Investigation of SOL Plasma Interaction with Graphite PFC Sun-Taek - - PowerPoint PPT Presentation
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
Seoul Nat’l Univ.
Plasma Application Laboratory
Plasma Application Laboratory Department of Energy Systems (Nuclear) Engineering Seoul National University, Korea
Seoul Nat’l Univ.
Plasma Application Laboratory
Structural deformation of graphite PFC Enhanced sputtering yield with incident angle
[1]
[1] Q. Wei et al., J. Phys. D: Appl. Phys., 41 (2008) 172002. [2] G. P. Maddison et al., Plasma Phys. Control. Fusion 48 (2006) 71.
§ Neutral densities of hydrocarbons (C, CD4) will be increased near the graphite PFC or SOL with
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?
100 nm
Seoul Nat’l Univ.
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D+ D2
+
D D2 C C+ CD4 CD4
+
C a-C:D film D+ D2
+
D+ D2
+
Irrad. Retention Phys. sputtering Chem. sputtering Self sputtering Redeposition Enhanced phys. sputtering due to graphite deformation Plasma (ne, Te) variation Dust formation
§ Condition of 1D simple SOL is considered. [3]
[3] P. Stangeby: The Plasma Boundary of Magnetic Fusion Devices (IOP Publishing, Bristol, 2000).
1. Deformation progress of graphite 2. Enhanced sputtering (hydrocarbon formation) in SOL 3. Variation of SOL plasma characteristics (ne, Te) by hydrocarbon
Seoul Nat’l Univ.
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[5] T. Hirai, et al., Material Transactions 46 (2005) 412. [6] B. Lee, et al., Fusion Sci. Technol., 37(2000) 110. [4] J. G. Bak et al., Contrib. Plasma Phys. 53 (2013) 69-74.
Device Plasma Characteristics Ion Irradiator (ECR source) ne ~ 2 × 1017 m-3, T
e ~ 5 eV,
Γi ~ 3.09 × 1021 m-2s-1 KSTAR SOL [4] ne ~ 2.5 × 1017 m-3, T
e ~ 4 eV,
Γi ~ 3.46 × 1021 m-2s-1
§ 1D Simple SOL Simulator
Device Thermal Flux Thermal Loader (Plasma torch) ≤ ~ 15 MW/m2 KSTAR divertor [5, 6] 3.5 ~ 4.5 MW/m2 ITER divertor [5, 6] 5 ~ 20 MW/m2
§ KSTAR/ITER Divertor Heat Load Simulator
Micro wave D2 inlet Turbo pump Target
< 300 V
DC Mode Pulse Mode
RB Switch C0
< 3 kV
A
Coolant Coolant N2 / Ar Coolant
Heat exchanger
R.P. ≤300 A
T.C. T.C.
T.C. gauge Pyrometer Target
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5 × 1016 cm-2 2 × 1017 cm-2
100 nm 100 nm 100 nm
(H2, Pulse mode, Energy = 1 keV, Ion flux = 2.07 × 1017 cm-2s-1) 5 × 1017 cm-2
100 nm 100 nm
5 10 15 20 25 30 35 40 45 50 10 20 30 40 50 60 70 80 90 100
Diameter of Conical Tip (nm) Dose (10
16 cm
10 20 30 40 50 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
ID/IG Dose (10
16cm
Pristine
1 × 1016 cm-2
§ Conical shaping on graphite surface is varied with plasma parameters (incident ion dose and energy è Ion Energy dose)
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1.89 × 1021 cm-2 3.79 × 1021 cm-2
200 nm 200 nm 200 nm
(D2, DC mode, Energy = 100 eV, Ion flux = 5.26 × 1017 cm-2s-1) 9.47 × 1020 cm-2 3.79 × 1021 cm-2 200 nm 1.89 × 1021 cm-2 200 nm
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100 eV, 3.79 × 1021 cm-2
200 nm
15 MW/m2, 5 min
200 nm
Ion Irradiation Thermal Load
1 μm
Effect of ion incidence à Conical shaping Effect of thermal load à Eroded surface
200 nm 200 nm
Thermal Load after Ion Irradiation
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0.8 MJ/m2 : Conical tip
100 nm 10 μm
2900 MJ/m2 [9] (KSTAR 2010 [10]): Spherical ptls Pristine
100 nm
§ Sputtering yield for plane surface can be calculated by J. Roth model [11] § Angle effect should be considered for the calculation of sputtering yield for conical tip and spherical particle § For conical tip : and spherical particle :
tot phys therm dam surf
Physical sputtering (C) Chemical sputtering (CH4)
[11] J. Roth et al., J. Nucl. Mater. 337–339 (2005) 970. [12] Q. Wei et al., J. Phys. D: Appl. Phys., 41 (2008) 172002.
2 2 2 2
( , ) sin cos exp cos exp[2(1 cos )] ( , 0) 2 Y E a Y E q q q q q q a æ ö = =
÷ = è ø
[12]
( , ) 1.3343 ( , 0) Y E Y E q q = = ( , ) 1.5973 ( , 0) Y E Y E q q = =
Sputtering yield is increased with structural deformation of graphite (energy dose, time) à Acceleration of erosion (sputtering) rate
[9] Y. Yu et al., Plasma Phys. Control. Fusion 54 (2012) 105006. [10] S. H. Hong et al., KSTAR 2010 tile SEM image.
§ Deformation progress of graphite with operation time : Plane surface à Conical tip à Spherical particle
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10 μm
5Spherical ptls
2 μm
4Eroded surface with
spherical ptls - 360 MJ/m2
1 μm
3Soot-like structure
1 μm
2Crack
Irradiated Ion Energy Dose (= Total Transferred Energy on PFC) = Ion Energy × Ion Dose
1
Energy Dose (Operation time)
ITER KSTAR
109 J/m2 1011 J/m2 107 J/m2 105 J/m2 103 J/m2
Structural Deformation
2 3 4 5 200 nm
1Conical tip
[10] S. H. Hong et al., KSTAR 2010 tile SEM image [13] S. J. Yang et al., Fusion Eng. Des. 87 (2012) 344-351.
ECR ECR ECR Thermal Torch
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Vbias = - 35 V, 110 A, 400 W, Z scan from the target center by LP 10 20 30 40 50 60 70 80 90 100 110 2.0x10
11
2.4x10
11
2.8x10
11
3.2x10
11
3.6x10
11
SUS Graphite
Electron Density (cm
Distance From Center (mm)
Target § KSTAR SOL simulator § D2, 1 mTorr § Coil current : 100, 110 A § Microwave power : 200, 400 W § Target bias : - 5, - 35, - 65, -95 V
Experimental Condition
Initial ne ↑ Te ↑
Initial values of Te and ne ↑ à Ion flux ↑ à Ychem ↑ + Yphys ↑ (enhanced deformation of graphite) à Density of carbon byproducts in the plasma ↑ à Enhance Te ↓ and ne ↑
0.94 0.95 0.96 0.97 0.98 0.99 1.00 1.01 1.02 Te,avg for graphite target/Te,avg for SUS target 1.00 1.02 1.04 1.06 1.08 1.10 1.12 1.14 1.16 ne,avg for graphite target/ne,avg for SUS target
100 A, 200 W,
100 A, 200 W,
110 A, 400 W,
110 A, 400 W,
10 20 30 40 50 60 70 80 90 100 110 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1
Electron Temperature (eV) Distance From Target (mm)
SUS
Graphite
Target
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§ Rate coefficients of dissociation and ionization of CD4 are larger than D2, respectively. § For example, rate coefficients of dissociation and ionization of CD4 are about 10-7 and 5 x 10-8 cm3s-1, and 2 x 10-8 and 5 x 10-9 cm3s-1 for D2 at electron temperature of 1 keV,
§ Decrease of Te and increase of ne was shown with CH4 percentage [14] and dust injection rate [15] in H2 plasma.
[15] [16]
[14] W. Möller et al, Appl. Phys. A 56 (1993) 527. [15] A. M. Dias. 2012, Modeling of Low Pressure Plasmas in CH4-H2 Mixtures, Master Thesis, UTL. [16] R. D. Smirnov et al., J. Nucl. Mater., 415 (2011) S1067.
à Graphite PFC in D2 plasma acts as a gas puff. source Quantitative evaluation of the effect of graphite deformation on sputtering yield (formation of hydrocarbon species, especially CD4) is required.
Seoul Nat’l Univ.
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Coil
Microwave Target
§ Cylindrical plasma (radius R and length L) with a strong axial magnetic field à Suppression of ion radial loss [17]
2 1/2 1/2
2 , 2 , 0.86{3 / 2 } , 0.8{4 / }
L R L i R i
A R A RL h L h R p p l l
= = + = +
eff L L R R L L
A h A h A h A = + =
CD4
§ Sputtering of graphite is considered as formation of CD4 using summation of physical sputtering yield (C) and chemical sputtering yield (CD4)
[3] P. Stangeby: The Plasma Boundary of Magnetic Fusion Devices (IOP Publishing, Bristol, 2000). [17] M. A. Lieberman, A. J. Lichtenberg: Principles of Plasma Discharges and Materials Processing. 2nd ed. New York: John Wiley & Sons; 1994.
[3]
Seoul Nat’l Univ.
Plasma Application Laboratory
[18]
2 2 2 2 2 4 4 4 4 4
0.5
D D D D i e D j k m l s D wall D i jkm ls CD CD CD i e CD j k m l s CD i jkm ls r r h e h j k m l s r wall r r h jkm ls
n I O k n n k n n k n n K n t n I O k n n k n n k n n t n k n n k n n k n n K n O t ¶ =
¶ ¶ =
¶ = +
§ Balance equation for D2 § Balance equation for CD4
where I is inflow, O is outflow, k is rate constant, J is gas inlet flow rate, V is chamber volume, vpump is pumping rate, γr is wall sticking coefficient, vthr is average thermal velocity and Ssurf is chamber surface area
Nonradical neutrals Ions Radical neutrals Sticking coefficient D2, CD4, C2D2, C2D4, C2D6, C3D8 D+, D2
+, D3 +, CD3 +, CD4 +,
CD5
+, C2D2 +, C2D4 +, C2D5 +
D, CD, CD2, CD3, C2D5 CD : 0.025, CD2 : 0.025, CD3 : 0.01, C2D5 : 0.01, D : 0.001 § Overall charge neutrality
i e i
n n =
§ Balance equation for other neutrals
3 1 17
[ ] 4.4 10 [ ]/ , / , / 4
r pump wall r thr surf
I cm s J sccm V O v n V K v S V g
´ = =
[18] I. B. Denysenko et al., J. Appl. Phys., 95, 2713 (2004).
§ Balance equation for ions
, , , 1
( )
s
N e iz i L L R R i B i cx ij i j j
Vn h A h A n u V k n n n
=
= + + å
2 1/2 1/2
2 , 2 , 0.86{3 / 2 } , 0.8{4 / }
L R L i R i
A R A RL h L h R p p l l
= = + = +
§ The power balance equation Pin = Pev + Pw § Energy lost to the electron-neutral collision process
, , 1 q ev e iz i L i i
P en V n e
=
=
, , , , , , 1
3 /
exc
N iz i L i iz i iz i exc k exc k elas e eff i k
m T M n e n e n e n
=
= + +
§ Loss of kinetic energy of charged species to the discharge walls
, 1
( )( )
g w i B i L L R R e i i
P en v h A h A
w w
e e
=
= + +
4
2 graphite CD D
A I Y V
+
= G
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§ Sputtering yield ↑ à n_CxHy/(n_H+n_H2) ↑ (ex) 0.51 % for Y = 0.0126 à 1.11 % for Y = 0.03520)
à Increase of the hydrocarbon (CxHy) portion in H2 plasma causes decrease of Te and increase of ne.
0.00 0.01 0.02 0.03 0.04 10
6
10
7
10
8
10
9
10
10
10
11
10
12
10
13
10
14
Neutral Density (#/cm
3)
Sputtering Yield
H H2 CH CH2 CH3 CH4 C2H2 C2H4 C2H5 C2H6 C3H8
0.00 0.01 0.02 0.03 0.04 3.5 3.6 3.7 3.8
1.08x10
11
1.12x10
11
1.16x10
11
1.20x10
11
Sputtering yield Condition SUS target 0.0126 17 eV 0.01777 40 eV 0.02204 100 eV 0.02941 100 eV w/ conical tip 0.03520 100 eV w/ spherical tip
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0.94 0.95 0.96 0.97 0.98 0.99 1.00 1.01 1.02 Te,avg for graphite target/Te,avg for SUS target 1.00 1.02 1.04 1.06 1.08 1.10 1.12 1.14 1.16 ne,avg for graphite target/ne,avg for SUS target
1 2 3 4
Experiment Plasma Condition YtotΓi (Ytot for Roth) 1 Γi = 8.03 x 1020 m-2s-1, Ei = 70 eV 1.74 x 1019 m-2s-1 2 Γi = 8.03 x 1020 m-2s-1, Ei = 100 eV 1.77 x 1019 m-2s-1 3 Γi = 1.68 x 1021 m-2s-1, Ei = 17 eV 1.54 x 1019 m-2s-1 4 Γi = 2.76 x 1021 m-2s-1, Ei = 40 eV 4.84 x 1019 m-2s-1
4
2 graphite CD D
A I Y V
+
= G § Inflow of CD4 is determined by YtotΓi.
1x10
19 2x10 19 3x10 19 4x10 19 5x10 19 6x10 19
0.92 0.94 0.96 0.98 1.00
i (m
Te,avg for graphite target/Te,avg for SUS target 1.00 1.02 1.04 1.06 1.08 1.10 1.12 ne,avg for graphite target/ne,avg for SUS target
§ Inflow of CD4 of which ionization occurs in the SOL plasma is increased with energy dose (Ion energy x Ion dose). à Graphite deformation causes and
enhances decrease of Te and increase of ne
Variation of SOL plasma characteristics with deformation progress of graphite is predicted.
Energy dose (operation time)
Seoul Nat’l Univ.
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1. For the H2 (or D2) plasma facing with graphite PFC, plasma characteristics, especially Te and ne, vary significantly by inflow of CD4 which is caused by reaction between hydrogen and carbon and enhanced by graphite deformation. 2. Effect of graphite deformation on SOL plasma characteristics is expected for the graphite PFC in fusion devices. 3. Hydrogen ion energy dose which considers reaction between hydrogen and carbon can be used to analyze the graphite deformation with the estimation of variation of sputtering yield. This also provides the estimation of etch rate of graphite PFC based on the analysis of deformation progress of graphite.