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 USA Seminar IPP Garching Garching, Germany 7 May 2002

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

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

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

Edge Pedestal and Fluctuation Diagnostics A. Hubbard

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

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 kHz Width  F/F ~ A. Mazurenko

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

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  m -3 –Clean wall conditions (boronization) Seen in both Ohmic and ICRF heated discharges Seen with both favorable and unfavorable drift direction. M. Greenwald

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  m -3 quite low for C-mod. ~0.15 n GW, low n e limit ~0.9  A. Mazurenko

EDA/ELM-free Operational Boundaries EDA favors high q 95 > 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

EDA/ELM-free Operational Boundaries EDA favors high q 95 > 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

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

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

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

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

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

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

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,  ~ /m 2 s Plumes from probe gas puffs show E r 0 at larger radii). 1 mm B. LaBombard

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

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

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

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

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

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

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

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

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

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