1. 2013 KAPRA & KPS/DPP Conference
Design and Fabrication of Electron Temperature Diagnostic System Using the Two Foil Absorbing Method in VEST
Jungmin Jo† , Jeong-jeung Dang, Younghwa An, K.J. Chung and Y.S. Hwang††
2013 KAPRA & KPS/DPP Conference
28th June, 2013
KAPRA, Chulwon, KOREA
NUPLEX, Dept. of Nuclear, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul , Korea †

2. Abstract
초 록
Versatile Experiment Spherical Torus(VEST) 장치에서 두 장 박막 감쇠 방법(two foil absorbing method)을 기반으로 한 전자온도 측정 시스템을 설계 및 제작하였다. 박막으로는 극자외선 이상의 에너지의 광자에 대해 투과성이 좋아 광학필터로 적합하고 구하기 쉬운 물질인 알루미늄을 사용하였고, eV의 전자온도를 측정하기 적합하도록 와 두께를 사용하였다. 한 번의 플라즈마 생성만으로 전자온도의 시간변화를 측정할 수 있도록 하기 위하여 Micro Channel Plate(MCP)의 영역을 분할하여 사용하였고, 분할된 영역 위에 각 박막을 놓아 광학필터로서 작용하도록 하였다. VEST 플라즈마에서 발생한 제동복사는 각각의 박막을 통과한 후, MCP를 통해 증폭되고 형광면에서 545nm의 가시광선으로의 변환되어, 초고속카메라를 통해 세기가 기록 되도록 설계하였다.
Abstract
Electron temperature(Te) diagnostic system has been designed and fabricated for Versatile Experiment Spherical Torus(VEST) by utilizing two foil absorbing method. The foil material has been chosen as Aluminium because of its high transmission rate over the range of EUV energy region and easy procurement. The thickness of the two foils are determined to be and respectively to be able to measure the Te range 10 to 100eV. Each foil used as an optical filter screens the each divided portion of the MCP detection area in order to measure the time variation of Te in a single shot. The induced bremsstrahlung radiation, passes through each foil, is amplified by MCP then converted into visible light of 545nm by phosphor screen, and consequently its intensity is recorded by fast camera.

3. Contents
Strong point of two foil absorbing method.
Theory – Two foil absorbing method
Detector system
Optical filter design

4. Strong point of two foil absorbing method.
Triple probe
Easy and simple method but perturb the Plasma and can not get core electron temperature because of the thermal damage.
Thomson scattering
Most prominent Te diagnostics, but the cost is too great and the system is too complicated.
Two foil absorbing method
Relatively simple and non perturbing method for electron temperature measurement.
By increasing the number of detectors and combined with tomography technique can get 2-D profile of electron temperature.

5. Theory – two foil absorbing method
Continuum radiation in fusion device
Origin
Bremsstrahlung radiation
Coulomb interaction between free electrons and ions
Governing plasma parameters
Spectral power density of the bremsstrahlung radiation (in thermal equilibrium plasmas)

6. Theory – two foil absorbing method
Spectral power density through the optical filter
With Two different optical filter
(T transmission functuion )
Intensity ratio – only function of Te. It can be used as electron temperature diagnostics

7. Detector system overview
MCP assembly
plasma
Filter 1,2
Bremsstrahlung radiation from the plasma
Phosphor screen
MCP
Fast camera
Data acquisition – Each recorded image pixel is classified in 256 steps 0(black) to 255(white)
depending on intensity. The intensity sum of each area is represented as number.

8. Detector system – MCP assembly (Hamamatsu corp. F2225-21PGF)
Micro Channel Plate
Radiation amplifier
Operation principle (similar with PMT)
Phosphor screen
Amplified electrons are converted into visible light
High voltage
Semiconducting layer
electrode
Primary radiation
Output electrons
Secondary electrons
Glass channel wall

9. Detector system – properties
1. MCP
Detection Properties
Size of micro channel - 12μm
Size of detection area - 42Φ
3. High Vacuum
High vacuum is necessary to prevent
discharge between MCP and phosphor
screen. (about )
MCP system has independent vacuum
system. (TMP, dry pump)
Type of radiation
Detection efficiency
U.V
300Å~1100Å
1100Å~1500Å
5~15%
1~5%
Soft X-ray
2Å~50Å
Hard X-ray
0.12Å~2Å
~1%
4. Collimator & foil holder
In order to each areas of MCP has similar
line of site, there is a long pipe between
VEST and MCP system.
Foil holder is on the drawing board.
2. Phosphor screen
Peak emission wavelength
545nm
10% afterglow time
1ms
~1100mm
보이는 플라즈마 크기 ~ 지름 100mm
MCP assembly

10. Expected measuring range for new diagnostic system
Present electron temperature in VEST
Edge region
~20eV
Core region(estimation)
30~40eV
10 to 100eV of Electron Temperature measuring range is suitable.
consideration
Filter properties(materials, transmission rate).
Filter thickness combination.
보이는 플라즈마 크기 ~ 지름 100mm

11. Optical filter design – interesting photon energy region
Electron temperature legend 순서 바뀜
Optical filter design – interesting photon energy region
Low frequency region photons(~visible light) have to be excluded because of characteristic radiation from the hydrogen plasmas.
Because of the spectral power density high frequency region photon is negligible. (over 700eV in VEST plasma)
The fraction of photon energy of over 700eV (compared with total bremsstrahlung radiation energy)
Electron temperature
percentage
50eV
0.011%
80eV
0.142%
100eV
0.453%
In thermal equilibrium at temperature T, electron velocity distribution function -> Maxwellian
VEST plasma electron temperature ~30eV
This percentage is lower than 0.01%
Extreme UV ~ 700eV
The bremsstrahlung radiation intensity
in the frequency region of

12. Optical filter design – materials
Requirements
1. Good transmission rate at the interesting photon energy region.
2. Filter out the abundance Hydrogen characteristic radiation from VEST plasma.
Low Z metal
Properly filtering low energy photon
In thermal equilibrium at temperature T, electron velocity distribution function -> Maxwellian
Candidates – Aluminum and Beryllium
Easy procurement and relatively cheap price
Aluminum

13. Optical filter design – Determination Filter thickness
Requirements
1. Properly measure 10 to 100eV range of electron temperature.
2. Because of the detector sensitivity Intensity ratio value couldn’t exceed eight.
Increase in thickness difference
– measurable range moved to high
temperature region
Measurable region
Increase in thickness of two foils (in same thickness difference)
– width of measurable range is decrease

14. Optical filter design – conclusion
To enlarge transmitted radiation power thin foil is chosen
0.8μm, 1.5 μm ( Commercial thin aluminum foil – 0.8, 1.5, 2, 3, 5, 6μm (cf. cooking foil – 15 μm) )
To cover 10 to 100eV electron temperature range 2set of thickness combination is used.
Thick foil is composed of thin foils’ combinations.
9.1μm=1.5μm x μm x 2
10.6μm=1.5μm x μm x 2
0.8 and 1.5 μm eV (ex. Intensity ratio ‘4’ for Te 30eV)
9.1 and 10.6μm eV (ex. Intensity ratio ‘6’ for Te 60eV)

15. References [1] Radiation processes in plasmas, G. Bekefi, 1966, wiley
[2] Principles of plasma diagnositcs, I. H. Hutchinson, 1987, cambridge press
[3]
[4] Nuclear Instruments and Methods, Vol. 162, 1979, pages 587 to 601
[5] Hamamatsu corp. manual