APS-DPP meeting, Nov. 2015, Savannah, GA, USA …...APS-DPP meeting, Nov. 2015, Savannah, GA, USA Y. Lin et al, CP12.023 “ICRF MC Flow Drive Study with Enhanced Wave Measurement

Y. Lin et al, CP12.023 “ICRF MC Flow Drive Study with Enhanced Wave Measurement by PCI on Alcator C-Mod” 1

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.

57th APS-DPP meeting, Nov. 2015, Savannah, GA, USA


Background


ICRF antennas on Alcator C-Mod

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 CP12.024 by Wukitch]. For data shown in this poster, J antenna had one-transmitter feeding the central two straps with less than 1 MW power.

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)


ICRF minority heating vs. mode conversion heating shown in E– field from TORIC


Mode conversion has been shown to enhance toroidal rotation

Mode Conversion: • ΔVφ ~ 90 km/s at 3 MW 50 MHz RF • Co-current direction.

Minority Heating:

• ΔVφ~ 35 km/s at 3 MW 80 MHz RF •Co-current direction. • Intrinsic plasma rotation

Y. Lin et al, IAEA and APS 2008


Scaling law from parameter scans Empirical scaling law

obtained from multiple-parameter regression for all the +90o and 180o data.

– Intermediate X[3He] – Optimized B field

Favorable scaling with PRF,

Ip, unfavorable vs. ne and RF frequency (or B).

8.09.05.03.1 −−∝∆ RFepRF fnIPV

Y. Lin et al, IAEA 2010


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 90,155004 (2003).

• The system has recently been upgraded to have higher sensitivity at high frequencies and better calibration.


Motivation: relating RF wave measurements to plasma Vtor

ICRF simulation • Wave field structure • Power partition

Plasma Rotation • Driven rotation • Intrinsic rotation • Momentum transport

PCI RF signals • Wave field • MC waves • Fast waves

RF power • Power deposition • Electron heating • Ion heating • Momentum input

Scaling law

Synthetic diagnostic

Correlation? Causality?


PCI Observation


PCI is in front of E antenna but some toroidal angles away from D and J


RF signals shown in PCI data

• 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 kR of the RF waves.

80 MHz RF signal from E antenna, shown in PCI spectra at ~880 kHz after heterodyne modulation


ICRF Mode Conversion

RF Antenna on the Low Field Side

n||2 = R cutoff

(LFS edge) n||

2 = S ion-ion hybrid

n||2 = L

cutoff

n||2 = R

cutoff (HFS edge)

Ion cyclotron resonance

Fast Wave Fast Wave

MC IBW

MC ICW

MC ICW

Fast Wave Fast Wave

Mode conversion to the ICW is a result of k|| up-shift caused by the magnetic shear at where Bpol ≠ 0

2)(

)(1

, )(

1

2

2

LRS

L

R

j j

pj

j j

pj

+=

Ω−−=

Ω+−=

∑

∑

ωωω

ωωω

Stix’ notations


Determine 3He level from MC locations

• 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.

• Bt0 = 8 T, D(3He) plasmas. At different 3He levels, the MC locations are different.

n||2 n||

2


PCI signals can also differentiate FW and MC waves

• At low 3He 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.

n||2 n||

2


At different 3He levels, the RF signals have different behavior vs. R

• Power partition among waves. • 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.

At 50 ms 3He puff, X[3He] = n3He/ne ≤ 5%, the RF signal is two orders of magnitude smaller than other cases. Fast wave has much smaller E|| than the MC waves at the same power. PCI RF signal ∝ div(E||)

X[3He] = 17%

X[3He] = 11%

14%

5%


FW and MC Waves can be separated in kR spectra

• Fast wave has a much longer wavelength than the MC waves; • MC ICW typically has kR ~ 3-8 cm-1; • MC IBW has kR ≥ 8 cm-1, and mostly out of the PCI kR resolution.

FAST WAVE

MC ICW

MC IBW

17% X[3He]

11% 14%

5%

Dispersion curves


Similar behavior observed for E antenna

• E antenna at 80 MHz, higher frequency than J antenna (78 MHz) MC at smaller R (higher B field);

• Also shows significant difference in kR at the two 3He levels.

• At higher 3He levels, the MC location moves out of the PCI detection window.

FAST WAVE

MC ICW

11% X[3He] 5%


Fast wave signals scale with RF power at X(3He) ~ 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.


Rotation vs. Mode Conversion


Rotation speed scales with the RF power for X[3He] ~ 11%

• ∆Vtor roughly scales with the input RF power, same as previously observed. • (Note: the plasma disrupted at 1.22 sec)

Vtor (km/s)

Total RF power (MW)

• Flow drive effect is observed in the plasma with X[3He] ~ 11% (L-mode plasma), but not in plasmas at other X[3He] levels;

• This is consistent with previous results that the mode conversion flow drive effect was found to peak around X[3He] ~ 10%.


Fast Wave R = 0.72 m

MC Waves R = 0.66 m

Power partition between FW and MC waves are complicated (J antenna)

ne

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.


Fast Wave @ R = 0.68 m

MC Waves @R = 0.64 m

Power partition between FW and MC waves are complicated (E antenna)

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 line-integrated.

RF power

ne


dV/dt decreases with rising fast wave signal amplitude

• 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[ 3He].

• Ptotal = PFW + PMC , PFW↓ PMC↑ • 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.

Fast Wave E antenna

Fast wave J antenna

dV/dt (km/s2)


Summary • Fast wave and mode converted waves have been studied in

more detail with upgraded PCI: – Waves spatial location and wavenumbers follow the 3He 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[3He] ~ 11%, but not in lower or higher X[3He] 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.

APS-DPP meeting, Nov. 2015, Savannah, GA, USA …...APS-DPP meeting, Nov. 2015, Savannah, GA, USA Y. Lin et al, CP12.023 “ICRF MC Flow Drive Study with Enhanced Wave Measurement

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