1. Enhanced D  H-mode on Alcator C-Mod presented by J A Snipes with major contributions from M Greenwald, A E Hubbard, D Mossessian, and the Alcator C-Mod Group MIT Plasma Science and Fusion Center Cambridge, MA 02139 USA Seminar IPP Garching Garching, Germany 7 May 2002

2. Global Features of EDA H-Mode EDA H-modes have:  Good energy confinement H 89 ~ 2  Low particle confinement no impurity accumulation  Low radiated power  No large ELMs  Steady State (>8  E ) Obtained with Ohmic or ICRF heating, 1 < P RF < 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 k R ~ 6 cm -1 ( ~1 cm) PCI measures k radially at top and bottom of plasma. for typical equilibria Frequency range 60-250 kHz Width  F/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 T e ( 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 q 95 > 3.5 in D q 95 > 2.5 (or lower) in H –Stronger shaping  > 0.35 – Higher L-mode target density n e > 1.2  10 20 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-freeHigher density, EDA Actual threshold may be in neutral density, local n e or gradient or collisionality (all are correlated; * ped < 1 at low n e, 5-10 at high n e ) 1.2  10 20 m -3 quite low for C-mod. ~0.15 n GW, low n e limit ~0.9  10 20 A. Mazurenko

11. EDA/ELM-free Operational Boundaries EDA favors high q 95 > 3.5 1 and moderate edge 150 < T e ped < 500 eV ELM-free plasmas are more likely at low q 95 and at lower densities and hence higher edge temperatures 0.6 MA < I p < 1.3 MA 4.5 T < B t < 6 T 1 MW < P RF < 5 MW 1 M. Greenwald, Phys. Plasmas 6, 1943 (1999) D. Mossessian

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

13. Edge Gradients Challenge MHD Limit Edge electron profiles from high resolution Thomson scattering –assume T i = T e 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 (P RF  5 MW, P  12 MPa/m) –Small ELMs when  N  1.2 D. Mossessian

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

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

16. Small ELMs appear at high input power Small, bipolar ELMs in D  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  P ped and T e ped  When T e ped  400 eV  broadband low frequency fluctuations observed in the pedestal region  QC mode reappears when edge is cooled  ELMs replace the QC mode at high pedestal T e D. Mossessian ELMyEDA

18. EDA/ELM-free Boundary in  P ped vs T e ped  QCM is not observed when T e >450 eV  ELMy regime exists in high T e, high  P ped 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,  ~ 10 22 /m 2 s Plumes from probe gas puffs show E r 0 at larger radii). 1 mm B. LaBombard

20. Particle Diffusion Increases with Quasi-Coherent Mode Amplitude Particle source calculated with Lyman-  emission, n e (r), and T e (r) Effective particle diffusion: D EFF = (Source - dN/dt)/  n As QC mode strength increases: – D eff increases –X-ray pedestal width (~D imp ) 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/cm 2 (~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 k r ~1.5 cm -1 10 cm above the outboard midplane Differs from Type III ELM precursor k r ~0.5 cm -1 seen on the limiter probes J. Snipes

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

24. QCM Toroidal Mode Structure  QCM is sometimes observed on a toroidal array of outboard limiter coils  When the outer gap  1 cm  Toroidal mode number 15 < n < 18  At q 95 = 5, for a mode resonant at the edge this implies 75 < m < 90 which is consistent with ~ 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,  very similar to EDA –Similar to small ELMs seen in EDA at high  N ? –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 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 X.Q. Xu, W.M. Nevins, LLNL

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, , * etc. One possibility is that EDA is related to drift ballooning turbulence. Diamagnetic stabilization threshold scales as m 1/2 /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: * ped > 2,  P ped < 1.2 x 10 6 Pa/(Wb/rad), T e ped <450 eV At higher  P ped, high T e ped QC mode is replaced by small grassy ELMs The observed fluctuations drive significant particle flux QCM’s are tentatively identified as resistive ballooning modes