KSTAR Conference, Mayhills Resort, Gangwon-do, Feb. 25, 2014 Investigation of SOL Plasma Interaction with Graphite PFC Sun-Taek Lim, Hyun-Su Kim, Younggil Jin, Jin Young Lee, Jae-Min Song and Gon-Ho Kim Plasma Application Laboratory Department of Energy Systems (Nuclear) Engineering Seoul National University, Korea Plasma Seoul Nat’l Univ. Application Dept. of Nuclear Eng. Laboratory
Motivation [1] Structural deformation of graphite PFC Enhanced sputtering yield with incident angle 100 nm § Neutral densities of hydrocarbons (C, CD 4 ) will be increased near the graphite PFC or SOL with operation time, and they can affect plasma characteristics (n e , T e ) like gas puff. [2] à Morphological variation of graphite with operation time should be considered in wall condition What is the effect of graphite deformation on graphite itself (erosion rate + PFC lifetime) and plasma characteristics? Plasma [1] Q. Wei et al., J. Phys. D: Appl. Phys., 41 (2008) 172002. Seoul Nat’l Univ. Application [2] G. P. Maddison et al., Plasma Phys. Control. Fusion 48 (2006) 71. Dept. of Nuclear Eng. Laboratory
Background § Condition of 1D simple SOL is considered. [3] LCFS SOL Plasma (n e , T e ) variation D + D 2 + D + D 2 + D + D 2 + CD 4 + C + Core (flow Irrad. a-C:D film C CD 4 C free) Phys. Chem. Self Dust Redeposition sputtering sputtering sputtering formation Enhanced phys. B sputtering due to D D 2 Graphite graphite deformation Retention 1. Deformation progress of graphite 2. Enhanced sputtering (hydrocarbon formation) in SOL 3. Variation of SOL plasma characteristics (n e , T e ) by hydrocarbon Plasma [3] P. Stangeby: The Plasma Boundary of Magnetic Fusion Seoul Nat’l Univ. Application Devices (IOP Publishing, Bristol, 2000). Dept. of Nuclear Eng. Laboratory
Experimental Set-Up 1D Simple SOL Simulator KSTAR/ITER Divertor Heat Load Simulator § § Coolant Micro D 2 inlet T.C. gauge wave Pyrometer T.C. N 2 / Ar Target Coolant Coolant Target Turbo A T.C. pump ≤300 A DC Mode Pulse Mode R B C 0 Heat exchanger R.P. < 300 V < 3 kV Switch Device Thermal Flux Device Plasma Characteristics Thermal Loader n e ~ 2 × 10 17 m -3 , T e ~ 5 eV, Ion Irradiator ≤ ~ 15 MW/m 2 (Plasma torch) Γ i ~ 3.09 × 10 21 m -2 s -1 (ECR source) KSTAR divertor [5, 6] 3.5 ~ 4.5 MW/m 2 n e ~ 2.5 × 10 17 m -3 , T e ~ 4 eV, KSTAR SOL [4] Γ i ~ 3.46 × 10 21 m -2 s -1 ITER divertor [5, 6] 5 ~ 20 MW/m 2 Plasma [5] T. Hirai, et al., Material Transactions 46 (2005) 412. Seoul Nat’l Univ. Application [4] J. G. Bak et al., Contrib. Plasma Phys. 53 (2013) 69-74. [6] B. Lee, et al., Fusion Sci. Technol., 37(2000) 110. Dept. of Nuclear Eng. Laboratory
Formation of Cone Shaped Graphite Surface : Ion Dose Effect (H 2 , Pulse mode, Energy = 1 keV, Ion flux = 2.07 × 10 17 cm -2 s -1 ) 5 × 10 16 cm -2 2 × 10 17 cm -2 1 × 10 16 cm -2 100 nm 100 nm 100 nm 1.8 5 × 10 17 cm -2 Diameter of Conical Tip (nm) 100 1.6 90 1.4 80 70 1.2 60 1.0 I D /I G 50 0.8 40 0.6 30 0.4 20 Pristine 100 nm 0.2 10 100 nm 0 10 20 30 40 50 5 10 15 20 25 30 35 40 45 50 16 cm -2 ) 16 cm -2 ) Dose (10 Dose (10 § Conical shaping on graphite surface is varied with plasma parameters (incident ion dose and energy è Ion Energy dose) Plasma Seoul Nat’l Univ. Application Dept. of Nuclear Eng. Laboratory
Collection of Conical Tips and Dust Formation Possibility (D 2 , DC mode, Energy = 100 eV, Ion flux = 5.26 × 10 17 cm -2 s -1 ) 9.47 × 10 20 cm -2 1.89 × 10 21 cm -2 3.79 × 10 21 cm -2 200 nm 200 nm 200 nm 1.89 × 10 21 cm -2 3.79 × 10 21 cm -2 200 nm 200 nm Plasma Seoul Nat’l Univ. Application Dept. of Nuclear Eng. Laboratory
Effect of Ion Incidence and Thermal Load on Graphite Morphology Ion Irradiation Thermal Load 100 eV, 3.79 × 10 21 cm -2 Effect of ion incidence à Conical shaping Effect of thermal load à Eroded surface 15 MW/m 2 , 5 min 200 nm 200 nm 200 nm 200 nm Thermal Load after Ion Irradiation Conical tips (partially) and eroded surface 1 μm Plasma Seoul Nat’l Univ. Application Dept. of Nuclear Eng. Laboratory
Increase of Erosion Rate - Angle Effect on Sputtering Yield 2900 MJ/m 2 [9] (KSTAR 2010 [10] ): Pristine 0.8 MJ/m 2 : Conical tip Spherical ptls 10 μm 100 nm 100 nm Deformation progress of graphite with operation time : Plane surface à Conical tip à Spherical particle § § Sputtering yield for plane surface can be calculated by J. Roth model [11] = + + + Y Y Y (1 DY ) Y tot phys therm dam surf Chemical Physical sputtering (CH 4 ) sputtering (C) § Angle effect should be considered for the calculation of sputtering yield for conical tip and æ ö q q 2 2 Y E ( , ) a sin spherical particle [12] = q = q - q 2 cos exp cos exp[2(1 cos )] ç ÷ q = a 2 Y E ( , 0) 2 è ø q q Y E ( , ) Y E ( , ) = = 1.5973 1.3343 § For conical tip : and spherical particle : q = q = Y E ( , 0) Y E ( , 0) Sputtering yield is increased with structural deformation of graphite (energy dose, time) à Acceleration of erosion (sputtering) rate Plasma [9] Y. Yu et al., Plasma Phys. Control. Fusion 54 (2012) 105006. [11] J. Roth et al., J. Nucl. Mater. 337–339 (2005) 970. Seoul Nat’l Univ. Application [10] S. H. Hong et al., KSTAR 2010 tile SEM image. [12] Q. Wei et al., J. Phys. D: Appl. Phys., 41 (2008) 172002. Dept. of Nuclear Eng. Laboratory
Structural Deformation of Graphite – Energy Dose (Operation Time) Irradiated Ion Energy Dose (= Total Transferred Energy on PFC) = Ion Energy × Ion Dose Structural Deformation 5 Spherical ptls - 2900 MJ/m 2 (KSTAR 2010) [10] 5 ITER KSTAR 4 Thermal 10 μm Torch 3 ECR 4 Eroded surface with 2 spherical ptls - 360 MJ/m 2 ECR 1 ECR 2 μm Energy Dose 3 Soot-like structure (Operation time) - 82.7 MJ/m 2 10 3 J/m 2 10 5 J/m 2 10 7 J/m 2 10 9 J/m 2 10 11 J/m 2 1 Conical tip 2 Crack - 0.23 MJ/m 2 (KSTAR GDC) - 12.4 MJ/m 2 (KSTAR SOL) [13] 1 μm 1 μm 200 nm Plasma [10] S. H. Hong et al., KSTAR 2010 tile SEM image Seoul Nat’l Univ. Application [13] S. J. Yang et al., Fusion Eng. Des. 87 (2012) 344-351. Dept. of Nuclear Eng. Laboratory
Decrease of T e and Increase of n e with Carbon Byproduct V bias = - 35 V, 110 A, 400 W, Z scan from the target center by LP 4.1 SUS SUS Electron Temperature (eV) 11 3.6x10 4.0 Graphite Graphite -3 ) Experimental Condition Electron Density (cm 3.9 11 3.2x10 § KSTAR SOL simulator 3.8 § D 2 , 1 mTorr 11 3.7 2.8x10 § Coil current : 100, 110 A 3.6 11 § Microwave power : 200, 400 W 2.4x10 3.5 § Target bias : - 5, - 35, - 65, -95 V 3.4 11 2.0x10 Target Target 3.3 0 10 20 30 40 50 60 70 80 90 100 110 0 10 20 30 40 50 60 70 80 90 100 110 Distance From Target (mm) Distance From Center (mm) T e,avg for graphite target/T e,avg for SUS target n e,avg for graphite target/n e,avg for SUS target 1.02 1.16 1.01 1.14 1.00 1.12 0.99 1.10 Initial values of T e and n e ↑ à Ion flux ↑ à 0.98 1.08 1.06 Y chem ↑ + Y phys ↑ (enhanced deformation of 0.97 1.04 0.96 graphite) à Density of carbon byproducts 1.02 0.95 1.00 in the plasma ↑ à Enhance T e ↓ and n e ↑ 0.94 100 A, 200 W, 100 A, 200 W, 110 A, 400 W, 110 A, 400 W, - 65 V - 95 V - 5 V - 35 V Initial n e ↑ T e ↑ Plasma Seoul Nat’l Univ. Application Dept. of Nuclear Eng. Laboratory
Requirement of Quantitative Evaluation of Graphite Deformation Rate coefficients of dissociation and ionization of CD 4 are larger than D 2 , respectively. § For example, rate coefficients of dissociation and ionization of CD 4 are about 10 -7 and 5 x § 10 -8 cm 3 s -1 , and 2 x 10 -8 and 5 x 10 -9 cm 3 s -1 for D 2 at electron temperature of 1 keV, respectively. [14] [15] [16] Decrease of T e and increase of n e was shown with CH 4 percentage [14] and dust injection rate [15] § in H 2 plasma. à Graphite PFC in D 2 plasma acts as a gas puff. source Quantitative evaluation of the effect of graphite deformation on sputtering yield (formation of hydrocarbon species, especially CD 4 ) is required. [14] W. Möller et al, Appl. Phys. A 56 (1993) 527. [15] A. M. Dias. 2012, Modeling of Low Pressure Plasmas in Plasma CH 4 -H 2 Mixtures , Master Thesis, UTL. Seoul Nat’l Univ. Application [16] R. D. Smirnov et al., J. Nucl. Mater., 415 (2011) S1067. Dept. of Nuclear Eng. Laboratory
Global Model for 1D SOL Plasma [3] Microwave § Cylindrical plasma (radius R and length L) with a strong axial magnetic field à Suppression of ion radial loss [17] Coil = + = A h A h A h A eff L L R R L L - - = p 2 = p = + l 1/2 = + l 1/2 A 2 R , A 2 RL h , 0.86{3 L / 2 } , h 0.8{4 R / } L R L i R i Sputtering of graphite is considered as formation of CD 4 § CD 4 using summation of physical sputtering yield (C) and Target chemical sputtering yield (CD 4 ) [3] P. Stangeby: The Plasma Boundary of Magnetic Fusion Devices (IOP Publishing, Bristol, 2000). Plasma [17] M. A. Lieberman, A. J. Lichtenberg: Principles of Plasma Discharges Seoul Nat’l Univ. Application and Materials Processing. 2nd ed. New York: John Wiley & Sons; 1994. Dept. of Nuclear Eng. Laboratory
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