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 “Hidden” Variables in the L/H transition a discussion Hendrik Meyer 6 th October 2009
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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
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 3
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?
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 4
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)
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 5
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
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 7
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.
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 8
Wall conditions and impuritiesWall conditions and impurities
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 9
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?
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 10
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
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 11
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
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 12
Divertor geometry, plasma shape, recycling and fuelling.
Divertor geometry, plasma shape, recycling and fuelling.
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 13
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
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.
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 33
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.
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 34
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
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 35
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
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 36
DiscussionDiscussion
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 37
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?
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 38
Thank you for your attention!Thank you for your attention!
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 39
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?
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 40
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
Hendrik Meyer, "Hidden" Variables in the L/H transition, 07.10.2009, ITPA Pedestal Topical Group meeting, PPPL (USA) 41
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.