Structure and Characteristics of the Quasi-Coherent Mode

Transcription

1 Structure and Characteristics of the Quasi-Coherent Mode in EDA H-mode Plasmas I. Cziegler, J. L. Terry, L. Lin, M. Porkolab,J. A. Snipes MIT Plasma Science and Fusion Center American Physical Society Division of Plasma Physics Philadelphia, Oct Nov. 3, 2006

2 Background and Motivation Enhanced Dα H-mode τ pelmfree > 0.2s τ EELMfree 80ms τ peda 0.1s τ EEDA > 60ms τ E transport 30ms EDA τexhibits increased particle p 30ms through pedestal L L Impurities do not accumulate No large ELMs: stability, low heat load EDA plasmas also exhibit a Quasi-Coherent Fluctuation at the plasma edge APS DPP 2006 (2)

3 The Quasi-Coherent Mode is believed to be responsible for the increased particle transport From earlier experiments: Close connection to transport Ballooning character from GPI Similar modes are sometimes present in other experiments (PDX, JFT2M, DIII-D) APS DPP 2006 (3)

4 Location of Current Diagnostics top view side view PCI chords gas puff gas puff Magnetics (up to 10 MHz) Phase contrast imaging Gas puff imaging photodiodes (1 MHz) APS DPP 2006 (4)

5 Normalized mode intensity QC Mode Localized to the Edge R Rsep (cm) R Rsep (cm) Earlier experiments showed a highly localized mode As a new measurement of the radial mode profile we swept the separatrix through the highest resolution part of the horizontal diode array APS DPP 2006 (5)

6 Harmonic Frequency Seen before on PCI Now Measured to be kθ2ω = 2kθQC with GPI Fundamental f = 65 ± 5 khz 1 kθ = 1.6 ±.3 cm Harmonic f = 132 ± 8 khz kθ = 3.4 ±.3 cm Coherent structure with 1 c p 2.5 km s 1 in the electron direction APS DPP 2006 (6)

7 Experiments to Test Consistency of Propagation Direction Usual operation: normal field, LSN electron diamagnetic direction (index) Edge flows change as magnetic normal field and current directions are B and I dir. reversed (ref: LaBombard) Measurements also done in: p Reversed B and Ip, USN Reversed B and Ip, LSN APS DPP 2006 (7)

8 Measurement with reversed magnetic field and plasma current in USN-mode Both X-point geometry and directions are flipped Same coherent structure as in normalfield, LSN mode APS DPP 2006 (8)

9 Wavenumber and Phase Velocity Measurement by Magnetic Pickup Coils from pick-up coils k B 0 Magnetics have toroidal resolution Wavenumber and direction predictable if magnetic topology is known according to GPI poloidal array coils k direction consistent between magnetic and GPI measurements APS DPP 2006 (9)

10 Measurement with reversed magnetic field and plasma current in LSN-mode from pick-up coils Result: EDA H-mode and QC mode are poorer in this H-mode unfavourable configuration kθ 1.3 cm 1 in the counter-current direction, ie in electron diamagnetic drift direction APS DPP 2006 (10)

11 QC Mode Poloidal Amplitude Variation X-point moved in three consecutive shots PCI samples both top and bottom of plasma APS DPP 2006 (11)

12 QC Mode Poloidal Amplitude Variation X-point moved in three consecutive shots PCI samples both top and bottom of plasma APS DPP 2006 (12)

13 QC Mode Poloidal Amplitude Variation X-point moved in three consecutive shots PCI samples both top and bottom of plasma APS DPP 2006 (13)

14 Normalized Mode Intensity QC Mode Poloidal Amplitude Variation X-point position Major Radius (cm) X-point moved in three consecutive shots PCI samples both top and bottom of plasma Qualitatively consistent with basic ballooning and BOUT prediction New feature found: role of X-point in overall poloidal structure Ref: Lin et al, Poster QP , Wed. PM APS DPP 2006 (14)

15 Summary Poloidal wavenumber measurements of the QC mode made in normal field, LSN reverse field, USN reverse field, LSN All consistently in the electron diamagnetic drift direction Observed poloidal variation is qualitatively consistent with ballooning drive Presence of higher harmonic and nontrivial spatial structure may indicate nonlinearity effects X-point effects on both sides are not accounted for in present models APS DPP 2006 (15)