Studies of Electron Temperature Fluctuations in the Core ... · diagnostics measure electron...

Studies of Electron Temperature Fluctuations in the Core of Alcator C-Mod Plasmas via Correlation ECE

C. Oi, C. Sung, N. Howard, A. E. White, J. Irby, R. Leccacorvi, R. Vieira, J. Rice, C. Gao

1

MIT – Plasma Science and Fusion Center (PSFC)

54th Annual Meeting Division of Plasma Physics October 29-November 2, 2012, Providence, RI

Curran Oi 54th APS-DPP Providence, RI 2012

Summary

• CECE diagnostic at Alcator C-Mod is able to measure Te fluctuations in the core of fusion plasmas

• fluctuations are believed to be responsible for anomalous transport seen in fusion plasmas

• Recent data shows higher Te fluctuation levels in linear ohmic confinement (LOC) than in saturated ohmic confinement (SOC) plasmas

2 Curran Oi 54th APS-DPP Providence, RI 2012

Anomalous transport motivates CECE diagnostic

• Transport in tokamaks is larger than predicted by neoclassical theory

• Turbulent fluctuations may be responsible for the anomalous transport

• Correlation electron cyclotron emission (CECE) diagnostics measure electron temperature fluctuations

• Electron temperature fluctuations can be compared to density fluctuations from other diagnostics (e.g. reflectometer, Phase Contrast Imaging (PCI), Two Color Interferometer (TCI))


Past attempts to measure Te fluctuations

• Past attempts to measure electron temperature fluctuations in C-Mod failed – None seen above statistical limit [Watts NF 2004]

• Nonlinear GYRO simulations used to design new CECE diagnostic [White PPCF 2011]

– Determined that spot size in plasma was too large

– New CECE diagnostic successful at measuring electron temperature fluctuations


C-Mod parameters Alcator C-Mod Parameters [Fusion Sci. Technol. 51 (2007)]


Parameter Value

R 0.67m

r 0.2m

BT 2-8T

Ip 0.3-2.0 MA

ne 0.1-10x1020/m3

Te~Ti 1-8 keV

ECE principles • The harmonics of the electron cyclotron frequency

• ECE uses fact that plasmas are optically thick at fec,1 (O-mode) and fec,2 (X-

mode) – Plasma acts like a blackbody at these harmonics

• In fusion plasmas hf << kT so

– hf ≈ 1.66 x 10-22 J and kT ≈ 1.38 x 10-15 J for a typical fusion plasma

(if hf << kT)

B = 5.4 T fec,2 ≅ 250 GHz


ECE principles

• B field in a tokamak proportional to 1/R – therefore ECE is proportional to 1/R

• ECE is well localized in plasmas confined by a toroidal magnetic field


ECE principles • The sensitivity of a single ECE signal to temperature

fluctuations

• Typical case has Bvid ~ 300 kHz and BIF ~ 500 MHz

• The fluctuations are about 1% so it it not possible to see them with a single ECE signal – Need to correlate two signals to get better sensitivity [Cima POP ‘95,

Sattler PRL ‘94, Sung RSI ‘12]

Bvid: video bandwidth BIF: IF bandwidth


Methods for noise suppression • Two correlation techniques

• CECE diagnostic at Alcator C-Mod uses spectral decorrelation

1) Spatial decorrelation 2) Spectral decorrelation

Spatial decorrelation Spectral decorrelation

Principle Large enough separation angle, α, thermal noise uncorrelated

Separate in frequency space, thermal noise uncorrelated

Advantage High radial resolution High poloidal resolution

Disadvantage Poor radial resolution Need two radiometers

Poor radial resolution


CECE correlation technique • Cross-correlation of two ECE signals which have uncorrelated

thermal noise and share common electron temperature fluctuations

• If , , and are uncorrelated with each other, then

• Sensitivity of diagnostic is improved via cross correlation

temperature fluctuations in signal x

thermal noise in signal x

Thermal noise is removed via cross correlation of two signals with uncorrelated thermal noise

Number of samples, Δt is the averaging time

In Alcator C-Mod, this reduces to ~0.3%


Hardware: In-vessel optics

• Mirrors mounted on outer wall of tokamak provide 1cm spot size in plasma


Hardware: CECE receiver

• Receiver installed near A port • RF section installed at front part of port to reduce waveguide length and

therefore reduce signal losses • RF and IF sections connected via SMA cable


Initial four-channel radiometer initially configured for Te measurements


CECE calibration results

• CECE measurements in agreement with profile radiometer (FRCECE) measurements

• Confirmed CECE could measure Te


Updated Radiometer Design for Te fluctuation measurements


CECE uses flexible quasi-optical system through ex-vessel modular arrangement

Horn Antenna

Lens for collimation window flat mirror parabolic mirror

RF section

• Linear diagram of CECE optical system

• CECE optical system can be considered as 1-D quasi-optical system with antenna, lens and mirrors.

• By changing the collimating lens in front of horn antenna, the focal point of parabolic mirror is changed due to beam spreading.

• Different lens can be used to change focal point without modification any in-vessel components.

• Flexible system through ex-vessel modular arrangement


In-vessel components Ex-vessel components

Improvements in optics allowed for electron temperature fluctuation data

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

1

2

3

4

5

6

Beam

dia

met

er (c

m)

rho

f=10cm f=7.5cm f=5cm

0.76 0.78 0.80 0.82 0.84 0.86 0.88 0.900

1

2

3

4

5

6

Beam

dia

met

er (c

m)

Rmid (m)

f=10cm f=7.5cm f=5cm

• Change of beam diameter depending on ex-vessel lens

• First data were obtained at r/a~0.85 after f=10cm lens was replaced with f=7.5cm lens.

• Changing f=7.5cm lens to f=5cm lens move focal point further inside. This change made it possible to measure fluctuations further inside of plasma (r/a<0.8).


High Pass Filtering / Testing for Fluctuation Measurements

• High pass filters used to remove DC and low frequencies from signal


Signals with and w/o high pass filter w/o high pass filter

with high pass filter


LOC/SOC: Intro and definitions

• In ohmic plasmas, LOC (linear ohmic confinement) regime occurs when confinement time increases linearly with density

• At a critical density, confinement time saturates (SOC regime)

• Characteristic turbulence modes differ between LOC and SOC regimes

• LOC dominated by TEM (trapped electron modes)

• SOC dominated by ITG (ion temperature gradient) modes

[contributed talks by Sung (BO7.00004), Rice (BO7.00005), Ennever (BO7.00006) C-Mod session Monday]


Rice POP 2012

LOC/SOC: Literature summary Author Year TEM LOC

and ITG SOC

Fluctuation Data Presented

in Paper

Growth Rates Presented in

Paper

Angioni 2005 Yes No No Esposito 2004 Yes No No

Lin 2009 No Yes Yes Rettig 2001 Yes Yes Yes Rice 2011 Yes Yes No

• Consensus that TEM turbulence dominant in LOC regime and ITG turbulence modes dominant in SOC regime

• Very few papers present fluctuation data or growth rates (only Rettig presents both)


LOC/SOC: Fluctuations from CECE measurements


ρ~0.8 ρ~0.8 LOC SOC

LOC SOC

23

LOC/SOC: Linear stability analysis LOC plasmas (ρ~0.8) SOC plasmas (ρ~0.8)

kyρs < 0.6, TEM is dominant in LOC, ITG is dominant in SOC kyρs > 0.6, TEM is dominant in both LOC and SOC

CECE can measure turbulence up to kyρs~0.3 (Green shading region). CECE measure fluctuations from TEM in LOC plasmas, and it was reduced

as ITG mode became dominant in SOC plasmas.

Curran Oi 54th APS-DPP Providence, RI 2012

Results: fluctuation levels in LOC/SOC • SOC shows lower fluctuation levels than LOC regime


Results: LOC/SOC fluctuation shots

Shot Rho ch 1&2

Ip [MA] Bt [T] navg [1020 m-3]

nl04 [1020 m-2]

Critical line averaged density/line averaged density

LOC/SOC

1120615013 0.84 1.02 5.45 0.65 0.4 1.59 LOC

1120615015 0.84 1.02 5.45 0.75 0.45 1.38 LOC

1120615017 0.84 1.02 5.45 0.8 0.45 1.29 LOC

1120615021 0.84 1.02 5.49 0.8 0.45 1.29 LOC

1120615024 0.84 1.03 5.45 1.1 0.65 0.95 SOC

1120615025 0.84 1.03 5.45 1.3 0.75 0.80 SOC

1120615026 0.84 1.03 5.45 1.35 0.8 0.77 SOC

• Shots and critical values for LOC/SOC fluctuation level plot (note: same Ip, Bt, and rho values)


Results: LOC/SOC fluctuation time series

• Time series shows fluctuation level higher in LOC than SOC during entire shot


Results: LOC/SOC time series shots

Shot Rho ch 1&2 Ip [MA] Bt [T] navg [1020 m-

3] nl04 [1020 m-2]

1120615022 0.84 1.02 5.45 1 0.56 1120615015 0.84 1.02 5.45 0.75 0.45 1120615025 0.84 1.03 5.45 1.3 0.75 1120615026 0.84 1.03 5.45 1.35 0.8

• Shot values for those used in time series plot of fluctuations • Similar Ip and Bt for all four shots used


Results: LOC/SOC time series <n> and Ip

• Line averaged density of four shots is higher for SOC plasmas than for LOC plasmas

• Plasma current Ip almost identical for all four shots


Conclusions

• CECE diagnostic at Alcator C-Mod is able to measure Te fluctuations

• Te fluctuation levels lower in SOC than LOC

• LOC plasmas are TEM dominant

• SOC plasmas are on the border between TEM and ITG dominant – Fall slightly on ITG side


Future Work

• Measure beam-pattern of CECE from mock-up CECE system in the laboratory

• Upgrade CECE system to measure fluctuations farther into plasmas

• Develop a synthetic diagnostic for CECE in

C-Mod


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