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 transition a discussion Hendrik Meyer 6 th October 2009 Hendrik Meyer 6 th October 2009

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 2 Contents What is a “hidden” variable? – Try to define what we don’t know! Wall conditions and impurities Divertor geometry, plasma shape, recycling and fuelling 3D effects – stellarators and RMPs (MHD, ripple) Plasma flows and the radial electric field Discussion

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 3 What is a “hidden” variable? – Try to define what we don’t know!  Concept of the “hidden” variable arose from the scatter of P LH with respect to the power threshold scaling and the variability of P LH 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. Conventional:More comprehensive: Also I p impurities Trapped particles  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?

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 4 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, , ITPA Pedestal Topical Group meeting, PPPL (USA) 5 The known knowns – there are not that many  There is evidence for a critical T e (B) or  T e (B) − Often hard to distinguish experimentally.  There is evidence for a critical T e (B) or  T e (B) − Often hard to distinguish experimentally.  The threshold power indicates that thermodynamic processes are important: P LH / (n S)  B (approx.)  This may be a critical T i (B) or  T i (B) DIII-D R.Groebner, PoP 8 (2001) 2272 At L/H transition JET Y.Andrew, PPCF 46 (2004) 337  AUG/JET: Transition at similar *,  *, , but different absolute T  No atomic processes.  AUG/CMOD: Transition not at similar *,  *,  atomic processes?

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 6 More known knowns – but maybe less well known?  H-mode can be initiated by a sheared E  B flow   E r changes before  n − Sufficient condition, but is it necessary?  see later.  H-mode can be initiated by a sheared E  B flow   E r changes before  n − Sufficient condition, but is it necessary?  see later. DIII-D K.H. Burrell et.al. PPCF 34 (1992) p.1859 Interval of measurement DIII-D R. Weynants et.al. PPCF 40 (1998) p.635

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 7 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 in limiter discharges is higher than in divertor discharges – or is it?  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.

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 8 Wall conditions and impurities

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 9 Standard shot on ASDEX Upgrade is a good indicator for wall conditions  Threshold reduces over time as wall and vacuum conditions improve.  B and Si coatings have no statistical significant effect.  Does not explain the scatter in the database.  Change of divertor reduces P LH by 20% 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, , ITPA Pedestal Topical Group meeting, PPPL (USA) 10 Low H-mode threshold with tungsten wall in ASDEX Upgrade  Trend for P LH /P scal to decrease with time: effect of tungsten? – Why?  Scatter only partly correlated with vessel vents and boronisation − Other factors: uncertainty due to P NBI modulation for example.  Scatter only partly correlated with vessel vents and boronisation − Other factors: uncertainty due to P NBI modulation for example. Different standard shot due to generator failure AUG F. Ryter

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 11 Li evaporation reduces P LH on NSTX Without Li evaporation:  P LH ~ 2.7 MW NBI − P heat /n e ~ MW m 3 Without Li evaporation:  P LH ~ 2.7 MW NBI − P heat /n e ~ MW m 3 NSTX With Li evaporation:  P LH ~ 1.4 MW NBI − P heat /n e ~ MW m 3 With Li evaporation:  P LH ~ 1.4 MW NBI − P heat /n e ~ MW m 3  P LH  n e from HHFW expts.  Normalize P LH by n e due to density differences between plasmas with and without Li evaporation.  Lithium evaporation produced plasmas with long ELM-free phases. − P NBi from 2 to 6 MW, 200 mg Li between shots.  Lithium evaporation produced plasmas with long ELM-free phases. − P NBi from 2 to 6 MW, 200 mg Li between shots. S. Kaye, R. Maingi et.al. 12 th H-mode workshop 2009

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 12 Divertor geometry, plasma shape, recycling and fuelling.

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 13 JET The divertor geometry is important  Lower X-point gives lower P LH on JET. JET Y Andrew et al. PPCF 46 (2004) p.A87 Strike points on horizontal plates Strike points on vertical plates  Effect only visible if strike points are on horizontal targets.  Difficult to distinguish effect of recycling from other changes.  Old studies: Lowest P LH when strike point sits on the septum! [Horton, PPCF, 42 (2000) A37] JET

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 14 DIII-D also sees lower P LH with lower X-point height  Same effect for He and H with NBI and ECH. – recycling, flows? DIII-D P. Gohil et.al. NF 49 (2009) accepted

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 15 A 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 −  L c > 2 m  Repetitive L-mode phases induced by change of the connection length on MAST. − 2 ms <  t < 5 ms −  L c > 2 m H. Meyer et.al. PPCF 50 (2008) p.53 j sat ....  Loss of bootstrap current pulls leg inwards  Shortening of leg  H-mode.  SOLPS calculations indicate a change of the parallel SOL flow with connection length MAST AUG  Does L c change as well with DIV IIb on AUG?

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 16 Also on NSTX indications for higher P LH with long divertor legs  SN discharge with low  fails to achieve H-mode with P NBI < 6 MW on NSTX − Mode locking occurs before LH transition.  SN discharge with low  fails to achieve H-mode with P NBI < 6 MW on NSTX − Mode locking occurs before LH transition. R. Maingi, H Meyer NSTX Research Forum (2007) 0.28s 0.22s NSTX (H-mode OK) NSTX (no H-mode)  Similar DN discharge has H-mode transition with P NBI < 2 MW.  Change to lower plasma shape (mostly) allows H- mode access in SN with P NBI < 2 MW as well NSTX  Effect of X-point height is also observed on Alcator C-MOD

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 17 t=0.22 sec Lowest P LH ~ 1.1 MW with  = 0.4  Discharges with higher  values have similar P LH ~ 2 MW.  = 0.40  = 0.55  = 0.70  = 0.4 NSTX R.Maingi et.al. 12 th H-mode workshop 2009  Lowest divertor leg length does not give lowest P LH –  L c ?

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 18 Change in Z X (  ) by 6cm improves H-mode on MAST  At similar P NBI dithering H-mode becomes sustained at higher . –  P LH could be 30%. MAST H. Meyer et.al. 12 th H-mode workshop 2009

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 19 The 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. M. Valovic et.al. PPCF 44 (2002) A175 A.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 E r  Lower density limit with HFS fuelling on COMPASS-D COMPASS-D

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 20 Pellets can initiate H-mode far away from usual H-mode operating regime  Density wave from pellet can trigger H-mode. M. Valovic et.al. J. of Phys.: Conf. Ser. 123 (2008) P. Gohil et.al PRL 86 (2001) 644  H-mode access far away from usual threshold conditions on DIII-D. MAST DIII-D

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 21 Plasma flows and the radial electric field

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 22 In DN configuration P LH is reduced H. Meyer et.al. NF 46 (2006) 64 H. Meyer et.al. PPCF 47 (2005) 843 AUG MAST

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 23 Different L-mode E r in U-SN, C-DN, and L-SN In L-mode:  No change of T e, n e, and T i between DN, LSN, and USN.  Inboard SOL decay length shorter in DN.  Most negative E r with  r sep  0 mm. In L-mode:  No change of T e, n e, and T i between DN, LSN, and USN.  Inboard SOL decay length shorter in DN.  Most negative E r with  r sep  0 mm. L-mode MAST AUG L-mode ErErErEr  Lowest E  B shear in USN in AUG, but not in MAST.  New measurements on MAST indicate DN field not always lower than L-SN field. H. Meyer et.al. NF 46 (2006) 64 H. Meyer et.al. PPCF 47 (2005) 843   E r ¼ -1 kV/m between DN and LSN on both devices.  SOLPS: Changes due to changes in SOL flow.

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 24 L/H transition at same SOL flow B.LaBombard et.al. NF 44 (2004) p.1047 CMOD

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 25 Lower P LH if SOL flow augments negative E r  SOL flow changes sign between USN and LSN independent of ion  B-drift direction. − Same flow direction contributes positively or negatively to E r depending on the I p direction.  SOL flow changes sign between USN and LSN independent of ion  B-drift direction. − Same flow direction contributes positively or negatively to E r depending on the I p direction.  P LH factor 2-3 higher with unfavourable ion  B- drift direction.  Results are consistent with USN/LSN comparison with respect to a critical  E r A. Hubbard PoP 14 (2007) CMOD

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 26 No change of  E r during power scan towards LH transition on MAST  “L-mode” E r shows considerable shear Measurement period MAST H. Meyer et.al. 12 th H-mode workshop 2009  Measurements can’t resolve dithers

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 27 Neither T e nor  E r changes significantly with   High  has marginally deeper E r well, but no change in  E r in L-mode. MAST  Measurements of T e at R-R sep = -4cm don’t support critical T e H. Meyer et.al. 12 th H-mode workshop 2009

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 28 On COMPASS-D no change of v  prior to the H-mode  Poloidal velocity is a marker for E r − Measurement on He + ions.  Poloidal velocity is a marker for E r − Measurement on He + ions.  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?  Bootstrapping into the H-mode. − Initiated by plasma potential fluctuations?  Bootstrapping into the H-mode. − Initiated by plasma potential fluctuations?

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 29  As net torque input is reduced the geodaesic acoustic mode (GAM) amplitude decreases GAM vanishes and ZF grows as L/H transition is approached DIII-D G.R. McKee NF 49 (2009) r/a = 0.9 co-injection phase balanced injection  GAM is replaced by low frequency zonal flow.  Shear in ZF may be the trigger of the H-mode GAM ZF

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 30 No change in  E r observed prior to L/H transition on TJ-II  Fast measurements using Doppler reflectometry T. Estrada et.al NF 50 (2009) in print  Reduction in n e fluctuations precedes increase of  E r  Reduction of n e fluctuations simultaneous with increase of low frequency fluctuations of sheared flow TJ-II

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 31 3D effects – stellarators and RMPs (MHD, ripple)

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 32 Stellarators may have a lower P LH 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-AS F. Wagner et.al PPCF 46 (2006) A217 F. Wagner et.al PoP 12 (2005)  CHS (Heliotron) has a higher P LH than the tokamak scaling.

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 33 n=1 error field delays LH transition on MAST  JET: P LH not affected by “resonant” n=1 perturbation or TF ripple. Increasing error field B Strong n=2 mode  DIII-D: No effect on P LH observed with resonant n=3 perturbation in D. A. Kirk 2 nd Transport Topical Group Workshop (2009) MAST  DIII-D: P LH 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, , ITPA Pedestal Topical Group meeting, PPPL (USA) 34 Application of n=3 Fields Results in Significantly Higher P LH  P LH increases from ~1.4 to 2.6 MW with higher n=3 current (~65% increase for P LH /n e )  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 P LH /n e significantly higher with higher applied n=3 NSTX S. Kaye, R. Maingi et.al. 12 th H-mode workshop 2009

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 35 LH 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? A. Kirk 2 nd Transport Topical Group Workshop (2009) Locked mode MAST

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 36 Discussion

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 37 Open 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 P LH. – What experimental/theoretical understanding is there to explain this? − Are SOL flows affecting  E r 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 P LH. – What experimental/theoretical understanding is there to explain this? − Are SOL flows affecting  E r or changes to the recycling the answer? – or both?  Are low frequency fluctuations of  E r the “hidden” variable? – What do we need for experimental proof?  What is needed to understand the effect of 3D physics on P LH ? − 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 P LH ? − 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, , ITPA Pedestal Topical Group meeting, PPPL (USA) 38 Thank you for your attention!

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 39 Postulate: E  B 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? DIII-D 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?

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 40 Gas puffing can remove the E r 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 COMPASS-D H-mode Gas on L-mode Gas off H- to L transition L- to H transition

Hendrik Meyer, "Hidden" Variables in the L/H transition, , ITPA Pedestal Topical Group meeting, PPPL (USA) 41 E r is influenced by scrape-off layer flows  Fluid modelling shows E r is changed by changes in the SOL flow pattern. [V. Rozhansky et.al. EPS (2005)] BSOLPS5.0 For AUG For MAST  Qualitative agreement with the experiment. V. Rozhansky et.al. PPCF 48 (2006) 1475