Current status of FIReTIP and its applications

Current status of FIReTIP and its applications

June-Woo Juhn (Seoul National U, Korea),K.C. Lee (UC Davis),

R. Kaita (PPPL)

Oct. 30th, 2010

NSTXNSTX Supported by

College W&MColorado Sch MinesColumbia UCompXGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton UPurdue USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU IllinoisU MarylandU RochesterU WashingtonU Wisconsin

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAEAHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

POSTECHASIPP

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech RepU Quebec

NSTXNSTX Current status of FIReTIP and its applications (Juhn) Sep. 27th, 2010 2

Outline

• FIReTIP status– Basic information about FIReTIP– Fringe jump error correction– 3-point density profile by the inversion

• FIReTIP applications– Early density distribution– Density fluctuations– ELM behavior


Far Infrared Tangential Interferometer/Polarimeter (FIReTIP) is a powerful diagnostic for many applications.

• Interferometer is the one of the refractive index measurements specialized to plasma density.

• A line-integrated density is obtained from phase information of the laser beam.

• Methyl alcohol (CH3OH) laser that emits 118.8μm far infrared ( = 2.52 THz ) beam which is favorite for interferometers in most mid-size tokamaks.

• A heterodyne interferometer provides direct phase difference between the probing beam and the reference beam

– This configure requires two laser beams with slightly different frequencies.– Laser beam amplitude doesn’t matter, provided sufficient signal to noise ratio for detector and electronics.

• Frequency shift from Stark-effect enables high intermediate frequency (IF) as about 5MHz which is larger than twice that of common Methyl alcohol lasers.

• High bandwidth of signal up to 4MHz is possible because both of high IF from Stark-effect and recently improved electronics.

2

1

2

1

d1082.2d4

e15

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2

2

l

l

l

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lnlnmc

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AIR

Plasma

),()/1,()/1( nrefractionpathbeamfvibrationmechanical

Light


Electronics in FIReTIP has been recently upgraded leading to better performance especially for fast bandwidth.

Phase Comparator

Mixer

Mixer

Mixer

Mixer

SBD

SBD

Probing Beam

Local Oscillator

Reference Beam Local Oscillator

Fringe Counter

I-Q Mixer

Index 1 2 I Q

Signal ~ Φ Φ (±2.5V) cos(Φ) sin(Φ)

Bandwidth ~1MHz ~4MHz

Sampling rate 1Ms/s 12 (10) Ms/s

Range -2.5 ~ 2.5V

Fringe Information

8 x 2π 2π

Remarks Switching each other not to be limited in 5V

Arctangent to obtain Φ

12

IQ

Basic information about the FIReTIP

output signals

Simple block diagram of Electronics*

SBD : Schottky barrier diode mixer

Mixer : Frequency Up-converter

*W-C Tsai, UC Davis


Both density-converted signals from the fringe counter and the I-Q mixer are in good agreement with their own features.

FC1

FC2

I

Q

Time [sec]

#136094

Raw signals from each outputVoltage[V]

Switching in turn

Time [sec]

Lin

e-av

erag

ed e

lect

ron

dens

ity[

m-3]

x1019

Density from raw signals

FCIQ

Phase difference φ

Arctangent(Q/I)

2

1

d1082.2 e15

l

lln


Line-integrated density up to 3 channels in mid-plane is being measured.

Layout of the FIReTIP beam channels.

#Tangency

Radius [cm]

One-way Path length

[m]

Tag names on the MDS Tree(TREE name = ‘microwave’)

1 32 3.07FC \den_firetipc1

IQ \den_fast_firetipc1

2 57 2.93FC \den_firetip










(1) CH.3, 5 and 6 of IQ signals are currently on operation after July 2010.

(2) FC signals (for faster analysis with fewer data points) will be available soon including CH.1.


Fringe Jump Error has been a serious problem in FIReTIP likely to most other interferometers.

There were hundreds of discharges suffering from fringe jumps (FJs). The number of fringe jumps in those discharges are from zero to thousands, provided FIReTIP system including laser power is acceptable.

Apparently, the whole waveform seems to become more absurd as the number of FJ increases. Even in some discharges with only several fringe jumps the difference between data from FIReTIP and Thomson scattering diagnostics is quite distinct so we are not able to use the original data.

Time [sec]

Lin

e-av

erag

ed e

lect

ron

dens

ity[

m-3]

x1019

FIReTIP

Thomson scattering

#135103

18 FJs

#136105

368 FJs

Indicators of fringe jumps when they appeared.


Most Fringe Jump Errors appear in FIReTIP have common characteristics.

1. They take place in a few microseconds usually, and up to a couple of tens microseconds.

2. Most of them have typical magnitudes, i.e. one-fringe (2π)-equivalent voltages or densities.

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For Channel 1

sec][100

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322

318avg e,

m

m

m

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Obviously not a plasma activity

It is easily characterized with common parameters, which is due to the recently upgraded fast electronic system by W.C. Tsai, UC Davis


Fringe Jump Errors were suppressed by post-processing of stored data.

Fringe jump corrected data compared with those of Thomson scattering diagnostics

(a) An example of full waveforms of each case: The corrected data show very good agreement with those of Thomson scattering

(b) Averages and deviations of the discrepancies from most shots after July 2009: They are distinctively reduced after correction .

By applying the algorithm, corrected FIReTIP data is compared with Thomson Scattering data which is reproduced along the beam path of FIReTIP CH.#1.


This algorithm works even in cases of up to a few hundreds of fringe jumps event.

Fringe jump corrected data compared with those of Thomson scattering diagnostic : FI means FIReTIP data, TS Thomson scattering diagnostic data respectively.

Time [sec]

368 Fringe jumps were reported and suppressed.

Even in this case of huge errors, corrected data has quite reasonable value compared with data without correction in dashed blue line. The correction algorithm has high opportunity to make the data reliable in most situations.

This algorithm has been routinely working since FY2010 and it is archiving the corrected FIReTIP data automatically in most cases without distinct problems.


FIReTIP data is being archived though PCS digitizer for real-time density control.

CH.3 Fringe counter (FC) data is being used after the fringe jump error correction.

5kHz digitizer is fast enough to control gas injection valves which operate at least in every 2ms.

PCS algorithm for density control has to be prepared.

#Tangency

Radius [cm]

One-way Path length

[m]

Tag name on MDS Tree(TREE name = ‘microwave’)

n(~ CH3)

85 2.64 FC \n

x1019

#139833~138939 x1019x1019

NSTXNSTX Current status of FIReTIP and its applications (Juhn) Sep. 27th, 2010

Interferometer basically provides only line-integrated density information.

• In many cases, density profiles are of interest than line-integrated ones.

• It can never be determined where the specific density variations take places, if one channel of interferometer data are only obtained.

• In order to have spatial resolution, It is required to combine the FIReTIP result with other diagnostics, OR to measure several channels together then reconstruct the profile.

12

#141325 CH.3

CH.5

CH.6


Linear interpolation between several points of measurement shows rough profile but good trends following the TS data.

13

Interferometer laser beam channels

Conventional slice & stack conceptMajor Radius R [m]

Major Radius R [m]

Original model profile

Reconstructed one

Density [arb. unit]

Major Radius R [m]

Original model profile

Reconstructed one

Density [arb. unit]


Linear interpolation between several points of measurement provides reasonable profile.

14

Major Radius R [m]

Pre-determined density profile [m-3]

x1020

Calculated interferometer signals on acutal channels of

FIReTIP [m-2]

x1019

Major Radius R [m]

Reconstructed density profile

[m-3]


Linear interpolation between several points of measurement shows rough profile but good trends following the TS data.

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Major Radius R [m]Major Radius R [m]

Pre-determined density profile [m-3]

x1020

Calculated interferometer signals

on each channel of FIReTIP [m-2]

x1019


[m-3]x1020


It is getting better as channel numbers increase.

• Although it is not practical in NSTX, the reconstruction with virtual 13 channels provides much better profile information, as expected, in entire cases that were tried before.

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• However, only 3~4 channels are practically available at the moment without huge sacrifice of reliability of measurements on each channel.

Major Radius R [m]


[m-3]


Only 3 points of measurements can reconstruct rough profiles but their trends looks good following the TS data.

17

#135695

• The MPTS diagnostic is able to provides much better density profile trace for the global plasma waveform.

• Instead, FIReTIP is capable of seeing much faster behavior than MPTS does.


One example is the early electron distribution .

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• As a well-known feature, the electrons are generated mainly at the HFS region during plasma ramp-up.

• The reason is though to be the combination of higher connection length and the higher inductive field strength in HFS of tokamak.

#135695


One example is the early electron distribution .

19


FIReTIP can roughly localize the density fluctuation

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#135161

Pay attention to this edge.


Termination of plasma is visible.

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#135163


FIReTIP can provide a particle point of view for plasma behaviors, for example, ELMs.

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• Dα signals has distinct relation to the electron density reduction in ELM events.

#132403

#132401

#132407

'\bayc_dalf_fscope'


Even ELM activity is still slow enough for FIReTIP.

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#132403

Dα

• ELMs are thought to take place in LFS of tokamaks. (Kamiya 2007)

This time


Another shot is also indicating the density loss at the HFS of NSTX rather than LFS.

24

#132401


3-D view of density profile shows ELM events clearly

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Time

HFS

LFS


4 channels makes the ELM behavior more clear.

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Layout of the FIReTIP beam channels.#132403

We have few discharges with 4

CH. 1

CH.3

CH. 5

CH. 6

Dα


A Filament rotation is measured in FIReTIP.

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#135163

Time [ms]


Filament peak points are measured and compared in each channel.

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CH.1

CH.3

CH.5

#135163

Δt13

Δt35Δt55

Δt53

Δt31


Line-integrated density up to 3 channels in mid-plane is being obtained.

Layout of the FIReTIP beam channels.

R

Travel length L=Rφ

φ

Rφ

L

L1

L2

L3

L4L5

L1

L2

L3

L4

L5

R [m]

• But measured peaks do not have similar relation from the calculated ones which means the filaments have complex behavior unlikely to assumed simple travel.

Calculated path lenth distribution according to traveling radius R


Summary

• Recently upgraded electronics from UC Davis enables the fast signal processing and this leads to the easy characterization and suppression of fringe jumps in FIReTIP.

– About 1MHz and 4MHz signal bandwidth from FC and IQ were achieved.

• Fringe jump errors are automatically corrected after almost every shot in FIReTIP normal operation.

• The correction algorithm is also applicable to the FIReTIP signals digitized by PCS for real-time density control and they are being archived together.

• 1st step of density profile has been carried out via conventional method with linear interpolation that compensates the limited number of channels.

– Comparing with MPTS data, the trend from FIReTIP data in each shot seems reasonable considering just 3 channels are available at the moment.

• The fast density profile provides a little bit of local information of fast variations such as early density distribution, density fluctuations and ELMs.

Current status of FIReTIP and its applications

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