Experiment: Mirror Langmuir Probe measurements reveal that the …labombard/Labombard... · 2013....
1
Mirror Langmuir Probe observations of edge plasma turbulence and the Quasi-Coherent Mode in Alcator C-Mod 1 B. 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. Zweben 4 2 MIT Plasma Science and Fusion Center 1 Supported by USDoE Coop. Agreement DE-FC02-99ER54512 3 Plasma University of Tromsø 4 Princeton Plasma Physics Laboratory Langmuir probe B θ magnetic probe ~ Target EDA H-mode 0.0 0.4 0.8 MA 0 1 2 10 20 m -2 0 1 2 3 4 5 Tesla 0 Freq [kHz] -2 -1 0.0 log 10 100 200 0.0 0.5 1.0 1.5 seconds 40 30 20 10 mm 1120712027 Plasma Current Line Density H α B T Phase Contrast Imaging Frequency Spectrum Magnetic Probe Position Quasi-Coherent Mode ~ Investigate structure of Quasi-Coherent Mode using Mirror Langmuir and Magnetic (B θ ) 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 ˜ n ˜ T e and relative phase angles ˜ B ˜ Phase propagation relative to V ExB , V de 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, I sat , T e and V f ‘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” F e e d b a c k : C o u p l i n g C a p a c i t a n c e D u m m y P r o b e L a n g m u i r P r o b e E r r o r F e e d b a c k S y n c h r o n i z e d w i t h V o l t a g e W a v e f o r m E r r o r S i g n a l P r o b e C u r r e n t S i g n a l + - E l e c t r o n T e m p e r a t u r e M i r r o r C u r r e n t S i g n a l F l o a t i n g P o t e n t i a l 1 / T e V f L o g ( I s a t ) I o n S a t . C u r r e n t C u r r e n t M o n i t o r M i r r o r L a n g m u i r P r o b e ( b i a s e d R F t r a n s i s t o r s ) M i r r o r P r o b e C u r r e n t L a n g m u i r P r o b e C u r r e n t P l a s m a R e a l - T i m e O u t p u t : 3 - S t a t e V o l t a g e S o u r c e F e e d b a c k : W a v e f o r m A m p l i t u d e + . 6 8 T e a b c a c t i m e 0 - 3 . 3 T e V s 3 - S t a t e V o l t a g e W a v f o r m c a b c o a x i a l c a b l e c o a x i a l c a b l e P r o b e V o l t a g e S i g n a l V s - Real-time T e and I sat signals are used to adjust voltage drive amplitude and coupling capacitance. Result: - Real-time I sat , T e , and V f 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 device 1 that adjusts its I-V response in real time to match that of an actual Langmuir probe What is a Mirror Langmuir Probe? Probe Voltage Waveform MLP Test Data -200 -100 0 100 volts -0.2 -0.1 0.0 0.1 0.2 amps -2 0 2 4 volts 0.0 0.5 1.0 1.5 2.0 μs -2 0 2 4 volts 0.9 μs Electron Collection Near floating Ion 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 600.08 600.12 600.16 600.20 ms eV 1120531021 NW 0.2 0.4 0.6 -100 -80 -60 -40 -20 20 40 60 80 Electron Temperature Floating Potential Ion Saturation Current 1120531021 NW amps volts eV 590 595 600 605 610 ms mm Distance into SOL 0.0 0.2 0.4 0.6 0.8 -100 -80 -60 -40 -20 0 20 40 60 80 -5 0 5 10 15 Electron Temperature Floating Potential Ion Saturation Current 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 of I sat , V f , and T e reported by Mirror Langmuir Probe 22,000 measurements of I sat , V f , and T e from a single electrode MLP waveforms from C-Mod fast-scanning probe Expanded time scale Immediate observation: I sat and T e fluctuations tend to track one another Real-time signals of I sat , V f , and T e reported by Mirror Langmuir Probe Use Fit data for remainder of poster 1 μs time resolution adequate to resolve plasma dynamics Post-processing computation of I sat , V f , and T e from I-V data yield nearly identical signals, but with no slew rate limitations Same data on expanded time scale Plasma potential = sh T e + V f Plasma density n = I sat /(2 Area C s ) q Vp+ Vp0 Vp- Ip+ Ip0 Ip- 160 155 165 170 175 180 Time (μs) after 0.6 seconds 1120531021 NW -300 -200 -100 0 volts -0.4 0.0 0.4 amps 0.2 0.4 0.6 amps -90 -70 -50 volts 50 70 90 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 recorded at high bandwidth Compute: Mirror Langmuir Probe -- a powerful new tool for investigating boundary n, T e , Φ profiles and turbulence Expanded time scale ~ 1.5 rad/cm; perturbation ~field-aligned [1] 1120712027 1.135 1.140 1.145 1.150 1.155 1.160 Freq [kHz] -1.0 0 log 10 Frequency Spectrum 1.135 1.140 1.145 1.150 1.155 1.160 [rad/cm] -1.0 0 log 10 [rad/cm] Poloidal Wavenumber Spectrum 1.135 1.140 1.145 1.150 1.155 1.160 seconds mm Probe Position 50 100 150 -4 -2 0 2 4 -4 -2 0 2 4 40 30 20 10 k Spectra from B θ pickup coils [1] J. Snipes, et al., PPCF 43 (2001) L23. ~ QCM has strong B θ , J // components ~ ~ 50kHz < f < 200kHz -0.10 0 0.10 mT 1.14680 1.14685 1.14690 1.14695 1.14700 Time (s) -1.0 -0.5 0 0.5 1.0 mT -10 -5 0 5 10 A/cm 2 B θ magnetic probe ~ QCM propagates in electron diamagnetic direction (lab frame) At location of QC mode layer: B r ~ 0.5 mT, J // ~ 5 A/cm 2 θ ) -4 -2 0 2 4 20 40 60 80 100 120 140 Frequency (kHz) -1.0 0.0 Cross-Power Spectrum, S(f,k -4 -2 0 2 4 (radians/cm) 20 40 60 80 100 120 140 Frequency (kHz) dr:12.mm ~ B r at mode layer ~Equivalent J // Measured Poloidal Field k k k B 0 Results from B θ probe: Langmuir probe Target EDA H-mode 0.0 0.4 0.8 MA 0 1 2 10 20 m -2 0 1 2 3 4 5 Tesla 0 Freq [kHz] -2 -1 0 log 10 100 50 150 0.0 0.5 1.0 1.5 seconds 40 30 20 10 mm Plasma Current Line Density H α B T Phase Contrast Imaging Frequency Spectrum Langmuir Probe Position Quasi-Coherent Mode 1120814028 N S W E Mirror Langmuir Probe investigation of 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 ˜ n ˜ T e South North 1120814028 1.195 1.200 1.205 1.210 0.0 0.2 0.4 MW P rad Freq [kHz] -1.0 0 log 10 Frequency Spectrum [rad/cm] -1.0 0 log 10 [rad/cm] Poloidal Wavenumber Spectrum seconds mm Probe Position 50 100 150 Spectra from North-South electrodes 10 5 0 -5 -15 -2 0 2 4 -4 -2 0 2 4 k MLP passes through mode layer -- reveals density fluctuation with frequency and wavenumber of QCM log 10 -4 -2 0 2 4 20 40 60 80 100 120 140 Frequency (kHz) -1.0 0.0 -4 -2 0 2 4 (radians/cm) 20 40 60 80 100 120 140 θ ) Cross-Power Spectrum, S(f,k k 1120814028 : 1.1964 s Mode exists near LCFS Probe perturbs plasma at peak insertion (see P rad 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 B θ 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: T e ~ 50 eV at LCFS (used here to set ρ = 0 location) - Profiles deeper into plasma are unreliable - QCM exists in region of positive E r (i.e. with ExB in ion dia. dir.) -2 0 2 4 6 8 10 ρ (mm) 0 1 2 10 20 m -3 0 1 0 40 20 60 eV 0 40 80 volts arb. units Density Electron Temperature Plasma Potential I sat Fluctuation Power 80 kHz < f < 120 kHz Profiles from East electrode 1120814028 - V dpe, V de are in opposite directions to V ExB in mode layer Quasi-Coherent Mode propagates at electron diamagnetic drift velocity in the plasma frame Frequency (kHz) 0 4 8 -1 0 log 10 -4 -2 0 2 4 6 8 10 kHz km/s km/s I sat Fluctuation Power -100 0 100 200 -5 0 5 10 15 ρ (mm) 1120814028 - V dpe, V de are stronger than V ExB in mode layer Velocities computed from East electrode profiles V de = T e r n b nB V dpe = r nT e b nB V ExB = b r B V de V dpe V ExB V ExB V dpe+ V ExB ) k θ (V dpe+ V ExB V de + V ExB ) k θ (V de + arb. units QCM - QCM propagates in e - dia. direction in the plasma frame QCM frequency is quantitatively consist with a k θ ~ 1.5 rad/cm mode propagating with velocity between V dpe and V de in the plasma frame. Radial extent of QCM fluctuation is mapped out separately by each electrode as they pass though layer 1120814028 1.194 1.200 1.206 Time (s) 10 5 0 -5 0.2 0.3 0.4 MW N S W W E E S N arb. units ρ (mm) Isat Pwr 80kHz < f < 120kHz East Probe Position P rad normalized Radial width of QCM is 3 mm Note: I sat power normalized to same peak value for in-going scan Time delay is seen among probes, consistent with their radial positions At peak insertion, P rad jumps up -- probe-induced impurity injection => electrodes likely affected; I sat power envelopes are different on out-going scan ~ ~ N S W E 0.0 0.2 0.4 0.6 0.8 1.0 -4 -2 0 2 4 6 8 10 ρ (mm) I sat Power 80kHz < f < 120kHz W E S N arb. units Ion Saturation Current A A B I sat scaled and ρ adjusted, such that profiles overlay amps B I sat power scaled, such that profiles overlay (using ρ values from ) A I sat and I sat Fluctuation Profiles Radial profiles of ion saturation current and QCM fluctuation envelope align from all four electrodes ~ ~ ~ I sat and I sat power profiles align, despite being recorded at different times by different probes Conclusion: 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 ˜ n = n exp ˜ ( ) / ˜ T e ] is ~1.5x larger than measured B ˜ n B ˜ n B ˜ n East 20 40 60 80 100 120 140 Frequency (kHz) -1.0 -0.5 0.0 20 40 60 80 100 120 140 Frequency (kHz) Phase Angle 0 2 2 - π - π π π log 10 [S(f, Φ )] 1.1964 s Cross Power Spectrum: Density and Potential ~ 10 m/s V r = ˜ n ˜ E / n B Potential lags Density with a phase angle of ~ 10 degrees => Drift wave => Drift wave Snapshot of QCM reveals large amplitude, ~in-phase, density, electron temperature and potential fluctuations 10 20 m -3 volts eV 200 250 Time (μsec) after 1.196 sec 300 350 400 1.6 1.2 2.0 40 50 60 70 80 90 100 110 120 Density Electron Temperature Plasma Potential Not a simple Boltzmann response T e ~ 45% n n ~ 30% T e T e ~ 45% -0.4 0.0 0.4 0.4 -0.4 0.0 0.4 -0.4 0.0 0.4 -0.4 0.0 n <n> nT e <nT e > ~ ~ ~ n <n> ~ Φ <T e > ~ Φ <T e > ~ 1 k // ∼ qR qR μ 0 k 2 T e ˜ J // ˜ n ˜ T e nT e ˜ T e 1 T e t ˜ A // // ˜ n ˜ T e nT e ˜ T e 1 B. Scott, PPCF, 39 (1997) 1635. Normalized electron pressure fluctuations exceed potential fluctuations by ~ 0.1 at their maxima Density fluctuations are smaller than Boltzmann Φ <T e > ~ < max ~ 0.1 ~ 0.1 But parallel electron force balance (Ohm’s law) in this case should include electron pressure and inductive E // associated with parallel current fluctuation 1 ˜ n ˜ T e nT e ˜ T e Plugging in values for QCM parameters yields: J // ~ ~ 6 A/cm 2 => Consistent with amplitude of J // inferred from magnetic probe. QCM is a electron drift-Alfven wave. ~ Non-Boltzmann response is consistent with measured EM character of the mode -- an electron drift-Alfven wave Mirror Langmuir Probe is a powerful new tool for investigating boundary n, T e , 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 ~k θ V dpe in plasma frame - EM signature - Drives transport directly across LCFS - Measured amplitude of plasma potential, electron pressure and parallel current fluctuations -1 0 log 10 kHz -100 0 100 200 V ExB ) k θ (V dpe+ V ExB ) k θ (V de + QCM -4 -2 0 2 4 6 8 10 ρ (mm) qR μ 0 k 2 T e ˜ J // ˜ n ˜ T e nT e ˜ T e 0.4 -0.4 0.0 0.4 -0.4 0.0 nT e <nT e > ~ ~ Φ <T e > ~ ~ 0.1 -4 -2 0 2 4 6 8 10 I sat Fluctuation Power ρ (mm) arb. units 1.14680 1.14685 1.14690 1.14695 1.14700 Time (s) -1.0 -0.5 0 0.5 1.0 mT -10 -5 0 5 10 A/cm 2 dr:12.mm ~ B r at mode layer ~Equivalent J // QCM is an electron drift-Alfven mode as determined by:
Transcript of Experiment: Mirror Langmuir Probe measurements reveal that the …labombard/Labombard... · 2013....
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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
0.0
0.4
0.8
MA
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1
2
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20 m
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Te
sla
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Fre
q [
kHz]
-2
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log
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0.0 0.5 1.0 1.5seconds
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mm
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
-200
-100
0
100
volts
-0.2
-0.1
0.0
0.10.2
amps
-2
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volts
0.0 0.5 1.0 1.5 2.0μs
-2
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volts
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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W
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Electron Temperature
Floating Potential
Ion Saturation Current
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W
amps
volts
eV
590 595 600 605 610ms
mm
Distance into SOL
0.00.20.40.60.8
-100-80-60-40-20
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-505
1015
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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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
1.135 1.140 1.145 1.150 1.155 1.160
Fre
q [
kHz]
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log 1
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Frequency Spectrum
1.135 1.140 1.145 1.150 1.155 1.160
[ra
d/c
m]
-1.0
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log 1
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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
0
0.10
mT
1.14680 1.14685 1.14690 1.14695 1.14700Time (s)
-1.0
-0.5
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1.0
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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Cross-Power Spectrum, S(f,k
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z)
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~ Br at mode layer
~Equivalent J//
Measured Poloidal Field
k
k k B 0
Results from Bθ probe:
Langmuir probeTarget EDA H-mode
0.0
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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
1120814028
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MW
Prad
Fre
q [
kHz]
-1.0
0
log 1
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Frequency Spectrum
[ra
d/c
m]
-1.0
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log 1
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[ra
d/c
m]
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
log 1
0
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z)
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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.)
-4 -2 0 2 4 6 8 10
-4 -2 0 2 4 6 8 10
-4 -2 0 2 4 6 8 10
-2 0 2 4 6 8 10ρ (mm)
0
1
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1020
m-3
0
1
0
4020
60
eV
0
40
80
volts
arb.
uni
ts
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)
0
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log 1
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kHz
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km/s
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
1.194 1.200 1.206Time (s)
10
5
0
-5
0.2
0.3
0.4
MW
N
S
WW
EE SN
arb
. un
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
~
~
N
S
WE
0.0
0.2
0.4
0.6
0.8
1.0
-4 -2 0 2 4 6 8 10ρ (mm)
Isat Power80kHz < f < 120kHz
W
E
SN
arb
. un
its
Ion SaturationCurrent
A
A
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
20
40
60
80
100
120
140
Fre
qu
en
cy
(kH
z)
-1.0
-0.5
0.0
20
40
60
80
100
120
140
Fre
qu
en
cy
(kH
z)
Phase Angle0
2 2- π - π π π
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
300 350 400
1.6
1.2
2.0
40
50
60
70
80
90
100
110
120
Density
Electron Temperature
Plasma Potential
Not a simple Boltzmann response
Te~ 45%
nn
~ 30% TeTe
~ 45%
-0.4
0.0
0.4
0.4
-0.4 0.0 0.4
-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)
-1.0
-0.5
0
0.5
1.0
mT
-10
-5
0
5
10
A/c
m2
dr:12.mm
~ Br at mode layer
~Equivalent J//
QCM is an electron drift-Alfven mode as determined by:
