Jungmin Jo, Jeong Jeung Dang, Young-Gi Kim, YoungHwa An, Kyoung-Jae Chung and Y.S. Hwang

Development of Electron Temperature Diagnostics Using Soft X-ray Absorber Foil Method in VEST

Department of Nuclear Engineering, Seoul Na-

tional University, Seoul 151-742, Korea

E-mail : yhwang@snu.ac.kr

†

The 2nd A3 Foresight Workshop on Spherical Torus (ST)Tsinghua University, Beijing, China

Jan. 7 2014

2/15

contents

1. Introduction2. Background Theory3. Overall system design4. Test experiments on VEST5. Conclusion & Future work

3/15

IntroductionVEST current diagnostic status

Plasma parameter Diagnostic Method Purpose Remarks

ne

Electrostatic ProbeRadial profile

of neTriple Probe

InterferometryLine aver-aged ne

94GHz

Te Electrostatic ProbeRadial profile

of Te

Triple Probe

Because of the thermal damage problem it is impossible to put electrostatic probe in core plasma region.

No diagnostics for core electron temperature.

Two Absorber Foil Method [1]• Relatively simple method for line integrated Electron temperature measurement.• It’s an application of Soft X-Ray diagnostics.

[1] F. C. Jahoda et al., phys. review, 119, 3(1960)

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IntroductionTwo Absorber foil method

[2]Delgado-Aparicio et al. J. Appl. Phys. 102, 073304 (2007)

Intensity ratio between A and B – function of Te only

Features of Two Absorber foil method1. Relatively simple method2. Good time resolution3. non-perturbative method

plasma

Thick filter

Thin fil-ter

Photon

e-

Photon

Photon

ion

PhotonLight Intensity A

Light Intensity B

Detector A

Detector B

[2]

5/15

1. Continuum radiation Coulomb interaction between free electrons and ions

Bremsstrahlung radiation (free – free transition)

2. Line radiation characteristic line radiation from ionized impurity

Background theoryRadiation mechanism of Soft X-ray in fusion device

+-- -

-

-

+-

+ -

In conventional fusion device the most dominant mechanism is Bremsstrahlung radiation be-cause of the high electron temperature.

Recombination radiation (free – bound transition)

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Spectral power density of the bremsstrahlung radiation (in thermal equilibrium)

Background TheoryContinuum radiation and Two Absorber foil method

Intensity ratio – function of Te only It can be used as electron temperature diagnostics

Spectral power density of the recombination radiation (in thermal equilibrium)

In the relatively low electron temperature, radiative recombination rate is increases

Recombination radiation spectral power density of ions ni with charge Zi to ions with charge Zi-1 Ion [7]

(T – transmission function )

With Two different thickness filters

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Background TheoryLine radiation and Two Absorber foil method

Two foil method and Characteristic line radiation

not function of Te only Al 0.8um, 1.5um

Effect of line radiation on intensity ratio

If there is line radiation which can transmit the filter set there is Overestimates in Te value

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Overall system design

1080 mm

VEST Plasma

Detector position

128 mm

Photodiode chamber (It has Independent vacuum system)

Al1.5μm

Al0.8 μm

detector Filter foil holder &

Al foil

In-vacuum component

Vacuum feedthrough

Signal processing

circuit Limit the line of sight

Extension SUS pipe

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Overall system design Filter – materials

Expected VEST core region plasma Te ~ 100eV Because of the relatively low Te, continuum Soft X-

ray radiation power will be small

transmission data - Center for X-ray Optics, http://www.cxro.lbl.gov

Requirements1. Good transmission rate at SXR region photon.2. Filter out abundant characteristic line radiation from hydrogen

Low Z metal

Aluminum

10/15

Overall system designFilter – thickness

Requirements Properly measure ~100eV electron temperature

Increase in thickness difference – measurable range moved to high temperature region

transmission data - Center for X-ray Optics, http://www.cxro.lbl.gov

Al 0.8 μm / Al 1.5 μm appropriate for ~100 eV Te measurements

0 100 200 300 4001

2

3

4

5

6

7 0.8um/1.5um 0.8um/1.9um 0.8um/2.3um

Inte

nsi

ty rat

io

Te (eV)

11/15

Overall system designFilter – impurity problems in thin foil set

In VEST the expected major impurity is Oxygen (tungsten limiter instead of graphite limiter)

Below the 50eV(photon energy) region there are characteristic lines of Oxygen(mainly from ionic Oxygen) so the filtered photon is not only from the continuum radiation but also line radiation.

So the overestimates is expected in measured Te Electron Tem-perature through the Two Absorber Foil Method.

404 406 408 410 412

0

20

40

60

0.0000

0.0005

0.0010

Ip (kA

)

time (ms)

Oxygen 7

77nm

lines (A

.U.)

0 200 400 600 800 1000

0.0

0.2

0.4

0.6

0.8

1.0

Tra

nsm

issi

on

Photonenergy(eV)

0.8um 1.5um

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Overall system designdetector

0 200 400 600 800 10000.00

0.05

0.10

0.15

0.20

0.25 transmitted power AXUV response

photonenergy (eV)

0.8u

m tr

ansm

itted

pow

er (A

.U.)

0.00

0.01

0.02

0.03

0.04

0.05

0.8um A

XU

V response(A

.U.)

Features linear and good quantum efficiency in Soft X-ray region Multi-element detector (16ch.) Relatively short rise time (500nsec)

AXUV 16ELG

Requirements Good quantum efficiency at SXR region Vacuum compatible

When consider the detector quantum efficiency there is enhancement in high energy photon region

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Overall system designInstallation on VEST

• Located on mid-plane of the VEST to diagnose core plasma.• Independent Vacuum system – Dry(oil free) pump , TMP base pressure ~5e-7(Torr)

Absorber foil holder and AXUV holder located inside the vacuum chamber

The holder has visible region light tight design

Signal lines from AXUV are twisted to prevent induc-tively coupled noise and also covered with copper braided wire to prevent EM wave noise

Signals are transferred via electrical vacuum feedthroughs (product of allectra)

Two different thickness(0.8 μm, 1.5 μm) Al foils are used and each are located in front of different AXUV channels

Al1.5um

Al0.8um

17mm

Ch12:Al1.5 μm

Ch5 :Al0.8 μm

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contents

1. Introduction2. Background Theory3. Overall system design4. Test experiments on VEST5. Conclusion & Future work

15/15

Test experiments on VEST

Te at the Plasma current flat top region : ~170eV

Te sustained almost constant during the plasma current lamp down region

-Plasma column size diminished -Loop voltage is still maintained -Also ECH heating constantly put into the plasma

Because of the impurity lines there is possibility for overestimates

Target plasma – ECH preionized ohmic plasma Heating power : ECH(6kW), Ohmic(~200kW)

Shot #7029

Yellow box : low signal to noise ratio region

405 406 407 408 409 410 4110

100

200

3002

4

6

80.0

0.1

0.2

0.3

0.4

0.50.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0

20

40

600

1

2

3

4

5

Te (eV)

time (ms)

ratio

SXR

(A

.U.)

Oxygen

777nm

(A

.U.)

Ip (kA

)

Vlo

op (V)

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Conclusion & future work

Conclusion Electron temperature diagnostic system using Two absorber foil method is successfully

installed in VEST. This diagnostics can be useful in relatively low impurity conditions Some overestimates in measurements expected as possibility for impurity line emission

existence

Future work Check the possible Impurity line emission and clarify the limits of use Use different thickness or materials of filters and crosscheck the absolute value and

evolution of Te

17/15

Back up slides

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TEST experiments on VEST

Target plasma – ECH preionized ohmic plasma

case A case B

Operating pressure

2.7E-5 (Torr)(1ms Hydrogen gas puffing with piezo electric valve)

3.6E-5 (Torr)(3ms Hydrogen gas puffing with piezo electric valve)

Heating ECH : 6kWOhmic : ~200kW

ECH : 6kWOhmic : ~200kW

Plasma current 58kA 51kA

Black line : case B, shot #7181Red line : case A, shot #7182

400 402 404 406 408 410 412 414-0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0

2

4

0

20

40

601.0x10-5

2.0x10-5

3.0x10-5

4.0x10-5

5.0x10-5

Oxy

gen

777n

m(A

.U.)

time(ms)

Vlo

op (V)

Ip (kA

)

pre

ssu

re (Torr

)

Because of the high impurity rate and high operating pressure relatively low Te expected in case B

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406 407 408 409 410 411 4120

50

100

150

200

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.50.0002

0.0004

0.0006

0.0008

0.00100

20

40

60

Te

time (ms)

hig

h p

ress

ure

SXR

low

pre

ssure

SXR

Oxy

gen

777n

m(A

.U.)

777nm

Ip (kA

)

TEST experiments on VEST

Yellow box : low signal to noise ratio region

Clear Te difference in the ramp up phase Case B has lower Te value as expected

The difference diminished as oxygen line signal difference diminished

Black line : case B, shot #7181Red line : case A, shot #7182

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Overall system designFilter – thickness

Because of the fabrication error in Aluminum foil, the measured Te value is unreliable. In this experiments, used Al foil thickness is especially thin so the percentage error will be large.

0 100 200 300 400 5001

2

3

4

5

6

7

8

inte

ns

ity

ra

tio

Te (eV)

0.784/1.53 0.816/1.47 0.8/1.5

0 100 200 300 400 5001

2

3

4

5

6

7

8

inte

ns

ity

ra

tio

Te (eV)

0.76/1.575 0.84/1.425 0.8/1.5

fabrication error 2% fabrication error 5%

At the same ratio value it correspond with wide range of Te

Large error bar in Absolute Te value

21/15

Overall system designFilter – thickness

Requirements Properly measure ~100eV electron temperature

Increase in thickness difference – measurable range moved to high temperature region

Increase in thickness of two foils (in same thickness difference) – measurable range moved to low temperature region

transmission data - Center for X-ray Optics, http://www.cxro.lbl.gov

Al 4.5 μm and Al 6.0 μm foil set good for detect around 100eV

However there is a some problem

Thin Thick

0 100 200 300 4000

1

2

3

4

5

6

7

8

9

10

inte

nsi

ty rat

io

Te (eV)

0.8um/1.5um 0.8um/1.9um 0.8um/2.3um

0 100 200 300 4000

1

2

3

4

5

6

7

8

9

10

inte

nsi

ty rat

io

Te (eV)

4.5um/5.2um 4.5um/5.6um 4.5um/6um

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Overall system designFilter – thickness

0 500 1000 1500

0.0

0.2

0.4

0.6

0.8

1.0

tran

smis

sio

n

Photonenergy(eV)

0.8um 4.5um

0 500 1000 1500 2000

0.0

0.2

0.4

0.6

0.8

1.0

Sp

ec

tra

l Po

we

r D

en

sit

y (

A.U

.)

Photonenergy (eV)

Te=50eV Te=100eV Te=200eV

In Signal intensity aspects

In relatively low Te condition, thick foil is hard to use because of the weak signal intensity.

0 200 400 600 800 1000 1200 1400

0.00

0.05

0.10

0.15

0.20

0.25

Sp

ectr

al P

ow

er D

ensi

ty (

A.U

.)

Photonenergy (eV)

0.8um 4.5um Te=100eV

0 100 200 300 4001

2

3

4

5

6

inte

nsi

ty rat

io

Te (eV)

0.8um/1.5um

Because of the low (expected) signal levelsfirstly the thinnest filter foil set is selected

Al 0.8 μm and Al 1.5 μm This foil set has relatively good resolution around Te=100eV~200eV region and possible to estimatethe VEST plasma Te levels.