Modeling Plasma Interactions with ITER Wall Materials: Erosion, Transport, Redeposition, Reerosion and Material Mixing, and Tritium Retention Kaoru Ohya Institute of Technology and Science, The University of Tokushima, Tokushima 77-856, JAPAN 1
(1) Introduction Contents (2) Modeling Long-Distance Transport of Carbon and Beryllium in an ITER edge Plasma (3) Modeling Tritium Retention in Tungsten Divertor Targets of ITER (4) Molecular Dynamics Study of Hydrogen and Hydrocarbon Interaction of Carbon and Beryllium Deposits 2
Related issues to plasma material interactions (1) Erosion of wall elements by physical and chemical sputtering Reduced life time of the walls (2) Eroded impurities can penetrate into the plasma Fuel Dilution and radiation cooling of core plasma (3) Redeposition and re-erosion of impurities Tritium retention of wall surface and bulk Erosion, transport, redeposition and re-erosion of impurities for realistic plasma and surface conditions 3
Global and Local Plasma Wall Interactions Divertor Wing (W) Dome (W) Mutual contamination between C and W Be deposition on C and W Vertical T arget ( C, W ) Global transport of impurities Codeposition with C and Be Local collision and thermal processes: Implantation, diffusion, trapping/ detrapping and surface recombination Be + Be first wall 4
Erosion Transport Redeposition Material mixing Models and assumptions in PWI codes Physical sputtering Chemical erosion Ionization (atoms) Ionization/dissociation (molecules) Forces driven ionized particle Reflection/ Sticking (atoms) Reflection/ Sticking(molecules) Model/assumption Yield Energy Angle Yield Energy Angle TRIM database Thompson Cosine Roth formula or Input Thermal Isotropic or cosine ADAS database Janev/Reiter data set for methane, ethane and propane families Magnetic, sheath, friction, thermal, cross-field diffusion, elastic collisions, Radial electric field TRIM database Input Coupling codes Static/dynamic BCA codes Static/dinamic BCA codes Molecular Dynamics codes or Database by MD Dynamic BCA codes 5
Modeling Long-Distance Transport of Carbon and Beryllium in an ITER edge Plasma 6
Model geometry of edge plasma and walls (Be) Plasma parameters in an ITER edge plasma with D and impurities (C and He) are taken from a B2/Eirene calculation [1]. [1] G.Federici et al., J.Nucl.Mater.29-293(21)26. Aside from sputtering by plasma ions, sputtering by charge exchange (CX) neutrals is taken into account at the first wall. Physical sputtering yield of C target in the divertor and Be first wall is calculated by using EDDY [2]. [2] K.Ohya, Phys.Scr. T124 (26)7. Due to high threshold energy for physical sputtering by D ions, sputtering of W baffle and dome is not taken into account. (CFC) (W) (CFC) Chemical sputtering of C target is calculated using Roth formulae [3]. Only CD 4 molecules are released from the target. [3] J.Roth et al., J.Nucl.Mater. 266-269(1999)1/337-339(25)97. Detailed description of impurity transport model is presented in ref. [4]. [4] K.Ohya et al., Phys.Scr. T138(21)141. 7
Ion and neutral energy (ev) Ion and neutral fluxes (m -2 s -1 ) Flux and energy of ions and CX neutrals Surface temperature, T s (K) 1 25 D + (L=1cm) 1 21 1 17 1 13 1 9 1 5 7 6 5 4 3 2 1 Inner region Outer region Dome Target First wall D + (L=3cm) D + DCX Ts First wall Target Dome D + (L=1cm) DCX (a) (b) 5 1 15 2 Poloidal distance (m) 12 1 8 6 4 Poloidal distributions of the flux of CX neutrals and of their mean energy along the grid edge are taken from ref. [5]. [5] R.Behrisch et al., J.Nucl.Mater. 313-316(23)338. Angular distribution of ions is influenced by gyro-motion of the ions; most probable angles of the distribution are 12 o ~18 o, which are much larger than the magnetic angles intersecting the wall. Average angle of magnetic field lines intersecting the first wall equipped with blanket modules is chosen to be 5 o, which results in an incident angle of ~21 o to the first wall. Ion flux at the first wall is assumed to decay exponentially from the grid edge to the wall. The decay length is taken to be 1 cm, 3 cm and 1 cm. 8
Sputtering fluxes (m -2 s -1 ) C, CD 4 and Be fluxes eroded from walls Dome Target 1 23 1 21 Inner region First wall C CD4 Be(ion,L=1cm) Outer region First wall Be(ion,L=3cm) Be(ion,L=1cm) Be(neutral) Target Dome Dominant erosion mechanism at the outer divertor target is physical sputtering. Asymmetric erosion between the inner and outer targets is observed, depending on the incident ion energy. 1 19 1 17 Erosion of the inner target is dominated by chemical sputtering, a maximum yield of which occurs at the strike point. 1 15 1 13 5 1 15 2 Poloidal distance (m) Erosion of the first wall is at least by factors of 1 1 smaller than that of the divertor targets. Localized gas puffing and recycling at the top of the first wall causes sputtering flux to be strongly decreased. If decay length of ion flux between the grid edge and the first wall is taken to be 1 cm, the sputtering flux by ions is high enough to be comparable to the flux by CX neutrals. With decreasing length, the ion flux is strongly reduced, showing complicated profile closely related to the local distance between the grid edge and the first wall. 9
Vertical distance (m) C, CD y and Be distributions in edge plasma Flight time integral 4 2-2 -4 Physical sputtering (a) (b) (c) C CD y Be Chemical sputtering Physical sputtering 4 6 8 4 6 8 4 6 8 Radial distance (m) Radial distance (m) Radial distance (m) A part of C atoms is promptly ionized and redeposit in the vicinity of the birthplace. The other part is transported away from it and some of them distribute out of the divertor. CD y is rather limited within the private flux region (PFR) of the divertor. Be atoms are ionized and subsequently transported along the magnetic field lines for a long distance, therefore, they distribute over the whole area of the machine. 1 28 1 26 1 24 1 22 1 2 The color represents the sum of the flight time of all particles traveling in each grid cell, per unit area in the poloidal cross-section. 1
Redeposition flux (m -2 s -1 ) C, D y and Be redeposition profiles Dome Target First wall 1 23 1 22 1 21 1 2 Inner region (a) C, CD y Outer region First wall C CD y Target Dome C atoms produce a very sharp profile on the outer target. CD 4 eroded from the position near the strike point produces a pronounced redeposition of neutral CD y directly on the dome. 1 19 1 18 1 17 1 23 1 22 1 21 1 2 (b) Be L=1cm L=3cm L=1cm Be redeposition in the divertor occurs rather more at the inner target. The asymmetric redeposition between the inner and outer divertor targets was found in ref. [6]. [6] K.Schmid, Nucl.Fusion 48(28)154. Be redeposition on the first wall is more in the outer side of the top of the first wall. 1 19 1 18 1 17 5 1 15 2 Poloidal distance (m) Be redeposition flux on the first wall by a factor of 1 smaller than the flux redeposited on the inner divertor target. 11
Net erosion/deposition flux (1 2 m -2 s -1 ) Net erosion and deposition profiles 4 2 Inner region Dome Target First wall (a) C Outer region First wall Target Dome Physical and chemical sputtering yields of original materials are used for the re-erosion yields for C (CD y ) and Be deposits. Net erosion and deposition profiles of C, CD y and Be, calculated as the flux difference between redeposition and re-erosion. -2-4.4.2 -.2 (b) Be L=1cm L=3cm L=1cm -.4 5 1 15 2 Poloidal distance (m) C deposits in the inner divertor are strongly re-eroded except for the dome where sputtering is negligibly small. Position near the strike point, as well as the dome, is a deposition zone whereas the position far from it is an erosion zone. Be deposits on the inner and outer targets are strongly re-eroded due to low threshold energy for physical sputtering. The top of the first wall and the inner dome are deposition zones. 12
T codeposition flux (1 19 m -2 s -1 ) Tritium codeposition profile on walls 4 3 2 Inner region Dome Target First wall (a) C, CD y Outer region First wall Target Dome Empirical formulae recently proposed by Doerner et al. [6] are used for atomic ratios of D to C and of D to Be. [6] R.P.Doerner et al., Nucl.Fusion 49(29)352. (The estimation of T retention, corresponding D data, are performed in this work.) 1 1 8 6 4 (b) Be L=1cm L=3cm L=1cm Using surface temperature and D energy on the inner and outer targets and the first wall, D/C and D/Be values are calculated as a function of the position. Net redeposition flux profiles are multiplied with D/C and D/Be profiles to obtain T codeposition profile. 2 5 1 15 2 Poloidal distance (m) Finally, assuming toroidal symmetry, total retention rate can be estimated from the calculated T codeposition profile. 13
Tritium retention rate in C and Be deposits Decay length *1 Divertor *2 First wall Total (cm) Inner target dome Outer target (a) Carbon deposition 1.26 [mgt/s].1 [mgt/s] 2.51 [mgt/s].12 [mgt/s] 3.89 [mgt/s] (b) Beryllium deposition 1.6.3.48.58 3.6.4.6.7 1.3.14 1.48 1.65 Dominant T retention in C occurs at the inner and outer divertor target, whereas it occurs at the first wall. Retention rate in Be is strongly influenced by decay length of plasma parameters from the grid edge to the first wall. Using a discharge duration of 4 s, the number of discharge after which an in-vessel T safety limit of 7 g is reached are estimated from the sum of the T retention rate in C and Be deposits, if the retention rate in W is negligibly low. It is predicted to be 295 395 discharges, depending on the decay length. 14
Conclusions (1) To predict tritium (T) retention in ITER wall components, an erosion and redeposition modeling of divertor targets (CFC) and a first wall (Be) is performed for an ITER edge plasma configurations. (1) Position near the strike point on the outer target is a deposition zone whereas position far from it is an erosion zone due to physical sputtering. (2) CD y eroded through chemical sputtering produces its pronounced redeposition on the dome. (3) Erosion of the first wall is strongly influenced by a decay length of local plasma parameters from the plasma edge to the wall. Eroded Be atoms distribute over the whole area of the machine, resulting in an influx of Be to the divertor. (4) T retention is dominated by C deposition on the inner and outer targets. Be deposition less contribute it, most of which occurs on the first wall. Total retention rate limits 295 395 discharges before T limit of 7 g is reached. 15
Modeling Tritium Retention in Tungsten Divertor Targets of ITER 16
Thermal Processes after a Collision Process Fick s law with source and trapping terms n i c j x, t c Tj x, t D j c j x, t G j x, t t t c i Tj i 1 C j ( x, t ) : j th solute concentration, D j : Diffussion coefficient for j th solute G j x,t : source term (range profile) ( x, t ) : concentration of j th solute trapped I th trapping site c i Tj Rate equation for trapping and detrapping i Te i x, t D c x, tc x, t t j j i x, t CTex, t C f c ( x, t) :jump distance, i j i x, t exp( E / ) Te i c 2 Tj T kt j i j i Tj : detrapping attempt frequency f i E T : the inverse trap saturability of j th solute fot the I th trapping site : detrapping energy of I th trap Boundary condition e.g., recombination limited c x K 2 j r 2 J K c x rc j x D j j K r : recombination coefficient t= t t t DIFFUSE Vacuum Sputtered atom E el (q 1 )-E sb Solid Recoiled atom E el (q 2 ) E 1 =E -E inel (L 1 )-E el (q 1 ) L 2 =-ln (E 1 ) Projectile ion E L 1 =-ln (E ) E 2 =E 1 -E inel (L 2 )-E el (q 2 ) / DIFFUSE DIFFUSE DIFFUSE N H BCA BCA BCA BCA BCA Time Time 17
Released D flux (cm -2 s -1 ) Areal D density (cm -2 ) Parameter Fitting with a TDS experiment 1 16 1 15 1 14 1 13 1 17 1 16 1 15 1 14 1 13 1 12 1 11 Implantation (3K) Outgass (3K) TDS (5K/s) (a) (b) Total Mobile Trap 1 Trap 2 ref + rec rec exp. Ttar 1 2 3 4 5 6 7 8 Time (s) 1 8 6 4 2 Ttar (K) Time evolution of the areal density of trapped D in W is shown along with that of the density retained as mobile atoms. In the experiment [1], a wrought W surface was irradiated by D 3 + ions with an energy of 1 ev/d and a flux of 2.5 1 15 cm -2 s -1. [1] C.Garcia-Rosales et al., J.Nucl.Mater.233-237(1996)83. (a) Diffusion (b) Surface recombination D (cm 2 s -1 ) E D (ev) E r (ev) K r (cm 4 K 1/2 s -1 ).39 2.5 1-7 -.59 1.2 1-25 (c) Trapping E T1 (ev) D trap1 /W E T2 (ev) D tarp2 /W.85.1 1.5.1 Density of mobile and trapped D atoms increases successively during implantation. After implantation, a part of mobile D atoms are released due to surface recombination. Trapped D atoms are kept to be retained in the bulk. At the early stage of the TDS phase, D atoms in the Trap 1 are released via mobile D atoms. At the delayed stage, D atoms in the deeper trap (Trap 2) are released. 18
E (ev), b (deg), Ttar (x1 K) Plasma Parameters used for calculation (cm -2 s -1 ) (cm -2 s -1 ) 15 1 5 15 1 5 Ttar b b Ttar E Outer target (a) Inner target -.1.1.2.3.4.5.6 E (b) Distance from strike point (m) 1 2 1 19 1 18 1 17 1 16 1 15 1 2 1 19 1 18 1 17 1 16 1 15 The plasma parameters in front of the targets are taken from a B2-EIRENE calculation [2], as a function of the distance from the strike point. [2] G.Federici et al., J.Nucl.Mater.29-293(21)26. The surface temperature depending on the position on the target is taken from [3], where the temperature were calculated assuming CFC, not W, with the thickness of 1 mm. [3] G.Federici et al., Plasma Phys. Control.Fus.45(23)1523. Typical duration of a discharge in ITER is 4 s. The surface temperature at each position is kept constant after discharge as well as during it. The trap concentration strongly depends on the material and additional traps may be produced in the near-surface region due to high D fluxes to the target, resulting in a depth-dependent concentration. Incident energy (T), angle (b) and flux () and target temperature (T tar ) as a function of the position on the inner and outer target. 19
Angle of incidence (deg) Angular distribution of incident D ions Incidence at Low energy and Shallow Angles Mean angle of incidence (deg) Sticking coefficient Angular distribution of the ions is calculated at different magnetic field angles using a particlein-cell simulation [5]. [5] K. Inai et al., J. Plasma Fusion Res. Ser. 8, 433 (29). 9 6 3.1.8.6.4 8 7 6 5 4 3 Fitting curve: 23.1-.221+.177 2 +5.68e-5 3 2 2 4 6 8 Magnetic angle, (deg) 7 o 85o Magnetic angle = o 86 o 87 o 88 o 89 o Sticking coefficient calculated using a molecular dynamics simulation [4], where the incident angle is 75. [4] K. Ohya et al., J. Nucl. Mater. 417, 637 (211). 1.1.1 T= 3 K T=1 K D on W.2 2 o 45 o 3 6 9 Incident angle q (deg) 3 6 9 Angle of magnetic field lines (deg) The angle is measured from the surface normal. The average angle of the distribution is used as incident angle, 12 ~18, depending on the magnetic angle, at each position on the target..1 1 1 1 Incident energy (ev) In the relevant energy range, the sticking coefficient, slightly depending on the surface temperature, is assumed to be.1 for the calculation of the implantation flux. 2
Areal density of retained D atoms (cm -2 ) Time evolution of retained D distribution Areal density of retained D atoms (cm -2 ) 1 18 During plasma exposure (a) 1 17 1 16 1 15 1 14 1 13 1 12 1 18 1 17 1 16 1 15 1 s Inner target 4 s After plasma exposure 1 s Dtrap/W=.1 4 s 1 3 1 3 (b) 1 18 4 s (a) 1 17 1 16 1 15 1 14 1 13 1 12 1 18 1 17 1 16 1 15 1 s Outer target 1 s During plasma exposure Dtrap/W=.1 After plasma exposure 4 s 1 3 1 3 (b) 1 14 1 13 Inner target Dtrap/W=.1 1 14 1 13 Outer target Dtrap/W=.1 1 12 -.1.1.2.3.4.5.6 Distance from strike point (m) 1 12 -.1.1.2.3.4.5.6 Distance from strike point (m) At the position where the temperature is high, the number of retained D atoms increases without any saturation. Most of D atoms are retained as mobile atoms. At the low temperature position, it tends to saturate where most of trap sites near the surface are occupied by implanted D atoms. After discharge (>4 s), most of D atoms are kept to be retained in the bulk, where they can diffuse 21 deeper.
Areal density of retained D atoms (cm -2 ) Trap concentration dependence of Deuterium Retention Retained distributions of D atoms along the inner and outer targets (a) just after a discharge (4 s) and (b)subsequently after it (1 s). 1 18 1 17 1 16 1 15 1 14 1 13 1 18 1 17 1 16 Inner target 4 s 1 s Outer target (a) Dtrap/W=.1 Dtrap/W=.1 Dtrap/W=.1 (b) Dtrap/W=.1 The number of retained D atoms at the position with low temperature strongly depends on the trap concentration in W. At the position with high temperature, a weaker dependence is obtained due to dominant retention of mobile atoms. 1 15 1 14 1 13 4 s 1 s Dtrap/W=.1 Dtrap/W=.1 -.1.1.2.3.4.5.6 Distrance from strike point (m) 22
Tritium retention (mg) Time evolution of Tritium Retention in Targets From the distribution of retained D atoms during and after discharge, the T retention in the inner and outer targets are estimated by taking the atomic mass difference between D and T into account, and assuming toroidal symmetry. 1 2 1 1 1-1 1-2 1-3 1-4 1-5 1-6 1 2 1 1 1-1 1-2 1-3 1-4 1-5 1-6 (a) Total Trap 2 Mobile During discharge Trap1 Total Mobile Trap 2 Trap1 During discharge (b) Inner target Dtrap/W=.1 After discharge After discharge Outer target Dtrap/W=.1 2 4 6 8 1 Time (s) In case of the inner target, dominant retention mechanism is the trapping in the deep trap (Trap 2) during discharge and most of the T atoms are kept in the trap even after discharge. Mobile T atoms dominate the T retention in the outer target due to its high temperature leading to detrapping from the trap and subsequent diffusion inside the bulk. The T atoms are retained ten times more in the outer target than in the inner target during discharge, whereas sufficiently after discharge the T retention is reduced due to surface recombination of mobile atoms. 23
Tritium retention (mg) Tritium Retention in Divertor Targets 5 4 3 2 1 Just after discharge Inner target Mobile Trap 1 Trap 2 Outer target.1.1.1.1.1.1 Trap concentration, Ttrap/W Tritium retention (mgt) after a discharge (4 s) in tunsgten. Trap concentration T trap /W Inner target Outer target (a) Just after discharge (4 s) Divertor T retention is dominated by mobile atoms in the outer target decreasing with time after a discharge, whereas in the inner target it is strongly enhanced due to an increase in the trap concentration. Finally, the number of discharges, after which an in-vessel T safety limit of 7 g is reached, is estimated from the sum of T retention of the inner and outer targets; T retention in other walls is not taken into account. Total.1 1.8 47.3 58..1 2.6 31.1 33.7.1 1. 28.5 29.6 (b) Subsequently after discharge (1 s).1 8.5 23.4 31.9.1 2. 12.8 14.9.1.6 11.1 11.7 The number of discharges is of the range between 12 and 24, depending on the trap concentration from.1 to.1. It is increased to the values between 22 and 6, if the T retention sufficiently after discharge (1 s) is used. 24
Conclusions (2) (1) Dominant retention mechanism for the inner target is the trapping in the deep trap and most of the retained T atoms are kept in the trap even after discharge. (2) Mobile T atoms dominate the T retention in the outer target due to its high temperature, leading to detrapping from the trap and subsequent diffusion inside the bulk. (3) The T retention after a discharge duration of 4 s is estimated to be tens of mg, strongly depending on the trap concentration in the bulk. It results in 1 4 discharges or more after which a T safety limit of 7 g is reached. It indicates that the T retention in a W target is about two orders of magnitude smaller than that for a CFC target. Nevertheless, Be used for the first wall is strongly eroded due to its low surface binding energy and a portion of eroded Be atoms migrates towards the divertor targets, although most of atoms redeposit on the other areas of the first wall. Codeposition mechanism for the Be deposits may dominate the T retention in the W target in both inner and outer regions. 25
Molecular Dynamics Study of Hydrogen Interaction of Carbon and Beryllium Deposits 26
Plasma Surface Interaction in Fusion devices (1) Erosion of plasma facing walls ---> Reduced life time of wall material (2) Transport of eroded impurities in plasmas ---> Fuel dilution and radiation cooling of core plasma (3) Redeposition of eroded particles ---> Tritium retention in redeposited layers and materials mixing Computer simulation of such erosion and redeposition requires: (1) transport of eroded impurities in edge plasmas, and (2) surface interactions of hydrogen and eroded impurities. Main purpose is to study realistic plasma surface interactions in fusion devices Molecular dynamics (MD) simulation is used for Local interactions of hydrogen isotopes with C, Be and W, and preparation of deposited C and material mixed layers formed on them. 27 27
Classical molecular dynamics (MD) codes Integrating equation of motions of constituent atoms The force on each atom calculated from the analytical derivation of appropriate interaction potential form. Coupling to an external bath (e.g., Langevin eq.) Periodic boundary condition (cell sides) : Small cell, 1 3-1 7 atoms : Excess heat dissipation in collisions with energetic atom : Top most atoms are free, but bottom most atoms are fixed. Potentials can be calculated by quantum chemistry or density functional, but Approximate parametrizations are used in classical scheme. bij b c R ji A V fij ( rij ) V ij ( rij ) Vij ( rij ) : Empirical bond order potential i j 2 Attractive term: Cutoff-function: D V R ( r) exp b 2S r r S 1 SD V ( ) exp 2 S r r A r b S 1 1, r R D, f c 1 1 ( r) r R D 2 2 sin 2, R r D,, r R D Repulsive term: 21 Bond-order function: Many-body term: 1 Fusion-related parameter sets for C-C, C-H : Brenner (199, 1992), REBO (22) and AIREBO (2) W-W, W-C, W-H : Juslin, Nordlund et al. (25) Be-Be, Be-C, Be-H, Be-W : Bjorkas, Nordlund et al. (29/21) ij b ij f ( r ) g ( q ) exp 2 r r c ik k( i, j) 2 2 c c Angular function:g( q) 1 2 2 2 d d h cosq ik ik ijk ij ijk ik ij 28 ik
W Erosion and Carbon Deposition on Tungsten C Carbon fluence φ C =1 1 2 m -2 (N=1) E=1eV 3 1 2 m -2 (N=3) 5 1 2 m -2 (N=5) incident energy E=1eV E=1keV 29
D areal density (1 15 cm -2 ) Preparation of hydrogenated carbon layer Crystal W Bombardment with 1 ev C atoms W C 1..8.6.4 E i D/(C+D).1eV (.37) 1 ev (.3) 3 ev (.22) 1 ev (.12) 3 ev (.6) deposition erosion Simultaneous bombardment with.25 ev C and.1-3 ev D atoms D.2-2. -1.5-1. -.5.5 1. 1.5 Depth (nm) An hydrogenated C layer is prepared by bombardment of a small crystal W bcc cell consisting 4 atoms by 1 ev C atoms. The D uptake in the C layer is performed by simultaneous bombardment with C and D atoms. The D impact energy (.1 3 ev) is changed to control the atomic ratio of D/(C+D) in the layer. 3
Emission probability Species reflected from deposited layers Emission probability Emission probability amorphous carbon CH 4 a-c:h W-C mixed CH 4 CH 4 1 CH 4 CH 3 a-c 1 CH 4 a-c:h H/(C+H)=.3 1 CH 4 CH 3 W-C mixed.1 CH 2 CH C.1 CH 3 CH 2 CH C.1 CH 2 CH C (c).1 1 1 1 Incident energy (ev) (d).1 1 1 1 Incident energy (ev) (e).1 1 1 1 Incident energy (ev) @ Both amorphization and hydrogen uptake in the amorphized C reduce emission of small hydrocarbons and carbon. @ Mixing of W with C reduces emission of both CH 4 (<3 ev) and C (>3 ev). 31
Deposition of Be on hydrogenated carbon Areal density (1 15 cm -2 ) Areal density (1 15 cm -2 ) Be coverage (%) Areal density (1 15 cm -2 ) Be coverage % 2% 2. 1.5 1..5 (a) C Number of D or Be 1 3 6 8 2. 1.5 1..5 (b) D C (w/o Be) Number of D 1 3 6 8 5% -2. -1. 1. Depth (nm) 2. C (w/o Be) 1.5 (c) Be -2. -1. 1. Depth (nm) 1 8 (d) 92% 1..5 Number of Be 1 3 6 8 6 4 2-2. -1. 1. Depth (nm) 2 4 6 8 Number of incident Be atoms Be atoms are deposited on a hydrogenated C layer by simultaneous bombardment with 1 ev Be and 1 ev D atoms. The Be deposition grows up with increasing number of incident Be atoms, where incident D atoms are codeposited as well. The percent coverage of Be is increased with increasing number of incidence, up to 92 %. 32
Fraction of bonds Fraction of bonds in hydrogenated carbon Fraction of bonds Fractions of different bonds with D atoms in the top surface at a depth above -.47 nm. 1..8.6.4 (a) D-C bond D-D bond D-Be bond Fractions of different bonds with C atoms in the top surface at a depth above -.47 nm. 1..8.6.4 (b) C-C bond C-D bond C-Be bond.2.2 2 4 6 8 1 Be coverage (%) 2 4 6 8 1 Be coverage (%) The fraction of D-Be bonds increases faster than for C-Be bonds with increasing Be coverage. The fraction of D-C and C-C bonds decreases. Analytic bond-order potentials of W-C-Be-H system was taken from that developed by the Helsinki group [1,2,3]. [1] N.Juslin et al., J.Appl.Phys. 98(25)123. [2] C.Bjorkas et al., J.Phys.:Condens.Matter. 21(29)4452. [3] C.Bjorkas et al., J.Phys.:Condens.Matter. 22(21)35226. Simulation cell was kept to a constant temperature by using Langevin thermostat [4]. [4] H.J.C. Berendsen, J.Chem.Phys. 81(1984)3684. 33
Probability of traveling at a depth Interaction depth in C and Be deposition C areal density (1 15 cm -2 ) Probability that a projectile D atom travels at a depth every 1 fs..25.2.15.1.5.25.2.15.1.5 (a) E i =5 ev (b) Be coverage C % 17% 5% 82% (w/o Be) Be coverage: 5% -2. -1.5-1. -.5.5 1. 1.5 Depth (nm) C (w/o Be) E i 1 ev 1 ev 5 ev 1 ev 2. 1.5 1..5 2. 1.5 1..5 Noncumulative bombardments with 1 D atoms with energies of 1 1 ev are performed and the same initial surface is used for each simulation. Incident atoms hit the top surface at random positions. Incident polar angle is 45 o, whereas the azimuthal angle is randomly selected from o 18 o. Target temperature is changed from 3 K to 12 K. Dominant interaction occurs within a hydrogenated C layer and Be deposition. Interaction layer tends to move from the C layer to the Be deposition layer with increasing Be coverage and decreasing D impact energy. 34
Emission yield Emission yield Emission yield C and CD y emission of hydrogenated carbon C emission yield 1.1.1 C CD CD2 CD3 T=3 K D/(C+D)=.3 (a) At 3 K, dominant emission species are small molecules. Larger molecules (CD 2 and CD 3 ) are emitted with increasing D/(C+D) ratio. C atoms are emitted through physical sputtering mechanism..1 1.1.1.1 1.1.1 T=12 K D/(C+D)=.3 T=8 K D/(C+D)=.3 (b) (c) C CD CD2 CD3 CD4.1 1 1 1 Incident D energy, E i (ev) C CD CD2 CD3 With increasing temperature, CD y emission is strongly enhanced. At 8 K, a maximum value of the emission yield is observed in the energy range of ev, where CD y s are more emitted with decreasing D impact energy. Clearly, D uptake in the C layer induces sputtering of C atoms at energies much less than the threshold energy for physical sputtering..1 1 1 1 Incident D energy, E i (ev) At 12 K, the numbers of emitted C and CD y increase 35 monotonously with increasing D energy. 1.1.1 C T= 3 K T= 8 K T=12 K (d) D/(C+D)= If there is no uptake of D in the layer (D/(C+D)=), hydrocarbon emission is very rare and sputtering of C atoms shows a clear threshold for physical sputtering.
D 2 /(D+D 2 ) D 2 /(D+D 2 ) D and D 2 emission of hydrogenated carbon 1..8.6 (a) D/(C+D)=.3 Molecular fraction shows a clear decrease with increasing temperature. A dip is observed at ~8 K due to the production of CD y s, as found in [5]. [5] J.W.Davis et al., J.Nucl.Mater. 22-222(1995)832..4.2 1..8.6.4.2 Ei=1 ev Ei=3 ev Ei=1 ev Be cov. % Be cov. 17% Be cov. 5% Be cov. 88% Be cov. 92% Ei=3 ev Ei=1 ev E i =1 ev D/(C+D)=.3 4 6 8 1 12 Temperature, T (K) (b) D 2 emission contributes CD 3 formation positively and negatively, depending on the temperature [6]. At relatively low temperature, CD 3 formation is enhanced by it, resulting in the dehydrogenation of the C layer. With further increasing temperature, D atoms recombine with adsorbed D atoms, which interrupts CD 3 formation. [6] J.Roth, J.Nucl.Mater. 266-267(1999)51. Although the D/(Be+D) ratio (~.4) is approximately unchanged, the number of emitted D 2 molecules increases with increasing Be coverage on C layer. 36
C emission yield CD 2 emission yield C and CD 2 emission of Be-deposited carbon 1.1 Be cov. % Be cov. 17% Be cov. 5% C T=8 K D/(C+D)=.3 (a) Emission of C and CD y is clearly suppressed by introducing Be deposition on C layer. The increase in the numbers of D-Be bonds causes C emission yield to be strongly reduced by a small deposition (<17%) of Be..1.1 1.1.1 Be cov. % Be cov. 17% Be cov. 5% CD 2 (b) T=8 K D/(C+D)=.3.1 1 1 1 Incident D energy, E i (ev) Reason for reduction of CD y emission can be explained by increasing numbers of D-Be and C-Be bonds, leading to the enhanced low-energy sputtering of Be and the formation of Be-C compound on the surface. These result in a strong decease in the number of C-D (D-C) bond relating to emission of CD y. Furthermore, the shift of dominant interaction layer from C layer to Be deposits suppress a production of C-D bonding with increasing Be coverage and decreasing D energy, whereas incident D atoms are bound mostly with Be atoms. 37
Be emission yield 1.1.1 Be emission of Be-deposited carbon T=8 K Be cov. 17% Be cov. 5% Be cov. 88% Be cov. 92% (a) When the target temperature is 3 K (or the Be coverage is high), Be sputtering shows a clear energy threshold for physical sputtering. With increasing temperature, sputtering occurs at energies of less than the threshold energy..1 1.1 Be cov. 17% T=3 K T=8 K T=12 K (b) Low-energy sputtering is resulted from a fast increase of the number of D-Be bonds in the vicinity of the surface at the early stage of Be deposition (<2%), as found in cumulative bombardment of Be metal [7]. [7] C.Bjorkas et al., New J.Phys. 11(29)12317..1.1 1 1 1 Incident D energy, E i (ev) However, with further increasing Be deposition, the number of C-Be bonds increase and the tight bonding, the bonding energy (~3.9 ev) of which is much higher than Be-Be bonds (~1 ev), suppress the sputtering. 38
Normalized CD y yield Normalized CD y yield of Be-deposited C Decrease in the emission yield is much faster than an increase in Be coverage on the surface. 1..8.6.4.2 1..8.6.4.2 T=8 K E i =1 ev Ei= Ei= (a) 1 ev 3 ev Ei= 1 ev Ei= 3 ev Ei=1 ev PISCES-B T= 3 K T= 6 K T= 8 K T=1 K T=12 K 2 4 6 8 1 Be coverage (%) (b) This result shows a good correlation with the mitigation of chemical erosion (i.e., the decrease in CD band light emission) of a C target exposed to a Be-seeded plasma in PISCES-B experiments [8]. [8] R.P.Doerner et al., Phys.Scr. T128(27)115. The reduction rate increases monotonically with decreasing D impact energy. This explain the ion energy dependence of decay time of chemical CD light emission observed [9]. [9] D.Nishijima et al., J.Nucl.Mater. 363-365(27)1261. The reduction rate changes in the different manner from the experiments with increasing surface temperature. The calculation indicates a maximum reduction rate at ~8 K where the CD y emission yield peaks. 39
Conclusions (3) Molecular dynamics simulation was performed for C and CD y emission due to D impact of 1 1 ev on hydrogenated C of different D contents and different temperatures. It also concerns the effect of Be deposition on the emission characteristics. (1) At 3 K, dominant emission species are small molecules for low D content whereas when increased the content, larger molecules dominate the species. With increasing temperature, CD y emission is strongly enhanced. At 8 K, a maximum value of the yield is observed in the energy range of ev, where CD 3 molecules are most emitted. (2) Molecular fraction in emission of D decreases with increasing temperature and a dip was observed due to the increased emission of CD y. (3) C and CD y emission is suppressed by introducing Be deposition on C. Reduction rate is much faster than an increase in the percent coverage of Be, showing a good correlation with the observed mitigation of CD light emission above a C target exposed to a Be-seeded plasma in PISCES-B experiments. Reduction rate increasing with decreasing D energy can explain the observed changes in the light emission. 4