Development of Electron Temperature Diagnostics Using
Soft X-ray Absorber Foil Method in VEST
Jungmin Jo, Jeong Jeung Dang, Young-Gi Kim, YoungHwa An, Kyoung-Jae Chung
and Y.S. Hwang†
The 2nd A3 Foresight Workshop on Spherical Torus (ST)
Tsinghua University, Beijing, China
Jan. 7 2014
Department of Nuclear Engineering, Seoul
National University, Seoul 151-742, Korea
E-mail : [email protected]
contents
1.
2.
3.
4.
5.
1/15
Introduction
Background Theory
Overall system design
Test experiments on VEST
Conclusion & Future work
Introduction
VEST current diagnostic status
Plasma parameter
Diagnostic Method
Purpose
Remarks
Electrostatic Probe
Radial profile
of ne
Triple Probe
Interferometry
Line
averaged ne
94GHz
Electrostatic Probe
Radial profile
of Te
Triple Probe
ne
Te
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)
2/15
Introduction
Two Absorber foil method
Detector A
Light Intensity A
Photon
Photon
Thin
filter
eplasma
Photon
Detector B
Light Intensity B
Photon
Thick
filter
ion
Intensity ratio between A and B – function of Te only
[2]
Features of Two Absorber foil method
1.
2.
3.
Relatively simple method
Good time resolution
non-perturbative method
[2]Delgado-Aparicio et al. J. Appl. Phys. 102, 073304 (2007)
3/15
Background theory
Radiation mechanism of Soft X-ray in fusion device
1. Continuum radiation
Coulomb interaction between free electrons and ions
 Bremsstrahlung radiation (free – free transition)
+
 Recombination radiation (free – bound transition)
+ -
2. Line radiation
characteristic line radiation from ionized impurity
+- - In conventional fusion device the most dominant mechanism is Bremsstrahlung radiation
because of the high electron temperature.
4/15
Background Theory
Continuum radiation and Two Absorber foil method
 Spectral power density of the bremsstrahlung radiation (in thermal equilibrium)
 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]
 With Two different thickness filters
(T – transmission function )
Intensity ratio – function of Te only It can be used as electron temperature diagnostics
5/15
Background Theory
Line 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
6/15
Overall system design
Detector
position
1080 mm
128 mm
VEST Plasma
Photodiode chamber (It has Independent vacuum system)
In-vacuum component
Extension
SUS pipe
Al
1.5μm
Signal
processing
circuit
7/15
Vacuum
feedthrough
detector
Al
0.8 μm
Filter foil holder
&
Al foil
Limit the line
of sight
Overall system design
Filter – materials
Requirements
1. Good transmission rate at SXR region photon.
2. Filter out abundant characteristic line radiation from hydrogen


Low Z metal
Expected VEST core region plasma Te ~ 100eV
Because of the relatively low Te, continuum Soft Xray radiation power will be small
Aluminum
transmission data - Center for X-ray Optics, http://www.cxro.lbl.gov
8/15
Overall system design
Filter – thickness
Requirements
 Properly measure ~100eV electron temperature
0.8um/1.5um
0.8um/1.9um
0.8um/2.3um
7
Intensity ratio
6
5
Increase in thickness difference
– measurable range moved to high temperature region
Al 0.8 μm / Al 1.5 μm
appropriate for ~100 eV Te measurements
4
3
2
1
0
100
200
300
400
Te (eV)
transmission data - Center for X-ray Optics, http://www.cxro.lbl.gov
9/15
Overall system design
Filter – impurity problems in thin foil set
1.0
0.8
Transmission
Oxygen 777nm lines (A.U.)
0.0010
0.8um
1.5um
0.0005
0.6
0.4
0.2
0.0000
0.0
60
0
200
400
600
800
1000
Photonenergy(eV)
Ip (kA)
40

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
Temperature through the Two Absorber Foil Method.
20
0
404
406
408
410
412
time (ms)
10/15
Overall system design
detector
Requirements


Good quantum efficiency at SXR region
Vacuum compatible
AXUV 16ELG
Features
 linear and good quantum efficiency in Soft X-ray region
 Multi-element detector (16ch.)
 Relatively short rise time (500nsec)
transmitted power
AXUV response
0.20
0.04
0.15
0.03
0.10
0.02
0.05
0.01
0.00
0.00
0
200
400
600
photonenergy (eV)
11/15
0.05
800
1000
0.8um AXUV response(A.U.)
When consider the detector quantum efficiency
there is enhancement in high energy photon region
0.8um transmitted power (A.U.)
0.25
Overall system design
Installation 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

Two different thickness(0.8 μm, 1.5 μm) Al foils are used
and each are located in front of different AXUV channels
17mm
Al
Al
1.5um 0.8um

Ch5 :
Al0.8 μm
Ch12:
Al1.5 μm
Signal lines from AXUV are twisted to prevent
inductively coupled noise and also covered with
copper braided wire to prevent EM wave noise
 Signals are transferred via electrical vacuum
feedthroughs (product of allectra)
12/15
contents
1.
2.
3.
4.
5.
13/15
Introduction
Background Theory
Overall system design
Test experiments on VEST
Conclusion & Future work
Test experiments on VEST
Vloop (V)
Shot #7029
5
4
3
Target plasma – ECH preionized ohmic plasma
2
1
Heating power : ECH(6kW), Ohmic(~200kW)
SXR (A.U.) Oxygen 777nm (A.U.)
Ip (kA)
0
60
40
20
0
0.0012
 Te at the Plasma current flat top region : ~170eV
0.0010
0.0008
0.0006
0.0004
 Te sustained almost constant during the plasma
current lamp down region
0.0002
0.5
0.4
0.3
-Plasma column size diminished
-Loop voltage is still maintained
-Also ECH heating constantly put into the plasma
0.2
0.1
0.0
8
ratio
6
 Because of the impurity lines there is possibility
for overestimates
4
Te (eV)
2
300
200
100
0
405
406
407
408
409
410
411
time (ms)
Yellow box : low signal to noise ratio region
14/15
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
15/15