1

Off-axis Fishbones in DIII-D

Bill Heidbrink

Leaders of the Experiment

G. Matsunaga, M. Okabayashi

Energetic Particle Working Group

R. Fisher, R. Moyer, C. Muscatello, D. Pace, W. Solomon, M. Van Zeeland, Y. Zhu

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Fishbones can trigger Resistive Wall Modes

Okabayashi

•Fast ions & toroidal rotation help stabilize RWM

•Fishbones cause reduction in both triggers RWM

•Low frequency, bursting instability with large effect on fast ions utilize new EP diagnostics

3

High Beta plasma with q0 > 1

1<qo<2

1.7 T

1.07 MA

H-mode

N=2.6

4e19 m-3

All beam angles

Classic (PDX) fishbone was an internal kink (q0<1)

4

Mode Frequency ~ 8 kHz Fluctuations detected on all EP

diagnostics

OkabayashiGlobal mode w/ large amplitude near q=2

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Outline

1. Orbit Topology

2. Mirnov Analysis

3. Loss Measurements

4. Wave Distortion and Phase Slippage

6

Fast-ion Loss Detector (FILD) measures lost trapped ions at

fishbone burst

Pace, Fisher

•Bright spot for ~80 keV, trapped fast ions

•Loss orbit resembles banana orbits deposited by perpendicular beams

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Perpendicular beams are born near the resonant frequency

•Er approximately adds to precession frequency like Doppler shift

•Counter-perp: ~9.5 kHz; Co-perp: ~6.7 kHz

•Initial mode frequency: ~8 kHz

•Modes w/o counter injection are different

Van Zeeland

•The p=0 curves represent the pre resonance

8

Counter-perp beam ions are expelled onto the loss orbit measured by FILD

Van Zeeland

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Fishbones are driven at the precession frequency of the trapped fast ions

Shinohara, Matsunaga

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Outline

1. Modes are driven by precession-frequency resonance

2. Mirnov Analysis

3. Loss Measurements

4. Wave Distortion and Phase Slippage

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Is the mode an energetic particle mode (EPM) or normal mode?

Normal Mode

nEP << ne

Wave exists w/o EPs.

Re() unaffected by EPs.

EPs resonate with mode, altering Im()

Energetic Particle Mode1

EP ~

EPs create a new wave branch

Re() depends on EP distrib. function

EPs resonate with mode, altering Im()

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Initial frequency depends on toroidal rotation & precession frequency

•Database of 388 bursts

•Scales with rotation near q=2 but not central rotation

•Best fit: nearly linear dependence (expected for both normal mode & EPM)

•Precession frequency proportional to E/Ip

•Data depends on E/Ip more weakly than fpre suggests normal mode?

13

Mode frequency chirps down (like classic

fishbone)

•Rotation frequency changes < 1 kHz but mode changes ~3 kHz

•Large frequency sweep suggests EPM

• f increases with fast-ion losses suggests EPM

14

Growth rate similar to classic fishbone

•Growth rate scales with mode amplitude•Considerable variation in decay rate

•Unlike PDX, average decay rate similar to growth rate

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Strong distortion of waveform observed late in burst

•Different from classic fishbone

•Distortion varies with position (on internal fluctuation diagnostics)

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Distortion greatest near maximum amplitude

•Use variation in half-period to measure distortion

•Other definitions give similar results

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Distortion occurs in every burst

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Distortion has (m,n)=(2,1) structure

TIME (ms)

•V

ER

TIC

AL P

OS

ITIO

N

•Fundamental sine wave has (3,1) structure

Okabayashi

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Outline

1. Orbit Topology: Modes are driven by precession-frequency resonance

2. Mirnov Analysis: Waveform similar to classic fishbone except for distortion

3. Loss Measurements

4. Wave Distortion and Phase Slippage

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•Total fast-ion loss rate inferred from slope of neutrons

•CER acquired in 0.5 ms bins

•Conditionally average 8 similar bursts

•Drop observed near q=2

•Losses act like a torque impulse—a negative beam blip (deGrassie PoP 2006)

•Magnitude reasonable

•<5% FIDA drops for R<208 cm

Non-ambipolar losses cause sudden drop in electric field toroidal

rotation

21

Losses increase with increasing mode amplitude

•Linear dependence predicted for convective losses (classic fishbone)

•Offset linear for convective with a threshold

•Quadratic for diffusive

•Fair fit to all 3 models

22

Losses have a definite phase relative to the mode

•BILD saturated on most bursts

•Relatively weak burst

•Like “beacon” measured for classic fishbones

•Phase consistent with Ex Bconvective transport

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All Loss Diagnostics Observe the “Beacon”

•Ion cyclotron emission (ICE) at 255o (midplane)

•Beam ion loss detector (BILD) at 60o (-12 cm)

•Neutral particle analyzer (NPA) at 225o ( ~ -35o)

•Fast ion loss detector (FILD) at 225o (R-1 port)

•Langmuir probe (ISAT) at 240o (-19 cm)

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BES signal has large spikes at peak mode amplitude

•BES channel near q=2

•Phase with mode preserved throughout burst

•BES amplitude grows dramatically as mode distorts

•Interpretation: BES signal is a combination of bipolar ne fluctuations and spikes of FIDA light as fast ions are expelled to high neutral density region at edge

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Outline

1. Orbit Topology: Modes are driven by precession-frequency resonance

2. Mirnov Analysis: Waveform similar to classic fishbone except for distortion

3. Loss Measurements: Fast ions lost in a convective beacon

4. Wave Distortion and Phase Slippage

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Phase of neutron oscillations slips relative to mode

•Fluctuations caused by motion of confined fast ions relative to scintillator

•Detrend neutron signal to observe oscillations clearly

•Initially fast ions oscillate with mode

•Phase slips over 360o

•No slip in internal fluctuations

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Phase slip occurs when distortion increases

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Phase slip is linearly proportional to frequency chirp

Okabayashi

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A proportionality constant of 2 is consistently observed

Okabayashi

•Rate of neutron drop also correlates with frequency chirp rate

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Does drag of the external kink on the wall cause phase slippage?

•Classic fishbone is an internal kink

•Classic fishbones had one angle of injection (greater anisotropy in velocity space)

Okabayashi

31

Phase slippage occurs when mass changes frequency faster than driving

frequencyWave & mass chirp together

Mass chirps faster

•Model wave & fast ions as a forced oscillator

•Chirping does not produce phase slippage when wave & particle chirp at same rate

•Suggests average precession frequency changes more than mode frequency non-resonant population causes opposite phase slippage

32

Speculation about the distortion & phase slip

Higher n modes are destabilized because... •Modes cross the linear threshold as the f.i. profile evolves•Nonlinear coupling

Is distortion important?•No? The n=1 predator-prey cycle determines evolution•Yes? Losses peak when distortion is greatest, suggesting an important role in fast-ion transport

Is neutron phase slip important?•No? Non-resonant (co-perp) confined trapped ions produce neutron signal•Yes? Strong correlation with distortion & losses suggest a causal relationship

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Comparison with Classic Fishbones

Off-axis

fpre resonance

f/f ~ 50%

Predator-prey burst cycle

Losses in “beacon”

Losses ~ linear w/ Bmax

Loss rate ~ chirp rate

Strong distortion of wave

Neutron oscillation slips in phase

Rapid vrot @ burst

Classic

fpre resonance

f/f ~ 50%

Predator-prey burst cycle

Losses in “beacon”

Losses ~ linear w/ Bmax

Loss rate ~ chirp rate

Weak distortion of wave

Neutron oscillation stays in phase

Not measured previously