Spa$al structure of low- frequency plasma fluctua$ons in a laboratory dipole experiment

Transcription

1 Spa$al structure of low- frequency plasma fluctua$ons in a laboratory dipole experiment J.L. Ellsworth, R.M. Bergmann, J. Kesner, MIT M. Davis, D.T. Garnier, M.E. Mauel, Columbia University

2 Introduc$on Levitated Supported Observa$on: Strongly 6 Levitated: Peaked Density Profile peaked plasma density profiles during levita$on. Hypothesis: Closed Profiles are Field Lines sustained 6 Supported: by turbulent Uniform Density Profile mixing. Goal: Evaluate the spa$al structure of the low frequency density!wave Transmitting Horn fluctua$ons terferometer Cords (b) Interferometer Measurements (c) Density and Number Radial Profiles erconducting Dipole Vacuum Vessel!Wave Receiving Horns 5 5 time (s) Figure Figure Supported Supported Radius Radius (m) (m)

3 Photodiode Array Diagnos$c 6- element photodiode array Built into an SLR camera body Detects visible light No filters are used Records the en$re shot Good frequency response up to 5 khz Signal is digi$zed at 5 khz Photodiode Plasma visible light emision + V Preamplifier BUF6 - + x OPA657 - INA + Amplifier - LF LF BUF6 +Vo -Vo

4 Diagnos$c Views N - - cm - - East [cm] PD Array PD Array + Interferometer Probe Array Top down view of the LDX vacuum chamber Diagnos$cs that measure plasma fluctua$ons include Two 6- channel photodiode arrays viewing from different toroidal loca$ons - chord microwave interferometer - element floa$ng probe array Sony camera views from the same port as PD array

5 Light Intensity (A. U.) Visible Light Emission is Propor$onal to Plasma Density n <n e L> [utorr/m ] 5 kw of mul$- frequency electron cyclotron hea$ng Visible light and line- integrated plasma density are measured on chords with minimum radii of 77 cm Mul$variable linear regression of the data gives <I>=.9 n +. <n e L> +. φ.6 where φ is the diamagne$sm

6 Light Intensity Neither the n=, nor the n= mode are observed in this shot. The data from the sony camera and the photodiode arrays agree well. Both the n= and n= modes are observed during this shot. The sony camera data and the photodiode arrays do not agree as well. Sony Camera Photodiode Array Photodiode Array Sony Camera Photodiode Array Photodiode Array Scaled, $me- averaged, chord- integrated visible light intensity from a Sony video camera, and each of the two photodiode arrays

7 Three Characteris$c Fluctua$ons Time- averaged power spectrum of photodiode array ( cm) between t=7 and 8 seconds, during 5 kw hea$ng power on shot 95 shows thee characteris$c features: n=, coherent (ω/δω ~ ) n=, quasi- coherent (ω/δω ~ 5) broadband turbulent fluctua$ons Although this shot has examples of each of the three type, they are not all present on all shots.

8 Diamagne$sm is the Best Predictor of which Fluctua$ons will be Observed!"!# $"!# %"!# &"!# '"!# (!"!# ($"!# (%"!# (&"!# ('"!# $!"!#!"#$%&'()%"**#%"(+#,-%%.( )*+,-*.#/*.*# 5!#/*.*# 5(#/*.*# 5!#6#5(#/*.*#!"!# $"!# %"!# &"!# '"!# ("!# )"!# *"!#!"#$#%&'()$*+$,-.* +,-./,#,,5# 67!#,,5# 67$#,,5# 67!#865#67$#,,5#

9 n= Mode Time trace of the line- integrated visible light intensity measured by photodiode array ( cm) shows a strong fluctuamon at 85 Hz. The mean of the signal has been subtracted. Frequency spectrum of the Mme- averaged cross phase between photodiode array ( cm) and photodiode array ( cm). The 85 Hz oscillamon is in phase indicamng n=. ms aker $me t=7 s

10 di/<i> [%] Radial Varia$on in Fluctua$on Shot 97 Amplitude for n= Mode. 6 8 Tangency Radius [cm] Line integrated data Time averaged for the period 6s < t < 9s. Deuterium plasma with 5 kw mul$- frequency ECRH Peaked near cm channel, but visible light diagnos$c may not be sensi$ve to density fluctua$ons in the plasma core.

11 S$ll Image of Plasma Shows Possible Ioniza$on Edge Shot 97 t=7 sec False Color Put photo of the plasma here.

12 Frequency [Hz] Frequency of n= Mode Decreases with Increasing Plasma Density DiamagneMsm [mwb] Frequency [Hz] 5 5 Frequency [Hz] Line Averaged Density [ 6 m - ] Density Peaking Factor n= not observed n= also observed

13 Amplitude [a.u] Amplitude [a.u] Amplitude of n= Mode Decreases with Increasing Plasma Density DiamagneMsm [mwb] Line Averaged Density [ 6 m - ] Amplitude [a.u] Density Peaking Factor n= not observed n= also observed

14 n= Primarily n=, although n= are some$mes visible. Visible light diagnos$c is not really sensi$ve to higher order modes. Edge probes measure a real rota$on of the plasma This rota$on appears to be associated with the n= mode. Oken, the same frequency is observed on both the probes and the chord integrated diagnos$cs But some$mes, there is radial varia$on in the n= mode frequency.

15 a.u. a.u. Edge Electrosta$c Probes also Measure Time- averaged Fluctua$on Power this Fluctua$on Probe array Channel Photodiode Array Channel 6 (solid) Line Average Density (dashed) Potential Cross Correlation Lag Time (ms) 9 S95 Cross- correla$on of each of the floa$ng probes in the probe array with the first probe in the array. Plasma rota$on rate is 888 Hz. See Adj. Poster by M. Mauel 5 s 9 s 6 8

16 Frequency of n= Mode Decreases with Increasing Plasma Density Frequency [Hz] DiamagneMsm [mwb] Frequency [khz] Frequency [khz] Line Averaged Density [ 6 m - ] Density Peaking Factor n= not observed n= also observed

17 Amplitude of n= Mode Decreases with Increasing Plasma Density Amplitude [a.u] DiamagneMsm [mwb] Amplitude [a.u] Amplitude [a.u] Line Averaged Density [ 6 m - ]....6 Density Peaking Factor QC only n= also observed

18 Gas Puff Experiment kw -6 Torr mwb < 6 m - > ECRH power Vessel IG Diamagetic Flux Chord average density Shot Time (sec) Frequency (khz) Time (sec) A large puff of gas is injected during $mes 7 s < t < 8 s. This causes the diamagne$sm to drop and an n= mode appears. As the diamagne$sm increases again aker the gas puff, the n= mode begins to return.

19 The n= Mode is Observed aker the ECRH is Turned Off phda_8 5 ECRH power Shot 97 khz Time (seconds) 97 The n= mode frequency decreases during the akerglow as the diamagne$c flux decreases. kw -6 Torr mwb < 6 m - > Vessel IG Diamagetic Flux Chord average density 6 8 Time (sec)

20 kw -6 Torr mwb < 6 m - > ECRH power Vessel IG Diamagetic Flux Diamagetic Flux Chord average density Compare Shots 95 and Time (sec) Shots have idenmcal RF heamng and neutral fueling. Line- Integrated Light Intensity [a.u] Small differences in neutral pressure and diamagne$sm Different density profiles Shot 8 has less visible light intensity Light Intensity [a.u] Shot R [cm] R min [cm] Shot 5

21 Radial Varia$on in the n= Frequency is Observed During Shot 98 Frequency of n= mode is about.5 khz on each channel for shot 95. Measured Spectra for Array Frequency of n= mode varies radially for shot 98. Measured Spectra for Array Frequency (khz) Frequency (khz) Minimum Radius [cm] 5. sec to 9. sec Minimum Radius [cm] Minimum Radius [cm] 5. sec to 9. sec Minimum Radius [cm] Color scale is the -me- averaged, chord- integrated fluctua-on power

22 Cross- Correla$ons of the Photodiode Array Signals During Shot 98 Measured Cross- Correla$on between Photodiode Array Channels Separated by 9 Toroidally Measured Cross-Correlation. Measured Cross- Correla$on between the Outermost Photodiode Array Channel and Each Channel with the Same Toroidal Loca$on Measured Cross-Correlation. Frequency (khz).67. Frequency (khz) Minimum Radius [cm] Minimum Radius [cm]. The n= mode is well correlated between the two arrays. The khz fluctua$on at the edge of the plasma is observed by most of the radially separated channels.

23 Synthe$c Diagnos$c We are interested in local informa$on, but the data is chord integrated Inversions are not always appropriate for fluctua$on data. Build a synthe$c diagnos$c that uses a model of the plasma fluctua$ons to see which models are consistent with the data Synthe$c diagnos$c uses the calibrated viewing chords for the photodiode array diagnos$c Accepts,, or - dimensional models

24 Fluctua$on Model Split the plasma into six sections of either equal radial extent, or equal magnetic flux Define an amplitude A k, frequency ω k, and mode number n k Fluctuation within the region is 6 5 f k (t, θ) =A k sin (ω k t n k θ) And total fluctua$on is the sum of the fluctua$ons from all six regions Ĩ(t, θ) = 6 f k (t, θ) k=

25 Synthe$c Diagnos$c Results () 5 Synthetic Spectra Model Parameters k A k ω k n k r min r max Frequency (khz) Minimum Radius [cm] An n= mode with a single frequency is consistent with the fluctua$on spectrum observed in shot 95.

26 Synthe$c Diagnos$c Results () 5 Synthetic Spectra Model Parameters k A k ω k n k r min r max Frequency (khz) Minimum Radius [cm] Two n= modes with constant amplitude and varied frequency are consistent with the fluctua$on spectrum observed in shot 98.

27 Summary Three types of low- frequency fluctua$ons are observed Broadband turbulence n= mode that corresponds to a global fluctua$on of the plasma n= mode that is closely related to the rota$on of the plasma The different types of fluctua$ons are observed under different plasma condi$ons Low frequency fluctua$ons are observed on every channel of the photodiode arrays. Determining the structure of the fluctua$ons is not straighrorward. More work is needed to fully understand the dynamics of the plasma.