CORRELATION ELECTRON CYCLOTRON

EMISSION DIAGNOSTIC FOR ALCATOR

C-MOD : DESIGN AND FIRST RESULTS

C. Sung1), A. E. White1), J. H. Irby1), R. Vieira1), R.

Leccacorvi1), C. Y. Oi1), W. A. Peebles2), X. Nguyen2)

1) MIT-Plasma Science and Fusion Center

2) UCLA

19th Topical conference High Temperature Plasma Diagnostics

May 6-10, 2012 Monterey, CA

Summary

Using GYRO simulations, a new CECE diagnostic for

C-Mod was designed with the guidance of the

simulations.

In 2011 campaign, CECE diagnostic was installed, and

first results were obtained.

From the first result, it was observed that CECE

radiometer performed as expected.

Introduction

Importance of turbulence diagnostics and

Correlation ECE in C-Mod

Transport in fusion devices is much larger than expected, and it has been

postulated that turbulent electrostatic fluctuations are responsible for this

anomalous transport.[1]

To understand turbulent transport, it is required to measure the

fluctuations of electron density, temperature and poloidal electric field

(and magnetic field, if fluctuations are electromagnetic)

Correlation Electron Cyclotron Emission (CECE) is a diagnostic

for electron temperature fluctuation measurements

New CECE will contribute to transport studies in C-Mod:

Electron heat transport in ohmic and L-mode plasmas

SOC/LOC rotation reversal

Fast ion driven coherent mode

Limitation of ECE for fluctuation

measurements Sensitivity of fluctuation measurement with a single ECE radiometer

2e vid

e IF

T B

T B Bvid : video bandwidth

BIF : IF bandwidth

For a profile radiometer, Bvid~1MHz and BIF~1.5GHz,

Considering the typical Te fluctuation level is ~1%, it is not possible

to measure fluctuations using a single ECE channel.

2~ 4%e vid

e IF

T B

T B

This sensitivity comes from thermal noise in ECE signals

We need to eliminate thermal noise to measure fluctuations.

Correlate 2 channels with uncorrelated thermal noise.

Cross correlation technique for

Te fluctuation measurements

Cross-correlation of two ECE channels, which have common temperature

fluctuation, 𝑇 𝑒

,1 1 ,1

,2 2 ,2

2

12 ,1 ,2 1 2 1 2

,

,

e e e

e e e

e e e e e

T T N T

T T N T

R T T T T N T N N N

: fluctuation signal in ch1,

: fluctuation signal in ch2,

If 𝑁 1, 𝑁 2 and 𝑇 𝑒 are uncorrelated each other,

𝑁 1 : noise signal in ch1

𝑁 2 : noise signal in ch2

1 2 1 2 0e eT N T N N N

Uncorrelated noise is eliminated through cross correlation

technique

2

,1 ,2 ~e e eT T T

Thus, condition for applying cross correlation technique will be,

Common temperature fluctuations in two ECE signals

Uncorrelated noise signals in two ECE signals

(In good radiometer, most of noise will be thermal noise.)

Thermal noise suppression techniques

Two diagnostic methods have succeeded in suppression of thermal noise

1) Spatial decorrelation 2) Spectral decorrelation

Spatial decorrelation Spectral decorrelation

Principle

Large enough separation angle, α,

noises from two sight lines are

uncorrelated

Separate in the frequency space,

two noises are uncorrelated

Advantage High radial resolution High poloidal resolution

Easy to implement

Disadvantage Poor poloidal resolution

Need two radiometers Poor radial resolution

CECE in C-Mod will use spectral decorrelation because of the limited port

space and high poloidal resolution.

[2] Watts, RSI (2004)

Design of CECE

Diagnostic in C-Mod

Past attempts in C-Mod for Te fluctuation

measurements

Result : no broadband Te fluctuation

above statistical limit is existed in C-Mod.

Possible explanations of the result

No core high frequency Te fluctuation

in C-Mod

Large spot size filters out high k

fluctuation measurement.

Poloidally asymmetric, off-midplane view

brings low fluctuation amplitude

[3]

Using correlation technique in the signal from high spatial resolution radiometer

(FRCECE) , core Te fluctuation measurement in C-Mod was attempted in the past.

We need to understand past null results for turbulent Te fluctuations in C-Mod

through gyro kinetic simulation[4], to guide a design of a new diagnostic for

Te fluctuation measurements.

Watts, NF (2004)

Understanding past attempts through

GYRO simulations

Possible reasons for past null result Prediction from GYRO simulations

No core high k Te fluctuation in C-Mod Broadband high frequency Te fluctuation

exists in C-Mod (kθρs<0.5, 1%)

Large spot size (~3cm) filters out high

k fluctuation measurement

Large spot size filters high k fluctuations,

whose wave length is shorter than spot size

Off-axis view might affect the result Variation of fluctuation level depending on

poloidal angle is small

[4]

Design constraints of CECE from GYRO

simulation

Constraints of CECE design from GYRO

1. Large spot size filters high frequency fluctuation

2. Radial correlation length is less than 1cm.

3. Including ExB shear flow, the frequency of

flluctuations is up to 300kHz

4. in the core

New optical system should be

designed for small spot size. (~1cm)

100MHz < BIF < 250MHz

Bvid > 500kHz, Bvid ~1MHz

0.5% 2.0% 0.4 0.9e

e

T

T

Sensitivity of CECE should be higher

than 0.5% (averaging time, ∆t> 0.32 s)

with ExB shear

w/o ExB shear

[4]

CECE system overview

CECE system with C-Mod plasma (shot :1120221014, t=1.0s)

CECE radiometer detects 2nd harmonic

X-mode EC radiation at low field side.

(Detection frequency : 232-248GHz)

CECE optical system is composed of in-

vessel stainless steel mirrors (flat and

parabolic mirror) and ex-vessel lens and

horn antenna.

CECE receiver has two parts.

RF section : High frequency

(~250GHz) components

IF section : Intermediate frequency

(2-18GHz) components

IF section

12’ sma cable

CECE optical system

: Quasi-optical design

Horn

Antenna

Lens

for collimation window flat mirror parabolic mirror

RF

section

Linear diagram of CECE optical system

Flat mirror change only beam’s direction

Approximate CECE optics to 1D

Gaussian beam propagation

(This approximation was verified in

CECE experiment in DIII-D [5])

Estimation of beam radius from Quasi-optics

It was found that the optical system

whose final beam diameter is about 1cm.

(d=1.3cm at ρ=0.2 kθ<4.8cm-1)

The mirrors and lens are designed

using the calculation result.

Flexible optical system through modular

ex-vessel arrangement

By changing the collimating lens in front of horn antenna, the focal point

of parabolic mirror is changed because of beam spreading.

ex) Changing focal length of lens from 10 to 7.6cm, focal point moves 3.2cm

further into the plasma.

Different lens can be used to change focal point without modification

any in-vessel components. (RF section designed with this consideration)

CECE receiver

: Initial 4-channel radiometer

Installation &

Test in the lab

CECE receiver :

Noise temperature measurement of IF section

Channel

(center f ,GHz)

Noff

[mV]

Non

[mV]

dV

[mV]

NF

[dB]

System

Temp [eV]

Sensitivity

[mV/eV]

1(8.00) 216.50 246.33 29.83 23.87 6.06 35.55

2(8.15) 25.06 41.92 16.86 16.99 1.23 20.05

3(8.05) 44.41 70.72 26.32 17.57 1.40 31.12

4(8.50) 97.86 129.29 31.43 20.20 2.59 37.44

* Noise temperature was calculated through interpolation from ENR at 8 and 9 GHZ

Using noise source, noise

temperature of IF section was

measured through Y-factor

method.

Noise temperature of IF section

is 1-6eV.

Installation of CECE system

: In-vessel optical components

Mirror

housing

Parabolic

mirror

Flat mirror

~10cm In-vessel components which are

stainless steel flat and parabolic

mirrors, are mounted on the outer wall.

The position of mirrors are aligned.

Installation of CECE system

: CECE receiver

RF section is installed in front of upper part of the A port to reduce losses.

IF section is installed near the A port in the cell.

Two sections are connected through low loss sma cable.

IF section

RF section

~45cm

First Results from CECE

Diagnostic in C-Mod

Initial results of CECE : calibration

CECE data are proportional to another ECE channel. (FRCECE

ch4, cross-calibrated to Michelson Interferometer in C-Mod and

has similar detection frequency to CECE channels.)

CECE channels are cross-calibrated to high spatial resolution

ECE radiometer (FRCECE, cross-calibrated to Michelson

interferometer) in C-Mod.

0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FRCECE ch4 (241.8GHz)

FR

CE

CE

ch

4 (

24

1.8

GH

z)

[ke

V]

CECE ch1 (242GHz) raw data [V]

Shot:1120210012

0.0 0.5 1.0 1.5 2.0 2.5 Time [sec]

2.0

1.5

1.0

0.5

0.0

Te [

ke

V]

Calibrated CECE data consistent with other

diagnostics in C-Mod

Verification of spectral decorrelation

technique

Spectral decorrelation technique was verified in CECE receiver by using

both plasma signal and noise source.

When frequency separation is larger than 100MHz, which is IF bandwidth,

cross correlation coefficient,Cxy(0) reaches near the statistical limit.

Thus, CECE can remove thermal noise through cross-correlation

technique,

0.0 0.1 0.2 0.3 0.4 0.5

0.0

0.2

0.4

0.6

0.8

1.0 with noise source

with plasma

(shot : 1120221012, t:0.8 ~ 1.3s)

ICX

Y(0

)If [GHz]

statistical limit

Shot : 1120221012, tme:0.8-1.3s

-6 -4 -2 0 2 6

Lag time [us]

Ch01(8.0GHz) & Ch04(8.05GHz)

Ch01(8.0GHz) & Ch02(8.08GHz)

Ch01(8.0GHz) & Ch03(8.50GHz)

4

Electronics noise might have masked

fluctuations in 2011 campaign

In the last campaign, noise from digitizer masked the fluctuation signal,

and it was removed in the last run day.

After digitizer noise was removed, electronics noise level before plasma

start-up is small compared to plasma signal, thus, it is ignorable in the

experiment.

(w/o plasma) (with plasma)

(w/o plasma) (with plasma)

Increased noise floor

due to electronics in

2011 Campaign

Flat noise floor

CECE optical system upgrade

0.2 0.4 0.6 0.8 1.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Be

am

dia

me

ter

[cm

]

2011

2012

The parabolic mirror used in 2011

campaign is focused to farther into the

plasma.

A new mirror was designed to focus at

ρ~0.8, d=0.7cm (w at horn is 0.27cm).

The new mirror was installed and will be

used in this campaign.

New CECE will measure Te fluctuation

data in C-Mod in the near future.

A new CECE diagnostic in C-Mod was designed under the constraints from

GYRO simulations

New CECE diagnostic can measure long wavelength, kθ≤4.8cm-1 (kθρs<0.5)

broadband temperature fluctuations (500-1000kHz) whose level is 0.5% in

typical video bandwidth and average time (>0.32s)

In 2011 campaign, CECE diagnostic was installed, and it was verified that

CECE radiometer is working properly.

In the next campaign, it is expected that CECE diagnostic will obtain

fluctuation data and used for turbulence studies in C-Mod.

Research supported by DE-SC0006419, DE-FC02-99ER54512-CMOD

and National Institute for International Education, Korea.

Reference [1] J. Friedberg, Plasma Physics and Fusion Energy (Cambridge University Press, 2007), p.497

[2] C. Watts et al, ‘Comparison of different methods of electron cyclotron emission-correlation radiometry for

the measurement of temperature fluctuations in the plasma core’, Review of Scientific Instruments, 75, 3177

(2004)

[3] C. Watts et al, ‘Upper limit on turbulent electron temperature fluctuations in the core of Alcator C-Mod’,

Nuclear Fusion, 44, 987 (2004)

[4] J. Candy and R.E. Waltz, ‘An Eulerian gyrokinetic-Maxwell solver’, J. Comput. Phys. 186, 545 (2003)

[5] A. W. White et al, ‘Feasibility study for a correlation electron cyclotron emission turbulence diagnostic

based on nonlinear gyrokinetic simulations’, Plasma Phys. Control. Fusion, 53, 115003 (2011)

[6] T. L. Rhode et al, ‘Quasioptical design of integrated Doppler backscattering and correlation electron

cyclotron emission systems on the DIII-D tokamak’, Review of Scientific Instruments, 81, 10D912 (2010)

[7] S. Sattler and H. J. Hartfuss, ‘Intensity interferometry for measurement of electron temperature fluctuations

in fusion plasmas’, Plasma Phys. Control. Fusion, 35, 1285-1306 (1993)

[8] G. Cima et al, ‘Correlation radiometry of electron cyclotron radiation in TEXTU (invited)’, Review of

Scientific Instruments, 66, 798 (1995)

[9] G. CIMA, “Correlation Properties of Black Body Radiation in the Context of the Electron Cyclotron

Emission of a Magnetized Plasma,” Il Nuovo Cim. D, 16, 359 (1994)

[10] G. Bekefi, ‘Radiation Processes in Plasmas’ ,Wiley, New York (1966)

[11] P. F. Goldsmith, ‘QUASIOPTICAL SYSTEMS : Gaussian Beam Quasioptical Propagation and

Applications’, IEEE, New York (1997)

[12] I. Hutchinson, ‘Principle of plasma diagnostics’ , Cambridge University Press, Cambridge (1987)