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High b p experiments in JET and access to Type II/grassy ELMs

High b 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). Scope of JET small ELM experiments. Obtain plasmas with: High confinement (H 98 ~1) with steady state core/edge

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High b p experiments in JET and access to Type II/grassy ELMs

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  1. High bp experiments in JETand 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 (H98~1) with steady state core/edge • Compatible with high density (n~0.8 nGR or more) • Acceptable ELM size (projected to ITER) • Max loss to divertor ~4MJ in ITER  3-5% Wped  ELM losses 5-10% Wped • 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 (q95>4) • High density (nped>70%nGR, *ped~1-2), N~1.8, p<1 • Also obtained at high Pin/bp – very high n (n*~0.8?) + high d QDN • Change of MHD stability  high n peeling/ballooning [Sips, PPCF 2002] d=0.43, b=3.5 88%ngr d=0.33, b=2.3 50%ngr d=0.43, b=3.2 83%ngr

  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 q95(6.54) • H-mode edge + ITB & Low density (n<0.5nGR) - *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 q95<4 and N~ 2. H98~1, n/nGr~1 • Definition: No type I ELMs + Increase of inter-ELM power losses enhanced broadband fluctuations in magnetics and density (WB)  Tped clamped, nped raise reduced Type I Type I-II nped,min~ 70% ngr

  6. JET: QDN and q95 • Contrary to AUG results: high q95reduces/closes access to mixed Type I-II regimes – QDNhas no significant effect bp~ 0.7 – 0.8

  7. JET: QDN and q95 (2) • q95: Type I III transition at low nped – no Type II • q95: 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: nped & Tped = constant! (nped*~2)- H98~1 • Low Pin: slow density peaking  radiative collapse • Increasing Pin  Type I ELMs + steady state plasma core No Type I ELMS, & pped=constant • Increasing Ip/Bt at constant q  operational space for Type II ELMsclosed between L-H transitionand Type I-III H-mode regime. • Type II  WB modes at ~10kHz + n fluctuations

  9. Grassy ELMs: High p H-modes • High  configuration, QDN (Dsep <1cm) – standard H-mode scenario (li~1.1) – 1.5-1.2MA/2.7T for high p • Results: • H98~1.2, n~0.9nGr and “grassy-like” ELMs obtained (q95~6.8, 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 pped) • High p Shafranov shift stabilisation  grassy ELMs? • Comparison with standard ELMy H-modes at low p • Comparison with high p, low li H-modes (qo>2, li~0.7, some with weak ion ITB)

  10. Overview of p scan (high li) • Standard Type I ELM activity up to p ~1.5 with H98~1 • From p ~1.6-1.7, regular H bursts disappear completely (H98 ~1.2) • Irregular “grassy” Ha signature

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

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

  13. p scan in low liH-modes - shapes • Early heating scenario – no sawteeth (qo~2) – li~0.8-0.85 • Plasma shape: k and d ~ high li H-modes, but SN (note that Dsep <1cm but 2nd x-point is not in vacuum) SN low li – QDNhigh libp~1.9 for both plasmas

  14. p scan in low liH-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 pped are not degraded (cfr Type III ELMs) • High-li, high bp: higher pedestal collisionality for similar pped • Lower limitfor n* for grassy ELMs existence not explored

  16. ELM losses of grassy ELMs • Grassy ELMs: • ELM energy losses <5% Wped(~15% for low li high p ) • Dn/nped- DT/Tped and DW/Wped below typical H-mode values

  17. Low b H-modes – q95 scan Low b H-modes – n scan ELM affected depth – reference • Low p H-modes (p <1): Type I ELM affected depth (LELM) unchanged with ELM size (n and q95) - Depth smaller only for Type III ELMs [Loarte PPCF 2002, PoP 2004]

  18. ELM affected depth – high bp • High p Grassy ELMs  reduction of LELM Change of MHD? • Correlation of LELM with pnot observed for the low li H-modes, High bp/high li H-modes High bp/low li H-modes

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

  20. bp – n*/q operational space • AUG sim – Type II q95 =4.2 • QDN – SN Type I-II q95=3 – 3.6 • QDN grassy, high bp – q95=6.7 • SN high bp Type I – q95=7.7

  21. AUG high bp (q95~6.3) SN vs QDN • ASDEX-upgrade equilibria – H-modes with bp~2

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

  23. Conclusions (1) • High plasma shaping (d, k, QDN)  common element to all small ELM experiments in JET: • At low bp, mixed Type I-II ELMs are observed in SN and QDN – increasing q95 closes off access to high nped and no Type II ELMs. • Type II: MHD/n broadband fluctuations: WB modes  increased transport • High bp: “threshold” similar to JT-60U, but • Grassy ELMs in QDN – high nped (n*~0.4) – high li • Type I ELMs in SN, lower nped (n*~0.2) – lower li. • Asdex-U results (improved H-modes and, more recently, high bp H-modes)  “continuum” between the two type of small ELMs (n*, 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 d QDN) with Asdex-U: Type II ELM phases correlated to enhanced MHD (and n) fluctuations (with n*~2). • Enhanced particle losses obtained – power losses still rather weak (Pin effect) • Increasing Pin or Ip/Bt Type I ELMs come back  • High n* and/or low Tped (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 bp • ELMs with low energy losses obtained in high p H-modes – with H98~1.2 and n/nGr~0.9 (demonstrated at q95~7) • 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 q95 a necessary condition? And QDN shape? • Low edge current/high shear required for Grassy ELM onset? • Difference in n*: does it explain the low vs high li difference in ELM behaviour observed in JET? • Future work: • (Higher Ip) experiments at low n*! as well as QDN  SN – explore lower q95 scenarios and systematically investigate role of li.

  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 Type I ELMs (62428) Type II ELMs (62430) Core MHD

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