Erosion and Deposition in Tokamaks Christian Schulz Institut für Energieforschung - Plasmaphysik Assoziation EURATOM- Forschungszentrum Jülich Trilateral Euregio Cluster

Outline Motivation Plasma wall contact Erosion mechanisms Deposition Amorphous carbon layers Summary/conclusion

Motivation No complete confinement in magnetic fusion devices (drifts, diffusion, ELMs…)  always interaction of plasma particles with wall material, even NECESSARY for removing He-ash (limiter, divertor) But also problematic: Erosion of wall elements  damaging, reduced lifetime Dilution and cooling of plasma by eroded particles Mixing of material  different properties Redeposition of eroded particles together with plasma species  tritium retention (safety limit for radioactivity)

D,C,O, … plasma-wall interaction: physical sputtering / reflection chemical erosion (CD 4 ) deposition from background plasma redeposition of eroded species local particle transport: ionisation, dissoziation friction (Fokker-Planck), thermal force Lorentz force diffusion C, W, … CD 4 C x+,W x+,… CD z 0,+ re-eroded/ reflected particles Surface (C, W, Mo, …) Plasma-Wall Interaction Ions gain energy in the sheath: Ei = 2 kTi + 3ZkTe

Erosion mechanisms  Physical Sputtering  Chemical Erosion  Chemical Sputtering  (Blistering, Radiation Enhanced Sublimation, Melting…)

Introduction: Definition of Erosion Yield Y Do not consider single projectile, but many of them: Erosion Yield Y: Average number of eroded particles per impinging projectile

Physical Sputtering Mechanism: Energy transfer from projectile to atom of the solid at surface D+D+ projectile sputtered particle W W0W0 collision cascade Impinging projectile initiates collision cascade in the solid  energy transfer to surface atom of solid, which can be released.

E = E m cos  2 4 M 1 M 2 (M 1 + M 1 ) 2 E 0 E m = =  E E 0 „kinetic factor“ Projectile energy Energy Transfer in a binary collision M2M2, E T M1M1, E 1 - E T E1 scattering angle Projectile Target  M2M2 M1M1

Physical Sputtering Different regimes of collision cascades Single collision: for light ions of low energies, only one collision before hitting surface atom, binary collision approximation (BCA) valid. Linear cascade: for ions of mean energies (> several 10 eV) collision cascade inside solid is generated including recoils, BCA. Collision spike: for ions of high energies, collisions between simultaneously moving particles, very dense collision cascade.

Physical Sputtering Main features of physical sputtering Occurs for all combinations of projectile – substrate Threshold energy E th. E in < E th : Y sputter = 0, overcome binding energy Sputtering yield Y sputter depends on energy and angle of incidence of projectile Sputtering yield Y sputter depends on projectile – substrate combination. Maximal energy-transfer-factor for head-on collisions:  = 4 M 1 M 2 /(M 1 + M 2 ) 2 No significant dependence on surface temperature Sputtered species: atoms or small clusters of substrate atoms

Physical Sputtering Energy dependence of sputtering yield (TriDyn calculation) E in < E th : Y = 0. Increasing E in  Y increases up to a maximum. Further increase of E in  Y decreases (collision cascade penetrates deeper into solid). E th ~ 3.4 eV

Y at first increases with increasing  in (at grazing incidence: more energy is deposited near to surface). After maximum yield  Y decreases continuously (reflection). Physical Sputtering Angular dependence of sputtering yield (TriDyn calculation)  in

Physical Sputtering Sputtering of rough surfaces At rough surfaces: distribution of local angles of incidence (in dependence on nominal angle of incidence) and redeposition of sputtered particles in “valleys”. Nominal angle of incidence (°) from M. Küstner et al., J. Nucl. Mat. 265 (1999) 22

Physical Sputtering Sputtering of layered systems Reflection of D on W is more effective than D on C  with W substrate below C layer: more D particles are reflected back to surface  larger sputtering yield C layer D+D+ W substrate

Physical Sputtering Sputtering of layered systems: carbon layer on a tungsten substrate Thin carbon layer on tungsten substrate is sputtered more effectively than solid carbon. SDTrimSP calculation projectile: D +

Chemical Erosion CCCCCCCC O+O+ O+O+ CCC CCCC CO2CO2 Definition: Thermal projectiles initiate chemical reactions with surface atoms Formation of volatile products, desorbed from solid in fusion devices mainly carbon and hydrogen isotopes important for chemical reactions

Chemical Erosion Mechanisms of H-atom induced chemical erosion J. Küppers, Surf. Sci. Rep. 22 (1995) 249 i)Hydrogenation of sp 2 to sp 3 hydrocarbon complex via intermediate radical stage sp x ii)Further irradiation: H 2 formation and desorption, sp x radical with broken bond is formed iii)If T surf large enough: chemical erosion of hydrocarbon complex via desorption, basic graphitic sp 2 configuration is formed

Chemical Erosion Main features of chemical erosion Occurs only for special combinations of projectile – substrate (most important in fusion research: hydrogen (isotopes) on graphite, oxygen on graphite) no (or very small) threshold energy strong dependence of erosion yield Y therm on surface temperature dependence of erosion yield Y therm on hydrogen content in solid synergetic effects due to energetic ions (  chemical sputtering) eroded species: molecules of projectile and substrate atoms (C x H y, CO x )

Chemical Erosion Chemical erosion yield in dependence on material properties soft (hydrogen-rich) carbon layers suffer from larger chemical erosion a-C:H amorphous hydrocarbon layer E. Vietzke, J. Nucl. Mat. (1987) 443

Chemical Sputtering Definition:due to ion bombardment a chemical reaction occurs, which produces a weakly bound particle which desorbs into the gas phase. D + (or D 0 +X + ) sputtered hydrocarbon graphite CxDyCxDy thermalised D Formation of C x D y Mechanism: Impinging D + ion penetrates into solid and forms hydrocarbon after thermalisation, which diffuses to surface and desorbs. Yield larger than for chemical erosion: radiation damage effects.

Chemical Sputtering Chemical sputtering yield in dependence on surface temperature maximal sputtering ~950K yield decreases at large T surf TEXTOR Surface temperature dependence of chemical sputtering yield is similar to the one of chemical erosion.

Chemical Sputtering Chemical sputtering yield in dependence on impact energy Calculated acc. to “Roth” formula: J. Roth, J. Nucl. Mat (1999) 51  H = 1·10 22 m -2 s -1 at E in < ~ 2 eV: only thermal erosion active at higher energies: damage-induced effects are dominant

Chemical Sputtering Chemical sputtering yield in dependence on impinging flux Chemical sputtering decreases with increasing flux J. Roth et al. Normalisation: 30 eV T surf (max) Flux dependence is predicted by model of thermal reaction cycle.

Deposition Eroded material will return to the wall (if not pumped out) after migration in the vessel Wether reflection, reaction or some kind of sticking Dependence of sticking on: –number of open bonds of radicals, –on surface state (temperature, soft/hard…), –on impact energy and angle of incidence –…

Tritium Retention Mechanisms of tritium retention Adsorption at surface: - binding of atoms or molecules at surface (Van-der-Waals, sharing of electrons) -saturates, transient due to weak bonding Implantation:- chemical bonding inside solid at location of thermalisation - saturates, permanent due to strong bonding Bulk diffusion:-at higher surface temperature (> ~1000K) -does not saturate, permanent -important for steady-state operation Codeposition:-fuel retention in deposited layers -does not saturate, permanent -dominating the long-term fuel retention

Prompt redeposition of eroded particles Mechanism: Larmor radius: r L = M·v  /Q·B Ionisation length: ion = v  / ion ·n e P prompt = ion /r L Prompt redeposition if P prompt < 1  large mass and large ionisation probability  B ion rLrL C+C+ W+W+ rLrL C0C0 W0W0 wall element

physical & chemical erosion/sputtering: Y ~ 1 - 3% re-deposited carbon layers can be effectively re-eroded Y ~ % D CxDyCxDy D CXDyCXDy CxDy+CxDy+ shadowed areas deposition of hydrogen- rich (~T) layers Tritium Retention Tritium retention by means of codeposition Long range migration possible

Erosion & deposition: flux balance Exposure of tungsten limiter to edge plasma plasma radius ● Particle flux  : D ions, C ions ● Plasma temperature (~ impact energy of D, C) decreases with increasing plasma radius   Tungsten test limiter after exposure to TEXTOR net erosion net deposition Depending on local balance of erosion and deposition flux: net-erosion or net-deposition

Amorphous carbon layers amorphous carbon enriched with hydrogen increasing thickness, no saturation soft (0,8 g/cm³) and hard films (2g/cm³) hydrogen-content 15-67%

Summary/Conclusion Material erosion, migration and deposition is a crucial issue for steady-state operation of fusion devices Choice of optimal configuration of wall materials Reduction of erosion for longer lifetime Develop efficient cleaning methods for deposit layers, especially in shadowed areas to reduce tritium retention

Conclusions Wall materials for ITER 700 m 2 Beryllium first wall - low Z (less critical for plasma), - oxygen getter 100 m 2 Tungsten baffles, dome - high Z (low physical sputtering) 50 m 2 Graphite CFC target plates - no melting, - but chemical erosion/sputtering (tritium retention via codeposition) ITER

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Sticking of Hydrocarbon Species Interaction of hydrocarbon radicals with a solid surface Reflection (R) CxHyCxHy CxHyCxHy CxHyCxHy CmHnCmHn Surface Reaction ⇒ Desorption (  ) Sticking (S) CxHyCxHy Surface Loss Probability:  =  + S ⇒  is upper limit for S S +  + R = 1

Plasma-Wall Interaction particle cycle Divertor / Limiter 10 keV 1 eV – 1 keV flow recycling cx Wall energy exhaust radiation convection heat load ions, electrons anomalous transport Erosion Ionisation hydrogen, impurities Ions gain energy in the sheath: Ei = 2 kTi + 3ZkTe -> enhanced physical sputtering Sheath potential:

Chemical Erosion Definition: Thermal projectiles initiate chemical reactions with surface atoms. Important in fusion research: bombardment of C surface with H atoms 2s-level can hybridise with the 3 2p-levels to form 4 sp 3 orbitals ⇒ diamond only 2 2p-levels involved in hybridisation: 3 sp 2 orbitals ⇒ graphite mixture of sp 2 and sp 3 ⇒ amorphous a-C:H Carbon: 6 electrons mspin 1s0  2s0  2p-1,0,1  sp 3 sp 2 Some basic properties of carbon:

Chemical Sputtering Chemical sputtering yield