ICRF Mode Conversion Flow Drive Studies with Improved Wave Measurement by Phase Contrast Imaging

Transcription

1 57 th APS-DPP meeting, Nov. 2015, Savannah, GA, USA ICRF Mode Conversion Flow Drive Studies with Improved Wave Measurement by Phase Contrast Imaging Yijun Lin, E. Edlund, P. Ennever, A.E. Hubbard, M. Porkolab, J.E. Rice, S.J. Wukitch and the Alcator C-Mod team MIT Plasma Science and Fusion Center, Cambridge, MA 02139, USA Work supported by US DoE Cooperative agreement DE-FC02-99ER54512 at MIT using the Alcator C-Mod tokamak, a DOE Office of Science user facility. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 1

2 Background Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 2

3 ICRF antennas on Alcator C-Mod Field-aligned 4-strap antenna at J port (78 MHz) Two 2-strap antennas at D-port (80.5 MHz) and E-port (80 MHz) Total RF source power: Four 2 MW transmitters. D and E antennas are each powered by one transmitter and provide up to 1.8 MW (together ~ 3.6 MW) RF power to plasma. J antenna was not able to provide full power in the 2015 campaign [see Poster CP by Wukitch]. For data shown in this poster, J antenna had one-transmitter feeding the central two straps with less than 1 MW power. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 3

4 ICRF minority heating vs. mode conversion heating shown in E field from TORIC Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 4

5 Mode conversion has been shown to enhance toroidal rotation Mode Conversion: ΔV φ ~ 90 km/s at 3 MW 50 MHz RF Co-current direction. Minority Heating: ΔV φ ~ 35 km/s at 3 MW 80 MHz RF Co-current direction. Intrinsic plasma rotation Y. Lin et al, IAEA and APS 2008 Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 5

6 Scaling law from parameter scans Empirical scaling law obtained from multipleparameter regression for all the +90 o and 180 o data. Intermediate X[ 3 He] Optimized B field Favorable scaling with P RF, I p, unfavorable vs. n e and RF frequency (or B). V P I n RF p e RF f Y. Lin et al, IAEA 2010 Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 6

7 Phase contrast imaging system (PCI) Plasma density fluctuations introduce phase variations to the laser beam. Laser phase variations are converted to intensity variations by a λ/4 phase plate. Acoustic-optical frequency shifter to modulate the laser beam to have a beat-frequency near the RF frequency (heterodyne scheme). RF waves can be measured in this setup at the beat frequency. E. Nelson-Melby et al, PRL 90, (2003). The system has recently been upgraded to have higher sensitivity at high frequencies and better calibration. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 7

8 Motivation: relating RF wave measurements to plasma V tor RF power Power deposition Electron heating Ion heating Momentum input Scaling law Plasma Rotation Driven rotation Intrinsic rotation Momentum transport ICRF simulation Wave field structure Power partition Synthetic diagnostic PCI RF signals Wave field MC waves Fast waves Correlation? Causality? Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 8

9 PCI Observation Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 9

10 PCI is in front of E antenna but some toroidal angles away from D and J Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 10

11 RF signals shown in PCI data 80 MHz RF signal from E antenna, shown in PCI spectra at ~880 khz after heterodyne modulation RF wave appears as a coherent signal in the PCI spectra (contour image in f and t); Signal amplitude is an indication of the wave E field amplitude; Signal phases from different PCI channels k R of the RF waves. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 11

12 ICRF Mode Conversion n 2 = R cutoff (HFS edge) n 2 = S ion-ion hybrid n 2 = L cutoff Ion cyclotron resonance n 2 = R cutoff (LFS edge) Fast Wave MC IBW Fast Wave MC ICW Fast Wave Fast Wave RF Antenna on the Low Field Side MC ICW Mode conversion to the ICW is a result of k up-shift caused by the magnetic shear at where B pol 0 Stix notations R = 1 L = 1 S = ( R + L) 2 j j ω 2 pj ω( ω + Ω ω 2 pj ω( ω Ω j, ) j ) Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 12

13 Determine 3 He level from MC locations n 2 n 2 PCI RF signal level contours vs. R and t. PCI has 32 channels, covering about a 10 cm window, 0.64 m < R < 0.74 m. B t0 = 8 T, D( 3 He) plasmas. At different 3 He levels, the MC locations are different. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 13

14 PCI signals can also differentiate FW and MC waves n 2 n 2 At low 3 He level, the signals are much weaker at similar RF power, suggesting that most of the signals are from fast wave; The PCI observation provides a quite useful constraint for ICRF simulation codes, like TORIC and AORSA. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 14

15 At different 3 He levels, the RF signals have different behavior vs. R X[ 3 He] = 17% X[ 3 He] = 11% At 50 ms 3 He puff, X[ 3 He] = n 3He /n e 5%, the RF signal is two orders of magnitude smaller than other cases. 14% 5% Power partition among waves. Fast wave has much smaller E than the MC waves at the same power. PCI RF signal div(e ) Geometric effect (supposition RF waves from different toroidal modes at the PCI location, some toroidal distance away from the antenna). PCI is line integrated. Out-of-phase RF signals could be averaged out along a vertical laser beam line. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 15

16 FW and MC Waves can be separated in k R spectra 17% X[ 3 He] 14% 11% 5% Dispersion curves FAST WAVE MC ICW MC IBW Fast wave has a much longer wavelength than the MC waves; MC ICW typically has k R ~ 3-8 cm -1 ; MC IBW has k R 8 cm -1, and mostly out of the PCI k R resolution. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 16

17 Similar behavior observed for E antenna 11% X[ 3 He] 5% FAST WAVE MC ICW E antenna at 80 MHz, higher frequency than J antenna (78 MHz) MC at smaller R (higher B field); Also shows significant difference in k R at the two 3 He levels. At higher 3 He levels, the MC location moves out of the PCI detection window. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 17

18 Fast wave signals scale with RF power at X( 3 He) ~ 5% RF signals are observed only in channels near R ~ 0.68 m. All fast wave. No mode conversion. For this plasma, the PCI signal level approximately scales linearly with RF power. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 18

19 Rotation vs. Mode Conversion Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 19

20 Rotation speed scales with the RF power for X[ 3 He] ~ 11% V tor (km/s) Total RF power (MW) Flow drive effect is observed in the plasma with X[ 3 He] ~ 11% (L-mode plasma), but not in plasmas at other X[ 3 He] levels; This is consistent with previous results that the mode conversion flow drive effect was found to peak around X[ 3 He] ~ 10%. V tor roughly scales with the input RF power, same as previously observed. (Note: the plasma disrupted at 1.22 sec) Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 20

21 Power partition between FW and MC waves are complicated (J antenna) Fast Wave R = 0.72 m n e MC Waves R = 0.66 m RF power The time traces have quite complicated features. Possibly plasma density has very strong effects in power partition. Need simulation to figure it out. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 21

22 Power partition between FW and MC waves are complicated (E antenna) Fast R = 0.68 m n e MC = 0.64 m RF power The PCI amplitude for the MC waves is not simple to interpret: MC waves are very local and have short wavelength, while PCI observation is lineintegrated. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 22

23 dv/dt decreases with rising fast wave signal amplitude Fast Wave E antenna dv/dt (km/s 2 ) Fast wave J antenna When the PCI FW signal increases, dv/dt starts to decrease, and vice versa. (e.g., t = 0.8, 0.9, and 1.0 sec) FW signals have been shown to broadly scale with RF power in the case of 5% X[ 3 He]. P total = P FW + P MC, P FW P MC RF power in MC waves may be positively correlate with the rotation drive force. More experiments (e.g., with better controlled plasma density) are necessary for finding a definite answer. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 23

24 Summary Fast wave and mode converted waves have been studied in more detail with upgraded PCI: Waves spatial location and wavenumbers follow the 3 He level; PCI FW signal amplitudes generally follow the RF power in minority heating plasma. Flow drive effects have been observed in the case of X[ 3 He] ~ 11%, but not in lower or higher X[ 3 He] levels. The rotation change in time seems to negatively correlate with the PCI observed FW amplitude positively correlated with the power to MC slow waves. More MC flow drive experiments will be carried out in the coming 2016 Alcator C-Mod experimental campaign. Y. Lin et al, CP ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod 24