ITER needs for power threshold to achieve good H-mode

1. ITER needs for power threshold to achieve good H-mode R Sartori

2. Outline • This presentation is based mainly on JET results+ ASDEX Upgrade results presented at this H-mode workshop (F Ryter) • What is good confinement in this context (power requirements for ITER) • Operational space for Type III ELMs • Power requirements for Type I ELMy H-modes

3. What is good confinement? ASDEX Upgrade data- F Ryter, H-mode workshop 2007 • “Good” confinement means  highest likelihood to achieve H98=1 • H98=1 is more likely in H-modes Type I ELMs than with Type III ELMs • Type III ELMs have on average lower confinement (H98~0.8)

4. What is good confinement? R Sartori PPCF 2004 G Saibene PPCF 2002 • ELM Type (i.e.Type I ELMs vs Type III ELMs) is the only key parameter in this context • Confinement can be optimised in other ways (e.g triangularity) or depends on other variables (density) • Type III ELMs follow similar trends as Type I ELMs but with overall lower H98

5. What is good confinement? Summary • ITER standard scenario requires H98=1  requires “Type I ELMy like” confinement • Confinement scaling laws are derived form a database dominated by Type I ELMy H-modes • Most devices also observe H-modes with Type III ELMs • H98 is lower in Type III ELMy H-modes in JET and ASDEX Upgrade •  • Is H98 overall lower with in H-modes Type III ELMs also in other devices? •  • In which conditions the H-mode has Type III ELMs (operational space)? • Is there an additional power requirement above L-H threshold power for transition to Type I ELMy regime ?

6. JET: Type III ELM operational space Boundary between Type III and Type I ELMy H-modes in pedestal ne-Te R Sartori PPCF 2004 ELMy H-mode, power scan ELMy H-mode, power scan Plasma with ITB Type III Teped ~ 1/nped at low density/collisionality Type I

7. JET: Type III ELM operational space Boundary between Type III and Type I ELMy H-modes in pedestal ne-Te ELMy H-mode, density scan L Horton, PPCF 1999 JET Te~ constant at high density/collisionality

8. JET: Type III ELM operational space Boundary between Type III and Type I ELMy H-modes in pedestal ne-Te Compound ELMs Interval of power exists where Type I and Type III ELMs coexist  compound ELMs Type I to III  Degraded confinement  loss of density

9. JET: Are all Type III ELMs the same? • Low and high density Type III ELMs • Common experimental observations • Same ELM frequency dependence on power ! • H factor degraded compared to Type I ELMs • Smaller ELM size than Type I ELMs • Lower power above the L-H threshold power • Effect of isotopic mass • Experimentally observed differences Low density  increase of density at constant power triggers Type III to I transition • Low density  Ip ramp down triggers Type III to I transition • Low density  confinement degradation is due to loss of density • High density  effect of collisionality • High density  confinement degradation mainly due to loss of temperature

10. JET, ASDEX: Collisionality JET ASDEX-U Sartori, IAEA 2004 F Ryter, H-mode Workshop 2007 Model based on resistive ballooning instability Type III ELMs operational space depends on collisionality? Low density behaviour of critical temperature (JET) suggests also a beta dependence JET Chankin, Saibene, PPCF 1999

11. JET, ASDEX: Normalised beta D McDonald, PPCF 2004 In JET Type III ELMs operational space is separated from Type I ELMs in normalised beta more than in ASDEX Upgrade

12. JET: Type III-Type I ELM threshold MarkII GB MarkII A Type I to Type III power threshold follows L-H like threshold scaling  Ip /density, Bt (and mass) dependence • PIN PL-H, with  ranging from ~1.3 to ~2.5 required for Type I ELMy H-modes . Value of  changes with triangularity (), density()/collisionality() • No scaling exists. No physics reason links the L-H and Type I threshold

13. JET: Type III-Type I ELM threshold D:T D:D D:T PIN>2.5 PL-H for low triangularity ne/nG=0.5(20% radiation, 40% dW/dt between ELMs) NTM limited for q95<2.4 Power required for transition to Type I ELMy H-mode decreased proportionally to isotope mass 4.5 MA/3.45T

14. Type III-Type I ELM threshold Summary • Is additional power above the threshold power required for Type I ELMy H-modes? • I think that there is no disagreement between JET and ASDEX- U results • JET Type-I ELMs requires powerlarger than ~1.3 to ~2.5 PL-H. Sufficient condition requires P> 2.5PL-H, but lower values are also possible • ASDEX Upgrade  this statement (JET) is sufficient (in ASDEX-U), but is not necessary, as Type-I ELMs also exist at lower values of P/PL-H. • It is possible to find lower values of P/PL-H required for Type I ELMs, but • how often ? • in which conditions? • (In JET it is possible to obtain H=1 at ne/nG=1…..) • Conditions required to achieve Type I ELMs with low P/PL-H in ITER need to be specified, understood and extrapolated from present data.

15. Type I/III transition: achievable density It is not always easy to achieve high density with good confinement. Increased power affects this behaviour?

16. Summary • Confinement • Both in JET and ASDEX Upgrade the confinement is statistically lower (~20%) in H-modes with Type III ELMs than in H-modes with Type I ELMs • Operational space • JET Type III ELMs at low and high collisionality • ASDEX Type III ELMS at high collisionality • No full understanding of physics or scaling of domain of existence for Type III ELMs • Power requirements • Which (if any) power above the threshold power for ITER? In JET the requirement P> 1.5PL-H is common and not conservative. And, for whichever reason, most machines do operate above this level. ASDEX-U? Other machines? • Density • Is there any link between the density that can be achieved with Type I ELM confinement and power requirements?

17. DD operation in ITER Access to H mode in DD at full field and current could be marginal

18. Future experiments 1- Dedicated experiments in each machine, for example variation of Bt, Ip, n to determine power required for Type I ELMs to clarify relation with L-H threshold if any Requires: Quasi steady phases, clear ELM classification, L-H threshold determination 2- Combined threshold/confinement experiment with N and  scans 3-Inter machine experiments

19. Proposed JET experiment bN Total number of discharges = 31 (q95 ~ 2.7-3) Push to highest bN in unfuelled conditions 2.2 Select Ip so that bN = 1.8 at ne = 0.7 nGW and explore ne range (4 levels) 2.0 Keep bN and explore ne range (3 levels) + exact n* match 1.8 Get discharge with Type I ELMs and best H-factor, bN and explore ne range (4 levels) 1.6 Keep bN and explore ne range (3 levels) + exact n* match 1.4 I1= 2.3 MA I2= 3.4 MA I3= 3.7-4.0 MA Ip(MA) 2.5 3.0 3.5 4.0 4.5

20. Confinement studies: dimensionless scaling  power requirements L-H/Type I threshold scaling Gyro-Bohm scaling Which loss power is required to keep the non-dimensional parameters  and * constant as * is decreased? G Petty, T Luce, NF 1997 If L-H or Type I threshold scaling has stronger negative * scaling than gyro-Bohm  dimensionally similar path could change to follow the L-H/Type I scaling instead of gyro-Bohm like scaling  increased power is required.

21. Type III-Type I ELM threshold MarkII A MarkII GB At low density  increase in density decreases the power threshold for Type I ELMs. Consistent with pedestal ne-Te boundary

22. Type III-Type I ELM threshold MarkII GB At low density  Ip ramp down at constant power produces transition to Type I ELMs (and Ip ramp up transition to Type III ELMs)

23. L Horton, PPCF1999 ASDEX-U- Pressure gradient with Type III ELMs can be as high as with Type I ELMs, but pedestal T higher with Type I ELMs

24. DIII-D, Osborne, EPS 1997 Type III ELMs at low density disappear above a critical pressure gradient which scales as Ip2