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 National University, Seoul , Korea † The 2nd A3 Foresight Workshop on Spherical Torus (ST) Tsinghua University, Beijing, China Jan

1 /15 contents 1.Introduction 2.Background Theory 3.Overall system design 4.Test experiments on VEST 5.Conclusion & Future work

2 /15 Introduction VEST current diagnostic status Plasma parameterDiagnostic MethodPurposeRemarks nene Electrostatic Probe Radial profile of n e Triple Probe Interferometry Line averaged n e 94GHz TeTe Electrostatic Probe Radial profile of T e 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)

3 /15 Introduction Two Absorber foil method [2]Delgado-Aparicio et al. J. Appl. Phys. 102, (2007) Intensity ratio between A and B – function of Te only Features of Two Absorber foil method 1.Relatively simple method 2.Good time resolution 3.non-perturbative method plasma Thick filter Thin filter Photon e- Photon ion Photon Light Intensity A Light Intensity B Detector A Detector B [2]

4 /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 theory Radiation mechanism of Soft X-ray in fusion device In conventional fusion device the most dominant mechanism is Bremsstrahlung radiation because of the high electron temperature.  Recombination radiation (free – bound transition)

5 /15  Spectral power density of the bremsstrahlung radiation (in thermal equilibrium) Background Theory Continuum 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 n i with charge Z i to ions with charge Z i-1 Ion [7] (T – transmission function )  With Two different thickness filters

6 /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 T e value

7 /15 Overall system design 1080 mm VEST Plasma Detector position 128 mm Photodiode chamber (It has Independent vacuum system) Al 1.5μm Al 0.8 μm detector Filter foil holder & Al foil In-vacuum component Vacuum feedthrough Signal processing circuit Limit the line of sight Extension SUS pipe

8 /15 Overall system design Filter – materials Expected VEST core region plasma Te ~ 100eV Because of the relatively low T e, continuum Soft X- ray radiation power will be small transmission data - Center for X-ray Optics, Requirements 1.Good transmission rate at SXR region photon. 2.Filter out abundant characteristic line radiation from hydrogen Low Z metal Aluminum

9 /15 Overall system design Filter – 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, Al 0.8 μm / Al 1.5 μm appropriate for ~100 eV T e measurements

10 /15 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 T e Electron Temperature through the Two Absorber Foil Method.

11 /15 Overall system design detector 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

12 /15 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  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)  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 Al 0.8um 17mm Ch12: Al1.5 μm Ch5 : Al0.8 μm

13 /15 contents 1.Introduction 2.Background Theory 3.Overall system design 4.Test experiments on VEST 5.Conclusion & Future work

14 /15 Test experiments on VEST  T e 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

15 /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

16 /15 Back up slides

17 /15 TEST experiments on VEST Target plasma – ECH preionized ohmic plasma case Acase 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 ECH : 6kW Ohmic : ~200kW Plasma current 58kA51kA Black line : case B, shot #7181 Red line : case A, shot #7182 Because of the high impurity rate and high operating pressure relatively low T e expected in case B

18 /15 TEST experiments on VEST Yellow box : low signal to noise ratio region  Clear T e difference in the ramp up phase Case B has lower T e value as expected  The difference diminished as oxygen line signal difference diminished Black line : case B, shot #7181 Red line : case A, shot #7182

19 /15 Overall system design Filter – thickness Because of the fabrication error in Aluminum foil, the measured T e 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 T e Large error bar in Absolute T e value

20 /15 Overall system design Filter – 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, Al 4.5 μm and Al 6.0 μm foil set good for detect around 100eV However there is a some problem Thin Thick

21 /15 Overall system design Filter – thickness In Signal intensity aspects In relatively low T e 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 Al 0.8 μm and Al 1.5 μm This foil set has relatively good resolution around T e =100eV~200eV region and possible to estimate the VEST plasma T e levels.