1. High  p experiments in JET and access to Type II/grassy ELMs G Saibene and JET TF S1 and TF S2 contributors Special thanks to to Drs Y Kamada and N Oyama (JAERI-Japan)

2. Scope of JET small ELM experiments Obtain plasmas with: –High confinement (H 98 ~1) with steady state core/edge –Compatible with high density (n~0.8 n GR or more) –Acceptable ELM size (projected to ITER) Max loss to divertor ~4MJ in ITER  3-5% W ped  ELM losses 5-10% W ped –Identify access conditions & potential for extrapolation Compare JET results other experiments  Asdex-U Type II ELMs and JT-60U “grassy” ELMs regimes

3. Small ELM regimes in Tokamaks: Type II ELMs in ASDEX-Upgrade Asdex-U: Type II ELMs found in standard H-modes –Quasi Double Null Configuration (QDN) [J Stober, NF 2001] –Favored by high shaping (  ~0.4) and q (q 95 >4) –High density (n ped >70%n GR, * ped ~1-2),  N ~1.8,  p <1 –Also obtained at high P in /  p – very high n ( *~0.8?) + high  QDN –Change of MHD stability  high n peeling/ballooning  =0.33,  =2.3 50%n gr  =0.43,  =3.2 83%n gr  =0.43,  =3.5 88%n gr [Sips, PPCF 2002]

4. Small ELM regimes in Tokamaks: Grassy ELMS – JT-60U JT-60U “grassy” ELMs: [Y Kamada PPCF 2002] –Threshold in  p (  p >1.7) –High  (~0.4  ~0.6)/ high q 95 (6.5  4) –H-mode edge + ITB & Low density (n<0.5n GR ) - * ped ~0.1 –Strong Shafranov shift stabilizes Type I ELMs  access to second stability

5. JET Type II Studies [G Saibene EPS 2003, CP Perez PPCF 2004] Mixed Type I-II ELMs obtained in SN (& QDN) plasmas (  ~0.45-0.5) at q 95 <4 and  N ~ 2. H 98 ~1, n/n Gr ~1 Definition: No type I ELMs + Increase of inter-ELM power losses  enhanced broadband fluctuations in magnetics and density (WB)  T ped clamped, n ped raise reduced Type IType I-II n ped,min ~ 70% n gr

6. JET: QDN and q 95 Contrary to AUG results: high q 95 reduces/closes access to mixed Type I-II regimes – QDN has no significant effect  p ~ 0.7 – 0.8

7. JET: QDN and q 95 (2) q 95  : Type I  III transition at low n ped – no Type II q 95  : average edge refuelling rate increases – not understood (both SN and QDN)

8. JET/AUG: is identity + QDN geometry the key to Type II ELMy H-modes? At identity parms: n ped & T ped = constant! ( ped *~2)- H 98 ~1 Low P in : slow density peaking  radiative collapse Increasing P in  Type I ELMs + steady state plasma core Increasing I p /B t at constant q  operational space for Type II ELMs closed between L-H transition and Type I-III H-mode regime. Type II  WB modes at ~10kHz + n fluctuations No Type I ELMS, & p ped =constant

9. Grassy ELMs: High  p H-modes High  configuration, QDN (  sep <1cm) – standard H- mode scenario (l i ~1.1) – 1.5-1.2MA/2.7T for high  p  Results: –H 98 ~1.2, n~0.9n Gr and “grassy-like” ELMs obtained (q 95 ~6.8, the only value explored so far) –Grassy ELMs are very small and irregular in size (H  ) and frequency (high) What makes these ELM small? (at high p ped ) –High  p  Shafranov shift stabilisation  grassy ELMs? Comparison with standard ELMy H-modes at low  p Comparison with high  p, low l i H-modes (q o >2, l i ~0.7, some with weak ion ITB)

10. Overview of  p scan (high l i ) Standard Type I ELM activity up to  p ~1.5 with H 98 ~1 From  p ~1.6-1.7, regular H  bursts disappear completely (H 98 ~1.2) Irregular “grassy” H  signature

11. High  p (high l i ): MHD bursty activity (low frequency only) Grassy ELMs: small MHD bursts at low frequency, no washboard modes 62413 -  p ~1.9 62406 -  p ~1.35 Type I: “Standard” MHD spectrum at lower  p (broadband ELM signature + wb modes inter-ELM) Grassy ELMs MHD signature similar to Type I ELMs but MHD bursts extend very little in frequency

12. MHD spectra with Grassy ELMs ASDEX-upgrade JET [J Stober, IAEA 2004]

13.  p scan in low l i H-modes - shapes Early heating scenario – no sawteeth (q o ~2) – l i ~0.8-0.85 Plasma shape:  and  ~ high l i H-modes, but SN (note that  sep <1cm but 2 nd x-point is not in vacuum) SN low li – QDN high li  p ~1.9 for both plasmas

14.  p scan in low l i H-modes – overview of results  pi increased from ~1.0 to 1.9  Type I ELMs observed up to the highest  p

15. Global parameters comparisons at high  p Global confinement similar at high  p Grassy ELM onset  confinement and p ped are not degraded (cfr Type III ELMs) High-l i, high  p : higher pedestal collisionality for similar p ped Lower limit for * for grassy ELMs existence not explored

16. ELM losses of grassy ELMs Grassy ELMs: –ELM energy losses <5% W ped (~15% for low l i high  p ) –  n/n ped -  T/T ped and  W/W ped below typical H-mode values

17. ELM affected depth – reference Low  p H-modes (  p <1): Type I ELM affected depth (L ELM ) unchanged with ELM size (n and q 95 ) - Depth smaller only for Type III ELMs Low  H-modes – n scanLow  H-modes – q 95 scan [Loarte PPCF 2002, PoP 2004]

18. ELM affected depth – high  p High  p Grassy ELMs  reduction of L ELM  Change of MHD? Correlation of L ELM with  p not observed for the low l i H-modes, High  p /high l i H-modesHigh  p /low l i H-modes

19. J edge and Grassy ELM onset Link between Grassy ELMs and high  p ? –Pedestal stabilisation by Shafranov shift (  s )  both high & low l i have similar  s –The current profile is much broader in the low l i pulses, for the same  p Higher edge current (or lower shear) at low l i may change the pedestal MHD stability  Type I Collisionality? High l i  *~0.4, Low l i  *~0.2 (JT-60U *~0.1) Caveat: high l i equilibrium near to DN, the low l i are pure SN

20.  p – */q operational space AUG sim – Type II q 95 =4.2 QDN – SN Type I-II q 95 =3 – 3.6 QDN grassy, high  p – q 95 =6.7 SN high  p Type I – q 95 =7.7

21. AUG high  p (q 95 ~6.3)  SN vs QDN ASDEX-upgrade equilibria – H-modes with  p ~2

22. AUG: “grassy” ELMs favored by QDN [J Stober, IAEA 2004] SN mixed Type I-Grassy SN QDN pure Grassy ELMs * ped similar for SN and QDN

23. Conclusions (1) High plasma shaping ( , , QDN)  common element to all small ELM experiments in JET: At low  p, mixed Type I-II ELMs are observed in SN and QDN – increasing q 95 closes off access to high n ped and no Type II ELMs. Type II: MHD/n broadband fluctuations: WB modes  increased transport High  p : “threshold” similar to JT-60U, but –Grassy ELMs in QDN – high n ped ( *~0.4) – high l i –Type I ELMs in SN, lower n ped ( *~0.2) – lower l i. Asdex-U results (improved H-modes and, more recently, high  p H-modes)  “continuum” between the two type of small ELMs ( *, but MHD?) with QDN still essential.

24. Conclusions (2) Role of QDN to obtain steady state Type II ELM pedestal: –Type II ELMs obtained in JET in an identity (= high  QDN) with Asdex-U: Type II ELM phases correlated to enhanced MHD (and n) fluctuations (with *~2). –Enhanced particle losses obtained – power losses still rather weak (P in effect) –Increasing P in or I p /B t  Type I ELMs come back  –High * and/or low T ped (resistive MHD) may be necessary to Type II ELMs onset and total Type I ELM suppression. –Insight in ELM physics (in particular role of magnetic geometry) but no direct extrapolability to hot plasmas.

25. Conclusions (3) “ Grassy” ELMs obtained at high  p –ELMs with low energy losses obtained in high  p H-modes – with H 98 ~1.2 and n/n Gr ~0.9 (demonstrated at q 95 ~7) –The reduction of ELM size correlated to shrinking of the ELM affected depth: change in MHD unstable modes? –In JET, high  p is not sufficient to obtain grassy ELMs The operational space for Grassy ELMs still to be explored –Is high q 95 a necessary condition? And QDN shape? –Low edge current/high shear required for Grassy ELM onset? –Difference in *: does it explain the low vs high l i difference in ELM behaviour observed in JET? Future work: –(Higher I p ) experiments at low *! as well as QDN  SN – explore lower q 95 scenarios and systematically investigate role of l i.

26. Type II ELM  Increased transport and MHD turbulence When pedestal ~identity (62430)  long Type II ELM phase associated with increased MHD turbulence (low frequency) n fluctuations up as well – Similar to Asdex-Upgrade Core MHD Type II ELMs (62430) Type I ELMs (62428)