Mirror Langmuir Probe observations of edge plasma turbulence and the Quasi-Coherent Mode in Alcator C-Mod1B. LaBombard,2 T. Golfinopoulos,2 D. Brunner,2 O.E. Garcia,3 M. Greenwald,2 J.W. Hughes,2 R. Kube,3 J.L. Terry,2 S. Zweben4

2MIT Plasma Science and Fusion Center 1Supported by USDoE Coop. Agreement DE-FC02-99ER54512 3Plasma University of Tromsø 4Princeton Plasma Physics Laboratory

Langmuir probe

Bθ magnetic probe~Target EDA H-mode

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log

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1120712027

Plasma CurrentLine Density

Hα BT

Phase Contrast Imaging Frequency Spectrum

Magnetic Probe Position

Quasi-Coherent Mode

~Investigate structure of Quasi-Coherent Modeusing Mirror Langmuir and Magnetic (Bθ) probes

Why study the QCM?A first-principles understanding of these modes is an important step towards unfolding the transport physics of the boundary layer.

Coherent edge modes, such as the QCM (in H-mode) and the “weakly coherent mode” WCM (in I-mode) play key roles in pedestal dynamics, including regulating particle and impurity confinement without ELMS.

Goal:

-resolved , , , k ñ T̃e and relative phase anglesB̃˜Phase propagation relative to VExB, Vde

Unambiguously identify QC mode type (drift wave, interchange, ...)

Radial width of mode layer

Measurements:

Experiment: Mirror Langmuir Probe measurements reveal that the Quasi-Coherent Mode is an electron drift-Alfven wave.

- A 3-state, fast-switching voltage waveform is applied to a Langmuir Probe.

- ‘Mirror LP’ responds to probe voltage, generates its own I-V response.

- By active feedback, Isat, Te and Vf ‘controls’ on MLP are adjusted to match that of Real LP.

Important: These data are obtained from a single LP electrode.

MLP bias: a “triple probe in the time domain”

Feedback:CouplingCapacitance

Dummy Probe

LangmuirProbe

Error FeedbackSynchronizedwith VoltageWaveform

Error Signal

ProbeCurrentSignal

+-

ElectronTemperature

MirrorCurrentSignal

FloatingPotential

1/Te

Vf

Log (Isat)

Ion Sat.Current

CurrentMonitor

Mirror Langmuir Probe(biased RF transistors)

MirrorProbeCurrent

LangmuirProbeCurrent

Plasma

Real-TimeOutput:

3-StateVoltageSource

Feedback:WaveformAmplitude

+.68Te

ab c

ac

time

0

-3.3Te

Vs

3-State Voltage Wavform

c

a

b

coaxial cable

coaxial cable

ProbeVoltageSignal

Vs

- Real-time Te and Isat signals are used to adjust voltage drive amplitude and coupling capacitance.

Result: - Real-time Isat, Te, and Vf signals which ‘mirror’ that of Real LP

[1] LaBombard, B. and Lyons, L., Rev. Sci. Instrum. 78 (2007) 073501;Lyons, L., Masters Thesis, EECS, MIT (2007).

- Optimum “triple probe bias” is applied to a single electrode

An electronic device1 that adjusts its I-V responsein real time to match that of an actual Langmuir probe

What is a Mirror Langmuir Probe?

Probe Voltage Waveform

MLP Test Data

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0.9 μsElectron CollectionNear floatingIon saturation

Bias States

Probe Current Response

TTL Timing Pulses:

Data Sample Timing

MLP waits for switching transients to die away before taking data samples

Bias Timing

Voltage

Current

Voltage changes by up to 360V,settling in less than ~ 150ns

TTL Timing - Bias

TTL Timing - Data Sample

MLP Fast-Switching Voltage Waveform: Samples I-V characteristic at three bias states in under 1 μs

amps

volts

600.08 600.12 600.16 600.20ms

eV

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Electron Temperature

Floating Potential

Ion Saturation Current

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mm

Distance into SOL

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Electron TemperatureFloating Potential

Ion SaturationCurrent

Data from a probe scan to the separatrix in an ohmic L-mode plasma

Fluctuations in signals are not noise!These are plasma fluctuations.

Real-time signals ofIsat, Vf, and Te reportedby Mirror Langmuir Probe

22,000 measurementsof Isat, Vf, and Te froma single electrode

MLP waveforms from C-Mod fast-scanning probe

Expanded time scale

Immediate observation: Isat and Te fluctuations tend to track one another

Real-time signals ofIsat, Vf, and Te reportedby Mirror Langmuir Probe

Use Fit data for remainder of poster

1 μs time resolution adequateto resolve plasma dynamics

Post-processing computationof Isat, Vf, and Te from I-Vdata yield nearly identical signals,but with no slew rate limitations

Same data on expandedtime scale

Plasma potential = shTe +VfPlasma density n = Isat /(2 Area Cs)q

Vp+ Vp0 Vp-

Ip+ Ip0 Ip-

160155 165 170 175 180Time (μs) after 0.6 seconds

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5310

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W

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volts

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volts

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eV

Real-time MLP

Fit

MLP

Electron Temperature

Floating Potential

Ion Saturation Current

MLP

Fit

Probe Current

Probe Voltage

1 μs

Post-processing Fit

I-V data are also recordedat high bandwidth

Compute:

Mirror Langmuir Probe -- a powerful new tool forinvestigating boundary n, Te, Φ profiles and turbulence

Expanded time scale

~ 1.5 rad/cm; perturbation ~field-aligned [1]

1120712027

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q [

kHz]

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Frequency Spectrum

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[ra

d/c

m]

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[ra

d/c

m]

Poloidal Wavenumber Spectrum

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Probe Position

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Spectra from Bθ pickup coils

[1] J. Snipes, et al., PPCF 43 (2001) L23.

~

QCM has strong Bθ, J// components~ ~

50kHz < f < 200kHz -0.10

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mT

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A/c

m2

Bθ magnetic probe~

QCM propagates in electron diamagnetic direction (lab frame)At location of QC mode layer: Br ~ 0.5 mT, J// ~ 5 A/cm2

θ)

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z)

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Cross-Power Spectrum, S(f,k

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qu

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cy

(kH

z)

dr:12.mm

~ Br at mode layer

~Equivalent J//

Measured Poloidal Field

k

k k B 0

Results from Bθ probe:

Langmuir probeTarget EDA H-mode

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sla

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log

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Plasma CurrentLine Density

Hα BT

Phase Contrast Imaging Frequency Spectrum

Langmuir Probe Position

Quasi-Coherent Mode

1120814028

N

S

WE

Mirror Langmuir Probe investigationof Quasi-Coherent Mode

Experimental setup:

˜Plunge probe across QCM mode layer

Does probe ‘kill the mode’? Use electrodes spaced in minor radius direction to assess probe perturbation effects

Goals:Record response Determine mode layer width (?)

, , , k ñ T̃e

South

North

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MW

Prad

Fre

q [

kHz]

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Frequency Spectrum

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m]

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[ra

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Poloidal Wavenumber Spectrum

seconds

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Probe Position

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MLP passes through mode layer -- reveals densityfluctuation with frequency and wavenumber of QCM

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θ) Cross-Power Spectrum, S(f,k

k

1120814028 : 1.1964 s

Mode exists near LCFS

Probe perturbs plasma at peak insertion (see Prad jump)

Must examine other electrodes to see if probe is perturbing mode...Post mortum: leading edge of probe head showed melt damage

Probe appears to pass through mode

Frequency, poloidal wave number and propagation in electron diamagnetic direction -- consistent with Bθ probe, PCI (and GPI)

East

Quasi-Coherent Mode lives at separatrix,in steep gradient region,with positive radial electric field

=> consistent with QCM kicking impurities out confined plasma onto open field lines

QCM spans LCFS

- From power balance: Te ~ 50 eV at LCFS (used here to set ρ = 0 location)

- Profiles deeper into plasma are unreliable

- QCM exists in region of positive Er (i.e. with ExB in ion dia. dir.)

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Density

Electron Temperature

Plasma Potential

Isat Fluctuation Power 80 kHz < f < 120 kHz

Profiles from East electrode

1120814028

- Vdpe, Vde are in opposite directions to VExB in mode layer

Quasi-Coherent Mode propagates at electrondiamagnetic drift velocity in the plasma frame

Freq

uenc

y (k

Hz)

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kHz

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Isat Fluctuation Power-100

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ρ (mm)

1120814028

- Vdpe, Vde are stronger than VExB in mode layer

Velocities computed from East electrode profiles

Vde =Te rn b

nBVdpe = r

nTe bnB

VExB =b r

BVdeVdpe

VExB

VExBVdpe+

VExB)kθ(Vdpe+VExBVde+

VExB)kθ(Vde +

arb.

uni

ts

QCM- QCM propagates in e- dia. direction in the plasma frame

QCM frequency is quantitatively consist with a kθ ~ 1.5 rad/cm mode propagating with velocity between Vdpe and Vde in the plasma frame.

Radial extent of QCM fluctuation is mapped out separately by each electrode as they pass though layer

1120814028

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its

ρ (m

m)

Isat Pwr 80kHz < f < 120kHz

East ProbePosition

Prad

normalized

Radial width of QCM is 3 mm

Note: Isat power normalized to same peak value for in-going scan

Time delay is seen among probes, consistent with their radial positions At peak insertion, Prad jumps up -- probe-induced impurity injection

=> electrodes likely affected; Isat power envelopes are different on out-going scan

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~

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its

Ion SaturationCurrent

A

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B

Isat scaled and ρ adjusted,such that profiles overlay

am

ps

B Isat power scaled,such that profiles overlay (using ρ values from )

A

Isat and Isat Fluctuation Profiles

Radial profiles of ion saturation current and QCM fluctuation envelope align from all four electrodes

~

~

~Isat and Isat power profiles align,despite being recorded at different times by different probesConclusion: QCM is not being attenuated by probe

Radial width of Quasi-Coherent Mode layer is ~ 3 mm FWHM

Simple Boltzmann electron response? Compute required to satisfy

ñ = n exp ˜( ) /T̃e ] is ~1.5x larger than measured

B

ñB

ñB ñ

East

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log 1

0[S(

f,Φ)]

1.1964 s

Cross Power Spectrum: Density and Potential

~ 10 m/sVr = ñẼ / n B

Potential lags Density witha phase angle of ~ 10 degrees

=> Drift wave

=> Drift wave

Snapshot of QCM reveals large amplitude,~in-phase, density, electron temperature and potential fluctuations

1020

m-3

volts

eV

200 250Time (μsec) after 1.196 sec

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Density

Electron Temperature

Plasma Potential

Not a simple Boltzmann response

Te~ 45%

nn

~ 30% TeTe

~ 45%

-0.4

0.0

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0.4

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-0.4 0.0 0.4

-0.4

0.0

n

nTe

~

~ ~

n

~

Φ

~

Φ

~1k// ∼ qR

qRμ0k 2 Te

J̃ //ñT̃enTe

˜

Te

1Te t

Ã// //ñT̃enTe

˜

Te

1B. Scott, PPCF, 39 (1997) 1635.

Normalized electron pressure fluctuations exceedpotential fluctuations by ~ 0.1 at their maxima

Density fluctuations are smaller than Boltzmann Φ

~<

max~ 0.1

~ 0.1

But parallel electron force balance (Ohm’s law) in thiscase should include electron pressure and inductiveE// associated with parallel current fluctuation1

ñT̃enTe

˜

Te

Plugging in values for QCM parameters yields: J//~ ~ 6 A/cm2

=> Consistent with amplitude of J// inferred from magnetic probe.

QCM is a electron drift-Alfven wave.

~

Non-Boltzmann response is consistentwith measured EM character of the mode -- an electron drift-Alfven wave

Mirror Langmuir Probe is a powerful new tool for investigating boundary n, Te, Φ profiles and turbulence

MLP is revealing new insights on Alcator C-Mod’s Quasi-Coherent Mode in ohmic EDA H-mode plasmas:

QCM spans LCFS region with a mode width of ~ 3mm

Summary

- Mode frequency at ~kθVdpe in plasma frame

- EM signature

- Drives transport directly across LCFS

- Measured amplitude of plasma potential, electron pressure and parallel current fluctuations

-1

0

log 1

0

kHz

-100

0

100

200 VExB)kθ(Vdpe+

VExB)kθ(Vde +QCM

-4 -2 0 2 4 6 8 10ρ (mm)

qRμ0k 2 Te

J̃ //ñT̃enTe

˜

Te

0.4

-0.4 0.0 0.4-0.4

0.0

nTe

~ ~

Φ

~

~ 0.1

-4 -2 0 2 4 6 8 10

Isat Fluctuation Power

ρ (mm)

arb.

uni

ts

1.14680 1.14685 1.14690 1.14695 1.14700Time (s)

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0

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1.0

mT

-10

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0

5

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A/c

m2

dr:12.mm

~ Br at mode layer

~Equivalent J//

QCM is an electron drift-Alfven mode as determined by: