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 : yhwang@snu.ac.kr

contents Introduction Background Theory Overall system design Test experiments on VEST Conclusion & Future work

Introduction VEST current diagnostic status Plasma parameter Diagnostic Method Purpose Remarks ne Electrostatic Probe Radial profile of ne Triple Probe Interferometry Line averaged ne 94GHz Te Radial profile of 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)

Introduction Two Absorber foil method plasma Thick filter Thin filter Photon e- ion Light Intensity A Light Intensity B Detector A Detector B Intensity ratio between A and B – function of Te only [2] Features of Two Absorber foil method Relatively simple method Good time resolution non-perturbative method NSTX twelve available Thomson scattering core Te measurements Spatial resolution Two detectors which have different thickness filters and sees same plasma Generally the filter materials are metal foil. So it is called Two absorber foil method. 이 측정은 온도가 keV 영역이나 우리 장치는 온도가 훨씬 낮지만 이 방법을 적용하고자 한다는 것이 이 연구의 중요한 동기가 될 것임. This result is from the large size conventional tokamaks which has high temperature. Apply these method to the small size fusion device which has relatively low temp. plasma is purpose of my research. [2]Delgado-Aparicio et al. J. Appl. Phys. 102, 073304 (2007)

Background theory Radiation mechanism of Soft X-ray in fusion device Continuum radiation Coulomb interaction between free electrons and ions Bremsstrahlung radiation (free – free transition) Line radiation characteristic line radiation from ionized impurity Recombination radiation (free – bound transition) + - + - + - In conventional fusion device the most dominant mechanism is Bremsstrahlung radiation because of the high electron temperature.

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

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 First term – approximate recombination to the lower states, with ground configuration n. xi – common ionization potential Second term – recombination to states with higher principal quantum number which are considered as hydrogenic Generally the line emission is negligible compare with the continuum radiation. It’s about below 1% of continuum radiation. When the line radiation transmit the foil and effects on the total radiation intensity the ratio is not a function of Te only. To detect line radiation intensity is difficult. Someone has no diagnostics for impurity and assume no impurity case they get overestimated Te value If there is line radiation which can transmit the filter set there is Overestimates in Te value

Signal processing circuit Overall system design 1080 mm VEST Plasma Detector position 128 mm Photodiode chamber (It has Independent vacuum system) Al 1.5μm 0.8 μm detector Filter foil holder & Al foil In-vacuum component Limit the line of sight Extension SUS pipe Signal processing circuit Vacuum feedthrough

Overall system design Filter – materials Requirements Good transmission rate at SXR region photon. 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 X-ray radiation power will be small Aluminum Expected VEST core region plasma Te ~ 100eV (it is CDX-U Te value, which has similar performance with VEST) transmission data - Center for X-ray Optics, http://www.cxro.lbl.gov

Overall system design Filter – thickness Requirements Properly measure ~100eV electron temperature 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 4.5마이크론 데이터는 필요없어 보입니다. 왼쪽의 thin flim 으로 충분해 보입니다. 0.8/1.5 0.8/2.3 두 경우면 충분히 커버가 되고 우선 0.8/1.5 경우를 먼저 적용했다고 설명하고 들어가면 될 것입니다. 두꺼운 것은 attenuation 때문에 쓰지 않았다고 하면 될 것입니다. transmission data - Center for X-ray Optics, http://www.cxro.lbl.gov

Overall system design Filter – 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 Temperature through the Two Absorber Foil Method. Because VEST plasma temperature not so high so metal impurities are relatively small. And VEST has tungsten limiter instead of graphite limiter so the portion of carbon impurity is relatively small. Dominant impurity is oxygen from water. 이 페이지로부터 다음 페이지의 detector를 이용하여 낮은 에너지의 측정이 산소이온에 의한 영향을 받으므로 그 영향을 줄이기 위해 높은 에너지 영역을 측정할 수 있는 detector 를 선정하였음을 다음 페이지에서 얘기하면 될 것임.

Overall system design detector Requirements Good quantum efficiency at SXR region Vacuum compatible Features linear and good quantum efficiency in Soft X-ray region Multi-element detector (16ch.) Relatively short rise time (500nsec) AXUV 16ELG When consider the detector quantum efficiency there is enhancement in high energy photon region AXUV Q.E. 그림 reference 달기 Lower photon energy region is easily affected by oxygen impurity lines. To mitigate this effect I choose detector which has high Q.E. at the high photon energy region.

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 Al 1.5um 0.8um 17mm Ch12: Al1.5 μm Ch5 : Al0.8 μm About 0.5degree separated 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)

contents Introduction Background Theory Overall system design Test experiments on VEST Conclusion & Future work

Test experiments on VEST Shot #7029 Target plasma – ECH preionized ohmic plasma Heating power : ECH(6kW), Ohmic(~200kW) 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 409.5 플라즈마 벽에서 떨어지는 시점 근처임 There is no diagnostic for comparison. So Verification experiments is carried out in various situation. Yellow box : low signal to noise ratio region

Conclusion & future work 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

Back up slides

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 : 6kW Ohmic : ~200kW Plasma current 58kA 51kA Because of the high impurity rate and high operating pressure relatively low Te expected in case B Black line : case B, shot #7181 Red line : case A, shot #7182

TEST experiments on VEST Black line : case B, shot #7181 Red line : case A, shot #7182 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 Yellow box : low signal to noise ratio region

Overall system design Filter – 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. fabrication error 2% fabrication error 5% 공정에러에 대한 부분은 백업 슬라이드로 보내는 것 고려 At the same ratio value it correspond with wide range of Te Large error bar in Absolute Te value