPs (MHD, ripple)


Stellarators may have a lower PLH than tokamaksStellarators may have a lower PLH than tokamaks

H-mode can be accessed on W7-AS with powers well below the scaling.− Large R/a low poloidal flow damping.− With 5/9 island divertor.

H-mode can be accessed on W7-AS with powers well below the scaling.− Large R/a low poloidal flow damping.− With 5/9 island divertor.

W7-ASW7-AS

F. Wagner et.al PPCF 46 (2006) A217F. Wagner et.al PPCF 46 (2006) A217

F. Wagner et.al PoP 12 (2005) 072509F. Wagner et.al PoP 12 (2005) 072509

CHS (Heliotron) has a higher PLH than the tokamak scaling.

CHS (Heliotron) has a higher PLH than the tokamak scaling.


n=1 error field delays LH transition on MASTn=1 error field delays LH transition on MAST

JET: PLH not affected by “resonant” n=1 perturbation or TF ripple. JET: PLH not affected by “resonant” n=1 perturbation or TF ripple.

Increasing error field B

Strong n=2 mode

DIII-D: No effect on PLH observed with resonant n=3 perturbation in D. DIII-D: No effect on PLH observed with resonant n=3 perturbation in D.

A. Kirk 2nd Transport Topical Group Workshop

(2009)

A. Kirk 2nd Transport Topical Group Workshop

(2009)

MAST

DIII-D: PLH increases with n=1 perturbation and in He with n=3 with q~3.6, but not with q=4.3. DIII-D: PLH increases with n=1 perturbation and in He with n=3 with q~3.6, but not with q=4.3.


Application of n=3 Fields Results in Significantly Higher PLHApplication of n=3 Fields Results in Significantly Higher PLH

PLH increases from ~1.4 to 2.6 MW with higher n=3 current (~65% increase for PLH/ne) PLH increases from ~1.4 to 2.6 MW with higher n=3 current (~65% increase for PLH/ne)

Motivated by JET ripple, DIII-D torque scan results.

Motivated by JET ripple, DIII-D torque scan results.

Apply n=3 braking to test effect on threshold power (non-resonant)− Braking applied prior to L-H

transition

Apply n=3 braking to test effect on threshold power (non-resonant)− Braking applied prior to L-H

transition

Found PLH/ne significantly higher with higher applied n=3

Found PLH/ne significantly higher with higher applied n=3

NSTX

S. Kaye, R. Maingi et.al. 12th H-mode workshop 2009S. Kaye, R. Maingi et.al. 12th H-mode workshop 2009


LH transition can be suppressed by n=2 error field on MASTLH transition can be suppressed by n=2 error field on MAST

Effect only observed if perturbation current is in the “wrong” direction interaction with EF? Effect only observed if perturbation current is in the “wrong” direction interaction with EF?

A. Kirk 2nd Transport Topical Group Workshop (2009)A. Kirk 2nd Transport Topical Group Workshop (2009)

Locked mode

MAST


DiscussionDiscussion


Open questionsOpen questions

What is needed experimentally to quantify the effect of wall/machine conditions?− Is it simply a matter of impurities in the edge, or has it to do with recycling? – or both?

What is needed experimentally to quantify the effect of wall/machine conditions?− Is it simply a matter of impurities in the edge, or has it to do with recycling? – or both?

The divertor geometry, ion-B drift direction and magnetic configuration (DN/SN) have profound repeatable effects on PLH. – What experimental/theoretical understanding is there to explain this?

− Are SOL flows affecting Er or changes to the recycling the answer? – or both?

The divertor geometry, ion-B drift direction and magnetic configuration (DN/SN) have profound repeatable effects on PLH. – What experimental/theoretical understanding is there to explain this?

− Are SOL flows affecting Er or changes to the recycling the answer? – or both?

Are low frequency fluctuations of Er the “hidden” variable? – What do we need for experimental proof?

Are low frequency fluctuations of Er the “hidden” variable? – What do we need for experimental proof?

What is needed to understand the effect of 3D physics on PLH?

− Are only non-resonant perturbations important? – What happens to flows and ZF?− Is there a concern for ITER?

What is needed to understand the effect of 3D physics on PLH?

− Are only non-resonant perturbations important? – What happens to flows and ZF?− Is there a concern for ITER?


Thank you for your attention!Thank you for your attention!


Postulate: EB shearing rate needs to exceed turbulence growth ratePostulate: EB shearing rate needs to exceed turbulence growth rate

Condition for existence of H-mode, but …−What generates the flow shear zonal flow (ZF)?−Causality: Is the flow shear also the trigger for H-mode?

Condition for existence of H-mode, but …−What generates the flow shear zonal flow (ZF)?−Causality: Is the flow shear also the trigger for H-mode?

RB

E

B

RB rBEturb

2

RB

E

B

RB rBEturb

2

DIII-DDIII-D

S. Coda et.al. EPS (1997)S. Coda et.al. EPS (1997)

Does the flow shear change prior to the transition or not?−How is the flow shear maintained (restored)?

Does the flow shear change prior to the transition or not?−How is the flow shear maintained (restored)?

What is the role of the background flow compared to n,m=0 low frequency velocity fluctuations, zonal flows?

What is the role of the background flow compared to n,m=0 low frequency velocity fluctuations, zonal flows?


Gas puffing can remove the Er shearGas puffing can remove the Er shear

Back transition to L-mode when shear is removed. LFS gas puff on MAST can inhibit H-mode.

Back transition to L-mode when shear is removed. LFS gas puff on MAST can inhibit H-mode.

H. Meyer et.al. Czech. Journ. Phys. 50 (20060)

p.1451

H. Meyer et.al. Czech. Journ. Phys. 50 (20060)

p.1451

COMPASS-DCOMPASS-D

H-modeH-modeGas onGas on

L-modeL-modeGas offGas off

H- to L transition

L- to H transition


Er is influenced by scrape-off layer flowsEr is influenced by scrape-off layer flows

Fluid modelling shows Er is changed by changes in the SOL flow pattern. Fluid modelling shows Er is changed by changes in the SOL flow pattern.

[V. Rozhansky et.al. EPS (2005)][V. Rozhansky et.al. EPS (2005)]

BSOLPS5.0BSOLPS5.0

For AUG For MAST

Qualitative agreement with the experiment. Qualitative agreement with the experiment.

V. Rozhansky et.al. PPCF 48 (2006) 1475V. Rozhansky et.al. PPCF 48 (2006) 1475

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