Pedestals and Fluctuations in C-Mod Enhanced D α H-modes

Transcription

1 Pedestals and Fluctuations in Enhanced D α H-modes Presented by A.E.Hubbard With Contributions from R.L. Boivin, B.A. Carreras 1, S. Gangadhara, R. Granetz, M. Greenwald, J. Hughes, I. Hutchinson, J. Irby, V. Klein 2, B. LaBombard, Y. Lin, E. S. Marmar, A. Mazurenko, D. Mossessian, T. Sunn Pedersen 4, M. Porkolab, J.A. Snipes, J. Terry, S. Wolfe, S. Wukitch, S. Zweben 3 and the Group MIT Plasma Science and Fusion Center 1 Oak Ridge National Laboratory 2 Univ. New Mexico 3 Princeton Plasma Physics Laboratory 4 Columbia University Research supported by US. Dept. of Energy APS-DPP Meeting Quebec, Canada 23 October 2000

2 Pedestals and Fluctuations in Enhanced D α H-modes Description of EDA H-Mode Pedestals Profiles Stability Time evolution Quasi-coherent edge fluctuations Conditions for achieving EDA Possible physical mechanisms

3 Global Features of EDA H-Mode EDA H-modes have: good energy confinement Low particle confinement Low radiation No large ELMs Steady State (>8τ E ) Obtained with both Ohmic and RF heating, P RF <5 MW, W<240 kj. Highly attractive reactor regime (no ELM erosion).

4 High Resolution Diagnostics of Edge Pedestals and Fluctuations Phase Contrast Imaging (12 ch, dr=3mm) { Edge Thomson scattering (22 points, dz=2.5 mm) Top edge xray array (38 chords, dz=1.1 mm) Scanning Langmuir probe Edge bolo array (20 ch, dr=2 mm) Helium beam (14 pts, dr=3 mm) ECE (9&19channels) Reflectometer ( 5 channels) Edge vis. brem. array (dr=1 mm) Outboard edge xray array (38 chords, dr=1.5 mm) Lyman α (20 channels, dr=2 mm) Scanning Langmuir probe

5 Pedestals are few mm wide, show differences in structure, position Density width 2-5 mm. Steepest T e region (~60 kev/m) few mm wide, Wider region with ~25 kev/m (2-3X core gradient). X-ray (impurity) pedestal at top of n e gradient, consistent with neoclassical pinch. Only the impurity width is significantly wider in EDA vs ELM-free; T e, n e pedestals show little difference. Only impurity width gets narrower at high I p. J. W. Hughes UP1.095 Thurs. am. Temperature Density Emissivity

6 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 β N 1.2 T e (kev) Pressure (kpa) P ' (MPa/m) ne Te Plasma Pressure (kpa) dp/dr (MPa/m) LCFS n e (10 20 m -3 ) D. Mossessian UP1.096 Thurs. am. Major Radius (m)

7 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).

8 Transients at LH transition consistent with sudden drop of χ in pedestal Rate of increase is largest near top of pedestal. dw ped /dt requires blocking most of the heat flux Q. No time delays in pedestal region, within ms resolution. Transients in core are slower, appear diffusive. Contributions of n, T are comparable at moderate n e. w ped (t)~ t 1/2, well fit by power balance analysis. Q/ A( χa χb) wped () t = wped ( 0) + t 1/2 1/2 π χ + χ χ ( ) b A A R/a=0.86 B. Carreras

9 At lower density, T e rise dominates Density rise is smaller. Measured neutral ionization rate also ~10x lower. T e pedestal higher (900 ev) Discharge is ELM-free Pressure rise about the same, still fit by the same transport model, χ s. At even lower n e, no density pedestal is formed and the H-mode is not sustained. This might be part of the reason for the low density limit

10 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 UP1.094 Thurs. am. Frequency (khz) Frequency (khz) Intensity (W/sr/m 2 ) Reflectometer Phase Contrast Imaging D α Time (s)

11 Phase Contrast Imaging measures k-poloidal ~ 6 cm -1 (λ~1 cm) 200 Frequency (khz) Frequency range khz Width F/F ~ A. Mazurenko UP1.105 Thurs. am k R, cm-1 PCI measures k radially at top and bottom of plasma. mainly poloidal component. kθ ρs 0.13

12 Probe Measurements Confirm Mode Drives Particle Transport Langmuir probes see mode when inserted into pedestal Φ (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 n e agreement with PCI 1 mm Propagation in electron diamagnetic direction Analysis of ne shows that the mode drives significant radial particle transport across the barrier, Γ~ /m 2 s Γ Plumes from probe gas puffs show E r < 0 at mode location. (E r > 0 at larger radii). B. Labombard BI1.006, this session ne

13 Mode Has a Strong Magnetic Component Pickup coil added to fastscanning probe. Frequency of magnetic component is identical to density fluctuations. 4 B~3 10 T implies mode current density in the pedestal ~10 A/cm 2 (~10% of edge j). Mode is NOT seen on wall, limiter probes (at least 1000x lower) J. Snipes

14 Rapid decrease of magnetic mode amplitude with R Decays as e -1.6 r Consistent with k θ ~1.6 cm -1. Differs from Type III ELM precursor, which has k θ ~0.5 cm -1, and is seen on limiter probes.

15 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 δ=> 0.35 Higher L-mode target density n e > m -3 Clean wall conditions (boronization) Seen in both Ohmic and ICRF heated discharges Seen with both favorable and unfavorable drift direction.

16 Higher density at L-H favours EDA Low density, ELM-free Higher density, EDA D α D α n e n e Actual threshold may be in neutral density, local n e or gradient or collisionality (all are correlated; ν* ped < 1 at low n e, 5-10 at high n e ) m -3 quite low for C-mod. ~0.15 n GW, low n e limit ~

17 A Continuum of QC Mode Amplitude, EDA Particle Transport is seen ELM-free to EDA transition can be progressive (unlike the L-H transition). Weakly enhanced particle transport is seen in discharges near EDA boundary. As QC mode strength increases: Rate of rise of n e drops (more steady state, D eff increasing). X-ray ped width (~D imp ) increases. Similar trends have been seen in scans of q, δ. EF steady EDA

18 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 Loarte, Snipes et al DO1.004, Mon. pm. 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, δ=very similar to EDA Similar to small ELMs seen in EDA at high β 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).

19 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,δ, ν* 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 wasfound for EDA in H than D. Initial scalings of QC mode characteristics show n ρ s kθ ρs n n Electromagnetic edge turbulence simulations by Rogers et al have shown a feature similar to QC mode, with kθ 2 π / p. Gyrokinetic simulations of growth rates (GS2 code) are in progress. A. Mazurenko UP1.105 Thurs. am. M.Greenwald, JO1.004, Tues pm H. Yuh, UP1.098 Thurs. am.

20 Summary EDA H-mode regime routinely combines good energy confinement and low particle confinement in steady state, without large ELMs. Edge pedestals have few mm widths, gradients above first stable limit; but stable with bootstrap currents. Time evolution of n, T indicates large, fast drops in D, χ. Quasicoherent pedestal fluctuations in density, potential and poloidal B are a key feature of EDA regime Mode has been shown to drive large particle transport. Safety factor, shaping and density/neutral pressure are key parameters to obtaining EDA.