Jungmin Jo, Jeong Jeung Dang, Young- Gi Kim, YoungHw a An, Kyoung -Jae Chung

1. The 2nd A3 Foresight Workshop on Spherical Torus (ST)Tsinghua University, Beijing, ChinaJan. 7 2014 • Development of Electron Temperature Diagnostics Using • Soft X-ray Absorber Foil Method in VEST Jungmin Jo, JeongJeung Dang, Young-Gi Kim, YoungHwa An, Kyoung-Jae Chung and Y.S. Hwang † Department of Nuclear Engineering, Seoul National University, Seoul 151-742, Korea E-mail : yhwang@snu.ac.kr

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

3. IntroductionVEST current diagnostic status • 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. Jahodaet al., phys. review, 119, 3(1960)

4. IntroductionTwo Absorber foil method Detector A Light Intensity A Photon plasma Photon e- Photon Thin filter Detector B Light Intensity B Photon ion Thick filter 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 [2]Delgado-Aparicioet al. J. Appl. Phys. 102, 073304 (2007)

5. Background theoryRadiation 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.

6. Background TheoryContinuum radiation and Two Absorber foil method • Spectral power density of the bremsstrahlung radiation (inthermal 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 Teonly It can be used as electron temperature diagnostics

7. 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 Tevalue

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

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

10. Overall system designFilter – 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 transmission data - Center for X-ray Optics, http://www.cxro.lbl.gov

11. 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 Temperature through the Two Absorber Foil Method.

12. Overall system designdetector • Requirements • Good quantum efficiency at SXR region • Vacuum compatible AXUV 16ELG • Features • linearand good quantum efficiency inSoft X-ray region • Multi-element detector (16ch.) • Relatively short rise time (500nsec) When consider the detector quantum efficiency there is enhancement in high energy photon region

13. 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 17mm Al 1.5um Al 0.8um • Two different thickness(0.8μm, 1.5μm) Al foils are used and each are located in front of different AXUV channels Ch12: Al1.5 μm Ch5 : Al0.8 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)

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

15. Test experiments on VEST Shot #7029 Target plasma – ECH preionizedohmicplasma Heating power : ECH(6kW), Ohmic(~200kW) • Te at the Plasma current flat top region : ~170eV • Tesustained 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 Yellow box : low signal to noise ratio region

16. 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. Back up slides

18. TESTexperiments on VEST Target plasma – ECH preionizedohmic plasma Because of the high impurity rate and high operating pressure relatively low Teexpected in case B Black line : case B, shot #7181 Red line : case A, shot #7182

19. TESTexperiments 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

20. 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. fabrication error 2% fabrication error 5% At the same ratio value it correspond with wide range of Te Large error bar in Absolute Tevalue

21. Overall system designFilter – thickness • Requirements • Properly measure ~100eV electron temperature Thick Thin 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 Al 4.5μm and Al 6.0μm foil set good for detect around 100eV However there is a some problem transmission data - Center for X-ray Optics, http://www.cxro.lbl.gov

22. Overall system designFilter – thickness In Signal intensity aspects In relatively low Te condition, thick foil is hard to use because of the weak signal intensity. Because of the low (expected) signal levels firstly the thinnest filter foil set is selected Al0.8μm and Al 1.5μm This foil set has relatively good resolution around Te=100eV~200eV region and possible to estimate the VEST plasma Te levels.