Enhanced D H-mode on Alcator C-Mod

presented by J A Snipeswith major contributions from

M Greenwald, A E Hubbard, D Mossessian, and the Alcator C-Mod Group

MIT Plasma Science and Fusion CenterCambridge, MA 02139 USA

Seminar IPP GarchingGarching, Germany

7 May 2002

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 (>8E)

• Obtained with 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

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 kR ~ 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

0.1sk A. Mazurenko

~ 2 Rk k

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 * > 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

• q95 > 3.5 in D

• q95 > 2.5 (or lower) in H– Stronger shaping

• > 0.35– Higher L-mode target density

• ne > 1.21020 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-free Higher density, EDA

•Actual threshold may be in neutral density, local ne or gradient or collisionality (all are correlated; *ped < 1 at low ne, 5-10 at high ne)

• 1.21020 m-3 quite low for C-mod. ~0.15 nGW , low ne limit ~0.9 1020

enen

DD

A. Mazurenko

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 MA4.5 T < Bt < 6 T

1 MW < PRF < 5 MW

1 M. Greenwald, Phys. Plasmas 6, 1943 (1999)

D. Mossessian

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-* region as EDA

ELM-free plasmas are more likely at low q95 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 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

(PRF5 MW, P12 MPa/m)– Small ELMs when N1.2

D. Mossessian

EDA Pedestal Pressure Increases with Ip

• Thomson pedestal electron pressure gradient in EDA increases strongly with plasma current

• Dashed curves are

1.7 0.42.8e p solp I P

J. Hughes

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

Small ELMs appear at high input power

Small, bipolar ELMs in Dat ~ 600 Hz

Plasma exhaust visible on divertor probe saturation current

ELMs observed in fast magnetic coil signal

D. Mossessian

QCM exists at moderate Pped and Teped

When Teped 400 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

ELMy EDA

EDA/ELM-free Boundary in Pped vs Teped

QCM is not observed when Te >450 eV

ELMy regime exists in high Te, high Pped 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, ~ 1022 /m2 s

• Plumes from probe gas puffs show Er < 0 at mode location.(Er > 0 at larger radii).

n E

n E

en1 mm

B. LaBombard

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

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)

4B ~ 3 10 T

J. Snipes

Magnetic QCM amplitude decreases rapidly with radius

• Scanning magnetic probe nearly reaches the LCFS

• Mode decays as

• Local QCM kr~1.5 cm-1

10 cm above the outboard midplane

• Differs from Type III ELM precursor kr~0.5 cm-1 seen on the limiter probes

~ ~ exp( ( ))B B k r rLCFSr LCFS

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

   kr 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 kR ~ 6 cm-1 along its vertical line of sight near the core

k k R R B B 1 2 2 12

2 1/ ( / ) ( / )

0k B

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 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

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 thatis 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 m1/2/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.

s

n

nn

0.1 0.2 sk

2 / pk

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, Pped < 1.2x106 Pa/(Wb/rad), Teped <450 eV

• At higher Pped, 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