Study of an Impurity Transport Boundary Layer in LHD Plasmas

Transcription

1 Study of an Impurity Transport Boundary Layer in LHD Plasmas Naoki Tamura NATIONAL INSTITUTE FOR FUSION SCIENCE 14 th Coordinated Working Group Meeting (CWGM) At the Golden Tulip Warsaw Centre Warsaw, Poland June 18, 2015

2 Outline of This Talk l Current Situation Regarding Impurity Transport Study in Large Helical Device (LHD) l Experimental Apparatus Ø Large Helical Device (LHD) Given to Prof. Ida Ø Tracer-Encapsulated Solid Pellet (TESPEL) l Experimental Results Ø Differences of Characteristics Between the Intrinsic Impurity and the Tracer Impurity Ø Existence of Impurity Transport Boundary Layer l Discussion l Summary NT - 14th CWGM, Jun. 18, /18

3 Outline of This Talk l Current Situation Regarding Impurity Transport Study in Large Helical Device (LHD) l Experimental Apparatus Ø Large Helical Device (LHD) Given to Prof. Ida Ø Tracer-Encapsulated Solid Pellet (TESPEL) l Experimental Results Ø Differences of Characteristics Between the Intrinsic Impurity and the Tracer Impurity Ø Existence of Impurity Transport Boundary Layer l Discussion l Summary NT - 14th CWGM, Jun. 18, /18

4 In LHD, Differences of Characteristics Between the Intrinsic Imp. and the Tracer Imp. Are Observed NT - 14th CWGM, Jun. 18, /18 l In the high-density regime of LHD, Ø Intrinsic impurities: strongly suppressed Ø Tracer impurities: accumulated l Ar gas puff (for the intrinsic imp.-simulated) exp. has indicated that there is a strong screening effect inside the last-closed flux surface (LCFS), not the disappearance of the influx of the intrinsic impurity Ø The tracer impurity deposited beyond the screening layer (namely, impurity transport boundary layer) will behave without noticing the existence of the layer accumulated l For gaining a full understanding the impurity transport in LHD, then in stellarator, the characteristics of such a layer should be fully-clarified

5 Outline of This Talk l Current Situation Regarding Impurity Transport Study in Large Helical Device (LHD) l Experimental Apparatus Ø Large Helical Device (LHD) Given to Prof. Ida Ø Tracer-Encapsulated Solid Pellet (TESPEL) l Experimental Results Ø Differences of Characteristics Between the Intrinsic Impurity and the Tracer Impurity Ø Existence of Impurity Transport Boundary Layer l Discussion l Summary NT - 14th CWGM, Jun. 18, /18

6 NT - 14th CWGM, Jun. 18, /18 TESPEL (Tracer Encapsulated Solid PELlet) is a Double-layered Impurity Pellet Polystyrene ball as a lid Polystyrene (C 8 H 8 ) n as an outer shell Tracer particles as an inner core (typically ~ 0.1 mm in size each) V V Mn Co Mn Co ~ 700 µm Great advantages of TESPEL l Direct local deposition of the tracers in the plasma is possible l Deposited amount of the tracers in the plasma can be known precisely l Relatively wide selection of the tracer materials is possible l Variable penetration depth of the tracers can be obtained owing to the flexible TESPEL size

7 Advantage of TESPEL Enables Us to Study Atomic-Molecular Processes Physics Extensively NT - 14th CWGM, Jun. 18, /18 For example, Au (gold) is used for searching for the soft X-ray radiation for water window Shell:polystyrene (C 8 H 8 ) n Plugged with a polystyrene ball OD = 900 µm Tracer: Au t ~ 300 µm

8 Summary of Impurities Utilized in the Study of the Impurity Transport Boundary Layer Intrinsic: Ti (from Ti gettering), Cr, Fe, Ni (from SUS (SUS316: Fe 72%, Cr 16%, Ni 10 %, Mo 2%) by sputtering) Z Element Cl Ar Ti V Cr Mn Fe Co Ni Cu Elements in between those are selected as tracers in the core of TESPEL; V, Mn, Co Cl is doped in the shell of TESPEL (for more shallower deposition by TESPEL) *Ar is injected as the intrinsic imitator Kα emission lines l Ar: 3.0 kev, V: 5.0 kev, Cr: 5.4 kev, Mn: 5.9 kev, Fe: 6.4 kev, Co: 6.9 kev Ø E(Kα) = 10.2 x (Z-1) 2 (ev) VUV emission lines V (Z=23) Mn (Z=25) Co (Z=27) Li-like 2s-2p (J: 1/2-3/2) V XXI (1.57 kev) Mn XXIII (1.88 kev) Co XXV (2.22 kev) Li-like 2s-2p (J: 1/2-1/2) Be-like 2s 2-2s2p (J: 0-1) V XX (1.49 kev) Mn XXII (1.79 kev) Co XXIV (2.12 kev) Na-like 3s-3p (J: 1/2-3/2) V XIII (0.34 kev) Mn XV (0.44 kev) Co XVII (0.55 kev) Na-like 3s-3p (J: 1/2-1/2) Mg-like 3s 2-3s3p (J: 0-1) V XII (0.31 kev) Mn XIV (0.40 kev) Co XVI (0.51 kev) NT - 14th CWGM, Jun. 18, /18

9 Experimental Setup for the TESPEL Injection Exp. for the Study of Impurity Transport in LHD Used Heating Power Tangential NBI: 5.2 ~ 9.4 MW Radial NBI: ~ 9.0 MW ECH (start-up assist.): 0.3/2.8 MW Circumstances No significant MHD No Transport Barriers Port 1-O PHA(Centre) (Iimp(t)) by S. Muto PHA(Peripheral) (Iimp(t)) by S. Muto TESPEL (Injection of Cl, V, Mn, Co) by N. Tamura FIR interferometer (ne(r, t)) by K. Tanaka SSGP(Injection of Ar) by J. Miyazawa Thomson Scattering (ne(r,t), Te(r,t)) AXUV diode arrays (Prad(r, t)) by N. Tamura VUV spectrometer (Iimp(t)) by C. Suzuki NT - 14th CWGM, Jun. 18, 2015 by I. Yamada, R. Yasuhara, H. Funaba Port 6-O Bolometers (Prad(r, t)) at Ports 3-O, 6.5-L, 8-O by B.J. Peterson are also used 9/18

10 Outline of This Talk l Current Situation Regarding Impurity Transport Study in Large Helical Device (LHD) l Experimental Apparatus Ø Large Helical Device (LHD) Given to Prof. Ida Ø Tracer-Encapsulated Solid Pellet (TESPEL) l Experimental Results Ø Differences of Characteristics Between the Intrinsic and the Tracer Ø Existence of Impurity Transport Boundary Layer l Discussion l Summary NT - 14th CWGM, Jun. 18, /18

11 NT - 14th CWGM, Jun. 18, /18 TESPEL Exp. Shows the Importance of Impurity Source Location and Collisionality on Impurity Transport Energy Spectra observed by PHA Plateau regime (n e = 3.4 x m -3 ) Intrinsic & Tracers Completely Suppressed Comparison with STRAHL calc. D = ~10 x neoclassical Pfirsch Schlüter (PS) regime (n e = 6.6 x m -3 ) Only Tracers l When the density is increased (collisionaltiy: Plateau PS), the Kα intensities from the tracers (V, Mn, Co) are increased, while those from the intrinsic impurities (Ti from Ti getter & Cr, Fe, Ni from SUS) are completely suppressed l Impurity source location and collisionality affect strongly the impurity transport property

12 NT - 14th CWGM, Jun. 18, /18 4-types of Impurity Injection Scheme Are Used to Study the Impact of Impurity Source Locations Radial profiles of TESPEL ablation emission Shallower deposition (i) Thick shell type (shell: C 8 H 8, tracers: V, Mn, Co) (ii) Thin shell type (shell: C 8 H 8, tracers: V, Mn) (iii) Tracer-Doped-Thin shell type (shell: C 8 H 6 Cl 2, tracers: V, Mn) (iv) Ar gas-puff

13 Cl-Doped-Thin Shell TESPEL Exp. Can Narrows the Range of the Impurity Transport Boundary Layer Kα emissions Cl Not observed, because it was! attenuated by 1mm Be filter Tracer deposited area in this exp. V & Mn: r eff /a 99 = 0.82 ~ 0.88 Cl: r eff /a 99 > 0.82 ü Tracer (V, Mn) Kα intensities shows longer impurity confinement time ü Cl Li-like emission decays faster than that calculated by STRHAL Line emissions in VUV # This exp. was performed in PS regime NT - 14th CWGM, Jun. 18, 2015 ü Li-like ion density profiles calculated by STRHAL indicate that Li-like ions of V & Cl will be affected by almost the same impurity transport property ü Thus, a suppressing mechanism at r eff /a 99 > 0.88 is necessary for explaining the results 13/18

14 NT - 14th CWGM, Jun. 18, /18 Ar Gas-Puff Exp. Suggests that the Influx of the Impurities from the Wall is Suppressed Around r eff /a 99 ~ 0.9 Ar Kα (He-like) Suppressed Plateau Ar Be-like increased & decreased gradually Ar Li-like PS increased gradually In the PS regime, ü Ar Kα (mainly He-like) emission is completely suppressed ü Ar Li- & Be-like line emissions are, however, clearly observed In order to evaluate the penetration depth of Ar particles, an ion distribution summation ratio (IDSR) is calculated 1 1 IDSR(Ion Distribution Summation Ratio) = IPS ( )/ IPlateau ( ) ü Ar ions reach up to around r eff /a 99 ~ 0.9 ρ 0

15 Outline of This Talk l Current Situation Regarding Impurity Transport Study in Large Helical Device (LHD) l Experimental Apparatus Ø Large Helical Device (LHD) Given to Prof. Ida Ø Tracer-Encapsulated Solid Pellet (TESPEL) l Experimental Results Ø Differences of Characteristics Between the Intrinsic and the Tracer Ø Existence of Impurity Transport Boundary Layer l Discussion l Summary NT - 14th CWGM, Jun. 18, /18

16 Impact of Impurity Source Location and Collisionality on Impurity Transport was Experimentally Observed Collisionality Plateau PS NT - 14th CWGM, Jun. 18, 2015 Impurity Source Location Inside Outside Decayed Not Decayed " Shortly Impurity deposited in the region " Core, but " with r eff /a 99 < 0.88 shows this featurer eff /a 99 > 0.9 Here, impurity will be accumulated due to the excess influx Edge surface layer M. Kobayashi et al" Fusion Sci. Tech. 58 (2010) 220. ü Accumulation of the impurity, which is born in the core (He-ash) and slipped through the screening effects, will be a critical drawback ü One of the promising measures against the impurity accumulation is the application of ECH, the impact of which was investigated preliminary 16/18

17 Outline of This Talk l Current Situation Regarding Impurity Transport Study in Large Helical Device (LHD) l Experimental Apparatus Ø Large Helical Device (LHD) Given to Prof. Ida Ø Tracer-Encapsulated Solid Pellet (TESPEL) l Experimental Results Ø Differences of Characteristics Between the Intrinsic and the Tracer Ø Existence of Impurity Transport Boundary Layer l Discussion l Summary NT - 14th CWGM, Jun. 18, /18

18 NT - 14th CWGM, Jun. 18, /18 Summary l In the high-density regime of LHD, there is a difference of the behavior between the intrinsic impurity and the tracer impurity Ø This observation clearly suggests the existence of the impurity transport boundary layer inside the last-closed flux surface (LCFS) l Experiment with the newly-developed tracer(cl)-doped thin-shell TESPEL and Ar gas-puff pins down the location of the impurity transport boundary layer, r/a ~ 0.9 l However, physical mechanisms for the impurity transport boundary layer inside LCFS still remain to be clarified