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 : [email protected]
†
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)
4/15
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)
6/15
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
7/15
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
8/15
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
9/15
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
nsity
ratio
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 (k
A)
time (ms)
Oxy
gen
777n
m li
nes
(A.U
.)
0 200 400 600 800 1000
0.0
0.2
0.4
0.6
0.8
1.0
Tran
smis
sion
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
powe
r (A.
U.)
0.00
0.01
0.02
0.03
0.04
0.05
0.8um AXUV 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
13/15
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
80.00.10.20.30.40.50.0002
0.00040.00060.00080.00100.0012
0
20
40
60012345
Te (e
V)
time (ms)
ratio
SXR
(A.U
.)
Oxy
gen 7
77nm
(A.U
.)
Ip (k
A)
Vloo
p (V
)
16/15
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
18/15
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
400 402 404 406 408 410 412 414-0.00020.00000.00020.00040.00060.00080.0010
0
2
4
0
20
40
601.0x10-5
2.0x10-5
3.0x10-5
4.0x10-5
5.0x10-5
Oxy
gen7
77nm
(A.U
.)
time(ms)
Vloo
p (V
)
Ip (k
A)
pres
sure
(Tor
r)
Because of the high impurity rate and high operating pressure relatively low Te expected in case B
19/15
406 407 408 409 410 411 4120
50100150200
0.0
0.5
1.0
1.5
0.0
0.5
1.0
1.50.00020.00040.00060.00080.0010
0
20
40
60
Te
time (ms)
high
pre
ssur
eSX
R
low
pre
ssur
eSX
R
Oxyg
en77
7nm
(A.U
.)
777nm
Ip (k
A)
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
nsity
ratio
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
nsity
ratio
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 TeLarge 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
nsity
ratio
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
nsity
ratio
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
sion
Photonenergy(eV)
0.8um 4.5um
0 500 1000 1500 2000
0.0
0.2
0.4
0.6
0.8
1.0
Spec
tral
Pow
er D
ensi
ty (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
Spec
tral
Pow
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
nsity
ratio
Te (eV)
0.8um/1.5umBecause 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.