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  1. Enhanced Da H-mode on Alcator C-Mod presented by J A Snipes with major contributions fromM Greenwald, A E Hubbard, D Mossessian,and the Alcator C-Mod Group MIT Plasma Science and Fusion Center Cambridge, MA 02139 USA Seminar IPP GarchingGarching, Germany7 May 2002

  2. Global Features of EDA H-Mode • EDA H-modes have: • Good energy confinement H89 ~ 2 • Low particle confinement no impurity accumulation • Low radiated power • No large ELMs • Steady State (>8tE) • Obtainedwith Ohmic or ICRF heating, 1 < PRF< 5 MW • Highly attractive reactor regime (no ELM erosion) • Similar to LPCH-mode (JET) and type II ELM regimes A. Hubbard

  3. Temperature and Density Profiles in EDA H-mode • Steep edge temperature and density gradients • Moderately peaked temperature profile• Flat density profile

  4. Quasi-Coherent Signature of EDA H-mode • Enhanced D emission in EDA H-mode • f ~100 kHz Quasi-Coherent density and magnetic fluctuations always found in EDA H-mode in the steep gradient edge • QC mode well correlated with reduced particle and impurity confinement • No large Type I ELMs found on C-Mod • Only small irregular ELMs sometimes found on top of the enhanced D emission M. Greenwald

  5. Edge Pedestal and Fluctuation Diagnostics A. Hubbard

  6. Quasi-Coherent Mode seen in Density Fluctuations in EDA H-modes • Quasi-coherent edge mode always associated with EDA H-Mode • After brief ELM-free period (~20 msec), mode appears • Frequency in lab frame decreases after onset ( ~100 kHz in steady state) • change in poloidal rotation • Reflectometer localizes mode to density pedestal Y. Lin

  7. Phase Contrast Imaging measures kR ~ 6 cm-1 (l~1 cm) • PCI measures k radially at top and bottom of plasma. • for typical equilibria • Frequency range 60-250 kHz • Width DF/F ~ 0.05-0.2 A. Mazurenko

  8. Steady Edge Pedestals in EDA • EDA pedestal characterized by steep pressure gradients • Pedestal parameters obtained from tanh fit to measured Thomson scattering profiles • Moderate pedestal Te (< 500 eV) and high collisionality n* > 2 • Steady-state conditions throughout ICRF pulse • Quasicoherent mode observed by reflectometer channel that views plasma region near the middle of the pedestal D. Mossessian

  9. Conditions Favoring EDA • EDA formation favored by: • Moderate safety factor • q95 > 3.5 in D • q95 > 2.5 (or lower) in H • Stronger shaping • d > 0.35 • Higher L-mode target density • ne > 1.21020 m-3 • Clean wall conditions (boronization) • Seen in both Ohmic and ICRF heated discharges • Seen with both favorable and unfavorable drift direction. M. Greenwald

  10. Higher density at L-H favours EDA Low density, ELM-free Higher density, EDA • Actual threshold may be in neutral density, local ne or gradient or collisionality (all are correlated; n*ped < 1 at low ne, 5-10 at high ne) • 1.21020 m-3quitelow for C-mod. ~0.15 nGW , low ne limit ~0.9 1020 A. Mazurenko

  11. EDA/ELM-free Operational Boundaries EDA favors high q95 > 3.5 1 and moderate edge 150 < Teped < 500 eV ELM-free plasmas are more likely at low q95 and at lower densities and hence higher edge temperatures 0.6 MA < Ip < 1.3 MA 4.5 T < Bt < 6 T 1 MW < PRF < 5 MW D. Mossessian 1 M. Greenwald, Phys. Plasmas 6, 1943 (1999)

  12. EDA/ELM-free Operational Boundaries EDA favors high q95 > 3.5 1 and high edge collisionality *ped > 2 ELMy H modes occupy the same q-n* region as EDA ELM-free plasmas are more likely at low q95 and at lower collisionality Collisionality n*ped calculated on 95% yn (top of the pedestal) D. Mossessian 1 M. Greenwald, Phys. Plasmas 6, 1943 (1999)

  13. Edge Gradients Challenge MHD Limit • Edge electron profiles from high resolution Thomson scattering • assume Ti = Te • Modeling shows gradients are ~30% above the first stability ballooning limit with only ohmic current. • Edge bootstrap current increases stability limit • No Type I ELMs (PRF5 MW, P12 MPa/m) • Small ELMs when bN1.2 D. Mossessian

  14. EDA Pedestal Pressure Increases with Ip • Thomson pedestal electron pressure gradient in EDA increases strongly with plasma current • Dashed curves are J. Hughes

  15. Time evolution of Te, ne pedestals studied using power ramps • RF input power continuously variable, ramped slowly up and down. • Te, ne measured with ms time resolution by ECE, bremsstrahlung array. • Strong hysteresis in net P. • H-mode threshold in Tedge is found. • Te pedestal varies in height and width with P • ne pedestal independent of P (above LH threshold). A. Hubbard

  16. Small ELMs appear at high input power Small, bipolar ELMs in Da at ~ 600 Hz Plasma exhaust visible on divertor probe saturation current ELMs observed in fast magnetic coil signal D. Mossessian

  17. QCM exists at moderate ÑPped and Teped ELMy EDA • When Teped³400 eVbroadband low frequency fluctuations observed in the pedestal region • QC mode reappears when edge is cooled • ELMs replace the QC mode at high pedestal Te D. Mossessian

  18. EDA/ELM-free Boundary in ÑPped vs Teped • QCM is not observed when Te >450 eV • ELMy regime exists in high Te, high Pped region D. Mossessian

  19. Probe Measurements Confirm Mode Drives Particle Transport • Langmuir probes see mode when inserted into pedestal(only possible in low power, ohmic, H-modes) • Amplitude up to ~50% in n, E • Multiple probes on single head yield poloidal k~4-6 cm-1, in agreement with PCI • Propagation in electron diamagnetic direction • Analysis of shows that the mode drives significant radial particle transport across the barrier, G~ 1022 /m2 s • Plumes from probe gas puffs show Er < 0 at mode location.(Er > 0 at larger radii). 1 mm B. LaBombard

  20. Particle Diffusion Increases with Quasi-Coherent Mode Amplitude •Particle source calculated with Lyman- emission, ne(r), and Te(r) • Effective particle diffusion: DEFF = (Source - dN/dt)/ n • As QC mode strength increases: • Deff increases • X-ray pedestal width (~Dimp) increases. M. Greenwald

  21. QCM has a strong magnetic component • Pickup coil added to fast-scanning Langmuir probe. • Frequency of magnetic component is identical to density fluctuations. • implies mode current density in the pedestal ~10 A/cm2 (~10% of edge j). • Mode is only observed within ~ 2 cm of the LCFS • Mode is NOT seen on the wall and limiter coils that are 5 cm outside the LCFS (at least 1000x lower) J. Snipes

  22. Magnetic QCM amplitude decreases rapidly with radius • Scanning magnetic probe nearly reaches the LCFS • Mode decays as • Local QCM kr~1.5 cm-110 cm above the outboard midplane • Differs from Type III ELM precursor kr~0.5 cm-1 seen on the limiter probes J. Snipes

  23. QCM Poloidal Mode Structure Ø   Frequency sweeps from > 200 kHz to ~ 100 kHz just after L-H transition ØStrong magnetic component only observed within ~2 cm of LCFS Økr k 1.5 cm-1 ( 4 cm) near the outboard midplane ØAssuming a field aligned perturbation with , k is expected to vary with position as consistent with PCI kR ~ 6 cm-1 along its vertical line of sight near the core J. Snipes

  24. QCM Toroidal Mode Structure Ø QCM is sometimes observed on a toroidal array of outboard limiter coils ØWhen the outer gap  1 cm ØToroidal mode number 15 < n < 18 ØAt q95 = 5, for a mode resonant at the edge this implies 75 < m < 90 which is consistent with <k> ~ 4 cm-1 Toroidal mode number J. Snipes

  25. Comparison with other ‘small ELM’ regimes EDA H-mode shares some characteristics of other steady regimes without large ELMS. • Low Particle Confinement regime on JET • Appears similar to EDA, but not easily reproduced. • Quasi-coherent Fluctuations on PDX • Fluctuations similar to those in EDA, present in short bursts in most H-modes. Coexisted with ELMs. • Type II or Grassy ELMs on DIII-D, JT60U, Asdex UG • Conditions in q, d very similar to EDA • Similar to small ELMs seen in EDA at high bN? • Does a quasi-coherent mode play a role in these regimes? • Quiescent H-Mode on DIII-D • Globally similar, but longer wavelength mode, different access conditions (esp density/neutrals). A. Hubbard

  26. LPCH-mode on JET Similar to EDA EDA H-mode in C-Mod LPCH-mode in JET J. Snipes

  27. Bout Simulations of the QCM X.Q. Xu, W.M. Nevins, LLNL • BOUT simulations find an X-point resistive ballooning mode that • is driven in the edge steep gradient region • has a similar magnetic perturbation amplitude and radial structure as the QCM • has a similar dominant k ~ 1.2 cm-1 at the outboard midplane as the QCM

  28. Physical origin of EDA, fluctuations • Since pedestal profiles are not much different in EDA, ELM-free H-modes, it seems likely to be the mode stability criteria which change with q,d, n* etc. • One possibility is that EDA is related to drift ballooning turbulence. Diamagnetic stabilization threshold scales asm1/2/q. A lower q threshold was found for EDA in H than D. • Initial scalings of QC mode characteristics show • Electromagnetic edge turbulence simulations by Rogers et al have shown a feature similar to QC mode, with . Gyrokinetic simulations of growth rates (GS2 code) are in progress. M. Greenwald

  29. Summary • EDA H-mode combines good energy confinement and moderate particle confinement in steady state, without large ELMs • Edge pedestals have few mm widths, gradients above first stable limit; but stable with bootstrap currents • Quasicoherent pedestal fluctuations QCM in density, potential and B are a key feature of EDA and only occur when: n*ped > 2, ÑPped < 1.2x106 Pa/(Wb/rad), Teped <450 eV • At higher ÑPped, high Teped QC mode is replaced by small grassy ELMs • The observed fluctuations drive significant particle flux • QCM’s are tentatively identified as resistive ballooning modes