1. HT-7 ASIPP Investigation on Z eff and impurities behavior with molybdenum limiter in lithium coating experiments on HT-7 tokamak Presented by Y.J.Chen Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China 2011.7.19 1 Workshop of 2011 Spring Experiment on HT-7

2. HT-7 ASIPP  Introduction -- Significance of Z eff and impurity control -- International development -- The principle of Z eff from V.B. and Define of averaged Z eff  Experimental Set-Up and major spectroscopy diagnostics  Lithium coating wall conditioning Experiment  Z eff and major impurities with lithium coating  Further more benefits for lithium coating -- Lower particle recycling -- Lower ratio of H/H+D  Conclusion and Discussion Outline 2

3. HT-7 ASIPP Introduction  For magnetically confined plasmas, How to control of impurity level is a critical issue for the fusion reaction in the future. the Z_effective is a significant parameter with indication of the total impurity level. Visible bremsstrahlung measurement is a powerful tool approaching to derive this key data.  Determination Zeff via Bremsstrahlung was first applied by Kadota on JIPP T-II in 1980, and soon used worldwide on many devices ， such as DIII-D, JET, FTU,ASDEX Upgrade, LHD etc, for the regardlessness of the number of impurity species in the plasma. The best region to measure the radiation is in the visible. 3

4. HT-7 ASIPP The Definition of an averaged Z eff 4 Statistics Quantum Mechanics

5. HT-7 ASIPP The principle of Zeff from V.B. 5 The emission coefficient of bremsstrahlung radiation Zeff can be expressed as: and the line averaged Zeff is defined as:

6. HT-7 ASIPP  Introduction -- Significance of Z eff and impurity control -- International development -- The principle of Z eff from V.B. and Define of averaged Z eff  Experimental Set-Up and key spectroscopy diagnostics  Lithium coating wall conditioning Experiment  Z eff and major impurities with lithium coating  Further more benefits for lithium coating -- Lower particle recycling -- Lower ratio of H/H+D  Conclusion and Discussion Outline 6

7. HT-7 ASIPP Experimental Set-up  The HT-7 superconducting tokamak has a major radius of R=1.22m and a minor radius of a =0.27m in the circular cross-section. From this spring the graphite limiters had been instead of molybdenum limiters in order to develop the lithium wall conditioning experiments  Basically, the HT-7 tokamak with the limiter configuration is normally operated under the basic parameters: Ip=100-250kA, the toroidal magnetic field Bt=2T, the central line-averaged plasma density ne=0.5-6e19m -3, central electron temperature Te=0.5-3.0keV and central ion temperature Ti =0.2-1.5keV 7

8. HT-7 ASIPP Key spectroscopy diagnostics 8 1 Two 35-channel Da (λ = 656.1 nm) array 2 The 6-channel Visible Bremsstrahlung emission 3 The fiber optic spectrometer 4 The 9-channel CIII (λ = 464.7 nm) array 5 Single channel impurity emission: OII (λ= 441.5 nm), OV (λ = 278.1 nm), MoI (λ = 386.4 nm) and LiI (λ = 670.8 nm).

9. HT-7 ASIPP  Introduction -- Significance of Z eff and impurity control --international development --The principle of Z eff from V.B. and Define of averaged Z eff  Experimental Set-Up and key spectroscopy diagnostics  Lithium coating wall conditioning Experiment  Z eff and major impurities with lithium coating  Further more benefits for lithium coating --lower particle recycling --lower ratio of H/H+D  Conclusion and Discussion Outline 9

10. HT-7 ASIPP Typical experiment of lithium coating 10 #113140 before lithium coating, #113191 after lithium coating

11. HT-7 ASIPP  Introduction -- Significance of Zeff and impurity control --international development --The principle of Zeff from V.B. and Define of averaged Zeff  Experimental Set-Up and key spectroscopy diagnostics  Lithium coating wall conditioning Experiment  Zeff and major impurities with lithium coating  Further more benefits for lithium coating --lower particle recycling --lower ratio of H/H+D  Conclusion and Discussion Outline 11

12. HT-7 ASIPP Zeff and major impurities with lithium coating 12

13. HT-7 ASIPP The impurity amount varies with n e 13 After lithiation After boronization boronization is further more effective to decrease the level of impurity

14. HT-7 ASIPP  Introduction -- Significance of Z eff and impurity control -- International development -- The principle of Z eff from V.B. and Define of averaged Z eff  Experimental Set-Up and key spectroscopy diagnostics  Lithium coating wall conditioning Experiment  Z eff and major impurities with lithium coating  Further more benefits for lithium coating -- Lower particle recycling -- Lower ratio of H/H+D  Conclusion and Discussion Outline 14

15. HT-7 ASIPP Further more benefits for lithium coating 15 In despite of much more reduced Z eff after boronization, lithium coating can get a very lower recycling of hydrogen and isotopes in the plasma edge and can get much more lower ratio of H/H+D after many times operation. The value of H/H+D can be decreased below 10%. It is benefit for ICRF minority ion heating Red --after boronization Blue -- after lithium coating

16. HT-7 ASIPP  Introduction -- Significance of Zeff and impurity control --international development --The principle of Zeff from V.B. and Define of averaged Zeff  Experimental Set-Up and key spectroscopy diagnostics  Lithium coating wall conditioning Experiment  Zeff and major impurities with lithium coating  Further more benefits for lithium coating --lower particle recycling --lower ratio of H/H+D  Conclusion and Discussion Outline 16

17. HT-7 ASIPP Conclusion  Impurities and Z eff are reduced obviously by lithium coating wall conditioning  It is indicated by statistics that reduced impurities and Z eff seems inverse proportional to the quantity of lithium coating  The impurity amount varies approximately linearly with ne, which is useful for Z eff determination and also indicated that boronization is further more effective to decrease the level of impurity  In despite of much more reduced Z eff after boronization, lithium coating can get a very lower recycling of hydrogen and isotopes in the plasma edge and can get much more lower ratio of H/H+D after many times operation. The value of H/H+D can be decreased below 10%. It is benefit for ICRF minority ion heating 17

18. HT-7 ASIPP Discussion 18 Why does the Zeff much more larger with the molybdenum limiter than with the graphite limiter? It comes into being from higher Z molybdenum PFC and limiter configuration in lower density region(1-3e19m-3) on HT-7 We are looking forward to doing the analysis of Zeff and impurities behavior on EAST with molybdenum PFC in lithium coating wall conditioning experiments in near future

19. HT-7 ASIPP Thank you ! Workshop of 2011 Spring Experiment on HT-7 20